Thermal modelling of a claw-pole car alternator: Steady-state computation and identification of free convection coefficients

Heating of machines is a key characteristic of their behaviour. Heat sources come mainly from energy losses during electromechanical conversions (electrical in copper, magnetic in iron, aerolic and mechanical in bearings). Temperatures changes can be described thanks to thermal conduction, convection and radiation equations. Thus it is necessary to build a thermal model adapted to the simulation and sizing of electrical generators. This paper proposes and describes a 3D nodal model with sixteen nodes, adapted to a claw pole, wire-wound and self air-cooled alternator. It permits the thermal characterization of the machine thanks to only four physical parameters, shown to be sufficient to deduce the model conductances. These properties are: the thermal exchange coefficient due to free convection at the external surface, free convection coefficients into the airgap and the cavity, the thermal equivalent conductivity within the bearings. They are shown to be non-linear and identified for steady-state working conditions and a non-rotating machine. The consistency and usefulness of both the model and identifications are checked thanks to numerical studies and measurements.