Dynamic current control using synchronous pulse-width modulation for permanent magnet machines

It is difficult to use the conventional fixed sampling pulse-width modulation (PWM) method in a motor drive system due to the harmonics and beat phenomenon when the number of switchings is low. Synchronous PWM is a solution to overcome these problems. However, it is difficult to implement a closed-loop current controller, since synchronous PWM only has merits when the switching frequency is synchronized with the operation speed. This paper proposes a closed-loop current controller for synchronous permanent magnet machines which are driven by synchronous PWM. The proposed controller uses the concept of variable time step control. The control variables are the sampling time interval and the output voltage magnitude. The proposed controller dynamically controls the dq-currents, while the steady-state merits of the synchronous PWM are maintained. This paper presents the design procedures and a dynamic analysis of the proposed current controller. The effectiveness of the proposed method is verified by experimental results.