Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

NOVEMBER 5 2014

Line HarmonicCurrents ndashAdjustableFrequencyControl Created

OCTOBER 13 2014

14 energy-efficiencyimprovementopportunities inpumpingsystems

SEPTEMBER 29 2014

8 Energy-EfficiencyImprovementOpportunitiesIn ElectricMotors

JUNE 18 2014

A Few WordsAbout SelectingAC InductionMotors forCement PlantApplications

Article Tags

coil contactor current direct on line dolmotor overload overload relay startertorque

Filed Under Category

Maintenance Motor

RSS Feed for Comments

N O V 04 2014Prafull Patel

S E P 29 2014

Rahul Kumar

S E P 24 2014

HardikSolanki

S E P 25 2014

E AFernandez

laquo Older Comments

RSS Feed for Comments

SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

FOLLOW EEP

Subscribe to Weekly Download Updates (free electrical softw are spreadsheets and EE guides)

Latest Technical Articles

N O VEM BE R 14 2014

7 Practical Tips ForInstalling a GoodMeasuring System

N O VEM BE R 12 2014

Characteristics of XLPEinsulated cables withreference to the UKstandards

N O VEM BE R 10 2014

34 Questions AndAnswers To Break theMyth About SF6 Gas InElectrical Equipment

Daily dose of knowledge and news fromElectrical Engineering World

ElectricalEngineeringDaily Dose

SUBSCRIBE

Science at Glance

Our mission is to be the leading provider ofscientific information in the field of power andengineering in general We publish we shareand we spread the knowledge

Youre welcome to read write and contributeto EEP in any way

Energy Efficency

We see the results of power usage all aroundus Every day the generation and usage ofenergy produces more pollution than any othersingle industry

Its our time to use energy on smart and moreefficent way EEP is on your side

Electrical Software

Electrical design without appropriate softwareand tools is impossible to imagine This sectioncovers professional engineering software fordesign MV and LV network and photometriccalculations voltage drop and short circuitcalculations etc

copy 2014 EEP - Electrical Engineering Portal All Rights Reserved | Privacy Policy | 32 queries in 0216 seconds Powered by CsanyiGroup

Home Technical Articles Electrical Software amp Guides EE Lectures Connect Google Custom Search

SHARE

GetPDF

TOP

Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

NOVEMBER 5 2014

Line HarmonicCurrents ndashAdjustableFrequencyControl Created

OCTOBER 13 2014

14 energy-efficiencyimprovementopportunities inpumpingsystems

SEPTEMBER 29 2014

8 Energy-EfficiencyImprovementOpportunitiesIn ElectricMotors

JUNE 18 2014

A Few WordsAbout SelectingAC InductionMotors forCement PlantApplications

Article Tags

coil contactor current direct on line dolmotor overload overload relay startertorque

Filed Under Category

Maintenance Motor

RSS Feed for Comments

N O V 04 2014Prafull Patel

S E P 29 2014

Rahul Kumar

S E P 24 2014

HardikSolanki

S E P 25 2014

E AFernandez

laquo Older Comments

RSS Feed for Comments

SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

FOLLOW EEP

Subscribe to Weekly Download Updates (free electrical softw are spreadsheets and EE guides)

Latest Technical Articles

N O VEM BE R 14 2014

7 Practical Tips ForInstalling a GoodMeasuring System

N O VEM BE R 12 2014

Characteristics of XLPEinsulated cables withreference to the UKstandards

N O VEM BE R 10 2014

34 Questions AndAnswers To Break theMyth About SF6 Gas InElectrical Equipment

Daily dose of knowledge and news fromElectrical Engineering World

ElectricalEngineeringDaily Dose

SUBSCRIBE

Science at Glance

Our mission is to be the leading provider ofscientific information in the field of power andengineering in general We publish we shareand we spread the knowledge

Youre welcome to read write and contributeto EEP in any way

Energy Efficency

We see the results of power usage all aroundus Every day the generation and usage ofenergy produces more pollution than any othersingle industry

Its our time to use energy on smart and moreefficent way EEP is on your side

Electrical Software

Electrical design without appropriate softwareand tools is impossible to imagine This sectioncovers professional engineering software fordesign MV and LV network and photometriccalculations voltage drop and short circuitcalculations etc

copy 2014 EEP - Electrical Engineering Portal All Rights Reserved | Privacy Policy | 32 queries in 0216 seconds Powered by CsanyiGroup

