Design and electromagnetic analysis of an 8/4-pole pole-changing winding

Abstract

Relevance: improving two-speed induction motors is an important task for electric drives used in machinery whose operating processes require two fixed rotational speeds. Separate stator windings limit slot-space utilization and increase the consumption of winding and insulation materials. Single-winding designs, however, must ensure coordinated air-gap flux densities, phase symmetry, and an acceptable magnetomotive force (MMF) harmonic spectrum at both pole numbers. These requirements necessitate the development of pole-changing windings that enable effective utilization of the motor’s active materials in both speed modes.

Aim: to develop and computationally substantiate a three-phase pole-changing winding with an 8/4 pole ratio in 48 stator slots, ensuring coordinated air-gap flux densities and phase symmetry at both pole numbers, followed by implementation in an experimental two-speed induction motor.

Methods: existing pole-changing winding configurations were analysed using the method of discretely specified spatial functions, based on the joint consideration of current distributions in windings with different pole numbers. The pole-changing winding was constructed using the basic Y-Δ/YY circuit, with selected coils assigned to additional branches.

Results: a three-phase pole-changing winding configuration with an 8/4 pole ratio in 48 stator slots was developed. Phase symmetry was achieved at both pole numbers. The winding factors were 0.691 in the four-pole mode and 0.876 in the eight-pole mode, while the corresponding differential leakage coefficients were 2.0% and 1.4%, respectively. The spatial magnetomotive force (MMF) distributions were approximately sinusoidal; harmonic analysis indicated the absence of even harmonics. The air-gap flux density ratio Bδ8/Bδ4 was 1.15. Based on the calculated design parameters, an experimental two-speed motor was manufactured at the private enterprise Energy Motors using the magnetic core of a standard 4A200L4U3 motor, with 48 stator slots, 38 rotor slots, and a stator stack length of 215 mm. Prototype fabrication confirmed the feasibility of implementing the proposed design.