Back-Iron Extension Thermal Benefits for Electrical Machines With Concentrated Windings

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 75kW electric vehicle (EV) traction motor, the tradeoffs involving the losses, fluxleakage, output torque, torque-quality and the peak winding temperature with back-iron extension (BIE) and without 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. Index Terms-Thermal management, machine cooling, power density, thermal analysis, thermal resistance network, slot cooling I. INTRODUCTION ITH the increasingly stringent emissions legislations, there is an unprecedented demand for transport electrification, be it for rail, marine, aerospace or automotive. For some applications this involves hybridization of the conventional engine-based systems, while for other applications all-electric architectures are developed [1, 2]. The core component in these electrified architectures is the electrical machine, and hence its performance improvement merits detailed research in order to achieve important

Equivalent Slot Thermal Conductivity and Back-iron Extension Effects on Machine Cooling

“…ESTC is variable for different electrical machines, the effects of which on the BIE thermal benefits are investigated in this research. Two existing stator segment based on a traction machine with applied BIE geometry and without are used as a case study [22], the correlation between ESTC and BIE thermal benefits is derived. Firstly, this section details the thermal model used for the ESTC effects analysis, followed by the thermal results and discussion.…”

“…The existing 75-kW, 12-slot, 8-pole permanent magnet machine with concentrated winding, applied to an electric vehicle (EV) is shown in Fig. 2(a) with relevant detailed data listed in [22]. Temperature sensors installed in the traction machine are able to give accurate peak winding temperature, while the temperature distribution can not be captured due to the limited numbers of thermocouples.…”

“…All the nodes in Fig. 3 are linked with conduction thermal resistance or convection thermal resistance calculated with formulas in [22]. Losses marked with [•] in Fig.…”

“…In this paper, a 2.2 mm wide BIE and having a depth equal to slot depth is proposed, accounting for multiple factors, including flux leakage and reduced slot fill factor, as with the electrical machine studied in [22]. Experimental tests conducted between the original segment and segment with BIE shows that 26.7% temperature reduction is achieved.…”

“…A simple, novel, low cost and effective way is proposed in [22] to improve the motor thermal performance by extending a part of back-iron along the centerline into the slot. A significant 26.7% temperature reduction is achieved with the experimental validation.…”