Discrete variable frequency soft starting on DSP-based voltage controller-fed IM drive

Thyristorized soft starters are being used as induction motor starters in fan or pump drives, etc, the principle of which is based on the control of variable voltage and constant frequency. One of the demerits of the 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. A new control strategy is proposed to improve electromagnetic torque at starting. According 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. Based on the fixed main circuit of soft starters, the strategy defines the triggering instants of soft starter thyristors to include or omit partial half cycles of the ac power supply. Therefore, the discrete variable frequency voltage and current applied to the motor are obtained. In fact, the new discrete frequencies are the ones of the fundamental components of the generated voltage and current. For all the subharmonic frequencies, in order to obtain most positive torque, the system is properly unbalanced, using the method of symmetrical components to choose the most positive sequence of the initial phase angle combination of three phases. The strategy similar to variable voltage variable frequency inverters gets the motor to produce low impact of current and high torque. Applying this strategy combining with the method of soft start allows the induction motor to start with full loads. The proposed strategy has been verified experimentally on a laboratory machine using a 32-b DSP-based soft starter-fed induction motor drive system, and proved to have good starting torque and current performance.