Electrical Drive Systems 324Industrial Motor Control

Dr. P.J Randewijk

Stellenbosch UniversityDep. of Electrical & Electronic Engineering

Stephan J. Chapman

Chapter 6.8 (5th Edition)

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Overviev

1 Industrial Motor ControlIntroductionSwitch GearIndustrial Motor Control CircuitsStarting three-phase induction motorsForward/Reverse Control of Three-phase InductionMotorsPlugging of Three-phase Induction MotorsReduced-voltage Starting Methods for Three-phaseInduction MotorsOutcomes of Industrial Control

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Industrial Motor Control

6.8 Starting Induction Motors & 8.8 DC MotorStarters

These two sections briefly describe the electricalcontrol of the three-phase induction – & DC machine,respectively . . .We will add a bit to this with spesific focus on:

The protection of the motors (& the electrical supply)The STARTING & STOPPING of motorsReduced-voltage starting methods for induction motorsThe FORWARD & REVERSE-contol for induction motorsAs well as electrical interlocking of theabovementioned. . .

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Industrial Switch Gear

Switch gear

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Industrial Switch Gear (cont.)

We will be making use of IEC symbols, not the ANSIsymbols used in Chapman.

Main Disconnect or Main Isolator without a fuse

Q01

2

3

4

5

6

Or a Fused Disconnect

Q01

2

3

4

5

6

Are used to isolate the Motor Control Centre (MCC),which controls a “big” motor or different small motors,from the supply, i.e, during maintenance

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Industrial Switch Gear (cont.)

Main Circuit Breaker

I > I > I >

Q01 3 5 13

14

These are mostly used instead of Main Isolator since italso protect the supply against faults, e.g. a short circuitin the MCC.The circuit breaker sometimes contain a small auxiliarycontact which can be used to determine whether thecircuit breaker is OPEN or CLOSEThese auxiliary contacts can be normally closed, i.e. NC,of normally open, i.e. NO. . .Auxiliary contacts are sometimes used for interlockingapplications

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Industrial Switch Gear (cont.)

The main objective of the circuit breaker is to disconnectthe supply “as quickly as possible” when a state of overcurrent occur, usually caused by a short-circuit(Itrip ≈ 5×Irated)

I >Further it can also protect the supply cable againstthermal overloading, e.g. when the supply current arejust above the rated current value for a long time –consist of a bi-metal strip (or electronics) which tries toimitate the thermal time constant of the electrical motor.

Isolator

Q21

2

3

4

5

6

13

14

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Industrial Switch Gear (cont.)

These are usually smaller than the main isolator and areused to isolate certain sections controlled by the MCC.It is usually placed at the electrical motor that requiresisolation.As a result the motor can be “disconnected” from thesupply for mechanical or electrical maintenance. . .

Thermal (Overload) Relays

Q1

Are used to protect the motor against continuesover-current to prevent the motor from failing as a resultof overheating.Consist of a bi-metal strip (or electronics) which tries toimitate the thermal time constant of the electrical motor.

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Industrial Switch Gear (cont.)

(Magnetic) Contactor

K1A1

A2

1

2

3

4

5

6

13

14

21

22

A Magnetic contactor is an electromechanical device witha set of three-phase(main) contacts which can beswitched through a small electromechanical winding(24 VGS/WS , 110 VWS , 230 VWS , ens.)

Motor Protection Circuit Breaker

I > I > I >

Q01 3 5 13

14

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Industrial Switch Gear (cont.)

This consists of a circuit breaker that provides shortcircuits protection as well as overloading protection. . .

Combination Starter or Compact Starters

I > I > I >2 4 6

Q11 3 5 13

14

1

2

3

4

5

6

13

14

A1

A2

K1

Combined with a mechanical contactor (everythingusually already assembled) . . .

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Industrial Switch Gear (cont.)

Z See the top row which are only motor protection circuitbreakers versus the bottom row which are alreadyassembled with a mechanical contactor,

Eaton Wiring Manual 2011

Pushbuttons

S00

21

22

S011

12

21

22

S113

14

23

24

Pushbuttons refer not only to the “face” of the buttons butalso to the contacts controlled by the buttons.The “face” of the button can be any colour (i.e. green,yellow, red, black, . . . ) depending on the function of thebuttonA legend (“START”, “STOP”, “FORWARD”, REVERSE”)are usually displayed on the button, or sometimes only asymbol . . .

