study of induction motor

7/31/2019 Study of Induction Motor

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Introduction to VizagSteel Plant

Type Public SectorFounded 1971

Headquarters Visakhapatnam, IndiaKey people P K Bishnoi, CMD

Industry Steel

Products

Plain Rounds, Squares, Flats, Billets,Pig Iron, Plain Wire Rods In Coils I,

Plain Wire Rods In Coils IIRevenue USD 1.36 billion

Slogan Pride of SteelWebsite http://www.vizagsteel.com/

Vizag Steel, also known as Visakhapatnam Steel Plant, is a steel company based in the outskirts ofVisakhapatnam, India. Its main plant is located in 26 kilometers from Visakhapatnam,Andhra Pradesh, it is among India's premier steel mills inIndia. It has also been conferredthe Mini Ratna status. Its vision is to be a continuously growing World Class company. Itsmission is to attain 10 MT liquid steel capacities through technological up-gradation.

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Various departments of VSP

The various important departments of VSP are created on the basis of its various importantproduction units. The most important departments are:

1. Coke Ovens

This department of VSP has three coke oven batteries for producing 3.26 million tons of dry coke.Each battery has 67 ovens of 7 meters height. Coking coal after selective crushing coal and properblending is subjected to destructive distillation (heating in absence of air) in the ovens. Afterheating for about 16 18 hours at a temperature of 1100oC, coke is obtained is used as a fuel andreducing agent in the Blast furnace.

Each battery is provided with facility of dry quenching of coke using Nitrogen for dry cooling ofcoke. The recovered heat is used for steam generation for plant consumption as well as powerusing two 7.5 MW Back Pressure Turbo Generators. Besides a group of by-product plants areprovided for recovery of Ammonia, Coal tar fuel, hard pitch and benzol.

2. Sinter Plant

Iron ore fines, coke breeze, limestone and dolomite along with recycledmetallurgical wastes are converted into an agglomerated mass at sinter plant. It has two 312 M2

sinter machines each with 420 M2 straight strand type cooler for annual production of 5.256 MTSinter. Sinter is 80% of the input charged in the blast furnace. Sinter machine is designed tooperate at the rate of 1.2 T/hr/M2 for 330 days in a year

3. Blast Furnace

Sinter along with iron ore and coke is charged to blast furnace to produce molteniron in presence of hot air blast. There are two Blast furnaces of 3200 M3 useful volumes, eachcapable of producing 1.3 Mt. of hot metal per annum. Charging to furnace is through Paul worth,Bell less top charging system.

Each furnace is designed for 2.5 ata top pressure and is provided with a set of fourhot blast stoves designed to supply air blast up to 1300o C. Three turbo-blowers, one for eachfurnace and one standby common to both furnaces are provided in Power Plant and Blower Housefor supplying Blast air in to the furnace. Each furnace is provided with 12 MW top pressurerecoveries Turbo Generator for generating Power. BF gas produced from furnaces is cleaned inGas cleaning plant comprising of dust catcher, scrubbers and high pressure ventures and isdistributed throughout the plant as fuel.

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4. Steel Melt Shop

Molten from along with scrap and lime stone are charged to three LD converters, each of 150 tonescapacity. The tap to tap cycle is 50 minutes for each converter. The LD gas generated during steel

making is recovered through gas cleaning plant and gas recovery plant and is used as fuel infurnace.

Six four-strand bloom caster with radial type continuous machine cast bloom frame liquid steelcontinuous casting of such large scale is for the first time in India. This avoids blooming andslabing mills thereby reducing energy consumption level and pollution.

A calcining and refractory plant of 100 tons/day capacity. Dolomite rotary kiln and five 325tons/day line rotary kiln is provided for production of flue mix and calcined dolomite required forsteel making. A brick plant is provided for production of tar-bonded dolomite bricks and rammingmass required for the plant.

5. Rolling mills

The rolling mill complex comprises of a Light and Medium Merchant mill (LMMM), a Wire RodMill (WRM) and a Medium Merchant and Structural Mill (MMSM). Each mill is equipped withrequired number of walking beam furnaces for heating of blooms / billets.

Introduction to Induction Motors

An AC induction motor orasynchronous motor is a type ofalternating current motor wherepower is supplied to the rotorby means ofelectromagnetic induction. This derives its name from

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the fact that AC currents are induced into the rotor by means of rotating magnetic field. Theinduction motor was first realized by Galileo Ferraris in 1885 in Italy.

