An investigation into pulse-width modulated AC electric drives with open-end winding machines

Abstract

Pulse-width modulated (PWM) ac drives have found numerous applications in industry and energy generation. Such drives offer advantages of higher efficiency and a wider range of operation as compared to line-connected machines. There are, however, certain disadvantages associated with PWM ac drives. These drives utilize bulky electrolytic capacitors in their power electronic sections which are costly. Also, the PWM inverter generates a switching common-mode voltage at the machine terminals. This causes spurious ground currents and harmful bearing currents through capacitively coupled paths to ground. Conventional ac machines used in electric drives are either star- or delta-connected and the machine has three terminals which are fed using a power electronic converter. In open-end winding machines, this star or delta connection is opened and the machine now has six terminals. These six terminals are then fed using two three-phase power converters. There are certain advantages to the open-end winding method, such as common-mode voltage reduction and increase in the voltage transfer ratio. Open-end winding ac drives have been investigated in this thesis. Different modulation strategies have been compared for the best performance in term of common-mode characteristics and output waveform quality. The inherent issue of circulating currents has been investigated and solutions have been proposed. Drive structures without dc-link capacitors have been proposed, analysed and their performance has been validated and evaluated.