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Industrial Switch Gear (cont.)

More than one set of contacts can be connected to abutton, i.e. NC and/or NO.

Pilot Lights

H1X1

X2

Pilot lights are usually mounted at or close to pushbuttonsto indicate operating condition, warnings or errors.

Control Relays

K3A1

A2

13

24

21

22

These are small relays for use in the control circuits andare not for switching three-phase machines.

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Industrial Switch Gear (cont.)

Are also used to isolate circuit operating at differentvoltages (i.e. 24 V|DC & 230 V|AC . . .Usually consist of a combination of normaly open (NO)and/or normaly close(NC) contacts

Timers or Timing Relays

K4A1

A2

17

18

27

28

35

36

45

46

ON-delay Timers (contacts 17&18 with there inverse,contacts 35&36)OFF-delay Timers (contacts 27&28 with there inverse,contacts 45&46)

+ N.B. Consider the “parachute” to determine whether itmoves slowely to the right or the left, as a result of “airresistance”Puls Timers

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Industrial Switch Gear (cont.)

Flasher TimersY −∆ timer relayUsually consist of a combination of normally open (NO)and normally close (NC) contacts

Z For a a graphic explanation on the operation of all thedifferent timer relays see: Eaton Wiring Manual 2011

Limit SwitchesThese switches switch when something touches it and itmoves far enough to switch the contactsUsually consist of a combination of normally open (NO)and normally close (NC) contacts

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Industrial Switch Gear (cont.)

Proximity SwitchesInductive proximity switchesCapacitive proximity switchesUltrasonic proximity switchesSeveral optic types. . .Usually consist of one or more normally open (NO)and/or normally close (NC) output

Z For a summary of different types of sensors seeEaton Wiring Manual 2011

L.W. The most industrial proximity switches contains electronicwithout contacts that switch, it produce a HIGH (i.e.usually 24 V|GS ) or a LOW (i.e. usually 0 V|GS ) output.

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Industrial Motor Control Circuits

Industrial Motor Control CircuitsElectrical motor circuits can be presented as:

Block diagramsSingle line diagramsWiring diagramsSchematic diagrams

We will focus on the last-mentioned.Chapman makes use of NEMA/ANSI simbole for theschematic diagramsIn South-Africa the IEC/DIN symboles are used for theschematic diagrams, these will be used in this course. . .

Z See Eaton Wiring Manual 2011 to compare the IEC/DIN &NEMA/ANSI symbols.

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Industrial Motor Control Circuits (cont.)

According to the IEC convention, schematic diagramsare divided into:

main circuit, i.e. the three-phase circuitcontrol circuit, usually 24 V DC or 24/110/230/400 V 1φAC

+ N.B. Remember the main and control circuit areconnected through relays or contactors, consequentlythe voltage of the control circuit can be anything.

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Starting three-phase induction motors

6.8 Starting three-phase induction motorsThree-phase cage rotor induction motors are usuallyconnected:

directly to the three-phase supply when started, orby making use of reduced-voltage starting methods.

The staring method depends on:the size of the motor,the capability of the three-phase supply &the type of load (mechanical).

The main advantages of Direct On Line starting are:Z simple enZ inexpensive.

The disadvantages of DOL starting are:Z the starting current can be 5 to 6 times the rated current

of the motor,Z the high starting current can cause voltage dips at the

supply. This might damage sensitive equipment&

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Starting three-phase induction motors (cont.)

Z mechanical shock can occur as a result of the suddenincrease in torque.

A three-phase motor cannot be connected “directly” tothe supply without protection for theZ three-phase supply &Z the three-phase motor.

To protect the supply against possible short circuitsfuses or, the more modern, circuit breakers are installedin series with the supply.The trip value of the circuit breaker needs to be set highenough for it not the trip as a result of the high currentassociated with the motor starting. . .Therefore the trip value is required to be more than 6times the rated current of the motor, preferably more,say, 10 times but lower than, say, 20 times. . .

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Starting three-phase induction motors (cont.)