Home Technical Articles Electrical Software amp Guides EE Lectures Connect Google Custom Search

SHARE

GetPDF

TOP

Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

NOVEMBER 5 2014

Line HarmonicCurrents ndashAdjustableFrequencyControl Created

OCTOBER 13 2014

14 energy-efficiencyimprovementopportunities inpumpingsystems

SEPTEMBER 29 2014

8 Energy-EfficiencyImprovementOpportunitiesIn ElectricMotors

JUNE 18 2014

A Few WordsAbout SelectingAC InductionMotors forCement PlantApplications

Article Tags

coil contactor current direct on line dolmotor overload overload relay startertorque

Filed Under Category

Maintenance Motor

RSS Feed for Comments

N O V 04 2014Prafull Patel

S E P 29 2014

Rahul Kumar

S E P 24 2014

HardikSolanki

S E P 25 2014

E AFernandez

laquo Older Comments

RSS Feed for Comments

SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

FOLLOW EEP

Subscribe to Weekly Download Updates (free electrical softw are spreadsheets and EE guides)

Latest Technical Articles

N O VEM BE R 14 2014

7 Practical Tips ForInstalling a GoodMeasuring System

N O VEM BE R 12 2014

Characteristics of XLPEinsulated cables withreference to the UKstandards

N O VEM BE R 10 2014

34 Questions AndAnswers To Break theMyth About SF6 Gas InElectrical Equipment

Daily dose of knowledge and news fromElectrical Engineering World

ElectricalEngineeringDaily Dose

SUBSCRIBE

Science at Glance

Our mission is to be the leading provider ofscientific information in the field of power andengineering in general We publish we shareand we spread the knowledge

Youre welcome to read write and contributeto EEP in any way

Energy Efficency

We see the results of power usage all aroundus Every day the generation and usage ofenergy produces more pollution than any othersingle industry

Its our time to use energy on smart and moreefficent way EEP is on your side

Electrical Software

Electrical design without appropriate softwareand tools is impossible to imagine This sectioncovers professional engineering software fordesign MV and LV network and photometriccalculations voltage drop and short circuitcalculations etc

copy 2014 EEP - Electrical Engineering Portal All Rights Reserved | Privacy Policy | 32 queries in 0216 seconds Powered by CsanyiGroup

Home Technical Articles Electrical Software amp Guides EE Lectures Connect Google Custom Search

SHARE

GetPDF

TOP

Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

NOVEMBER 5 2014

Line HarmonicCurrents ndashAdjustableFrequencyControl Created

OCTOBER 13 2014

14 energy-efficiencyimprovementopportunities inpumpingsystems

SEPTEMBER 29 2014

8 Energy-EfficiencyImprovementOpportunitiesIn ElectricMotors

JUNE 18 2014

A Few WordsAbout SelectingAC InductionMotors forCement PlantApplications

Article Tags

coil contactor current direct on line dolmotor overload overload relay startertorque

Filed Under Category

Maintenance Motor

RSS Feed for Comments

N O V 04 2014Prafull Patel

S E P 29 2014

Rahul Kumar

S E P 24 2014

HardikSolanki

S E P 25 2014

E AFernandez

laquo Older Comments

RSS Feed for Comments

SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

FOLLOW EEP

Subscribe to Weekly Download Updates (free electrical softw are spreadsheets and EE guides)

Latest Technical Articles

N O VEM BE R 14 2014

7 Practical Tips ForInstalling a GoodMeasuring System

N O VEM BE R 12 2014

Characteristics of XLPEinsulated cables withreference to the UKstandards

N O VEM BE R 10 2014

34 Questions AndAnswers To Break theMyth About SF6 Gas InElectrical Equipment

Daily dose of knowledge and news fromElectrical Engineering World

ElectricalEngineeringDaily Dose

SUBSCRIBE

Science at Glance

Our mission is to be the leading provider ofscientific information in the field of power andengineering in general We publish we shareand we spread the knowledge

Youre welcome to read write and contributeto EEP in any way

Energy Efficency

We see the results of power usage all aroundus Every day the generation and usage ofenergy produces more pollution than any othersingle industry

Its our time to use energy on smart and moreefficent way EEP is on your side

Electrical Software

Electrical design without appropriate softwareand tools is impossible to imagine This sectioncovers professional engineering software fordesign MV and LV network and photometriccalculations voltage drop and short circuitcalculations etc

copy 2014 EEP - Electrical Engineering Portal All Rights Reserved | Privacy Policy | 32 queries in 0216 seconds Powered by CsanyiGroup

