Control Strategies for the Identification and Reduction of Cogging Torque in PM Motors

In this work two different methods for the identification of the cogging torque in three-phase Permanent Magnet (PM) motors are presented. The cogging torque map carried out from these methods is then used in a Field Oriented Control (FOC) algorithm to mitigate the ripple in speed and torque. The first method uses a high bandwidth position control loop in a FOC structure, in which a slow and accurate positioning of the rotor is implemented and the cogging torque map is extracted from the q-axis current. Second method uses Load Torque Observer (LTO), which is able to observe various disturbances like load, friction but also the cogging torque. These two methods are experimentally verified and compared. After having identified the most significant harmonics in the cogging torque waveform, they have been suitably added to the q-axis current reference of a FOC in order to mitigate the ripple in torque and speed caused by cogging torque. The effectiveness of the cogging torque reduction algorithm was experimentally verified by significant minimization of the speed ripple.