In the event of a short-circuit the supply current tends to∞ and is only limited by the impedance of the supplywires (which usually is very small).The aim of the circuit breaker is then to open as quicklyas possible to prevent possible damage to the supplywires. . .In the case where the motor is operated above its ratedcurrent (say at 150% of its rated value) the motor canstill fail thermally (i.e. burnout) since the circuit breakerprovides no protection. . .To protect the motor, Thermal (Overload) relays arerequired to be installed in series with the supply and thecircuit breaker.A thermal (Overload) relay’s trip time constant tries toemulate the thermal time constant of the motor what itneeds to protect.

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Starting three-phase induction motors (cont.)

i.e. for a small over-current, the relay will take longer totrip than for a higher over-current.

Direct On Line switching using contactorsIn most cases the control panel and the motor are nottogether, but a distance apart.A more elegant manner is to make use of a pushbuttonand contactor to switch the motor “ON” and “OFF”.This means that:

the electrical panel of the motor and the motor do notneed to be togetherthe motor can have more than one “START” and/or“STOP” buttonthe motor can be controlled simultaneousness throughmore than one button and/or sensors that are equippedwith auxiliary contacts

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Starting three-phase induction motors (cont.)

In the main circuit a (mechanical) contactor is nowplaced in series with the circuit breaker (or fuse) and theoverload relay.The control circuit is responsible for activating the(mechanical) contactor (energise).

Z A NEMA/ANSI schematic diagram of the main- encontrol circuit for a typical DOL application are shown inFig. 6–37

L.W. Chapman makes use of NEMA/ANSI symbols en drawsthe main- and the control circuit (“control station”)together, from left to right in the so-called ladderformat. . .

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Starting three-phase induction motors (cont.)

Z A schematic diagram of the main and control circuit of atypical DOL application according to the IEC/DINconvention are shown in Eaton Wiring Manual 2011

L.W. According to the IEC/DIN convention, the main and thecontrol circuit are drawn separately, from top tobottom. . .

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Starting three-phase induction motors (cont.)

Main (three-phase) circuitFuses were traditional used for short-circuit protectionwith separate thermal (overload) relays to protect themotorThese days Motor Protection Circuit Breakers are used,these provide short-circuit as well as thermal (overload)protection. . .

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D.O.L. IEC Main Circuit

Using Fuses & a thermal (overload) relay

L1 phase “A”

L2 phase “B”

L3 phase “C”

F1 fuse for short-circuit protection

F2 thermal (overload) relayterminals 1–6 are the terminals of the maincontacts and 13 & 14 are those of the N/Oaxillary contacts

K1 mechanical three-phase contactorterminals 1–6 are the terminals of the maincontacts and 13 & 14 are those of the N/Oaxillary contacts with A1 & A2 the terminals ofthe winding of the contactor used in thecontrol circuit

M1 the three-phase induction machine

1

2

3

4

5

6

F1

F2

L1L2L3

1

2

3

4

5

6

1

2

3

4

5

6

13

14

13

14

A1

A2

K1

M1 M3∼

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D.O.L. IEC Main Circuit

Using a Motor Protection Circuit Breaker

L1 phase “A”

L2 phase “B”

L3 phase “C”

Q1 the motor protection circuit breaker with buildin short-circuitand thermal (overload)protectionterminals 1–6 are the terminals of the maincontacts and 13 & 14 are those of the N/Oaxillary contacts

K1 mechanical three-phase contactorterminals 1–6 are the terminals of the maincontacts and 13 & 14 are those of the N/Oaxillary contacts with A1 & A2 the terminals ofthe winding of the contactor used in thecontrol circuit

M1 the three-phase induction machine

I > I > I >2 4 6

L1

L2

L3

Q1

1 3 5 13

14

1

2

3

4

5

6

13

14

A1

A2

K1

M1 M3∼

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D.O.L. Control Circuit

Control CircuitThe control are implemented using (momentary)pushbuttons.A START button switches on a contactor.A axillary contact of the contactor serves as a selfsealing contact to keep the contactor pulled in after theSTART button is releasedA STOP button in series met N/C-contacts switches offthe contactor when the it is pushedAccording to the law(in R.S.A. it is the Factories act of1985, i.e. Law’85) a E-STOP button is required to cut allthe power to the motor, when it is pushed.

