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This paper 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 three-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 three-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 threeleg 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 three-leg inverter to validate the proposed framework.

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Winding layout plays a crucial role in enabling variable-pole operation in an induction machine (IM). Several winding design alternatives, which consist of toroidal and distributed single-and double-layer windings, have been shown to increase speed range and improve partial load efficiency in traction applications. These windings have different polechanging capability, end-winding length, leakage, harmonic content, and inverter requirements. This paper compares these winding alternatives with a generalized variable-pole machine design framework that captures the impact of winding selection on key performance metrics such as losses, volume and torquespeed envelop. This framework shows that the core aspect ratio, defined as ratio of stack length to rotor diameter, selected to minimize losses depends on whether a distributed or toroidal winding is used. When a toroidally-wound IM is designed with a low aspect ratio, it can provide the largest torque-speed envelop with highest efficiency over a wide speed range. An experimental toroidally-wound IM driven by an 18-leg converter is used to validate the design framework. The experimental setup is configured externally to emulate a single-layer winding and to show benefits gained from the extra pole-changing flexibility of a toroidal winding.

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