Hybrid Rectangular Bar Wave Windings to Minimize Winding Losses of Permanent Magnet Machines for EV/HEVs over a Driving Cycle

Abstract

The utility of thick rectangular wire wave windings in electrical machines for electrical vehicle/hybrid electrical vehicles (EV/ HEVs) have gained increasing interest due to its high slot filling factor, short end-winding, good manufacturability and excellent heat dissipation capability, etc. [1]. More EV makers tend to adopt this kind of windings in their new generation EV motors [2] [3]. However, significant eddy current losses may be induced in the thick bars especially at a high speed operation, thus reducing the machine efficiency and may cause winding local overheating. The conductor in a slot suffers from the circumferential flux caused by the armature (blue) and the radial flux caused by the magnet (red), as shown in Fig. 1 (a). The magnitude of radial flux density caused by the magnet and the circumferential flux density caused by the armature along the center line is shown in Fig. 1 (b). It can be seen that the flux density dramatically decreased as the distance to the stator bore increases. This means the conductors in the slot near the slot opening are exposed to a relatively large variable field by magnet and armature and significant AC losses may occur. Some methods have been reported to reduce the AC losses in the existing papers [4]–[6]. Reference [4] shows that the AC losses can be reduced by keeping the conductors away from the slot opening or using the magnetic wedge. Reference [5] and [6] reduce the AC losses by choosing an optimal winding layout and stranding the conductors, respectively. This paper proposes a novel hybrid rectangular bar wave winding consisting of copper bars and aluminum bars to reduce the winding losses. The aluminum bars are located close to the slot opening to suppress the induced AC losses for its high resistivity. It shows that the total winding losses can be reduced with the hybrid winding compared to with the conventional copper winding, especially at high speeds and light loads. Besides, the weight and cost can be also reduced. Further, an optimization method is proposed to minimize the winding losses over a given driving cycle. The areas of the copper and aluminum bars and the distance of conductor to the slot opening are optimized using this method.