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D.O.L. IEC Contol Circuit

Generic Control Circuit

L+ the +24 V control circuit voltage

M the 0 V (ground) of the control circuit voltage

F0 (not shown) a single-phase circuit breakeror fuse for short-circuit protection of thecontrol circuit are sometimes placed inseries with the contol circuit (usually in a110/230/400 V 1φ AC control circuit)

F2/Q1 (terminals 13 & 14) a axillary contact ofF2/Q1 to switch off the contactor K1 ifF2/Q1 trips

S0 contacts of STOP pushbutton (perconvention always N/C)

S1 contacts of START pushbutton (perconvention always N/O)

S2 contacts of E-STOP emergency stoppushbutton (always N/C)

K1 the winding of the mechanical contactor(terminals A1 & A2)terminals 13 & 14 are the axillary contactsof the mechanical contactor used for selfsealing (self sealing contacts)

F2/Q1

S2 E-STOP

S0 STOP

K1S1 START

M

L+

11

12

11

12

13

14

13

14

43

44

A1

A2

K1

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Forward/Reverse Control

Forward/Reverse Control using contactorsTo change the direction a three-phase induction machinerotates two of its phases needs to be changed, thuschanging the phase sequence from (say) ABC to CBA.This can be accomplished by making use of twocontactors, one for the Forward or CW rotation and onefor Reverse or CCW rotation.

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Forward/Reverse IEC Main Circuit

FORWARD/REVERSEIEC Main Circuit

L1 phase “A”

L2 phase “B”

L3 phase “C”

Q1 the motor protection circuit breakerwith build-in short-circuit andthermal (overload) protection13 & 14 are the terminals of theN/O axillary contact

K1 the mechanical three-phasecontactor(say for CW rotation)13 & 14 is the terminals of the N/Oaxillary contact21 & 22 is the terminals of the N/Caxillary contact

K2 the mechanical three-phasecontactor(say for CCW rotation)13 & 14 is the terminals of the N/Oaxillary contact21 & 22 is the terminals of the N/Caxillary contact

M1 the three-phase induction machine

Z Eaton Wiring Manual 2011

I > I > I >2 4 6

L1

L2

L3

Q1

K1 K2

1 3 5 13

14

1

2

3

4

5

6

13

14

21

22

A1

A2

1

2

3

4

5

6

13

14

21

22

A1

A2

M1 M3∼

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Forward/Reverse IEC Control Circuit

IEC Control CircuitAgain we are looking for a solution that does not makeuse of mechanical latching, since it is not that safe. . .A system needs to be designed that if “something” fails,the system will fail to a safe operating condition. . .Furthermore, the use of a E-STOP as a STOP is a BADdesign. . .A design with mechanical latching also makes itimpossible to have more than one FWD, REV or STOPbutton as well as to make use of safety devices.Therefore control is done using (momentary)bpushbuttons with “electronic” latching. . .

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Forward/Reverse IEC Control Circuit (cont.)

The following pushbuttons are therefore required:A FWD button to activate contactor K1A REV button to activate contactor K2A STOP button to switch-off both contactors whenpushed, andA E-STOP button according to law

A N/O axillary contacts of contactors K1 & K2 serves asself sealing contact to keep each of the contactors inwhen the FWD or REV button is pressed andreleased. . .A N/C axillary contacts of contactors K1 & K2 serves asprotection against the possibility of both contactors K1 &K2 being switched on simultaneously. . .’n N/C axillary contacts of pushbuttons S1 & S2 servesas protection against the possibility of both buttons S1 &S2 being pressed simultaneously resulting in K1 & K2momentarily switching on simultaneously and causing ashort-circuit. . .

