Effects of time-varying loads on rotor fault detection in induction machines

The authors address the problem of motor current spectral analysis for the detection of nonidealities in the airgap flux density in the presence of an oscillation or position-varying load torque. An analysis of the effects of position-varying loads on the current harmonic spectrum is presented. The load torque-induced harmonics are shown to be coincidental with rotor fault-induced harmonics when the load varies synchronously with the rotor position. Since the effect of the load and fault on a single stator current harmonic component is spatially dependent, the fault-induced portion cannot be separated from the load portion. Therefore, an online detection scheme which measures the spectrum of a single-phase current must rely on monitoring those spectral components which are not affected by the load torque oscillations. With this in mind, the detection of broken bars is still possible, since the current typically contains higher order harmonics than those induced by the load