FEA study of proximity effect in hairpin windings of a PMSM for automotive applications

Abstract

The electrification of the vehicle fleet is increasing the demand for highly eÿcient and cheap electric traction motors. One way of meeting this demand could be to use hairpin windings in the motors. This technique has recently gained interest in literature and is also used by some manufacturers. The hairpin winding uses rectangular copper bars in order to achieve a high fill factor. This has beneficial e˙ects for thermal properties and also gives an increased power density. However, the larger cross sectional area increases the frequency dependant eddy current loss. The conventional machine winding using round random winding consists of many small circular conductors, therefore the frequency dependent loss is reduced. However, the non frequency dependent loss is higher since it has a lower fill factor. The primary aim of this thesis is to investigate the frequency dependant copper loss in a hairpin interior permanent magnet synchronous machine. The study is based on a machine that was recently bought by the division of Electric Power Engineering at Chalmers University of Technology. The study of copper losses also looks at the impact of changing some design parameters, namely, stator slot opening and the number of conductors. The study is performed by finite element method simulations in Ansys Maxwell. The thesis separates di˙erent phenomena contributing to the total copper loss by isolating di˙erent e˙ects by simulating several modified motor models. The results are then compared to a simplified analytical calculation. The study shows that the major part of the losses are caused by the proximity e˙ect between the conductors in one slot. A comparison is made by simulating the same machine design with the two di˙erent types of winding. This shows that the hairpin winding is to prefer at lower frequen-cies while the random winding is better at higher frequencies. To be more general a frequency sweep of 1-2000Hz is performed. The losses at the maximum speed, from the data sheet of this machine, show an increase in loss of 93.2% for the hairpin and 3.7% for the round random winding. At base speed the increase of loss is 16.8% for hairpin and 0.9% for round random winding. Since the fill factor decides the non frequency dependent loss the round random winding starts at a higher loss. The loss distribution between conductors in one slot containing conductors from one phase shows that the distribution of loss becomes more uneven the more the speed increase. At the maximum design speed the conductor closest to the yoke has around 13% of the total loss of the slot and the conductor closest to the air gap holds 42%.