Home Technical Articles Electrical Software amp Guides EE Lectures Connect Google Custom Search

SHARE

GetPDF

TOP

Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

NOVEMBER 5 2014

Line HarmonicCurrents ndashAdjustableFrequencyControl Created

OCTOBER 13 2014

14 energy-efficiencyimprovementopportunities inpumpingsystems

SEPTEMBER 29 2014

8 Energy-EfficiencyImprovementOpportunitiesIn ElectricMotors

JUNE 18 2014

A Few WordsAbout SelectingAC InductionMotors forCement PlantApplications

Article Tags

coil contactor current direct on line dolmotor overload overload relay startertorque

Filed Under Category

Maintenance Motor

RSS Feed for Comments

N O V 04 2014Prafull Patel

S E P 29 2014

Rahul Kumar

S E P 24 2014

HardikSolanki

S E P 25 2014

E AFernandez

laquo Older Comments

RSS Feed for Comments

SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

FOLLOW EEP

Subscribe to Weekly Download Updates (free electrical softw are spreadsheets and EE guides)

Latest Technical Articles

N O VEM BE R 14 2014

7 Practical Tips ForInstalling a GoodMeasuring System

N O VEM BE R 12 2014

Characteristics of XLPEinsulated cables withreference to the UKstandards

N O VEM BE R 10 2014

34 Questions AndAnswers To Break theMyth About SF6 Gas InElectrical Equipment

Daily dose of knowledge and news fromElectrical Engineering World

ElectricalEngineeringDaily Dose

SUBSCRIBE

Science at Glance

Our mission is to be the leading provider ofscientific information in the field of power andengineering in general We publish we shareand we spread the knowledge

Youre welcome to read write and contributeto EEP in any way

Energy Efficency

We see the results of power usage all aroundus Every day the generation and usage ofenergy produces more pollution than any othersingle industry

Its our time to use energy on smart and moreefficent way EEP is on your side

Electrical Software

Electrical design without appropriate softwareand tools is impossible to imagine This sectioncovers professional engineering software fordesign MV and LV network and photometriccalculations voltage drop and short circuitcalculations etc

copy 2014 EEP - Electrical Engineering Portal All Rights Reserved | Privacy Policy | 32 queries in 0216 seconds Powered by CsanyiGroup

Home Technical Articles Electrical Software amp Guides EE Lectures Connect Google Custom Search

SHARE

GetPDF

TOP

Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

NOVEMBER 5 2014

Line HarmonicCurrents ndashAdjustableFrequencyControl Created

OCTOBER 13 2014

14 energy-efficiencyimprovementopportunities inpumpingsystems

SEPTEMBER 29 2014

8 Energy-EfficiencyImprovementOpportunitiesIn ElectricMotors

JUNE 18 2014

A Few WordsAbout SelectingAC InductionMotors forCement PlantApplications

Article Tags

coil contactor current direct on line dolmotor overload overload relay startertorque

Filed Under Category

Maintenance Motor

RSS Feed for Comments

N O V 04 2014Prafull Patel

S E P 29 2014

Rahul Kumar

S E P 24 2014

HardikSolanki

S E P 25 2014

E AFernandez

laquo Older Comments

RSS Feed for Comments

SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

FOLLOW EEP

Subscribe to Weekly Download Updates (free electrical softw are spreadsheets and EE guides)

Latest Technical Articles

N O VEM BE R 14 2014

7 Practical Tips ForInstalling a GoodMeasuring System

N O VEM BE R 12 2014

Characteristics of XLPEinsulated cables withreference to the UKstandards

N O VEM BE R 10 2014

34 Questions AndAnswers To Break theMyth About SF6 Gas InElectrical Equipment

Daily dose of knowledge and news fromElectrical Engineering World

ElectricalEngineeringDaily Dose

SUBSCRIBE

Science at Glance

Our mission is to be the leading provider ofscientific information in the field of power andengineering in general We publish we shareand we spread the knowledge

Youre welcome to read write and contributeto EEP in any way

Energy Efficency

We see the results of power usage all aroundus Every day the generation and usage ofenergy produces more pollution than any othersingle industry

