Influence of Rotor Residual Flux on the Measurement of Inductance as an Impending Fault Indicator

-During normal operation of a three-phase AC induction motor, the magnetic flux in a healthy rotor forms a sinusoidal pattern. Typically, in healthy rotors, little residual flux will remain after the motor is de-energized. However, as rotor anomalies such as cracked or broken bars develop, a net residual flux may be established. This additional flux appears as a variation in the residual flux after the motor is de-energized. To analyze these variations, a modified version of the standard practice inductance measurement is used. Plotting all three phases of inductance as the rotor is rotated will show any variations, and thus, anomalies associated with the rotor. This paper will discuss how residual flux influences the waveforms generated by plotting inductance and suggests, by analyzing changes in them, impending faults in induction motors may be detected. A. Inductance Measurements on Motors Inductance testing of motors, a standard practice, measures the total impedance of the motor, which includes the impedance of the stator windings and the reflected rotor impedance. To measure the inductance, impedance is measured on each phase by applying a lowvoltage AC signal of known frequency to the winding and measuring the current that is passed through the windings. Resistance of the windings is measured by applying a DC voltage to the windings and measuring the current. From these measurements, inductance is then calculated using: