Reduction of On-Load Terminal Voltage Distortion in Fractional Slot Interior Permanent Magnet Machines

On-load terminal voltage distortion can significantly reduce the flux weakening operation region of electrical machines. This paper investigates the effectiveness of several harmonics reduction methods, e.g., rotor skewing, rotor pole shaping, and stator tooth-tip shaping, on the terminal voltage distortion reduction in fractional slot interior permanent magnet machines. It is found that skewing can only reduce the peak terminal voltages, but not total harmonic distortion, while the average torque is also notably lower. However, on-load terminal voltage distortions can be greatly reduced by rotor pole or stator tooth-tip shaping with only a slight average torque drop. It is found that the local magnetic saturation is largely alleviated based on the magnetic permeance and flux path modification. Finally, these analyses are experimentally validated based on the measured torque speed curves of two prototype machines having the same stator, but different rotors with or without rotor pole shaping.