1 Abstract— This paper addresses the analysis of the main thermal phenomena in brushless permanent magnet machines such as demagnetization of the magnets due to the heat created by losses. There are investigated several methods to mitigate complex loss components, i.e. magnet losses and AC copper losses. The efficiency of the cooling system, with natural or forced convection is also presented. Index Terms-- Brushless permanent magnet motors, thermal analysis, cooling system, convection, radiation, conduction, magnet losses, proximity losses. I. NOMENCLATURE ρ = electrical resistivity α = thermal coefficient of the electrical resistivity µ = magnetic permeability k h.e = hysteresis and eddy-current loss coefficient τ = magnet full pole pitch width k = thermal conductivity II. INTRODUCTION HE brushless permanent magnet machines (BPM) represent the electrical machinery topology with the highest torque density. In the last three decades, the development and manufacturing of BPMs have seen a very significant interest from various industrial fields: hybrid and electric vehicles, renewable energy generation, aerospace, home appliances, etc. Being driven by the rare earth elements extraction and processing, high energy magnets are used widely in the BPM manufacturing. Theoretically seen as an everlasting source of energy within the electrical machine system, the permanent magnet materials may be irreversible demagnetized and hence loosing energy due to the thermal stress and high faulty electrical loads. M. Popescu is with Motor Design Ltd., Ellesmere, U.K. (e-mail: mircea.popescu@motor-design.com). D. A. Staton is with Motor Design Ltd., Ellesmere, U.K. (e-mail: dave.staton@motor-design.com). D. G. Dorrell is with University of Technology, Sydney, Australia (e- mail: David.Dorrell@uts.edu.au) D. Hawkins is with Motor Design Ltd., Ellesmere, U.K. (e-mail: dougie.hawkins@motor-design.com). Figure 1 Demagnetization curves for NdFeB type magnet [1] There are two main high energy magnet materials that are currently employed, NdFeB and SmCo types. Fig. 1 show a typical set of demagnetization curves for NdFeB magnet. When the magnet’s operation point is below the knee point of the demagnetization curve, the magnet is irreversibly demagnetized. The thermal stress on the permanent magnets is created by the losses dissipated in the machine. One can protect thermally the permanent magnets by mitigating local losses as the induced eddy-currents losses in the magnet blocks, or via an efficient cooling system. Depending on the application, cooling systems with natural convection (totally enclosed non-ventilated), or forced convection (air or liquid cooling), or radiation cooling (case of BPMs operating in vacuum environment) can be employed. Thermal analysis of electric motors is generally regarded as a more challenging area of analysis than electromagnetic analysis in terms of the ease of constructing a model and achieving good accuracy. The thermal analysis of an BPM motor is a 3- dimensional problem, with complex heat transfer phenomena to solve such as heat transfer through complex composite components like the wound slot, temperature drop across interfaces between components and complex turbulent air flow within the end-caps around the end- winding that includes rotational effects. This study presents various solutions for an efficient thermal management of the permanent magnets in BPMs. The mechanism for the main loss components in BPMs is Study of the Thermal Aspects in Brushless Permanent Magnet Machines Performance Mircea Popescu, Senior Member, IEEE, David A. Staton, Member, IEEE, David G. Dorrell, Senior Member, IEEE and Douglas Hawkins T