Analysis of direct torque controlled BLDC motor with reduced torque ripple

Direct Torque Control (DTC) of brush less 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. In the constant torque region the control of amplitude of the stator flux is found to be difficult due to the sharp dips in the flux trajectory. A pre stored 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. This conventional DTC fails to concentrate on the torque ripple problem due to the variation in switching frequency. The introduction of null vectors causes significant change in the torque response without deteriorating the overall performance of the system. The validity of the scheme is verified from the simulation results.