Principle of Operation

An induction motor works on the principle of electromagnetic induction. When the stator windingsare energized with a polyphase supply they create a rotating magnetic field pattern which sweepspast the rotor. This changing magnetic field pattern induces emf in the rotor conductors. Thisinduced emfs give rise to current in the rotor conductors as they are short-circuited. These currentsinteract with the rotating magnetic field created by the stator and in effect cause a rotational motionon the rotor in the direction of rotating magnetic field.For these currents to be induced, the speed of the physical rotor must be less than the speed of therotating magnetic field in the stator, or else the magnetic field will not be moving relative to therotor conductors and no currents will be induced. If by some chance this happens, the rotortypically slows slightly until a current is re-induced and then the rotor continues as before. Thisdifference between the speed of the rotor and speed of the rotating magnetic field in the stator iscalled slip. It is unit less and is the ratio between the relative speed of the magnetic field as seen bythe rotor (the slip speed) to the speed of the rotating stator field.

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Construction

The stator consists of wound 'poles' that carry the supply current to induce a magnetic field thatpenetrates the rotor. In a very simple motor, there would be a single projecting piece of the stator(a salient pole) for each pole, with windings around it; in fact, to optimize the distribution of the

magnetic field, the windings are distributed in many slots located around the stator, but themagnetic field still has the same number of north-south alternations. The number of 'poles' canvary between motor types but the poles are always in pairs (i.e. 2, 4, 6, etc.).Induction motors are most commonly built to run on single-phase orthree-phase power, but two-phase motors also exist. In theory, two-phase and more than three phase induction motors arepossible; many single-phase motors having two windings and requiring a capacitor can actually beviewed as two-phase motors, since the capacitor generates a second power phase 90 degrees fromthe single-phase supply and feeds it to a separate motor winding. Single-phase power is morewidely available in residential buildings, but cannot produce a rotating field in the motor (the fieldmerely oscillates back and forth), so single-phase induction motors must incorporate some kind ofstarting mechanism to produce a rotating field. They would, using the simplified analogy of salientpoles, have one salient pole per pole number; a four-pole motor would have four salient poles.Three-phase motors have three salient poles per pole number, so a four-pole motor would havetwelve salient poles. This allows the motor to produce a rotating field, allowing the motor to startwith no extra equipment and run more efficiently than a similar single-phase motor.

There are three types of rotor:Squirrel-cage rotor

The most common rotor is a squirrel-cage rotor. It is made up of bars of either solid copper (mostcommon) or aluminum that span the length of the rotor, and those solid copper or aluminum strips

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can be shorted or connected by a ring or some times not, i.e. the rotor can be closed or semi-closedtype. The rotor bars in squirrel-cage induction motors are not straight, but have some skew toreduce noise and harmonics.Slip ring rotor

A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are connected to slip

rings. When these slip rings are shorted, the rotor behaves similarly to a squirrel-cage rotor; theycan also be connected to resistors to produce a high-resistance rotor circuit, which can bebeneficial in startingSolid core rotor

A rotor can be made from solid mild steel. The induced current causes the rotation.

Windings in Induction Motors

Stator Winding:

Schematic diagram for a three-phase, Y-connected stator winding

Generally stator windings are made out of copper or aluminum. The diagram shows that eachphase has one or more parallel paths for current flow. Multiple parallel paths are often necessarysince a copper cross-section large enough to carry the entire phase current may result in anuneconomical stator slot size. Each parallel path consists of number of coils in series. Each coilconsists of a number of turns of copper conductors formed into a loop. The rationale for selectingnumber of parallel groups, number of coils in series and number of turns in a coil is at thediscretion of the designer based on the requirement of winding resistance and load. The winding

groups have to be inserted into the slots in such a way that the 3-phases should be displaced by anangle of 120o electrical in space.The winding may be either single layer or double layer.

The final major component of stator winding is electrical insulation. Unlike copper conductors andmagnetic steel, which are active components in making a motor, the insulation is passive.Insulation is provided with the primary purpose of preventing short circuits between theconductors or to ground. Without the insulation, copper conductors would come in contact withone another or with the grounded stator core, causing the current to flow in undesired paths andpreventing proper operation of the machine. In addition, indirectly cooled machines require the

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insulation to be a thermal conductor, so that the copper conductors do not over-heat. The insulationsystem must also hold the copper conductors tightly in place to prevent movement.

3-phase winding diagram of the stator winding of an induction motor

Types of Stator Winding Construction:

A. Random Wound Coils:Commonly also referred to as mush wound coils, this type of coil is mostly used on low Voltage

windings. In short, the winding consists of a number of enamel coated round section copper wires,randomly wound in an elliptical shape. These wires are inserted into the semi-closed slot, one (ormore than one wire) at a time. A slot liner is used to protect the wires from damage due to sharpedges on the laminated core. A top cap is inserted prior to inserting the wedges. The top capsprotect the wires from damage during the wedging process. The slot wedge is inserted from theside of the core with a special tool. This tool presses down the top cap and thus also the coils thereby allowing the wedge to be easily inserted. The wedges are inserted along the whole lengthof the core, and extend a short distance past the core edge. They provide mechanical stability to thestator windings.

