Direct Torque Control with duty cycle control strategy for permanent magnet motors with non-sinusoidal back EMF

Abstract

Direct Torque Control (DTC) of brushless DC (BLDC) motor having trapezoidal back emf with a reduced torque ripple is presented in this paper. Unlike conventional six step PWM technique, a quasi square wave current is obtained by selecting the voltage vectors from the pre defined switching table, which results in faster torque response compared to the conventional scheme. A back emf versus position look up table is used to avoid the low frequency oscillations as a result of which, a better torque response is obtained while the amplitude of stator flux linkage is deliberately kept constant by ignoring the flux control in constant torque region. The conventional DTC fails to concentrate on the torque ripple problem due to the variation in switching frequency. By adjusting the duty ratio of the active vector selected from the switching table, the torque ripple can be reduced and it causes significant change in the torque response without deteriorating the overall performance of the system. In this paper the validity of the scheme is verified from the simulation results.