A Current Control Scheme of Brushless DC Motors Driven by Four-Switch Three-Phase Inverters

Based on the brushless dc motor driven by a four-switch three-phase inverter (FSTPI), a current control scheme is proposed to reduce the current ripple of both the normal conduction region and the commutation region. Assuming c-phase winding is connected to the middle point of a dc-link capacitance, in the normal conduction region when a-phase and b-phase windings conduct, the current of c-phase may not be zero because of c-phase back electromotive force. The proposed strategy adds two regulating vectors into each control cycle based on the traditional PWM scheme, and controls the c-phase current to be zero by controlling the working time of the regulating vectors in each control cycle. In the commutation region, the noncommutated phase or the outgoing phase switch is modulated by comparing the change rate of the incoming and outgoing phase currents to maintain the noncommutated phase current constant. Compared with the traditional current control strategy, good control effect is kept in both the normal conduction region and the commutation region by accurately controlling the working time of the voltage vectors. The proposed strategy does not need to adjust the parameters of the controller, and it is simple and easy to implement. The experimental results prove the correctness and effectiveness of the control strategy.