Third harmonic current injection to reduce the pulsating torque of a five-phase SPM machine

Five-phase Surface-mounted Permanent Magnet (SPM) machine offers the possibility of using the third harmonic of current to improve the average torque. In this paper, the use of third harmonic to compensate the pulsating torque and thus to reduce the vibrations transmitted to the shaft is explored. According to the multimachine theory, the torque of a five-phase star-connected SPM machine is distributed among two virtual machines: the Main Machine (MM) that corresponds to the fundamental of the current and the virtual Secondary Machine (SM) that corresponds to the third harmonic of the current. With this approach, it is shown that the first harmonic of the electromagnetic torque can be almost eliminated for a particular level of third harmonic current injection: this ratio depends on the back-emf spectrum and the resulting control makes the SM operating as generator to absorb the vibrations due to the MM operating as motor. This method is evaluated for the design specifications of a marine propeller where very low ripples at low speed are required. According to Finite Elements analysis, the expected significant reduction of the torque ripple factor is confirmed: more than two times with reference to the sinus control of the MM for the same average torque. The time simulations of the drive show the positive effect of this damping control strategy.