Figure 1: (a) Total air gap MMF waveform

Matthew Magill with Adviser P. T. Krein

Conventional polyphase machine windings were designed to set the operating speed and minimize spatial harmonics when operated under utility excitation. As power electronic sources have replaced line feeds in most applications, refinements in stator and rotor slot design shapes have been considered for increased performance. The stator winding should also be investigated, because typical pre-determined winding patterns limit machine excitation to temporal properties (amplitude, phase, and frequency of time-domain waveforms). Spatial manipulations of air gap magnetomotive forces (MMF) are lost.

Figure 1: (b) Harmonic content for 2- and 6-pole operation under stator current loading constraints

Figure 1: (b) Harmonic content for 2- and 6-pole operation under stator current loading constraints

This work explores the stator winding structure and the fundamental constraints it imposes on electric machine performance in an effort to design an electric machine that fully leverages the flexibility available with a power electronics-based source. Current work involves exploiting temporal and spatial control of machine excitations for electrically-controlled pole changing, active damping, improved fault tolerance, and space harmonic reduction.
Electronically-controlled pole changing is enabled through increased control of individual slot currents. This allows a single winding structure to produce flux patterns with multiple magnetic pole counts. An 18-slot stator with nine fully-pitched concentrated coils (coil ends are separated by nine slots) with separate control of each coil excitation, for example, may be run in either 2- or 6-pole operation (Fig. 1). The MMF waveforms and associated harmonic spectrum (in mechanical radians) for each operation are provided in Figs. 1a and 1b, respectively. Multi-pole operation has potential advantages in highly dynamic applications where a wide speed range is required and there are significant machine and drive losses.

This research is supported by the Grainger Center for Electric Machinery and Electromechanics.