Its our time to use energy on smart and moreefficent way EEP is on your side

Electrical Software

Electrical design without appropriate softwareand tools is impossible to imagine This sectioncovers professional engineering software fordesign MV and LV network and photometriccalculations voltage drop and short circuitcalculations etc

copy 2014 EEP - Electrical Engineering Portal All Rights Reserved | Privacy Policy | 32 queries in 0216 seconds Powered by CsanyiGroup

Home Technical Articles Electrical Software amp Guides EE Lectures Connect Google Custom Search

SHARE

GetPDF

TOP

Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

NOVEMBER 5 2014

Line HarmonicCurrents ndashAdjustableFrequencyControl Created

OCTOBER 13 2014

14 energy-efficiencyimprovementopportunities inpumpingsystems

SEPTEMBER 29 2014

8 Energy-EfficiencyImprovementOpportunitiesIn ElectricMotors

JUNE 18 2014

A Few WordsAbout SelectingAC InductionMotors forCement PlantApplications

Article Tags

coil contactor current direct on line dolmotor overload overload relay startertorque

Filed Under Category

Maintenance Motor

RSS Feed for Comments

N O V 04 2014Prafull Patel

S E P 29 2014

Rahul Kumar

S E P 24 2014

HardikSolanki

S E P 25 2014

E AFernandez

laquo Older Comments

RSS Feed for Comments

SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

FOLLOW EEP

Subscribe to Weekly Download Updates (free electrical softw are spreadsheets and EE guides)

Latest Technical Articles

N O VEM BE R 14 2014

7 Practical Tips ForInstalling a GoodMeasuring System

N O VEM BE R 12 2014

Characteristics of XLPEinsulated cables withreference to the UKstandards

N O VEM BE R 10 2014

34 Questions AndAnswers To Break theMyth About SF6 Gas InElectrical Equipment

Daily dose of knowledge and news fromElectrical Engineering World

ElectricalEngineeringDaily Dose

SUBSCRIBE

Science at Glance

Our mission is to be the leading provider ofscientific information in the field of power andengineering in general We publish we shareand we spread the knowledge

Youre welcome to read write and contributeto EEP in any way

Energy Efficency

We see the results of power usage all aroundus Every day the generation and usage ofenergy produces more pollution than any othersingle industry

Its our time to use energy on smart and moreefficent way EEP is on your side

Electrical Software

Electrical design without appropriate softwareand tools is impossible to imagine This sectioncovers professional engineering software fordesign MV and LV network and photometriccalculations voltage drop and short circuitcalculations etc

copy 2014 EEP - Electrical Engineering Portal All Rights Reserved | Privacy Policy | 32 queries in 0216 seconds Powered by CsanyiGroup

Home Technical Articles Electrical Software amp Guides EE Lectures Connect Google Custom Search

SHARE

GetPDF

TOP

Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

NOVEMBER 5 2014

Line HarmonicCurrents ndashAdjustableFrequencyControl Created

OCTOBER 13 2014

14 energy-efficiencyimprovementopportunities inpumpingsystems

SEPTEMBER 29 2014

8 Energy-EfficiencyImprovementOpportunitiesIn ElectricMotors

JUNE 18 2014

A Few WordsAbout SelectingAC InductionMotors forCement PlantApplications

Article Tags

coil contactor current direct on line dolmotor overload overload relay startertorque

Filed Under Category

Maintenance Motor

RSS Feed for Comments

N O V 04 2014Prafull Patel

S E P 29 2014

Rahul Kumar

S E P 24 2014

HardikSolanki

S E P 25 2014

E AFernandez

laquo Older Comments

RSS Feed for Comments

SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

FOLLOW EEP

Subscribe to Weekly Download Updates (free electrical softw are spreadsheets and EE guides)

Latest Technical Articles

N O VEM BE R 14 2014

7 Practical Tips ForInstalling a GoodMeasuring System

N O VEM BE R 12 2014

Characteristics of XLPEinsulated cables withreference to the UKstandards

N O VEM BE R 10 2014

34 Questions AndAnswers To Break theMyth About SF6 Gas InElectrical Equipment

Daily dose of knowledge and news fromElectrical Engineering World

ElectricalEngineeringDaily Dose

SUBSCRIBE

Science at Glance

Our mission is to be the leading provider ofscientific information in the field of power andengineering in general We publish we shareand we spread the knowledge

