Effect of motor current sampling method on motor torque ripple in high torque permanent magnet synchronous machine drives

High torque low speed direct drive permanent magnetic synchronous machines (PMSM) powered by variable speed drives are increasingly used in applications traditionally reserved for standard induction motors with gear-boxes. Our application experience shows that in such motors resonant frequency of the common mode current path can have unusually low value (comparable with the inverter switching frequency) which is for at least a decade lower than that normally expected in standard induction machines. Thus the drive inverter switchings can excite large common mode currents which appear as a strong low frequency disturbance in the motor currents causing significant errors in the motor current measurements and related motor torque ripple induced by the drive itself. Rejection of such strong low frequency disturbance in the motor current measurements appears to be far more problematic that the rejection of the standard motor current switching ripple or high frequency noise caused by the inverter switching. Our goal in this paper is to point out to this issue and investigate practically available options appropriate for its mitigation. It will be shown and experimentally demonstrated that the averaging sampling method is suitable for this purpose.