Rotor Slot Permeance Harmonics under Distorted Air-Gap Flux Waveform for Cage Induction Motors
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The air-gap flux waveform for a cage induction machine is not sinusoidal in shape and contains various harmonics, including the phase-belt harmonics, slot magnetomotive force (mmf) harmonics, slot permeance harmonics, and saturation harmonics. The slot mmf and permeance harmonics affect the stray losses considerably. Among the saturation harmonics, the third harmonic is usually the most significant one unless it is suppressed by allowing circulating current to flow. The magnitude and phase angle of the saturation harmonics compared to those of fundamental flux waveform depend on whether teeth or core (back iron) section is more saturated. The various amount of third harmonic air gap flux with different phase angle is intentionally injected into a cage induction motor to examine the effects of distorted flux waveform on rotor slot permeance harmonics. For the same air gap fundamental flux, the waveform which has the higher air gap peak flux also produce higher rotor slot permeance harmonics. The stator core losses, no-load rotor high frequency losses, no-load losses, and stray load losses under distorted air gap flux waveforms are presented and discussed.