Investigation of an Integrated Evaporative Cooling Mechanism for an Outer-Rotor Permanent Magnet Machine

Abstract

This paper investigates an integrated evaporative cooling mechanism for an outer-rotor, axial flux permanent magnet machine. The success of the cooling mechanism relies on two parts: an internal means to transports heat from the windings to the casing, and an efficient heat sink to reject the heat from the casing to ambient. The paper demonstrates how a heat sink on the casing dictates the size of the active components in the machine and influences the choice of the working fluid. An experimental investigation on a wick-assisted cooling mechanism was performed. This mechanism was found to be adequate for an evaporatively cooled electrical machine. The effects of the wick were compared with immersion cooling and it was found that the wick prevents a temperature overshoot and keeps the winding at the boiling temperature once a self-sustaining steady-state condition is reached. However, the fluid dynamics inside the machine move across different flow regimes as the motor is accelerated. This plays an important role in the design of the wicking geometry. The paper presents a general systems integration and shows how different limiting factors come into play during different operating points of the machine.

NOMENCLATURE

a	=	aspect ratio
A	=	surface area, m2
ho	=	film thickness, m
l	=	cylinder height, m
L	=	latent heat of vaporization, J/kg-K
	=	mass flow rate, kg/s
P	=	pressure, Pa
Q	=	heat flow rate, W
r	=	acceleration, m/s2
R	=	radius, m
Rth	=	thermal resistance, K/W
Re	=	Reynolds number
Vf	=	fluid volume, m3

Greek Symbols

θ	=	contact angle,
μ	=	dynamic viscosity, m3/kg
π	=	pi, 3.142
ρ	=	density, m3/kg
ω	=	rotational speed, rad/s
σ	=	surface tension, N/m

Subscripts

c	=	condenser
C	=	capillary
e	=	evaporator
l	=	liquid
∞	=	ambient

Additional information

Notes on contributors

Robert Camilleri

Robert Camilleri graduated in mechanical engineering from the University of Malta in 2005, and holds a M.Sc. in gas turbine technology from Cranfield University. He has subsequently worked as a research engineer at Seifert Systems Ltd, specializing in electronics cooling and heat exchangers. He joined the Energy and Power Group in 2010 as a research assistant, where he investigated novel evaporatively cooled electrical machines. He is completing his D.Phil. in engineering science at the University of Oxford with his thesis focusing on conjugate heat transfer in high current density electrical machines.

Malcolm McCulloch

Malcolm McCulloch is an associate professor in the Department of Engineering Science at the University of Oxford and heads the Energy and Power Group. He has active research programs in the three sectors of domestic, transport, and renewable generation. The domestic sector is one of the largest energy sectors in the United Kingdom. A key research area is the development of smart feedback metering that provides useful information to enable behavior change. This work lead the spin-off Intelligent Sustainable Energy, of which he is both a founder and nonexecutive director. This has merged to form Navetas Energy Management. In the transport sector, research is ongoing in developing power trains for electric and hydrogen vehicles. A successful project was that of the Morgan LifeCar—the first ever hydrogen sports car. This project lead to the development of high-efficiency, low-weight motors using new materials—the yokeless and segmented armature motor. This has resulted in the Oxford spin-off company Oxford Yasa Motors, of which he is also a founder and nonexecutive director. He is also the director of the Institute for Carbon and Energy Reduction in Transport, based at the Oxford Martin School. In renewable generation, novel lightweight low speed direct-coupled generators are being developed along with a transverse axis tidal turbine. He has more than 100 publications and 15 patents and patent applications.