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Aim :

Explain rotor fault in induction motor

Theory :

Although induction motors are reliable electric machines, they are susceptible to

many electrical and mechanical types of faults. Electrical faults include inter-turn

short circuits in stator windings, open-circuits in stator windings, broken rotor

bars, and broken end rings, while mechanical faults include bearing failures and

rotor eccentricities.

1.Broken rotor bars / Broken end rings2.Rotor eccentricities

Percentage of failure by componentFailed Component

Percentage of failures (%)

IEEE-IAS EPRIBearings Related 44 41

Windings Related 26 36

Rotor Related 8 9

Others 22 14

1. Broken rotor bars / Broken end ringsRotor cage fault (broken rotor bar/end-ring) accounts forapproximately 5

10% of

all induction motor failures . ForMV motors, the rotor cage fault is even more

common thanthat of small motors due to the extensive thermal stresses on

therotor. Normally, broken rotor bar can be caused by the followingreasons :

1) thermal stresses due to thermal overload; overheatingof the rotor cage can

cause thermal expansion and thusmechanical stresses;

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2) magnetic stresses caused by electromagnetic forces, un-balanced magnetic

pull;

3) dynamic stresses due to shaft torques;

4) environmental stresses due to contamination, abrasion ofrotor material;

5) mechanical stresses due to loose laminations, etc.

For MV motors, the broken rotor bar/end-ring faults are com-monly caused by

the high thermal stresses during starting.Therefore, the MV motors can only be

started for much lesstimes than small motors, before a rotor cage failure

happens.Once cracked or broken, rotor bars develop in the motors, therotor cage

normally cannot be repaired. The rotor cage fault canalso lead to shaft vibration

and thus bearing failures and air gapeccentricity, etc. Therefore, early detection

of the broken rotorbar/end-ring is essential not only for the rotor protectionbutalso for reducing other types of motor failures.

As shown in Fig. the squirrel cage of an induction motor consists of rotor bars and

end rings. A broken bar can be partially or completely cracked. Such bars may

break because of manufacturing defects,frequent starts at rated voltage, thermal

stresses, and/or mechanical stress

caused by bearing faults and metal

fatigue .

A broken bar causes several effects

in induction motors. A well-know

effect of a broken bar is the

appearance of the so-called

sideband components . These

sidebands are found in the power

spectrum of the stator current on

the left and right sides of the

fundamental frequency

component. The lower side

bandcomponent is caused by

electrical and magnetic

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asymmetries in the rotor cage of an induction motor , while the right sideband

component is due to

consequent speed ripples

caused by the resulting

torque pulsations . The

frequencies of these

sideband are given by:

fb = (12s)f

where sis the slip in per unit

and fis the fundamental

frequency of the stator

current (power supply). The sideband components are extensively used for

induction motor fault classification purposes . Other electric effects of broken

bars are used for motor fault classification purposes including speed oscillations ,

torque ripples, instantaneous stator power oscillations, and stator current

envelopes .

2.

Air-Gap Eccentricity

Air gap eccentricity is the

condition when the air gap

between the stator and the rotor

is unequal. Severe air gap

eccentricity may lead to

unbalanced magnetic pull and

eventually result in the stator to

rotor friction, which can cause

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severe damage to the stator and rotor core.

There are two types of air gap eccentricity, namely, static and dynamic

eccentricity. Intrinsic static eccentricity even exists for newly manufactured

motors . The unbalanced magnetic pull, caused by static eccentricity, may lead tobent rotor shaft, bearing failures, dynamic eccentricity, and eventually stator to

rotor rub, causing a major breakdown of the motors. For MV motors, as the air

gap is relatively smaller in per-unit values compared to small motors, a relatively

small eccentricity maylead to a severe motor failure. Therefore, the detection of

airgap eccentricity at early stage is essential for the protection ofthe motor

system.

The effect of dynamic eccentricity or combined eccentricity is the appearance of

characteristic sideband frequency components is the stator current spectrum,

which are given by

Wherefsis the stator supply frequency,pis the number of polepairs , andsis the

slip. As the fault becomes more severe, the amplitude of the indicative frequency

coefficients gets also higher values.