Youre welcome to read write and contributeto EEP in any way

Energy Efficency

We see the results of power usage all aroundus Every day the generation and usage ofenergy produces more pollution than any othersingle industry

Its our time to use energy on smart and moreefficent way EEP is on your side

Electrical Software

Electrical design without appropriate softwareand tools is impossible to imagine This sectioncovers professional engineering software fordesign MV and LV network and photometriccalculations voltage drop and short circuitcalculations etc

copy 2014 EEP - Electrical Engineering Portal All Rights Reserved | Privacy Policy | 32 queries in 0216 seconds Powered by CsanyiGroup

Home Technical Articles Electrical Software amp Guides EE Lectures Connect Google Custom Search

SHARE

GetPDF

TOP

Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

NOVEMBER 5 2014

Line HarmonicCurrents ndashAdjustableFrequencyControl Created

OCTOBER 13 2014

14 energy-efficiencyimprovementopportunities inpumpingsystems

SEPTEMBER 29 2014

8 Energy-EfficiencyImprovementOpportunitiesIn ElectricMotors

JUNE 18 2014

A Few WordsAbout SelectingAC InductionMotors forCement PlantApplications

Article Tags

coil contactor current direct on line dolmotor overload overload relay startertorque

Filed Under Category

Maintenance Motor

RSS Feed for Comments

N O V 04 2014Prafull Patel

S E P 29 2014

Rahul Kumar

S E P 24 2014

HardikSolanki

S E P 25 2014

E AFernandez

laquo Older Comments

RSS Feed for Comments

SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

FOLLOW EEP

Subscribe to Weekly Download Updates (free electrical softw are spreadsheets and EE guides)

Latest Technical Articles

N O VEM BE R 14 2014

7 Practical Tips ForInstalling a GoodMeasuring System

N O VEM BE R 12 2014

Characteristics of XLPEinsulated cables withreference to the UKstandards

N O VEM BE R 10 2014

34 Questions AndAnswers To Break theMyth About SF6 Gas InElectrical Equipment

Daily dose of knowledge and news fromElectrical Engineering World

ElectricalEngineeringDaily Dose

SUBSCRIBE

Science at Glance

Our mission is to be the leading provider ofscientific information in the field of power andengineering in general We publish we shareand we spread the knowledge

Youre welcome to read write and contributeto EEP in any way

Energy Efficency

We see the results of power usage all aroundus Every day the generation and usage ofenergy produces more pollution than any othersingle industry

Its our time to use energy on smart and moreefficent way EEP is on your side

Electrical Software

Electrical design without appropriate softwareand tools is impossible to imagine This sectioncovers professional engineering software fordesign MV and LV network and photometriccalculations voltage drop and short circuitcalculations etc

copy 2014 EEP - Electrical Engineering Portal All Rights Reserved | Privacy Policy | 32 queries in 0216 seconds Powered by CsanyiGroup

Home Technical Articles Electrical Software amp Guides EE Lectures Connect Google Custom Search

SHARE

GetPDF

TOP

Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

NOVEMBER 5 2014

Line HarmonicCurrents ndashAdjustableFrequencyControl Created

OCTOBER 13 2014

14 energy-efficiencyimprovementopportunities inpumpingsystems

SEPTEMBER 29 2014

8 Energy-EfficiencyImprovementOpportunitiesIn ElectricMotors

JUNE 18 2014

A Few WordsAbout SelectingAC InductionMotors forCement PlantApplications

Article Tags

coil contactor current direct on line dolmotor overload overload relay startertorque

Filed Under Category

Maintenance Motor

RSS Feed for Comments

N O V 04 2014Prafull Patel

S E P 29 2014

Rahul Kumar

S E P 24 2014

HardikSolanki

S E P 25 2014

E AFernandez

laquo Older Comments

RSS Feed for Comments

SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

FOLLOW EEP

Subscribe to Weekly Download Updates (free electrical softw are spreadsheets and EE guides)

Latest Technical Articles

N O VEM BE R 14 2014

7 Practical Tips ForInstalling a GoodMeasuring System

N O VEM BE R 12 2014

Characteristics of XLPEinsulated cables withreference to the UKstandards

N O VEM BE R 10 2014

34 Questions AndAnswers To Break theMyth About SF6 Gas InElectrical Equipment

Daily dose of knowledge and news fromElectrical Engineering World

ElectricalEngineeringDaily Dose

SUBSCRIBE

Science at Glance

Our mission is to be the leading provider ofscientific information in the field of power andengineering in general We publish we shareand we spread the knowledge