B. Formed Coils

Commonly referred to as set diamond coils, formed coils are manufactured complete as a soft coilbefore inserting them into the stator core. Formed coils are mostly used on medium and highVoltage (above 1000 Volt) motors, but they can also be used on high power (above 300 kW), lowVoltage motors. The formed coil construction requires the stator slot to be completely open, andnot semi-close like that used for random wound coils. The function of the stator slot wedge issimilar to that of the random wound coils, with the important exception that not only non-magnetic

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(plain) wedges are used, but also magnetic wedges. Plain wedges are used on 2 pole motors and onslower motors (more than 2 poles) magnetic wedges are most commonly used. These magneticwedges are used to simulate a semi-closed slot like the slots used for random wound coils. Themagnetic wedges act as a path for the magnetic fields and thus effectively reduce the slot openingfrom the slot width to approximately half the slot opening.

Rotor Windings:

Windings are present only in Slip-ring Induction motors while squirrel cage motors use only areun-insulated copper bars which are short circuited at the end of rotor core. Phase wound rotorwindings are made up of individual coils in a manner similar to the stator windings. The threeindividual phase windings are usually connected in wye or sometimes in delta to form three phasewinding. Leads from the terminals of the rotor windings are brought out for external connectionsby means of slip rings which rotate with the winding and which mate with the brushes, which arestationary. This permits variable resistance to be inserted externally into the rotor winding, forincreasing the rotor torque, for reducing the starting current, and for controlling the operatingspeed.

Rotating Magnetic field

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A rotating magnetic field is a magnetic field which changes direction at a constant angular rate.This is a key principle in the operation of the alternating-current motor.Nikola Tesla claimed in hisautobiography that he identified the concept of the rotating magnetic field in 1882.

In a three phase induction machine, there are three sets of windings phase A winding, phase Band phase C windings. These are excited by a balanced three-phase voltage supply. This would

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result in a balanced three phase current. Equations 1 3 represent the currents that flow in thethree phase windings. They have a 120o time lag between them.iap = Im cos 2_.50.t (1)ibp = Im cos(2_.50.t 120) (2)icp = Im cos(2_.50.t 240) (3)

Further, in an induction machine, the windings are not all located in the same place.They are distributed in the machine 120o away from each other. The windings have their axesseparated in space by 120o. When currents flow through the coils, they generate mmfs. Since mmfis proportional to current, these waveforms also represent the mmf generated by the coils and thetotal mmf. Further, due to magnetic material in the machine, these mmfs generate magnetic flux,which is proportional to the mmf. The net result of all the fluxes is a travelling flux wave.

Slip of an Induction Motor

An AC (Amplitude Current) induction motor consists of two assemblies - a stator and a rotor. Theinteraction of currents flowing in the rotor bars and the stators' rotating magnetic field generate atorque. In an actual operation, the rotor speed always lags the magnetic field's speed, allowing therotor bars to cut magnetic lines of force and produce useful torque.This speed difference is called the slip. The slip increase with load and is necessary for torqueproduction. Slip speed is equal to the difference between rotor speed and synchronous speed.Percent slip is slip multiplied by 100. When the rotor is not turning the slip is 100 %.

Slip is calculated using:

Where s is the slip.ns = Synchronous speednr= Rotor Speed

Rotor Speed nr= ns(1-s)

Operation and control of Induction Motors:

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Starting of Induction Motors:

Three Phase Direct-on-line starters:

The simplest way to start a three-phase induction motor is to connect its terminals to the line. This

method is often called "direct on line" and abbreviated DOL.

In an induction motor, the magnitude of the induced emfin the rotor circuit is proportional to the

stator field and the slip speed (the difference between synchronous and rotor speeds) of the motor,

and the rotor current depends on this emf. When the motor is started, the rotor speed is zero. The

synchronous speed is constant, based on the frequency of the supplied AC voltage. So the slip

speed is equal to the synchronous speed, the slip ratio is 1, and the induced emf in the rotor is

large. As a result, a very high current flows through the rotor. This is similar to a transformer with

the secondary coil short circuited, which causes the primary coil to draw a high current from the

mains.When an induction motor starts DOL, a very high current is drawn by the stator, in the order of 5

to 9 times the full load current. This high current can, in some motors, damage the windings; in

addition, because it causes heavy line voltage drop, other appliances connected to the same line

may be affected by the voltage fluctuation. To avoid such effects, several other strategies are

employed for starting motors.

