Nature and assessments of torque ripples of permanent-magnet adjustable-speed motors

Torque ripple of permanent magnet motors can be classified into four types depending on the nature of their origin. The four types are pulsating torque, fluctuating torque, reluctance cogging torque, and inertia and mechanical system torque. Pulsating torques are inherently produced by the trapezoidal back-EMFs and trapezoidal currents used in certain permanent magnet adjustable-speed motors. The torque ripples caused by pulsating torques may be reduced by purposely produced fluctuating counter torques. Air-gap torque assessments are conducted on a sample motor. Experimental results agree with theoretical expectations.