Harmonic current effect on torque density of a multiphase permanent magnet machine

Analytical calculation method of electromagnetic torque in time domain for multiphase permanent magnet machines is presented. And the accuracy of the proposed method is verified by FEM. The effectiveness of the harmonic current injection to boost torque density is confirmed by analytical results and also proven by FEM simulation. The torque characteristics of a nine-phase permanent magnet machine with various combinations of back-EMF and current waveforms are compared. It is shown that significant torque enhancement can be achieved by harmonic current injection if the phase back-EMF is designed appropriately. Further, the torque ripple is also affected by the back-EMF waveforms. Considerations on phase number choosing and the merits and demerits of multiphase permanent magnet machines with harmonic current injection are discussed.