Starting Inrush Current Mitigation during Reswitching of Three-Phase Induction Motors by Discontinuous Phase-Controlled Switching

In many industrial applications, frequent and repeated switching of three-phase induction motors is required. If reswitching takes place while the speed of the rotor is significant, a large transient-negative toque may appear at the shaft of motor. It depends upon the difference of voltage magnitude U, between supply voltage V, and induced emf, E of the stator winding, and the phase angle $\delta$ between V and E. At this condition, both starting inrush current and the transient-negative toque which tries to block or oppose the movement of rotor, are extremely high (as high as six-seven times of the rated value). Soft switching with reduced applied voltage and reduced starting current is achieved by switching angle control of a three-phase AC regulator. But the starting operation takes more time, and thus reswitching is delayed. Moreover, it becomes difficult to start the motor with a high constant-toque load coupled with rotor due to the need of high starting current. It was shown earlier that with the help of non-simultaneous switching of AC regulators, both starting current as well as starting torque can be controlled individually. However, these switching methods require complex control system due to the use of vector or d-q based control systems or artificial neural network (ANN) based controllers, which require computation based on the motor parameters. In this work it is shown that only on the basis of U, the inrush current and the transient-negative torque can be reduced at reswitching condition. Here, the switching is performed in any power cycle when U becomes zero. With the help of a feedback control signal of U, a zero-voltage switching (ZVS) based controller circuit is developed to control the switching angle of a three-phase AC regulator. The experimental records show the reduced starting inrush current as well as the switching transient period of the motor, and the proposed circuit works satisfactorily. It improves the power quality (PQ) of the line as the period of disturbance due to starting current is reduced too.