Youre welcome to read write and contributeto EEP in any way

Energy Efficency

We see the results of power usage all aroundus Every day the generation and usage ofenergy produces more pollution than any othersingle industry

Its our time to use energy on smart and moreefficent way EEP is on your side

Electrical Software

Electrical design without appropriate softwareand tools is impossible to imagine This sectioncovers professional engineering software fordesign MV and LV network and photometriccalculations voltage drop and short circuitcalculations etc

copy 2014 EEP - Electrical Engineering Portal All Rights Reserved | Privacy Policy | 32 queries in 0216 seconds Powered by CsanyiGroup

Home Technical Articles Electrical Software amp Guides EE Lectures Connect Google Custom Search

SHARE

GetPDF

TOP

Home Technical Articles Article Categories Contact us Subscribe to articles Subscribe to downloads El Enginering Guidesl EE General guidesl Power Substationsl Schneider Electric CTsl Siemens Basics of EEl ABB Drives Guidesl Industry Automationl Relay controlprotectionl Alternative Energy

Electrical Software MS Excel Spreadsheets Electrical Design Docs Engineering Resourcesl Video Lectures ndash Electrical Engineeringl PLC Programming Trainingl Electric Testing and Maintenance (VIDEO)l Network Theorems and Lawsl -

Home Tec hn ical A r tic les D ir ec t O n L ine (DO L) M o to r S tar ter

P o sted A PR 22 2012 by JIG UP A RMA R in MA INTEN A N C E MO TO R w ith 30 C O MM EN TS

Recommended EE articles

About Author

Jignesh Parmarjiguparmar - Jignesh Parmar has completed his BE(Electrical) fromGujarat University He is member of Institution of Engineers(MIE)India Membership NoM-1473586He has more than 12 yearsexperience in Transmission -Distribution-Electrical Energy theftdetection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution) He is Presently

associate with one of the leading business group as a Assistant Manager at AhmedabadIndiaHe has published numbers of Technical Articles in Electrical Mirror Electrical India LightingIndia Industrial Electrix(Australian Power Publications) Magazines He is FreelancerProgrammer of Advance Excel and design useful Excel base Electrical Programs as per IS NECIECIEEE codes He is Technical Blogger and Familiar with English Hindi Gujarati Frenchlanguages He wants to Share his experience amp Knowledge and help technical enthusiasts tofind suitable solutions and updating themselves on various Engineering Topics

30 Comments

Leave a Comment

Tell us what youre thinking we care about your opinionand oh not to forget - if you want a picture to show with your comment go get a free Gravatar

NA ME

EMA IL

WEBSITE

WE BELIEV E Y OURE A N ENGINEER BUT NEV ERTHELESS

six minus = 1

SIGN ME UP FOR THE NEWSLETTER

Direct On Line (DOL) Motor Starter

Different starting methods are employedfor starting induction motors becauseInduction Motor draws more startingcurrent during starting To preventdamage to the windings due to the highstarting current flow we employ differenttypes of starters

The simplest form of motor starter for theinduction motor is the Direct On Linestarter The Direct On Line Motor Starter(DOL) consist a MCCB or Circuit BreakerContactor and an overload relay forprotection Electromagnetic contactorwhich can be opened by the thermal

overload relay under fault conditions

Typically the contactor will be controlled by separate start and stop buttons and an auxiliarycontact on the contactor is used across the start button as a hold in contact Ie the contactoris electrically latched closed while the motor is operating

Principle of Direct On Line Starter (DOL)To start the contactor is closed applying full line voltage to the motor windings The motor willdraw a very high inrush current for a very short time the magnetic field in the iron and then thecurrent will be limited to the Locked Rotor Current of the motor The motor will develop LockedRotor Torque and begin to accelerate towards full speed

As the motor accelerates the current will begin to drop but will not drop significantly until themotor is at a high speed typically about 85 of synchronous speed The actual starting currentcurve is a function of the motor design and the terminal voltage and is totally independent ofthe motor load

The motor load will affect the time taken for the motor to accelerate to full speed and thereforethe duration of the high starting current but not the magnitude of the starting current

Provided the torque developed by the motor exceeds the load torque at all speeds during thestart cycle the motor will reach full speed If the torque delivered by the motor is less than thetorque of the load at any speed during the start cycle the motor will stops accelerating If thestarting torque with a DOL starter is insufficient for the load the motor must be replaced with amotor which can develop a higher starting torque

