Cogging torque estimation for sensorless PMSM

A major issue for mass produced permanent magnet synchronous motors (PMSM) is the minimization of acoustic noise. A significant source of this noise is cogging torque. Design related cogging torque that does not vary between motors can be compensated by a predetermined control scheme. However cogging torque caused by variation in the manufacturing process will vary from motor to motor and thus needs to be compensated online on a motor by motor basis. While schemes have been published for cogging torque compensation for motors with encoders, to date no schemes have been proposed for use with sensorless drives (without position sensors). As sensorless drives make up a large proportion of the mass produced PMSM market, online estimation and compensation of cogging torque without position feedback deserves investigation. The success of a compensation scheme will be based on the accuracy of the cogging torque estimation. This paper presents a scheme for estimating the dominant cogging torque harmonics using a back-EMF estimator sensorless controller. Experimental results show that over the expected range of system variation in viscous friction, the estimate will vary by less than 20%. Analysis shows that if this estimate was used in an iterative cogging torque cancellation scheme then the critical harmonics would be reduced to less than 10% in 2 iterations.