Thermal Modeling of Directly Cooled Electric Machines Using Lumped Parameter and Limited CFD Analysis

This paper presents a practical approach to model thermal effects in directly cooled electric machines. The main focus is put on modeling the heat transfer in the stator winding and to the cooling system, which are the two critical parts of the studied machines from a thermal point of view. A multisegment structure is proposed that divides the stator, winding, and cooling system into a number of angular segments. Thereby, the circumferential temperature variation due to the nonuniform distribution of the coolant in the cooling channels can be predicted. Additionally, partial computational fluid dynamics (CFD) simulations are carried out to model the coolant flow in the cooling channels and also on the outer surface of the end winding bodies. The CFD simulation results are used as input to the analytical models describing the convective heat transfer to the coolant. The modeling approach is attractive due to its simplicity since CFD simulations of the complete machine are avoided. The proposed thermal model is evaluated experimentally on two directly cooled induction machines where the stator winding is impregnated using varnish and epoxy, respectively. A good correspondence between the predicted and measured temperatures under different cooling conditions and loss levels is obtained.