Determination of rotor-stator heat exchange coefficients in the case of totally enclosed machines: Application to an integrated starter-generator

Some applications use totally enclosed electrical machines; in these cases, the use of forced internal air cooling via fans becomes impossible. The major part of losses must be evacuated through the stator. Moreover, some machines such as the induction or wound rotor synchronous machines create rotor losses which have to be first evacuated through the air gap. Even a permanent magnet machine, included in a more complex system like the integrated starter generator (ISG), may have equivalent rotor losses to be evacuated through the air gap. An accurate determination of the rotor-stator heat exchange transfer phenomenon is important for the temperature determination and afterward for the machine design. This can be established using the "Taylor-Couette flow" which plays an important role, especially in variable speed applications. This paper proposes to describe this phenomenon, to model it on the basis of nonlinear thermal resistances (mainly linked to the rotation speed and air gap temperature) and to validate this proposition by comparing the analytical approach to the experimental results issued from a permanent magnet ISG.