A Design of Rotor Bar for Improving Starting Torque by Analyzing Rotor Resistance and Reactance in Squirrel Cage Induction Motor

There were many attempts to design the rotor bar shape to increase the starting torque in squirrel cage induction motor, and it is well known that deep slot of rotor bar increases the starting torque. However, this deep slot increases the leakage flux so that it hurts the running efficiencies. To increase the running efficiency, it is necessary to keep shallow slot of rotor bar. This paper presents a new design of rotor bar to improve the starting torque without decreasing motor efficiency. The influence of current density of rotor bar on starting torque and rotor copper loss is analyzed so as to design the shapes of rotor bar. In the design, the starting torque was improved by increasing the rotor resistance at startup, and the rotor reactance was maintained during operation. The result showed the improvement of the starting torque without reducing the running efficiency.