Effects of Rotor Geometry on Equivalent-Circuit Parameters of Variable-Pole Induction Machines

Variable-pole induction machines (IM) are a viable alternative to permanent magnet-based solutions for electric vehicle drive trains, owing to their cost effectiveness and reliability. The variable pole-count allows the IM to deliver high torque to accelerate at low speeds and have low torque to cruise at high speeds. Low magnetizing inductance affects losses in the low-speed/high-pole operation by drawing a high magnetizing current. In the high-speed/low-pole operation, the rotor-slot leakage inductance impacts the performance. These parameters depend on the machine and slot geometry and influence the attainable torque under different pole counts. This paper investigates the effects of rotor-slot geometry on the equivalent-circuit parameters of a variable-pole IM. An analytical model to compute machine parameters from the geometry is developed. The model shows a tradeoff between magnetizing inductance, rotor leakage, and rotor resistance in variable-pole operations. Six feasible rotor-slot variations are used to investigate the effects of rotor-slot geometry on the parameters of a 36-slot, 28-bar IM driven by an 18-leg inverter under 9-phase/2-pole, 4.5-phase/3-pole and 3-phase/6-pole operations.