Closed-Loop Soft Starter Firing Angle Control Method Using Only Current Feedback

A soft starter is commonly utilized in various industrial induction motor systems to reduce the inrush current during motor startup. The conventional voltage ramp-up method, used in the typical soft starter, gradually increases the voltage over the startup period. However, this method can result in a significant voltage increase as the induction motor approaches its rated speed, resulting in a rapid torque increase compared to the beginning part of the startup phase. This sudden torque increase can potentially cause physical damage to the induction motor system due to a transient current overshoot, which in turn can result in resonance. Additionally, the changes in the parameters of the induction motors can lead to the variations in torque during startup, especially because of the air gap permeance. Therefore, a firing angle control method for the soft starter that maintains a constant torque, facilitating a gradual increase in speed, is desirable. This paper proposes a novel firing angle control method, to achieve a constant torque and a gradual speed increase, using only current feedback. This paper is the first time that a change in air gap permeance is considered and compensated for in controlling the firing angle of the soft starter. The air gap permeance variation is modeled as an effective inertia in the transient response of the induction motor. Experimental results demonstrate that the proposed method effectively reduces rapid torque increases and speed variations compared to the conventional voltage ramp-up method, through its application to a 30kW industrial induction motor system.