Protection of Induction Motor

8/3/2019 Protection of Induction Motor

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PROTECTION OF

INDUCTION MOTOR

SUBMITTED TO:

Mr. Prateek SinghalElectrical & Electronics Dept.

HCST Mathura

SUBMITED BY:

Devendra Singh(0806421021)Dhirendra Singh(0806421022)

Divyanshu Singh (0806421023)

Diwakar Dubey (0806421024)


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CONTENTS: Introduction.

Abnormal Operating Condition.

Protection Requirements. Protection of Low Voltage IM.

Protection of Large Motor.

Overload Protection of IM.

Protection Against Unbalance.

Protection Against Single-phasing.

Phase Reversal Relay.


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CONTENTS: Phase to Phase Fault Protection.

Stator to Earth-fault Protection.

Fault in Rotor Winding. Reference .


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INTRODUCTIONThe type of protection used for a particular motor

depends upon the switchgear used for its control.

Basically two type of protection are providedfor every motor, and these are-

Thermal Overload Protection. AND

Short Circuit Protection.

The switchgear used for motor control

classified as following two category-


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CONT.... Contactor starter with HRC fuse and thermal over

current relay.

Circuit breaker and associated protective relays.Contactor and fuse are used for motor up to

approximately 150kw.Whereas for large motor circuitbreakers are used.


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ABNORMAL OPERATING

CONDITIONSThe abnormal conditions can be classified as

follow-

Mechanical Overload.(CAUSES)a) Sustained overloads

b) Stalling

c) Prolonged starting or locked rotor

Abnormal Supply Conditions.(CAUSES)a) Loss of Supply Voltageb) Unbalanced supply Voltage

c) Phase Sequence reversal of Supply Voltage


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CONT....d) Over Voltagee) Under Voltagef) Under frequency

Fault in Starting Supply/Circuit.(CAUSES)a) Interruption in Phasesb) Blowing of fuse/Single Phasingc) Short Circuit in Supply Cable

Internal Fault in Motor itself. (CAUSES)

a) Phase to Phase Faultb) Phase to Earth Faultc) Failure to Phase (open circuit)d) Mechanical Failure


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PROTECTION REQUIREMENTSThe cost of protective system of motor should be

within about 5% of motor cost.

The choice of motor protection scheme depend

upon the followings:

Size of motor , rated voltage , kW.

Type: squirrel-cage OR wound rotor.

Type of starter , switchgear and control gear.

Cost of motor and driven equipment.

Type of load , starting current , possible abnormalconditions , etc.


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PROTECTION OF LOW VOLTAGE

IM(BELOW 1000V AC)

The important point regarding this, are- SCHEME OF STARTING CIRCUIT:

This can be understand by the help of following

fig.


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CONT.... BIMETAL OVERLOAD DEVICE:

These are very popular. In case of 3-phase motor

triple pole bimetal relays are generally employed. Thebimetal strips are either heated by current flowingthrough them or by special heater coil through whichmotor current f low.

Incase of bigger motors they are connected insecondary circuit of CTs.

Bimetal strip can be self setting type or handresetting type.


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CONT....While selecting the bimetal-overload devices formotor protection, the following aspect should be

considered------

a) Characteristic of Relay, Characteristic of motor.

b) Nature of loading.

c) Type of Starting, Starting Current.

d) Protection against overload.e) Protection against single phasing.


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CONT.... SHORT CIRCUIT PROTECTION BYHRC FUSES:

Fault current exceeding 10 times the rated current

can be considered as short circuit current and theseshould be covered by Short Circuit Protecting Devices(SCPD). The SCPD may be in one of the followingform-------

a) HRC Fuseb) Short circuit release opening the circuit-breaker

c) Instantaneous high set overcurrent relay trips the CB


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PROTECTION OF LARGE MOTORLarge motors are provided with protection

against following:

1. Fault in winding and associated circuit .2. Reduction of loss of supply voltage.

3. Excessive overload.

4. Phase unbalance and Single phasing.

5. Phase reversal.6. Switching overvoltage.

7. Surge.


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CONT.... Type of relay available for motor protection:

1. Thermal protection only.

2. Thermal protection, Instantaneous overcurrentprotection .

3. Thermal, Instantaneous Three phase Overcurrent,Instantaneous unbalance, single phasing.

4. Thermal, Instantaneous Three phase Overcurrent,Instantaneous unbalance, single phasing andinstantaneous Earth fault.


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CONT....In the protection of large IM all the relays are such

that the time reduces with increase in current.

