Influence of Wire Layer Structures on the Thermal Behavior in Electrical Machine Slots

The demand for new winding technologies for electric traction drives leads to various winding types. Especially round and rectangular wire windings need to be differentiated: Round wires are advantageous in terms of frequency-dependent losses but possess lower fill factors than the rectangular wire windings. An important issue concerns the thermal conductivity of the winding in the stator slots, particularly when different wire geometries and layer structures are faced. A high thermal conductivity is essential in order to dissipate the generated heat in the windings through the laminated core into the cooling medium. The amount of heat transport out of the windings in the slots determines the continuous and peak power operation in an electrical machine. The practical proof of thermal conductivity at different wire layer structures and its coherences has not yet been investigated in terms of a realistic slot structure. Against this background, a test bench including a slot model and a suitable experimental setup is developed in this paper. Hence, slots with different wire types and layer structures are examined at the test bench and compared to each other. The main influencing factors investigated are the wire geometry, layer structure, compression of the windings and the use of insulation paper. Furthermore, the outcome of the measurements is compared to known analytical lumped circuit slot models. The results of the investigations are intended to give reference points about the realistic differences in the thermal conductivity of different slot structures and can be used to design electrical machines and their production systems. The strength of the influencing factors on thermal conductivity is demonstrated and assessed.