Hysteresis Control and Constant-Switching Strategy in Direct Torque Control Drive - A Comparative Analysis

Comparative analysis between to control strategies for direct torque control (DTC) of current converter fed (CSI) induction motor drive is presented. The well-known DTC strategy based on hysteresis type torque controller is compared with new proposed DTC algorithm. Contrary to the direct torque control of CSI drive presented in the known literature (without experimental results), proposed algorithm is based on constant switching frequency with modification of the inverter optimal switching table. The main idea is to develop robust, but simple for implementation algorithm without rotational transformation and parameter dependences that exist in vector control of the same drive. With such a solution, problems detected under basic DTC using torque hysteresis comparator are avoided (significant torque Pulsations, requirement for filtering of the estimated torque and adaptation of hysteresis bandwidth in depend of motor speed). Analysis is performed using simulated mathematical model. Than, experimental verification is done through implementation in the realized drive prototype with a standard thyristor type frequency converter. Both simulation and experimental results confirm the performance of the proposed control algorithm.