Current Limitation and Speed Drop Minimization in Optimal-Efficiency of Induction Motors

In conventional direct torque control (DTC), the stator flux is usually kept constant by controlling the x-axis component of the stator voltage in the stator flux reference frame. The torque is then controlled by the y-axis component of stator voltage. In this scenario, the stator current does not exceed its permissible value. However, in the so-called optimal efficiency mode, the induction motor operates with reduced flux-level at light loads. The reference flux increases with the increase in the load torque. Consequently, stator current overshoots from its threshold, and the inverter’s rated power needs to be increased. When the load torque increases suddenly, speed will drop significantly as the flux level had been initially reduced. In this paper, the origin of this phenomenon and the overshoot current are described using the equivalent circuit model of an induction motor. A current limiting strategy is then proposed for the DTC drive with a torque controller. Also to improve dynamic response, an algorithm for optimum distribution of stator current is developed that includes flux and torque components. This algorithm produces the maximum feasible output torque and minimizes the speed drop. Numerical simulation verifies the efficiency and efficacy of both proposed methodologies.