Inverter Design Considerations for Variable-Pole Induction Machines in Electric Vehicles

This article proposes a generalized inverter design framework for a variable-pole induction machine (IM). It quantifies the advantages of pole changing and a high number of inverter legs on converter efficiency and size. The framework is used to design an 18-leg drive that reconfigures a six-pole IM to four-and two-pole while increasing torque capability at maximum speed by a factor of 2.2 compared to a conventional 3-leg fixed-pole design. The framework also shows that a three-leg drive must be oversized by a factor of 1.5 to reach the same torque capability using an identical-sized machine. The proposed 18-leg drive has 50% less losses and requires 62% less dc-link capacitance compared to a 3-leg converter. The framework is used to propose a loss minimization method for the combined machine and converter, with pole count as an operational degree of freedom at partial load and high speed. As a result, variable-pole operation reduces combined machine and drive losses by up to 45% compared to a conventional three-leg drive with the same IM. An 18-leg experimental GaN-based 890 VA inverter driving a toroidally wound IM was designed, built, tested, and compared to a 3-leg inverter to validate the proposed framework.