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Forward/Reverse IEC Control Circuit

FORWARD/REVERSEIEC Control Circuit

Q1 (terminals 13 & 14) an axillary contactof Q1 to switch off the contactorsK1/K2 if Q1 trips

S00 E-STOP Emergency Stop

S0 STOP pushbutton

S1 FWD pushbuttonterminals 21 & 22 are the axillarycontacts to interlock S2

S2 “REV” drukknoppieterminals 21 & 22 are the axillarycontacts to interlock S1

K1 terminals A1 & A2 are the contactor’swinding for say “CW” rotationterminals 13 & 14 are the contactsused for self sealingterminals 21 & 22 are the axillarycontacts used for interlocking K2

K2 terminals A1 & A2 of the contactor’swinding for say “CCW” rotationterminals 13 & 14 are the contactsused for self sealingterminals 21 & 22 are the axillarycontact used for interlocking K1

Z Eaton Wiring Manual 2011

Q1

E-STOP – S00

STOP – S0

K2

REV – S2

S1

K1

FWD – S1

S2

K2 K1

K1 K2

M

L+

11

12

11

12

11

12

21

22

13

14

21

22

13

14

13

14

21

22

13

14

21

22

A1

A2

A1

A2

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“Plugging”

Plugging of Three-phase Induction MotorsTo quickly stop a three-phase induction machineplugging is used.In effect two phases of the induction machine areinterchanged, exactly the same as for forward/reversecontrol.The main circuit therefore look exactly the same as forforward/reverse control, with the only difference acentrifugal-type switch used to stop the motor when thespeed reach 0 rpm, otherwise the motor starts rotatingin the opposite direction.

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Plugging IEC Main Circuit

Plugging IEC Main Circuit

L1 phase “A”

L2 phase “B”

L3 phase “C”

Q1 the motor protection circuit breakerwith build-in short-circuit andthermal (overload) protection13 & 14 are the terminals of theN/O axillary contacts

K1 the mechanical three-phasecontactor(for say CW rotation)13 & 14 is the terminals of the N/Oaxillary contacts21 & 22 is the terminals of the N/Caxillary contacts

K2 the mechanical three-phasecontactor(for say CCW rotation)13 & 14 is the terminals of the N/Oaxillary contacts21 & 22 is the terminals of the N/Caxillary contacts

M1 the three-phase induction machine

F1 the centrifugal switch with a set ofN/O & N/C axillary contacts

I > I > I >2 4 6

F1 (n > 0)

L1

L2

L3

Q1

K1 K2

1 3 5 13

14

1

2

3

4

5

6

13

14

21

22

13

14

21

22

A1

A2

1

2

3

4

5

6

13

14

21

22

A1

A2

M1 M3∼

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Plugging IEC Control Circuit

Plugging IEC Control CircuitThe following pushbuttons are required:

A START button to activate contactor K1A PLUG button to activate contactor K2A STOP button to switch off both the contactors whenpressed, andAn E-STOP button according to the law

A N/O axillary contact of contactors K1 & K2 serves asself sealing contacts to hold in the contactors after theSTART or the PLUG button have been pressed andreleased. . .A N/C axillary contacts of contactors K1 & K2 serves asprotection against the possibility of both contactors K1 &K2 switched on simultaneously. . .The N/C axillary contact of F1 is used to assure that K1will only switch on (using the “START” button) if themotor is stationary.

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Plugging IEC Control Circuit (cont.)

The N/O axillary contact of F1 is used to assure that K2will only switch on if the motor is rotation and the “PLUG”button are presses, it also help to switch off K2 when thespeed of the motor reaches 0 rpm.’n N/C axillary contacts of pushbuttons S1 & S2 servesas protection against the possibility of both buttons S1 &S2 pressed simultaneously resulting in K1 & K2momentarily switching on simultaneously and causing ashort-circuit. . .

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Plugging IEC Control Circuit

Plugging IEC Control Circuit

Q1 (terminals 13 & 14) an axillarycontact of Q1 to switch off thecontactors K1/K2 if Q1 trips

S00 E-STOP Emergency Stop

S0 STOP pushbutton

S1 FWD pushbuttonterminals 21 & 22 are the axillarycontact to interlock S2

S2 Plug pushbuttonterminals 21 & 22 of the axillarycontact to interlock S1

K1 terminals A1 & A2 of thecontactor’s winding for say “CW”rotationterminals 13 & 14 are the selfsealing axillary contactterminals 21 & 22 the axillarycontacts used for interlocking K2

K2 terminals A1 & A2 of thecontactor’s winding for say“CCW” rotationterminals 13 & 14 are the selfsealing axillary contactterminals 21 & 22 of the axillarycontacts for interlock K1

