New strategy to improve electromagnetic starting torque for thyristor controlled induction motors

A demerit of conventional soft starters is that a small reduction in voltage gives rise to a considerable drop in electromagnetic torque, which may cause the motor with loads failure to start. This paper presents a control strategy to improve starting torque. Without changing main circuit of the conventional soft starters, the strategy defines triggering instants of thyristors to include or exclude half cycles of the ac power supply. Therefore, the resultant discrete frequency voltage and current applied to the motor are obtained. For all the frequencies, in order to obtain maximum positive torque, the system is properly unbalanced, choosing the initial phase angle combination of the most positive sequence of three phases. The strategy, similar to that in variable-voltage variable-frequency inverters, makes the motor produce low current impact and high torque. The strategy has been verified experimentally on a laboratory machine using a 32-bit DSP-based soft starter-fed induction motor drive system, and proved to have good starting torque and current performance.