Current Ripple Reduction in Synchronous Machines driven from Multi-Functional Converter Systems

An automotive electric power steering synchronous machine driven from a multi-functional converter has been analyzed and optimized. A multi-functional converter system integrates a front-end boost converter to increase the inverter voltage of an electrical machine. The integration leads to new requirements for the machine due to dc currents and increased high-frequency ripple currents in the motor windings. In this paper the high-frequency ripple currents of a synchronous machine with multifunctional converter is investigated. The paper contains analyses of the high-frequency ripple currents with and without low frequency ac modulation and hence, when operated at high and low rotational speed. An interleaved switching scheme is proposed to reduce the high-frequency ripple currents and is evaluated for the given application. Experimental results are in good correlation with theoretical predictions and demonstrate the reduction of the high-frequency phase and star-point ripple currents if a good coupling between the machine windings is achieved.