This paper describes the thermal analysis of an external-rotor brushless permanent-magnet motor used in a traction application. The machine has a nonstandard open-slot stator lamination and is wound with a single-layer concentrated "modular" winding. This structure potentially has several heat transfer advantages compared to the more traditional double-layer concentrated winding. An analytical lumped-parameter thermal model has been developed for the machine to gain an understanding of the main cooling mechanisms and to investigate the potential heat transfer advantages. The thermal advantage of the modular winding over the more conventional 1.5-slot-per-pole design is presented. Results from the developed thermal model have been validated experimentally on a prototype wheel hub motor.