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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.