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1 Design and Optimization of SMPM Electric Machines Incorporating Direct Winding Heat Exchange

S.A. Semidey*, J.R. Mayor † *The G.W.W Woodruff School of Mechanical Engineering, Georiga Institute of Technology, 813 Ferst Dr, Atlanta, GA, 30332, USA, andrew.semidey@gatech.edu †The G.W.W Woodruff School of Mechanical Engineering, Georiga Institute of Technology, 813 Ferst Dr, Atlanta, GA, 30332, USA, rhett.mayor@me.gatech.edu

Keywords: Thermal modeling, Thermal management, electric machine, PM machine, optimization Abstract This work presents a novel advanced cooling technique termed the direct winding heat exchanger (DWHX). In the infinite reservoir case study, the torque density based on motor volume was 48.9 N-m/L and 38.4 N-m/L for a fluid temperature of 20°C and 90°C respectively. The torque density based on electromagnetic volume was 105.7 N-m/L and 82.9 N-m/L for a fluid temperature of 20°C and 90°C respectively. 1. Introduction The push towards hybrid electric vehicle (HEV) and electric vehicle (EV) power trains has created a need for high torque density electric machines. In addition to the expansion of the HEV and EV passenger car fleet, there are other applications that require high torque density machines such as off road construction equipment, freight trucks, and electric actuators for flight control surfaces in aircrafts. Currently, the limiting factor of torque density in these cases is the thermal degradation of the windings. To overcome this issue, advanced cooling technologies are required. The techniques to improve the thermal transport processes in small scale (<100kW) electrical machines has been focused mainly on improving the internal and external air flow across the electrical machine in order to increase the effective heat transfer coefficient and thereby remove more heat from the machine [1, 2]. Nakahama et al used visualization experiments to understand flow patterns inside an open type motor and suggested corrections to flow separation issues that improved heat transfer [3]. Micallef et al [4] and Mugglestone et al [5] studied the flow around the end windings of the electric machine in order to improve the heat transfer from the end windings to the frame. These methods are limited because they rely on the winding heat to be conducted through the stator. Water cooling has typically been used in large motor applications but there has been some recent activity investigating water cooling for smaller scale motors. Direct Lamination Cooling (DLC) was used by Rippel et al to increase the current density in an 75kW induction machine by passing coolant directly through channels in the stator as seen in [6]. This work changes the flux paths inside the stator. Also DLC primarily removes heat from the stator and relies on conduction to cool the windings. Rahman et al presented a 42kW axial flux machine with integrated aluminum water cooling ring for HEV traction drive applications as seen in [7]. This application is an obvious cooling technique applied to a specific electric machine topology. Phase change cooling is typically applied to thermal management of large scale electrical machines on the order of several hundred megawatts [8]. Zhang et al. investigated the use of evaporative cooling for under water applications as seen in [9]. The method for applying evaporative cooling involved submerging the stator in coolant which lead to pool boiling. The coolant was condensed on the surface of the frame which was cooled by a water jacket. The location of the condenser depends on the free surface which depends on the orientation of the machine. The authors noted that a difficult challenge, large temperature gradients, arises from the increased heat transfer from evaporative cooling. Advanced cooling in electric machines is limited and there is little knowledge in designing electric machines with advanced cooling. An integrated thermal-electromagnetic model could provide great insight into the design of advanced cooled electric machines. The coupling of thermal and electromagnetic simulations has been very limited to this point. Lazarezic et al. showed a coupling between magnetic equivalent circuit and a simplified thermal circuit. The magnetic circuit was used to calculate the losses that could then be simulated in the thermal circuit. Both circuits were calibrated using experimental data [10]. Similarly, Alberti et al. showed a coupled magnetic equivalent circuit and thermal circuit [11]. In this work the authors calibrated there models using FEA simulations. Mezani et al. showed a combined approach that used an FEA electromagnetic simulation to calculate losses that were simulated in a thermal circuit [12]. Marignetti et al. used an FEA electromagnetic simulation to calculate losses that were then thermally simulated in an FEA simulation [13]. Dorrell et al. presented a combined analysis of an induction machine using SPEED and MotorCAD [14]. SPEED is an electromagnetic simulation software package that utilizes magnetic equivalent circuits. The authors simulated the induction machine in speed and through ActiveX were able to import the modeled machine directly into MotorCAD for thermal simulation. Most of these integrated models are empirically fit to the specific electric machine and are not useful is the design of advanced cooled electric machines. An integrated thermal-electromagnetic design tool that accounts for advanced cooling technology does not exist. This paper will present a novel advanced cooling technique. This