Improve direct torque control performance of induction motor with duty ratio modulation

Since 1980´s Direct Torque Control has get more and more attentions, but there still are many problems need to be solved in this field. One of these is how to minimize the torque ripple and current harmonics. As we all know the out put voltage in DTC contains much harmonics for applying hysteresis band in this kind of system. In this paper the torque ripple in DTC is analyzed in detail with little signal model. And it is found that the torque ripple can be divided into two parts, one is only related to the motor parameter and the other is related to not only the applied voltage but also the rotor angular velocity. Based on the analysis, a duty ratio modulation method is proposed that only sends the voltage vector in part of the sampling period, while in the rest time of the sampling period zero voltage vector is sent. The duty ratio modulation algorithm, which is induced from the torque ripple analysis, allocates the sampling period. Simulation results indicate the effectiveness of the proposed torque ripple minimizing method.