Variable Time Step Control for Six-Step Operation in IPMM

The low frequency current ripple is major concern in high power drives and high speed drives especially in six-step operation, because low frequency current ripple induces excessive heat and lower system efficiency. However, the low frequency current ripple is inevitable in implementing six-step operation at a low frequency ratio (a ratio of the sampling frequency to the fundamental frequency, F_ratio) because switching signal cannot be updated at exact point. Therefore, this paper proposes a variable time step control method to minimizes the low frequency current ripple for six-step operation of interior permanent magnet machine(IPMM), operated at a low frequency ratio. This paper also presents the system transfer function with linearization to analyze stability. Additionally, since there is no flux weakening controller and current reference generator, control structure is simple. Various simulations are conducted to verify the effectiveness of the proposed method.