Research on displacement angle in the control system of motor soft starter

The rule of displacement angle vs. speed in a three-phase induction motor is investigated. Based on the operating principle of thyristor voltage controller, the cause of phase current oscillation during soft starting with a voltage regulator, is analyzed. It is proposed that the displacement angle closed loop control should be introduced in induction motor soft starter. Through analyzing, it is pointed out that the thyristor firing angle should be composed of two parts. One is obtained from the predetermined voltage control rule, and the other is the increase of dynamic regulating angle with the variation of displacement angle. The experiments prove that the proposed closed loop control method can avoid current oscillation during soft starting, and energy saving operation of the motor under light load can be realized by using the displacement angle detection in real time.