Adaptions of space vector modulation for AC motor drives

Abstract

The control of three-phase induction motors through the use of power electronics is now widely accepted by industry. Manufacturers of such equipment are concentrating on reducing system costs and solving application issues. Traditionally two PWM power converters have been required for the efficient four-quadrant control of a three-phase induction motor. Reduced cost implementations are possible by utilising a four-switch topology to control two phases of the motor directly, with the third phase of the motor connected to the midpoint of a split DC link. For low power systems this can be coupled to a single-phase reversible rectifier which also offers bidirectional power flow, sinusoidal input current shaping with near unity power factor, and a well regulated DC link voltage. The design and implementation of a current-forced controller for such a rectifier is developed in this thesis, where the novel use of periodic sampling techniques has led to improved sinusoidal current shaping and power factor control. This thesis describes how the operation of the four-switch inverter at low output frequencies causes a cyclic imbalance ripple in the DC link, and how the output waveform quality is sensitive to this. A technique called Asymmetric Space Vector Modulation (ASVM) is developed that can dynamically eliminate the output distortion caused by such DC link imbalance. Simulations and experimental testing are used to assess the effectiveness of the technique and identify any limitations. The PWM control of three-phase inverters leads to the generation of voltage components that are common to all phases. Their high frequency harmonics are the primary cause of electromagnetic compatibility issues involving such equipment, including premature motor bearing failure caused by the passage of electric current. Two adaptations of the traditional Space Vector Modulation algorithm are presented that modify the common-mode output characteristics of the inverter. The technique of Null State Avoidance (NSA-SVM) achieves a 66% reduction in common-mode output voltage. A new technique called Zero Average Common-Mode (ZACM-SVM) is proposed which seeks to eliminate the low frequency components from the common-mode voltage, allowing more cost effective filters to be designed specifically for the higher frequency elements that remain.