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Modeling of convection heat-transfer at the end-windings for thermal networks using lumped-parameters has been widely discussed in the literature. Unfortunately, the resulting coefficients are highly influenced by the end-winding shape, the area in the vicinity of the end-windings, the cooling method of this region (if any), as well by the size and power of the machine. This makes it extremely difficult for the designer to choose a suitable coefficient for the thermal analysis during the design stage of an electric motor. A methodology to obtain the end-windings convection heat-transfer coefficients for fractional-slot concentrated winding permanent-magnet synchronous motors is proposed in this article. Machine designs with both internal and external rotors will be considered. The experimental tests required for the model characterization are described in detail. In this article, general expressions of the convection heat-transfer coefficients between the end-windings and the housing end-caps are proposed for both internal and external rotor designs. Differences observed with the results reported in the literature are also discussed.

Torque density of electric motors can be increased if the rotor is placed in the exterior. However, this makes heat extraction more difficult. Study of the thermal behavior at the design stage of the machine can be addressed using FEM and CFD methods. However, they are computationally expensive, and consequently not suitable for the evaluation of multiple designs. A short-time consuming lumped parameter thermal model for external rotor permanent magnet synchronous machines is presented in this work. It allows a fast evaluation of multiple machine design options, being suitable for its use in analytical machine design tools. In addition, comparative analysis of the thermal performance of internal and external rotor designs is provided. The developed model is validated by commercial software achieving good results.

Electric motors for traction applications are traditionally designed with internal rotors. However, torque density can be increased with the rotor in the outside. Two permanent magnet synchronous machines (PMSM) with fractional slot concentrated windings (FSCW) and internal and external rotors, designed with same electromagnetic properties and volume restrictions, are compared to prove which one gives the best performance. The external rotor machine increases the torque density by 24.9% at the expense of higher torque ripple.