New strategy to improve electromagnetic torque at starting in thyristor controlled induction motors

One of the demerits of conventional soft starters is that a small reduction in voltage produces a considerable drop in electromagnetic torque, which may cause the motor with loads failure to start. This paper presents a new control strategy to improve torque at starting. Without changing the main circuit of the conventional soft starters, the strategy defines the triggering instants of thyristors to include or exclude half cycles of the ac power supply. Therefore, the discrete frequency voltage and current applied to the motor are obtained. The new discrete frequencies are the fundamental frequencies of the generated voltage and current. Due to the relationship that the electromagnetic torque of the induction motors is inversely proportional to the ac power supply frequency, reducing the supply frequency can improve the starting torque. For all the discrete 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. Therefore, applying this strategy allows the motor to start with full loads. The proposed 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.