A novel volts/hertz control method for an induction motor to improve the torque characteristics in the low speed range

The volts/hertz controlled induction motors are widely used in industries because of their simple control system configurations. In such motors, the influence of stator resistance drop becomes large in the low speed range, and compensation of this voltage drop is necessary to prevent a decrease in the developed torque. In this paper, a novel volts/hertz control method to improve the torque characteristics in the low speed range is presented. This method is based on the online calculation of rotor flux information from the reactive power of the motor. The calculated result is controlled so as to coincide with its reference signal. As a result, a linear torque to slip frequency characteristic is obtained. Since no knowledge of stator resistance is required, the torque characteristics become insensitive to stator resistance variations due to temperature variations. Experimental results are included to confirm the effectiveness of the proposed control method