Closed-loop Stator Flux Control Method for Soft Starters to Prevent Rotor Speed oscillations in Induction Motors Drive

This paper presents a closed-loop stator flux control method for a soft starter to prevent speed oscillations. A soft starter controls starting current of an induction motor by gradually decreasing the firing angle. However, when the conventional firing angle control method is applied to the induction motor with a low inertia rotor and load, the stator flux magnitude rapidly increases as the rotor speed reaches the rated speed. This rapid increase in stator flux magnitude results in speed oscillations. To overcome the issues, in this paper, a stator flux magnitude closed-loop control method is proposed to prevent the rapid increases of the stator flux. First, the situation where speed oscillations can occur with low inertia is analyzed, using the torque-speed curve and the equations of the stator flux and rotor current in the transient state. Then, the closed-loop controller is designed by considering the relationship between the stator flux magnitude and the input voltage. The proposed stator flux control method allows the stator flux magnitude to increase at a moderate speed and prevents speed oscillation at start-up. The MATLAB/SIMULINK simulations using the model of a lightly loaded induction motor are performed and show the effectiveness of the proposed method.