The acceleration torque is the torque developed by the motor minus the load torque and willchange as the motor accelerates due to the motor speed torque curve and the load speedtorque curve The start time is dependent on the acceleration torque and the load inertia

DOL starting have a maximum start current and maximum start torque

This may cause an electrical problem with the supply or it may cause a mechanical problemwith the driven load So this will be inconvenient for the users of the supply line alwaysexperience a voltage drop when starting a motor But if this motor is not a high power one itdoes not affect much

Parts of DOL Starters

Contactors amp Coil

Magnetic contactors are electromagnetically operated switchesthat provide a safe and convenient means for connecting andinterrupting branch circuits

Magnetic motor controllers use electromagnetic energy for closingswitches The electromagnet consists of a coil of wire placed on aniron core When a current flow through the coil the iron of themagnet becomes magnetized attracting an iron bar called thearmature An interruption of the current flow through the coil of wirecauses the armature to drop out due to the presence of an air gapin the magnetic circuit

Line-voltage magnetic motor starters are electromechanical devices that provide a safeconvenient and economical means of starting and stopping motors and have the advantage ofbeing controlled remotely The great bulk of motor controllers sold are of this type

Contactors are mainly used to control machinery which uses electric motors It consists of a coilwhich connects to a voltage source Very often for Single phase Motors 230V coils are usedand for three phase motors 415V coils are used The contactor has three main NO contactsand lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the controlcircuit A contact is conducting metal parts which completes or interrupt an electrical circuit

l NO-normally openl NC-normally closed

Over Load Relay (Overload protection)

Overload protection for an electric motor is necessary to prevent burnout and to ensuremaximum operating life

Under any condition of overload a motor draws excessive current that causes overheatingSince motor winding insulation deteriorates due to overheating there are established limits onmotor operating temperatures to protect a motor from overheating Overload relays areemployed on a motor control to limit the amount of current drawn

The overload relay does not provide short circuit protection This is thefunction of over current protective equipment like fuses and circuit breakersgenerally located in the disconnecting switch enclosure

The ideal and easiest way for overload protection for a motor is an element with current-sensingproperties very similar to the heating curve of the motor which would act to open the motorcircuit when full-load current is exceeded The operation of the protective device should be suchthat the motor is allowed to carry harmless over-loads but is quickly removed from the line whenan overload has persisted too long

Normally fuses are not designed to provide overloadprotection Fuse is protecting against short circuits (overcurrent protection) Motors draw a high inrush current whenstarting and conventional fuses have no way ofdistinguishing between this temporary and harmless inrushcurrent and a damaging overload Selection of Fuse isdepend on motor full-load current would ldquoblowrdquo every timethe motor is started On the other hand if a fuse werechosen large enough to pass the starting or inrush currentit would not protect the motor against small harmfuloverloads that might occur later

The overload relay is the heart of motor protection It has inverse-trip-time characteristicspermitting it to hold in during the accelerating period (when inrush current is drawn) yetproviding protection on small overloads above the full-load current when the motor is runningOverload relays are renewable and can withstand repeated trip and reset cycles without need ofreplacement Overload relays cannot however take the place of over current protectionequipment

The overload relay consists of a current-sensing unit connected in the line to the motor plus amechanism actuated by the sensing unit which serves directly or indirectly to break the circuit

Overload relays can be classified as being thermal magnetic or electronic

1 Thermal Relay As the name implies thermal overload relays rely on therising temperatures caused by the overload current to trip the overloadmechanism Thermal overload relays can be further subdivided into two typesmelting alloy and bimetallic

2 Magnetic Relay Magnetic overload relays react only to current excessesand are not affected by temperature

3 Electronic Relay Electronic or solid-state overload relays provide thecombination of high-speed trip adjustability and ease of installation Theycan be ideal in many precise applications

Wiring of DOL Starter

1 Main Contact

l Contactor is connecting among Supply Voltage Relay Coil and Thermal Overload Relayl L1 of Contactor Connect (NO) to R Phase through MCCBl L2 of Contactor Connect (NO) to Y Phase through MCCBl L3 of Contactor Connect (NO) to B Phase through MCCB