Such type of protection can be understand byfollowing fig---


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OVERLOAD PROTECTION OF IM The over load protective devices can be grouped as:

Those which respond to motor current.

Those which respond to winding temperature.The current sensing overload protecting devices

can sense the following abnormal conditions---

1. Overloads and undervoltage .

2. Single phasing.

3. Locked rotor , stalling.


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CONT....4. Heavy starting.

5. Continuous overload.

6. Heavy breaking.

And the following condition can be sensed only

by embedded thermal device-----

Temperature rise due to various cause like,

Higher ambient temperature. Failure of cooling etc.


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CONT....The characteristic of IM heating and Replica Relay

can be understand by following fig..


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PROTECTION AGAINST UNBALANCEThe voltage supplied to three phase IM can be

unbalanced due to any of the following reasons:1. Single phase load on distribution service line.

2. Blow out fuse in power factor correcting plant.

3. Short circuit within or out side the motor.

4. Phase failure by blown fuse.(single phasing).

Unbalanced voltage cause the flow of Negative

Sequence current, this result as the heating the

motor winding, which may reach above the safe

limit.


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CONT....The unbalanced voltage protection based upon

the following method:

1. Bimetallic relay arranged to trip faster for unbalancedcurrent.

2. Single phase relay sensing overcurrent in heavily loadedphase.

3. Phase unbalanced relay.


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PROTECTION AGAINST SINGLE

PHASING(PHASE FAILURE)A 3-phase IM continuous to run even if one of the

supply line is disconnected. This cause the

unbalanced stator currents.The negative sequencecomponent of the unbalanced current cause the

heating of rotor and temperature rise.

Single phasing is the extreme unbalanced for

3-phase motor. In this condition the current in the

healthy phase increase by\/3 times. This increase

the heating in motor winding.


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CONT....The connection of single-phasing preventer can be

understand by following fig..


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PHASE REVERSAL RELAYPhase reversal occurs when supply connection arechanged, this cause the running of the motor in

wrong direction .

In some application phase reversal is dangerous

like Crane, Trams etc. Thats why we need phase

reversal relay.

The phase Reversal Relay based onelectromagnetic principal comprises a disk motor

driven by magnetic system actuated by

secondaries of two line CTs or VTs.


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PHASE TO PHASE FAULT

PROTECTIONThe phase to phase fault short circuit in stator

winding cause burn-out of coil. Hence the motor

should be disconnected from supply very quickly.Fast Overcurrent Relay are provided for

phase to phase short circuit protection. Although

it has some limitation.

The working of Overcurrent Relay can be

understand by: Starting characteristics of squirrel-

cage IM co-ordinated with Overcurrent Relay for


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CONT....phase fault. Which is as following


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STATOR EARTH-FAULT PROTECTIONEarth-fault protection is set to disconnect the

motor from supply as early as possible so that the

damage to winding and lamination is minimum.Zero sequence current transformer (ZSCT) or

core balance type protection is very convenient

method of protection of motor from Earth Fault.

The connection of Core Balance CT(ZSCT) for

earth-fault protection can be understand


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CONT....by help of following fig..


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FAULT IN ROTOR WINDINGIn slip-ring IM , Rotor Fault is possible. This could

be of following types:

Inter-turn Fault. AND Ground or Earthing.

PROTECTION-----

To avoid this the Neutral Point of Supply,

should be Earthed at every Voltage level. Cascade

failure of motor can occur if supply neutral is not

Earthed.


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REFERENCESwitchgear Protection & Power System.

By: Sunil S. Rao

Wikipedia.Power System Protection & Switchgear.

By: Badri Ram

D N Vishwakarma


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THANK YOU.

Protection of Induction Motor

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