The Seamless Transition From Discrete Frequency Control to Phase Control Method Using Soft Starter

Soft starters are used in a variety of industries to safely apply grid voltage to drive induction motors. The soft starter protect the motor from overcurrent and torque that occurs during starting of the motors by regulating the voltage applied to the motor. There are various methods to implement this soft starter, and among them, the discrete frequency control (DFC) method allows for simple operation, but research to limit the motor peak current is still insufficient. In this paper, a new method to limit the current and start the induction motor using the DFC was proposed. The proposed method calculates the feedforward component of the firing angle according to the speed command and add it to the output of additional controller to limit the current. The proposed method has the advantage of seamless transition to the phase control method when the speed command increases and can be implemented without structural change from the conventional phase control method. Using this method, it is implemented without motor parameter information, and the system efficiency is improved. The effectiveness of proposed method was verified through simulations and experiments.