Torque Control of PMSM and Associated Harmonic Ripples

Vector control techniques have made possible the application of PMSM motors for high performance applications where traditionally only dc drives were applied. The vector control scheme enables the control of the PMSM in the same way as a separately excited DC motor operated with a current-regulated armature supply where then the torque is proportional to the product of armature current and the excitation flux. Similarly, torque control of the PMSM is achieved by controlling the torque current component and flux current component independently. Torque Control uses PMSM model to predict the voltage required to achieve a desired output torque or speed. So by using only current and voltage measurements (and rotor position in sensor controled machine), it is possible to estimate the instantaneous rotor or stator flux and output torque demanded values within a fixed sampling time. The calculated voltage is then evaluated to produce switching set to drive the inverter supplying the motor. PMSM torque control has traditionally been achieved using Field Oriented Control (FOC). This involves the transformation of the stator currents into a synchronously rotating d-q reference frame that is typically aligned to the rotor flux. In the d-q reference frame, the torque and flux producing components of the stator current can separately be controlled. Typically a PI controller is normally used to regulate the output voltage to achieve the required torque. Direct Torque Control (DTC), which was initially proposed for induction machines in the middle of 1980’s (Depenbrock, 1984 and 1988; Takahashi, 1986), was applied to PMSM in the late 1990's (French, 1996; Zhong, 1997). In the Direct Torque Control of the PMSM, the control of torque is exercised through control of the amplitude and angular position of the stator flux vector relative to the rotor flux vector. Many methods have been proposed for direct torque control of PMSM among which Hysteresis based direct torque control (HDTC) and Space Vector Modulation direct torque control (SVMDTC). In 2009 Adam and Gulez, introduced new DTC algortim for IPMSM to improve the performance of hysteresis direct torque control. The algorithm uses the output of two hysteresis controllers used in the traditional HDTC to determine two adjacent active vectors. The algorithm also uses the magnitude of the torque error and the stator flux linkage position to select the switching time required for the two selected vectors. The selection of