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 ontrolling the x-axis component f the stator voltage in the stator flux reference frame. The torque is then controlled by he y-axis component of stator oltage. In this scenario, the stator current does not exceed its permissible value. owever, in the so-called optimal fficiency mode, the induction motor operates with reduced flux-level at light loads. The eference flux increases with he increase in the load torque. Consequently, stator current overshoots from its hreshold, and the inverter’s rated ower needs to be increased. When the load torque increases suddenly, speed will drop ignificantly as the flux level ad been initially reduced. In this paper, the origin of this phenomenon and the vershoot current are described using he equivalent circuit model of an induction motor. A current limiting strategy is then roposed for the DTC drive with torque controller. Also to improve dynamic response, an algorithm for optimum istribution of stator current is eveloped that includes flux and torque components. This algorithm produces the aximum feasible output torque and inimizes the speed drop. Numerical simulation verifies the efficiency and efficacy of both roposed methodologies.