A spectral method of speed ripple analysis for a fault-tolerant six-phase squirrel-cage induction machine

This paper proposes a method to analyze the speed ripple of a six-phase squirrel cage induction machine based on the non-stationary stator current characteristics. Some frequency components associated with the machine space harmonics are present in the stator current spectrum and depend on the rotor slip. The time evolution of these frequencies can be examined to evaluate the effect of torque ripple generated by the negative sequence components when one or several stator phases are lost. The information on the time evolution can be obtained using the spectrogram for time-frequency analysis or several consecutive simple fast Fourier transforms (FFT) as well. A six-phase squirrel-cage induction machine of 90 W, 20 V, 2-poles in normal operation and in faulted mode with two lost phases has been used to validate the proposed method.