Wye-Delta starters:

An induction motor's windings can be connected to a 3-phase AC line in two different ways:

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wye in U.S, star in Europe, where the windings are connected from phases of the supply to

the neutral;

delta, where the windings are connected between phases of the supply.

A delta connection of the machine winding results in a higher voltage at each winding compared toa wye connection (the factor is ). A wye-delta starter initially connects the motor in wye,

which produces a lower starting current than delta, then switches to delta when the motor has

reached a set speed. Disadvantages of this method overDOL starting are:

Lower starting torque, which may be a serious issue with pumps or any devices with

significant breakaway torque

Increased complexity, as more contactors and some sort of speed switch or timers are

needed

Two shocks to the motor (one for the initial start and another when the motor switches

from wye to delta)

Variable-frequency drives:

Variable-frequency drives (VFD) can be of considerable use in starting as well as running motors.

A VFD can easily start a motor at a lower frequency than the AC line, as well as a lower voltage,

so that the motor starts with full rated torque and with no inrush of current. The rotor circuit's

impedance increases with slip frequency, which is equal to supply frequency for a stationary rotor,so running at a lower frequency actually increases torque.

Autotransformer starters:

Such starters are called as auto starters or compensators, consists of an auto-transformer. A part of

the supply voltage is applied to the stator terminals at the time of starting, by means of an auto-

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transformer. After the motor has accelerated near to its operating speed, auto-transformer is

disconnected and full line voltage is applied to the induction motor by connecting it directly across

the supply mains.

Speed control of Induction motors

In a number of industries motors must satisfy very strict speed characteristic requirements bothwith respect to the range and smoothness of control and also with respect to economical operation.The speed of the D.C motor can be adjusted between a wide range with good efficiency andregulation, but in induction motors speed cannot be varied without loosing efficiency and goodspeed regulation.The speed of the induction motor is given by the expressionN = 120*f*(1-s)/pThus there are factors i.e, supply frequency, number of poles and slipHence for the speed control we have to change at least one factor.

Methods of speed control are distinguished according to the main action of the motor:1. From the stator side

2. From the rotor side

a) Various methods of speed control from the stator side are1. Variation of supply frequency2. Variation of applied voltage3. By changing the number of poles

b) Various methods of speed control from the rotor side are

1. By changing the resistance in the rotor circuit2. By introducing into the rotor circuit an additional E.M.F of the same frequency as thefundamental E.M.F of the rotor.

a) From the stator side:

1. Speed control by variation of supply frequency:

This method of speed control provides wide speed control range with gradual variation of thespeed throughout this range. The auxiliary equipment requires for this purpose results in high costfirst, increased maintenance and lowering of the overall efficiency. Thats why; this method is notemployed for general purposes. With the advent of power electronic drives, this method of speed

control is the most popular method used in the speed control of induction motors in the industry.If an induction motor is to be operated at different frequencies with practically constant values ofefficiency, power factor, over load capacity and a constant absolute slip, then with the ironunsaturated, it is essential that the supply voltage varied with the change in frequency according tothe equation

V/V = (f/f) * (T/T)Where v and T are the voltage and torque corresponding to the frequency f and v and T to thefrequency f.

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For constant torque T = T

2. Speed control by variation of supply voltage

This is a slip control method with constant frequency variable supply voltage. In this method ofspeed control of induction motors, the voltage applied to the stator is varied for varying the speed.For this type the equation is given by

I2 = E2 / (R2/s) 2 + (X2)2

And rotor current with reference to stator is given byI2 = V / (R2/s) 2 + (X2)2

Where the R2 and X2 are rotor resistance and stand still reactance per phase as referred to thestator, s is slip and V is supply per phase.In general this method is used only on loads where the torque required drops off considerably asthe speed is reduced such as with small squirrel cage motors driving fans.

The variable voltage may be obtained by means of either saturable reactors, variac, or tap

changing transformers.3. Speed control by changing the number of poles

This method is applicable to the squirrel cage motors because a cage winding automatically reactsto create the same number of poles as the stator. This method of speed control is generally notpracticable with wound rotor motors as in such machines this method involve considerablecomplications of design and switching since the interconnections of both primary and secondarywould have to be changed simultaneously in a manner to produce the same number of poles in bothwindings. Other wise, negative torque will be developed by certain of the rotor conductor belts.The number of pole pairs in the stator can be changed as followsBy using multiple stator windingsBy using consequent pole techniqueBy using pole amplitude modulation techniquea) From the rotor side:

1. Speed control by variation of Rotor Resistance

This type of control is only possible with wound rotor induction motors i.e, this methodcannot be applied to the squirrel cage motor. Wound rotor motors are usually started by connectingstarting resistances in the secondary circuit which are shorted as the motor speeds up. This methodof speed control has characteristics similar to that of dc shunt motor speed control by means ofresistance in series with the armature.

This method of speed control has the following disadvantages1. Reduction in speed is accompanied by reduction in efficiency.

2. Double dependence of speed not only on rotor resistance R2 but on load as well withlarge resistance in the rotor circuit, the speed varies considerably with the variation intorque.

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3. The external rotors are comparatively bulky and expensive as they have to dissipate a gooddeal of power without getting overheated

This method is not suited for controlling the speed at constant torque. But it is widely used inintermittent operation.

Protection of Induction Motors

There are two main risks for an overheated motor: Stator windings insulation degradation and rotorconductors deforming or melting. Insulation lifetime decreases by half if the motor operatingtemperature exceeds thermal limit by 10C.

Overload Protection:

Three-phase motors are designed in such a way that overloads must be kept below the machinethermal damage limit. The motor thermal limits curves consist of three distinct segments, which

are based on the three running conditions of the motor: the locked rotor or stall condition, motoracceleration and motor running overload.The primary protective element of the motor protectionrelay is the thermal overload element and this is accomplished through motor thermal imagemodeling. This model must account for all thermal processes in the motor while motor is starting,running at normal load, running overloaded and if motor is stopped. The algorithm of the thermalmodel integrates both stator and rotor heating into a single model.

Thermal Overload Relay

Differential Protection:

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This protection function is mostly used to protect induction motors against phase-to-phase faults.This function requires two sets of CTs, one at beginning of the motor feeder, and the other at thestar point. Differential protection may be considered the first line of protection for internal phase tophase or phase to ground faults. In the event of such faults, the quick response of the differentialelement may limit the damage that may have otherwise occurred to the motor.The differential protection function can only be used if both sides of each stator phase are broughtout of the motor for external connection such that the phase current going into and out of eachphase can be measured. The differential element subtracts the current coming out of each phasefrom the current going into each phase and compares the result or difference with the differentialpickup level. If this difference is equal to or greater than the pickup level, a trip will occur.

Biased Differential protection method allows for different ratios for system/line and neutral CTs.This method has a dual slope characteristic. To prevent a mal-operation caused by unbalancesbetween CTs during external faults. CT unbalances arise as a result CT accuracy errors or CTsaturation.

Ground Fault Protection:

Damage to a phase conductors insulation and internal shorts due to moisture within the motor arecommon causes of ground faults. A strategy that is typically used to limit the level of the ground

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The overall result of an under voltage condition is an increase in current and motor heating and areduction in overall motor performance.

OvervoltageProtection:

When the motor is running in an overvoltage condition, slip will decrease as it is inverselyproportional to the square of the voltage and efficiency will increase slightly. The power factor willdecrease because the current being drawn by the motor will decrease and temperature rise willdecrease because the current has decreased. As most new motors are designed close to thesaturation point, increasing the V/HZ ratio could cause saturation of air gap flux causing heating.The overall result of an overvoltage condition is an increase in current and motor heating and areduction in overall motor performance.

Over/Under Voltage relay

Mechanical Jam:

The mechanical jam element is designed to operate for running load jams due to worn motorbearings, load mechanical breakage and driven load process failure. This element is used todisconnect the motor on abnormal overload conditions before motor stalls. In terms of relayoperation, the Mechanical Jam element prevents the motor from reaching 100% of its thermalcapacity while a Mechanical Jam is detected. It helps to avoid mechanical breakage of the drivenload and reduce start inhibit waiting time.

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Maintenance and Repair of Induction Motors:

Testing of Induction Motors:

Various tests are performed on the induction motor to determine its efficiency and operatingcharacteristics. They are:

1. No-load test :

This test is conducted on over hauling motor or completely assembled motor. In this testthe three phase supply is connected to the terminal and currents in the phases are notedwith the help of a clamp meter.

1. Speed is determined with the help of tachometer

2. Temperature of the motor is checked

3. Bearing conditions checked

4. Vibrations are checked

The no load test is performed with different values of applied voltage below and above ratedvoltage, while the motor is running light (without load).

2. Blocked rotor test :

This test is performed to determine the short circuit current when voltage less than rated

voltage is applied to stator. In this test rotor is held firmly (rotor windings are shortcircuited at slip rings in case of wound rotor motor) and stator is connected across supplyof variable voltage.