NO Contact (-||-)

l (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)l Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is

connected to Common wire of StartStop Button

NC Contact (-||-)

l (95-96) is a normally closed NC contact (opens when the thermal overloads trip ifassociated with the overload block)

2 Relay Coil Connection

l A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermalover Load Relayrsquos NC Connection (95)

3 Thermal Overload Relay Connection

l T1T2T3 are connect to Thermal Overload Relayl Overload Relay is Connecting between Main Contactor and Motorl NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and

Common Connection of StartStop Button

Wiring Diagram of DOL Starter

Working principle of DOL StarterThe main heart of DOL starter is Relay Coil Normally it gets one phase constant from incomingsupply Voltage (A1)when Coil gets second Phase relay coil energizes and Magnet of Contactorproduce electromagnetic field and due to this Plunger of Contactor will move and MainContactor of starter will closed and Auxiliary will change its position NO become NC and NCbecome (shown Red Line in Diagram)

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Maincontactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2)Now Coilenergizes and Magnetic field produce by Magnet and Plunger of Contactor move MainContactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) fromNO to NC

Release Start Button

Relay coil gets supply even though we release Start button When We release Start PushButton Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) ndash Auxiliarycontactor (54)-Stop Button-96-95-Relay coil (shown Red Blue Lines in Diagram)

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply ofRelay coil is broken Plunger moves and close contact of Main Contactor becomes OpenSupply of Motor is disconnected

Motor Starting Characteristics on DOL Starterl Available starting current 100l Peak starting current 6 to 8 Full Load Currentl Peak starting torque 100

Advantages of DOL Starter1 Most Economical and Cheapest Starter2 Simple to establish operate and maintain3 Simple Control Circuitry4 Easy to understand and trouble‐shoot5 It provides 100 torque at the time of starting6 Only one set of cable is required from starter to motor7 Motor is connected in delta at motor terminals

Disadvantages of DOL Starter1 It does not reduce the starting current of the motor2 High Starting Current Very High Starting Current (Typically 6 to 8 times the FLC of the

motor)3 Mechanically Harsh Thermal Stress on the motor thereby reducing its life4 Voltage Dip There is a big voltage dip in the electrical installation because of high in-

rush current affecting other customers connected to the same lines and therefore notsuitable for higher size squirrel cage motors

5 High starting Torque Unnecessary high starting torque even when not required by theload thereby increased mechanical stress on the mechanical systems such as rotor shaftbearings gearbox coupling chain drive connected equipments etc leading topremature failure and plant downtimes

Features of DOL startingl For low- and medium-power three-phase motorsl Three connection lines (circuit layout star or delta)l High starting torquel Very high mechanical loadl High current peaksl Voltage dipsl Simple switching devices

Direct On Line Motor Starter (DOL) is suitable for

l A direct on line starter can be used if the high inrush current of the motor does not causeexcessive voltage drop in the supply circuit The maximum size of a motor allowed on adirect on line starter may be limited by the supply utility for this reason For example autility may require rural customers to use reduced-voltage starters for motors larger than10 kW

l DOL starting is sometimes used to start small water pumps compressors fans andconveyor belts

Direct On Line Motor Starter (DOL) is NOT suitable for

l The peak starting current would result in a serious voltage drop on the supply systeml The equipment being driven cannot tolerate the effects of very high peak torque loadingsl The safety or comfort of those using the equipment may be compromised by sudden

starting as for example with escalators and lifts

Direct Online Motor Starter - Square D

DOL part -Contactor

DOL part - Termal Overload Relay

Direct On Line Starter - Wiring Diagram

DOL - Wiring scheme

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Article Tags

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SUBMIT COMMENT

Respected sir

how select fuse for 3 phase motor

(reply)

sir please tell me about the sizing of overload relay(ie how to selectoverload relay for a motor of specific capacity)

(reply)

Dear sirwe are drilling machine manufacturer I need help we have fitted 3-Phase15HP motor in machine controlled with ReverseForward Switch now wewish to add Emergency Push Button Stop where should I give connectionfor it what other components we require for it (someone suggested mepushbuttton with NC connector- but the NC has only 2 Nodes as 3-phasemust have 3 nodes in it) kindly help meThanks in advance

(reply)

Just connect a mushroom-type industrial grade emergency stop button inseries with the stop push in the control circuit of the motor Pleaseconsult a certified technician engineer for its proper installation andadherance to all safety normsI guess the esteemed author of this page will also support my suggestion

(reply)

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