Abstract
This paper proposes a novel, low-cost, effective way to improve the thermal performance of electrical machines by extending a part of the back-iron into the slot. This modification helps in reducing the thermal resistance path from the center of the slot to the coolant; however, its thermal benefits must be clearly evaluated in conjunction with the electromagnetic aspects, due to the higher iron losses and flux leakage, and furthermore such an extension occupies space, which would otherwise be allocated to the copper itself. Taking a case study involving an existing 75-kW electric vehicle traction motor, the tradeoffs involving the losses, flux leakage, output torque, torque quality, and the peak winding temperature with and without back-iron extension are compared. Finally, experimental segments of the aforesaid motor are tested, verifying a significant 26.7% peak winding temperature reduction for the same output power with the proposed modification.
Highlights
W ITH the increasingly stringent emissions legislations, there is an unprecedented demand for transport electrification, be it for rail, marine, aerospace or automotive
While material developments are an enabler for improved performance, often new materials come with an increased cost premium
When the BIE depth is small, the output torque is not really affected no matter how wide the back-iron extension is. This is within expectation, as it is difficult for flux leakage to be significant with a shallow back-iron extension as verified by Fig. 6 (b), which plots the maximum flux density in the air-gap corresponding to the lines in Fig. 6 (a) for a BIE width of 4mm
Summary
W ITH the increasingly stringent emissions legislations, there is an unprecedented demand for transport electrification, be it for rail, marine, aerospace or automotive. One major bottleneck in the thermal improvement of motors with indirect cooling is the equivalent thermal conductivity perpendicular to the winding orientation in the slot which is poor due to the multiple layers of insulation involved (typically the insulation build up consists of wire enamel, slot impregnation resin, air-pockets, and the slot liner) [10, 11] In light of this some novel cooling methods targeting directly the heat directly inside the slot have been proposed. A novel heat path has been proposed and experimentally validated in[5], where a piece of thermally conductive material is inserted into the slot of a low frequency machine, benefiting approximately from a 40% hotspot temperature reduction for the same current loading This method adds extra eddy current losses, which might nullify the effect the heat path brings, especially with higher frequency machines as in the case of automotive traction.
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