3. Voltage Ratio test :

This test can only be performed on a wound rotor motor by exciting the stator winding atthe rated voltage and rated frequency. The ratio of stator emf to rotor emf is determinedand it is tested whether emf is induced in the rotor at standstill condition.

5. Heat run test:

The life of the insulation of the electrical equipment depends up on the temperatureattained during operation. The objective of this test is to find out the actual maximumtemperature attained while the machine is operating under certain load conditions. Thetemperature is measured both while the motor is operating and after its shutdown.It is sometimes called temperature rise test.

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6. Insulation Testing:

Insulation testing evaluates the integrity of the insulating medium. This usually consists

of applying high potential to the motor under test and determining the leakage current

that flows under test conditions. Excessive leakage current may indicate a deteriorated

condition or impending failure of insulation.

7. Insulating resistance testing:

The quality of insulation is evaluated based upon the level of insulation resistance.

The instrument used to measure insulation resistance is known Megger. The measuredinsulation resistance is a variable, depending on temperature and other environmentalfactors. So all readings must be corrected to a standard temperature for the class ofequipment under test.

8. Polarity Test:

Usually six terminals from the three phases of an induction motor are available. Thepolarity markings can be found out by a polarity test on the induction motor. Polarity test inthe field can be conveniently carried out by using a dc battery, a switch and a dc volt meter.The switch on the primary side is closed, the primary current increases, and so do the fluxlinkages of both the windings, inducing emfs in them. The positive polarity of this inducedemf in the primary is at the end to which the battery is connected (according to Lenzslaw).The end of secondary which simultaneously acquires positive polarity, as determinedby the dc voltmeter is the similar polarity end. The reverse happens on opening of theswitch i.e. the similar polarity end is that end which acquires negative potential.

9. Inductance Test:

Rated AC voltage is applied to each and every phase of winding and amount of currentflowing is measured with the help of clamp meter.

Z=V/I

V=sinusoidal voltage applied between phase and neutral

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I= current flowing through the winding

XL = (Z22-R22)

XL=2 f L

L = XL / (2 f)

10. Surge Test:

This test will be performed to know the earth fault, inter turn short, reverse coil connectionand phase to phase short.

This test is conducted on surge kit. This kit can supply voltage to the phase windingsindividually. It consists of a C.R.O display in it, in order to observe the waveforms of thecurrent passing through the winding. A surge voltage is applied to the winding terminals.The surge consists of a train of impulses.

11. Pole formation:Only two phases are connected to the terminals of the three-phase supply. Then onewinding will be kept unexcited, hence continuous magnetic field will not be developed, andthen the number of poles formed can be measured.

Faults in an Induction Motor

The three-phase induction motors are used in many industrial applications due to their reliability,low cost and high performance. The induction motors need to be maintained regularly to preventthe occurrence of faults. When a fault occurs, the motor needs to be repaired.

The popular ac motor performance is affected by following type of faults:

Electrically related faults (33%): The faults come under this classification are over/undervoltage, over load, phase reversing, unbalanced voltage, single phasing and earth faultMechanically related faults (32%): The rotor winding failure, stator winding failure andbearing faults are most occurring mechanical fault in three-phase induction machine

Environmentally related faults (15%): The external moisture, contamination and ambienttemperature also affect the induction motor performance. The vibration of machine also affectsthe performance of induction machine under various operations

Electrical related faults are frequently occurring faults in three-phase induction machine which willproduce more heat on both stator and rotor winding. This leads to reduce the life time of inductionmachine.

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To protect the machine from more heating due to these electrical faults, a reliable protectionscheme is to be applied. In the existing protection scheme, each and every individual faults requireseparate protective relay like earth fault relay and over current relay, etc. which will be morecostly. To overcome this, a low cost, reliable, integrated protection scheme for three-phaseinduction motor is developed using PIC 16F877 micro controller.

Electrical Faults in Motor:

1) Unbalanced Supply Voltages:

Unbalanced voltage comprises of two opposing components, a positive sequence component thatproduces the wanted positive torque, a negative sequence component that produces unwantednegative torque. As the amount of unbalance in the supply voltage increase, the positive sequencevoltage decreases and negative sequence voltage increases. The percentage of unbalance defined

by NEMA:(1)

Causes:Open delta transformers, unbalanced loading, and unequal tap settings, high resistanceconnections, Shunted single phase load, unbalanced primary voltage and defective transformer.

Effects

Reduction in motor efficiency, increase in stator and rotor copper losses, temperature rise, serious

reduction in starting torque, nuisance and overload tripping, premature failure of motor winding,excessive and unbalanced full load current.

