Importance of MMF harmonics

What is it about?

In this paper, implementation of fractional‐slot concentrated windings (FSCWs) into an interior permanent magnet (IPM) machine, designed by using the same geometrical and operational specifications as Toyota Prius 2010 IPM machine, has been presented. In detail, the Toyota Prius's IPM equipped with integer‐slot distributed windings has been considered as a reference machine. Then the geometry of the stator has been adequately modified in other five models to equip FSCWs and adopt different magnetomotive force (MMF) reduction methods. It has been validated that the FSCW technique causes a significant increase in the rotor losses due to increase in the rate of the total harmonic distortion of the MMF harmonics. To reduce the effect of these harmonics, different MMF harmonic reduction methods including phase‐winding coils with different number of turns, multilayer winding with phase shifting, and stator with flux barriers have been investigated.

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Why is it important?

This paper investigates the current state‐of‐the‐art of the MMF harmonic reduction techniques for FSCW configurations in IPMs, including multilayer winding with phase shift, different number of turns per coil side (uneven turn numbers per coil side) and stator flux barriers. Merits and demerits of each method have been discussed and verified by numerical analyses by FEM. The MMF cancellation methods given below are adopted for the FSCW IPM machine. a) The different number of turns per coil side (uneven number of turns per coil side) method; b) The multilayer winding with phase‐shifting method; c)The flux‐barrier method including the parametric analysis of the flux barrier's position and dimensions. The, obtained key findings can be summarized as follows. (1) It is validated that very short end‐windings and low end‐winding copper loss are achieved by adopting the FSCW technique. (2) The FSCW technique cause a dramatic increase in the eddy losses including the rotor core and PM losses. (3) Since the leakage flux density of the IPM machine designed by using the FSCW technique is much higher than that of the IPM machine designed by using the ISDW technique, the obtained average torque of the FSCW IPM machine is lower. (4) Adopting the multilayer winding configuration increases the complexity of the winding structure and causes to obtain lower fundamental winding factor and consequently low torque production capability. (5) Adopting the uneven number of turns per coil causes a decrease in the average torque and a slightly decrease in PM loss, but it also leads an increase in the efficiency. In addition, it also causes to less effective utilization of the slots. (6) Adopting multilayer and phase‐shifting method and using flux barriers on the stator is not favourable in terms of average torque, output power, and efficiency. (7) It has been shown that the considered MMF reduction methods have very limited effect on the MMF harmonics of the chosen 12S/8P combination, which has no subharmonics. It has been revealed that since the large amount of MMF harmonics of the FSCWs cause a significant increase in the total losses, the temperature of the magnets is quite higher under the same operating condition. In addition, it has also shown that reduction of the number of stator slots causes a remarkable reduction at the heat dissipation area of the windings. (8) The IPMs equipped with FSCWs require more cooling equipment.