Performance comparison for six-step voltage and constant back EMF control strategies for PMSM

Six-step voltage or constant back EMF control strategies can be applied to a permanent magnet synchronous motor (PMSM) in the flux weakening region in order to extend the motor´s speed range. These two control strategies are compared on the basis of maximum torque vs. speed envelope, maximum current requirement, current and torque ripple, torque response and implementation complexity. The six-step voltage control strategy is shown to provide a significantly larger maximum torque vs. speed envelope, while the current requirements of both control strategies are in the same order. However, the six-step voltage control strategy exhibits higher current and torque ripples. It is shown that the magnitude of this torque ripple is relatively small and can be tolerated in most applications. The torque response of the constant back emf control strategy is higher than that of the six-step voltage control strategy. Analytical derivations of the maximum current and maximum speed in each case are provided. A procedure for deriving the instantaneous phase current and torque waveforms in steady state under the six-step voltage control strategy is presented. Experimental work in support of the key theoretical claims is provided