High voltage unbalance factor leads to lower efficiency and higher power factor. Negativesequence voltage component has little effect on the power factor when compared to positivesequence voltage. Single phasing occurs when one phase of the three-phase supply is open. Single-phasing condition is the worst case of voltage unbalance. If a three-phase motor is running with thesingle phase condition, it will attempt to deliver its full horsepower of the load. The motorcontinuously trying to drive the load, until the motor burns out or until the properly sized overloadelements make the motor off.

2) Single Phasing:

Causes

This is caused when one of the supply lines get disconnected. In such a condition the motorcontinues to operate single phase, provided the load does not exceed 57.7% of the normal rating.

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The amount of phase current unbalance is a very good indication of the turn-turn insulationconditions. Turn-turn insulation failure is a prelude to most insulation failure in motors andnormally occurs before a fault propagating to a turn to ground failure. Ground fault current leads toinsulation failure in motor; therefore, a considerable amount of attention is given to the groundcurrent levels available in the system.

Effects of Ground Fault

Hazards for human safety, thermal stress due to fault current, voltage stress, interference withtelecommunication, interruption of power supply.

7) Phase to phase faults and Inter-turn fault:

These faults are rare be cause enough insulation is provided between phases. Inter-turn faultsinvariably develop into earth faults, so it is considered not to provide separate protection for inter-turn faults.

Induction Motor Stator Rewinding:

The life of a three-phase stator winding can be shortened dramatically when the motor is exposedto unfavorable operating conditions - electrical, mechanical or environmental. The failure ofwinding may be due to open in one phase of the power supply to the motor, phase-to-phase fault,turn-to-turn fault, Winding grounded at edge of slot, etc.In case of failure of winding, it needs to be rewound to continue the motor on service. The motormust be rewound for the same number of poles as it was designed for.

Rewinding Process:

1. No initial electrical testing is necessary because the motor winding failure is easily visible.2. The motor is first roasted in an oven up to 500 degrees F to soften the copper coils so that theycan be cut out easily.3. The copper coils are then stripped off on one side of the stator.4. While removing the copper coils from the other side, a note of all the information regarding thewinding of coils like no. of poles, no. of turns, no. of groups per phase, no of layers per slot,thickness of each coil, etc. is made.5. The stator frame and laminations are cleaned completely after stripping.

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6. The stator is painted with red insulating enamel and is now ready to be rewound.7. Coil forms are pre-made based on the winding specifications. Each coil contains multiple wiresthat are grouped together.8. Each slot is lined with NOMEX Type 410 insulating paper and then new coil forms areinserted into the slots and covered with insulated paper again.9. Two surge ropes are installed on each end and each coil is tied to it. These ropes mechanicallysecure the coils when subjected to magnetic forces.10. The slot insulation is trimmed, folded and a Glastic G10 top-stick wedge is tapped into placefrom each end. These wedges securely hold the coils in the slots.

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11. The motor is dipped and baked again to harden the insulation material.12. The motor is reassembled again. The reassembly process begins with the rotor being driftedinto the stator. The fan shrouds, heaters, end bells and junction boxes are installed next.13. Finally the motor is bench tested to make sure all three phases have the same current drawn andit is within the original specifications of the motor.

Overhauling

Overhauling of electric motor means renewal of worn out ball bearings, cleaning of winding/internal with CTC, inspecting the winding with Megger, inspecting of rotor with growler,inspecting for any bend in shaft, inspection of bearing housing, grease route, correcting anydefects, ensuring it is ready for further operation of another period. Rebalancing of running partsis recommended. The motor should be water proof after the overhaul. Anti-corrosive paint shouldbe applied externally.

The procedure of overhauling is as follows:

1. Disassemble the fan assembly and remove the motor to the shop for overhaul.2. Disassemble the motors. Thoroughly clean and examine all parts.3. Protect machined surfaces.4. Inspect and dimensionally measure motor components including end bells, frame, shaft, sleevebearings, keyways, fan and running surfaces for wear eccentricity and other defects. Submit acondition report with readings and conditions as found.5. Completely strip and clean the armature, field coils and inter-pole coils as applicable.

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6. Rewind the motor using new wire of the proper size and type as specified by the motormanufacturer or equivalent.7. Perform insulation resistance test, DC High Pot or equal, on newly rewound motor and submitreport to owner's engineering representative.8. Replace bearings and grease seals, or sealed bearings as original and lubricate if required.

