Current Control Of Brushless DC Motors Driven By Four-Switch Three-Phase Inverters

The four-switch three-phase inverter is proposed to drive motor.Two series capacitance are used in four-switch three-phase inverter to substitute one phase bridge arm in six-switch three-phase inverter,so the number of switches is reduced.The four-switch three-phase inverter has the advantages of low switch conduction losses and simple drive circuit.While when bipolar PWM modulation is used in a permanent magnet brushless DC motors control system driven by four-switch three-phase inverter,if the phase connected to the middle point of DC-link capacitor does not conduct,the circuit is formed by non-conduction phase winding and the other two phases through DC-link capacitor.So possible distortion of three phase currents will occur because of back-EMF.In addition,in the commutation region,the line voltage between two active phases changes according to the working states,which cause the unbalance of change rates of incoming phase and outgoing phase currents,will leads to the ripple of non-commutated phase current.This article first analyzes the running characteristics of four-switch three-phase inverter driven permanent magnet brushless DC motor system,demonstrates the cause of current fluctuations when the phase connected to the busbar capacitance midpoint is the non-conducted phase by mathematical model,analyzes the change trend of the phase current under different voltage vector,two adjust vectors can respectively control the non-conduction phase current increase and decrease rapidly are obtained.Based on permanent magnet brushless DC motors driven by four-switch three-phase inverter,a current control scheme is proposed to reduce the curre nt ripple of both normal conduction region and commutation region.This current control scheme use double PI controller to control the speed loop and current loop.When the phase connected to the middle point of DC-link capacitance,on the base of bipolar PWM modulation,the proposed strategy adds two regulating vectors into each control cycle.Through building mathematical model,the duty cycle formula of regulating vector are obtained on the base that change rate of non-conduction phase current is zero,so that the non-conduction phase current is controlled to be zero by correctly control the working time of regulating vector.In the commutation region,the proposed control scheme compares the changerate of incoming phase and outgoing phase currents and obtains the formula of modulation duty cycle on the base that change rate of non-commutated phase current is zero.The non-commutated phase or the outgoing phase switch is modulated by to maintain the non-commutated phase current constant.Compared with traditional current control strategy,good control effect is kept both in the normal conduction region and the commutation region by accurately controlling the working time of voltage vectors.The proposed strategy do not need to adjust the parameters of the controller and it is simple and easy to implement.And there are three voltage vectors in each control cycle of the proposed method which cause less volatile of current,and the control cycle is fixed.In addition,in view of the back-EMF waveform of trapezoidal wave and sine wave,this paper discusses respectively.Methods in this paper are applicable under back-EMF of all kinds of shape between trapezoidal wave and sine wave back-EMF waveform.Finally,the permanent magnet brushless DC motor experimenta l platform is built and the experiments under different conditions are completed on so as to verify the effectiveness of the method is not influenced by load and speed.And the experiments under proposed method is compared with the other method that applied on permanent magnet brushless DC motor driven by four-switch three-phase inverter and the advantage of the proposed method is proven.