Improved thermal modelling of induction motors for design purposes

The refinement of the methodologies for the electromagnetic design of induction motors and the increasing employment of these machines in highly variable operation conditions leads to promotion of the efforts in order to improve their thermal modelling: to this end, the circuit approach is still valid, because it allows an easier computer implementation and reduces calculation time and computer resources. Sometimes the use of thermal networks shows some discrepancies between experimental and simulation results, mainly due to the following causes: inadequate discretization of losses sources and of nonisothermal parts; and uncertain knowledge of thermal parameters. The authors analyze these problems, particularly with reference to: discretization level of the machine, both in axial and in radial direction; characterization of the ventilation conditions between frame and ambient, in the case of totally enclosed fan cooled machines; and improvement of the circuit modelling of the thermal exchange cooling air-machine. For design calculations, a special program suitable for the steady state operation analysis is described: its results show a satisfactory agreement with experimental results and allows an easy variation of the design