Influence of Coil Pitch and Stator-Slot/Rotor-Pole Combination on Back EMF Harmonics in Flux-Reversal Permanent Magnet Machines

Harmonics of back electromotive force (back EMF) always have great effects on torque ripples in flux-reversal permanent magnet (FRPM) machines. In this paper, the back EMF in FRPM machines with different magnetization modes and coil pitches is studied. Then, a simple analytical method is developed to judge whether an FRPM machine with an arbitrary slot-pole combination has compensation to eliminate phase back EMF even-order harmonics or not. On this condition, four different types of machines, i.e., FRPM machine with compensation (magnetization mode 1), FRPM machine with compensation (magnetization mode 2), FRPM machine without compensation (magnetization mode 1), and FRPM machine without compensation (magnetization mode 2), are summarized to derive the winding factors when coil pitches varied from 1 to 3. Thus, the influence of the coil pitch on back EMF and its harmonic components as well as their phase angles can be studied by both analytical expression and 2-D finite element analysis. It is found that the optimal magnetization mode and coil pitch can help to increase the winding factor and hence enlarge the fundamental back EMF. Finally, the prototypes are fabricated and the corresponding experiments are conducted to further verify the effectiveness of derived analytical expressions.