A Current Balancing Method of Two-Phase Soft Starter for Three-Phase Induction Motor Drive System Without Current Sensor

This paper proposes a firing angle control algorithm for two-phase soft starters. Soft starters are widely utilized to control the starting current of three-phase induction motors by regulating the firing angle of the thyristors. The two-phase soft starter, in which only two-phase currents are controlled by thyristors with the other phase directly connected to the induction motor, has been proposed by researchers to reduce the cost and energy consumption. However, when the thyristors in each phase are controlled using the same firing angle algorithm as in the three-phase soft starter, the current in each phase becomes unbalanced because of the voltage asymmetry between the two controlled phases and the other uncontrolled phase. To overcome this current imbalance, various firing angle algorithms for the two-phase soft starter have also been proposed. However, these algorithms require an additional current sensor. In this paper, a new current balancing method for the two-phase soft starter is proposed which does not require the current sensors. In the proposed algorithm, the initial firing angles and the slopes of the firing angles are calculated using a model of the two-phase soft starter system. The experimental results in a 2HP three-phase induction motor show that the induction motor starts smoothly without a large current imbalance in the two controlled phases.