Influence of on-load voltage distortion on torque-speed characteristic of interior permanent magnet machines

The on-load voltage distortion in interior permanent magnet (IPM) machine refers to the terminal voltage distorted by armature reaction. In this paper, the mechanism of the on-load voltage distortion have been investigated with the aid of frozen permeability method based on both fractional and integer slot IPM machines. It reveals that the local magnetic saturation caused by armature reaction will distort the on-load back-EMFs and dq-axis inductances, which consequently leads to terminal voltage distortion. Due to the geometric feature of IPM rotors, the armature reaction fluxes become more difficult to circulate with the increase of current angle, which further enhances the fluctuation of dq-axis inductances and is found to be the main reason for the terminal voltage distortion in flux weakening operations. Then, the variations of both on-load back-EMFs and dq-axis inductances due to armature reaction are investigated. Consequently, the torque ripples under different current angles are analysed. Meanwhile, the flux weakening performances under fixed DC link voltages are also calculated, which shows the terminal voltage distortion may largely reduce the flux weakening operation region and cause the control of the drive system difficulty. Finally, a 42-slot/8-pole IPM machine is tested to validate the analyses, which also reveals that the phase currents would be distorted if ignoring the peak voltage caused by on-load voltage distortion.