Duty Ratio Restriction Strategies of Space Vector PWM for Power Amplifiers of AMBs

Abstract

The performance of power amplifiers for active magnetic bearings (AMB) is closely related with their main power circuits and control algorithms used. A three-phase bridge circuit is selected as a main power circuit of an AMB power amplifier. Through a space vector pulse width modulation (SVPWM) algorithm, two electromagnet coils can be driven simultaneously by the three-phase bridge circuit. However, when the coil current command signal for the amplifier fluctuates sharply, a specially designed duty ratio restriction strategy is required. In this paper, two duty ratio restriction strategies, which are Period Bisection Strategy and Proportional Reduction Strategy, are proposed for the situation that the coil current command signal exceeds the tracking range in one PWM period. Experiments show that when the change rate of the coil current command signal is large, the Proportional Reduction Strategy makes the distortion of the current in each coil (i.e. the difference between the required current change and the actual current change in one PWM period) proportional to the change rate of the command signal. And the Period Bisection Strategy significantly reduces or even eliminates the distortion of the current in the coil with a slower change rate of the command signal, but at the cost of increasing the distortion of the current in the coil with a faster change rate of the command signal.