9. Renew all assembly hardware and gaskets on covers and inspection plates10. Reassemble and dynamically balance the rotor and motor assemblies. Owner's on siteengineering rep to witness11. Reassemble the motor using new contractor provided; shaft sleeves, gaskets and fasteners,bearings.12. All surfaces of the fan, fan housing and motor frames exterior shall be blasted and painted withtwo coats of primer and one finish coat13. Upon completion of all repairs, inspection and approval by the owner's engineering rep, returnassembly to the ship.14. Reinstall motor assemblies using new fasteners and gaskets as original.15. Reconnect the motor and perform operational test for rotation in presence of owner's

engineering representative.16. Test under operational conditions for minimum of one hour after temperatures stabilizes.Record current voltage, speed and frame/bearing temperature rise at 15-minute intervals. Submitall reports and prove work satisfactory to the owner's engineering representative.17. All interference's and earlier removals shall be reinstalled.

General Safety precautions

The following precautions related to electrical equipment are to be strictly followed in anyindustry:

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1. Work on electrical installations is to be taken up by a person duly Authorized by acompetent person.

2. Permit-to-work/shutdown is to be taken before working on any energized system /equipment.

3. The following operation should be carried out before issuing work permit.

a) Follow established procedure of using Blue formats for mechanical work permitsand Pink formats for Electrical work permits.

b) Ensure that the device is not running before issue of shutdown / work permit.

c) The concerned circuit breaker / switch should be made off.The Breaker/cassette should be drawn out / fuses should be removed whereverapplicable.

d) Permission switch, if any should be put in off position. Control supply should beswitched off.

e) Caution Boards should be fixed at concerned places. In case of multiple workpermits provide separate tags for each work permit.

f) Earthing should be done at relevant places depending upon the nature of work.

g) It should be checked if there is any possibility of back feeding and the same has tobe isolated before earthing.

h) Wherever shutdown is to be issued for one section of DSL, stoppers are to be fixedon the rail, to prevent bridging of dead and live D.S.L of cranes due to travel of othercrane in the bay.

4. Before starting the job, the absence of power is to be ensured by using test lamp, multimeter, panel voltmeter etc. Proper working of the above instruments shall be ensuredbefore using them to check absence of power.

5. The equipment can be made live only after the work permit is returned back duly signed bythe receiver. If multiple work permits are issued all such permits required to be returnedand cancelled before normalizing.

6. Once a work permit has been retuned duly signed, the equipment is unsafe to work. Even ifthe equipment is off, a fresh work permit should be taken to carry out any work on theequipment.

7. The person issuing the shutdowns should enter in the shutdown register, work permit no.and other details, including other permits issued on the same equipment and the operations

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carried out. The status of OFF/ON should be clearly entered in the register. The shutdownregister should at any time show full status of equipment under shutdown, work permitsissued, work permits, returned etc., unless all the work permits on equipment are cancelled,the said equipment will not be made ready for taking into service.

8. Use only 3-lamp connected in series as a test lamp.9. Use electrically insulated tools e.g., screwdrivers, and pullers.

10. Ensure there is no possibility of feed back from parallel feeders, inter connected ,buscouplers, bus section switch, temporary connections etc.,

11. Always be alert on the job. Alertness prevents accidents. And also persons are to be alertabout material or tools falling from height.

12. Do not close or open a switch or fuse slowly or hesitatingly, do it quickly and positively.

13. Cultivate the habit of turning your face away when opening a circuit breaker / knife switch.

14. As far as possible, use only one hand because hand-to- hand if 2 hands are used, electricshocks are most serious.

15. Avoid the possibility of exposing your eyes to electric arc.

16. The leads of a multimeter must be of different colors to avoid short circuiting.

17. Use fuse grips for handling fuses.

18. When H.T/L.T circuit breakers/contactors are drawn into Service position, electrical handgloves and rubber mats shall be used without fail.

19. If any feeder is switched ON with cover open there is possibility of injury on account offlash over. Hence before switching ON, the cover of the feeder must be firmly closed.

Conclusion:

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This report deals about the Study of Induction Motors. We are in the modern world. Theutility of electricity and electrical equipment is much more increased. To know the usage andperformance of the electrical machines, studying the theory of operation and procedures of

electrical machines are not enough.In addition to the theoretical knowledge, practical knowledge is also needed to know

the operating procedure, characteristics of the different machines to deal and use the equipment inoptimum level in any industry or organization.

In this regard, we physically observed the different parts of the induction motor, theirstarting and control methods, repair procedures, maintenance and testing of Induction Motors inthe Machine building and area repair shop of the Sinter Plant, Visakhapatnam Steel Plant.

We conclude that, rather then class room study; in this project we have gained thepractical knowledge about the Induction Motors and in turn enhanced our theoretical knowledge.

study of induction motor

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