Discrete time direct torque control of induction motors using back-EMF measurement

The authors present a fixed switching frequency pulse width modulated (PWM) inverter based induction motor direct torque control. The proposed method controls motor torque and stator flux amplitude and is developed in discrete time to allow implementation on microcontrollers or digital signal processors (DSP). The proposed control requires knowledge of a small number of induction motor parameters, viz only stator resistance and leakage inductance. The discrete time direct torque control (DTDTC) requires voltage and current measurements to calculate the back-EMF. Using the DTDTC dead beat control of motor torque and stator flux amplitude is approximated. The control method is not limited to a specific PWM voltage inverter; triangular wave or space vector modulation can be used. The effect of calculation delay is studied and a modified control method is proposed to take into account this calculation delay. A method to take into account the inverter current and voltage limits is proposed. The authors demonstrate the validity of the proposed method using simulation and experimental results.