F1 Centrifugal switch

Q1

E-STOP – S00

STOP – S0 (Coast Stop)

K2

PLUG – S2

S1

K1

START – S1

F1

S2

K2

F1 Centrifugal

K1

K1 K2

M

L+

11

12

11

12

11

12

21

22

13

14

21

22

13

14

21

22

13

14

13

14

21

22

13

14

21

22

A1

A2

A1

A2

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Reduced-voltage Starting Methods

Reduced-voltage Starting Methods for Three-phaseInduction Motors

The supply current when starting a three-phaseinduction machine is usually very high, since the slip is 1(s = 1).By reducing the supply voltage at start-up, the start-upcurrent is also reduced:

∵ I1 ∝ Vφ

The disadvantage of this is since

τm ∝ I12 ∝ Vφ

2

the start-up torque of the induction machine might be tosmall to bring the load up to speed.

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Reduced-voltage Starting Methods (cont.)

8 External Stator Resistor Starting methodThis method is very simple but results in major I2Rlosses during start-up.

Z See Fig. 6–39

Z Or the IEC example in the Eaton Wiring Manual 2011

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Reduced-voltage Starting Methods (cont.)

7 Autotransformer starting methodThis method is more efficient than the above-mentioned,but are more expensive since autotransformers cost waymore that simple resistors.

Z See Fig. 6–36

Z Or the IEC example in the Eaton Wiring Manual 2011

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Reduced-voltage Starting Methods (cont.)

3 External Stator Reactor starting methodVery simple – control is simialer to that of External StatorResistor starting methodBut with little/no losses during start-up & relatively cheap(i.e. more expensive than External Stator Resistorstarting method, but way less expensive thanAutotransformer starting method)

3 Slip ring induction motor with external resistorstarting

A slip ring induction motor is more expensive than a cagerotor induction motor + there are major losses in theexternal rotor resistors during starting.

Z See IEC example in Eaton Wiring Manual 2011

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Reduced-voltage Starting Methods (cont.)

3 Y-Delta starting (wye-delta)Are often used since it is easy and inexpensive – it is thecheapest way to start a induction motor using reducedvoltage sarting method. Can only be used for inductionmotor of which the normal operating configuration isdelta.

Z See Fig. 6–35

Z Or see the IEC example in Eaton Wiring Manual 2011

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Reduced-voltage Starting Methods (cont.)

4 Soft startThis is the best metord since the voltage graduallyincrease, all the other methods have step increases inthe voltage.Can be used on wye or delta connected inductionmotors.The cost is currently still higher that the cost of thestar-delta method, but the cost of soft starters aredecreasing. . .

Z For more information on soft starters seeEaton Wiring Manual 2011

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Outcomes of Industrial Control

For Industrial Control you now need to:know the difference between a:

Main Isolator (Main Disconnect)Main Circuit BreakerFuse DisconnectIsolatorThermal (Overload) Relay(Mechanical) contactorMotor Protection Circuit Breaker

know the difference between a normally open (N/O) anda normally cloce (N/C) contactknow that a axillary contact isknow the IEC/DIN diagram symbols of the following:

IsolatorFuseCircuit BreakerThermal (Overload) Relay(Mechanical) contactor

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Outcomes of Industrial Control (cont.)

Motor Protection Circuit BreakerPushbuttons (N/O and N/C)Pilot Lights

know what is meant by Direct On Line (D.O.L.)connection.understand the advantages of using a (mechanical)contactor to switch a D.O.L. motor instead of by hand.understand how a contactor can be electricallyinterlocked by using a self sealing contact.know how to design and draw, using IEC symbols, aD.O.L. starter main- and control circuit with pushbuttonsand self sealing contacts, that makes use of electricalinterlocking.

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Outcomes of Industrial Control (cont.)

know how to design and draw, using IEC symbols, aforward/reverse main- and control circuit starter usingpushbuttons and self sealing contacts, what providessufficient protection against the possibility of the forwardand the reverse contactors switching on simultaneously,by using electrical interlocking.know how to design and draw, using IEC symbols, a“plugging” main- and control circuit.understand what reduced-voltage starting means andknow the advantages and disadvantages.know the different reduced voltage starting methodstogether with their advantages and disadvantages.

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