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Rotor fault in induction motorAlthough induction motors are reliable electric machines, they are susceptible to

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

Bearings Related 44 41

Windings Related 26 36

Rotor Related 8 9

Others 22 14

1. Broken rotor bars / Broken end ringsAs 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 band

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component is caused by electrical

and magnetic 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 s is the slip in per unit and f

is 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 currentenvelopes .

2. Air-Gap EccentricityThe air-gap eccentricity can occur in the form of dynamic, static, or combined

eccentricities . The effect of dynamic eccentricity or combined eccen-tricity is the

appearance of characteristic sideband frequency components is the stator current

spectrum, which are given by

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Where fs is the stator supply frequency,p is the number of pole pairs , and s is the

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

coefficients gets also higher values.