Thermal loading can induce mechanical stresses in the windings of electrical machines, especially those impregnated with epoxy resins, which is mostly the case in modern traction motors. Although designers look for cooling techniques that give better performance in terms of the power density and efficiency of the machine, several thermal cycles can lead to fatigue and the degradation of the copper insulation, epoxy and consequently the windings. In this paper, the performance of different cooling techniques has been compared based on the temperature distribution and the mechanical stress induced in the windings. Three-dimensional finite element thermo-mechanical models were built to perform the study. Two different variants of water jacket cooling, two configurations of direct coil cooling and two cases of combined water jacket and direct coil cooling methods have been considered in the paper. The results show that the combined water jacket and direct coil cooling perform the best in terms of the temperature and also the mechanical stress induced in the windings. An experimental set-up is built and tested to validate the numerical results.

A measurement setup is designed and constructed to experimentally investigate the thermohydraulic performance of a novel direct rotor cooling method for an interior permanent magnet synchronous machine, where oil is pumped through weight saving channels of the rotor iron close to the magnets. The setup consist of a stationary rotor assembly where heat is dissipated in an aluminum sleeve surrounding the rotor outer surface. Measurements are performed for different oil inlet temperatures, flow rates and heat losses. It is shown that tangential temperature variations can be neglected and axial variations are as expected. The conductive resistance within the rotor assembly is similar for each measurement point as the differences are within the measurement uncertainty. The convective resistance decreases with an increasing flow rate, inlet temperature and heat dissipation. From correlations in scientific literature, it is expected that the convective resistance will further reduce with increasing rotational speed of the rotor. A comparison of these results to a standard hollow shaft cooling method shows a great potential of the novel direct rotor cooling method to further increase the power density of electric machines.

The increasing concern about global warming has led to a search for more sustainable solutions for energy use in the residential, industrial and transport sectors. In the automotive industry, a way...

Some applications expose the electric motor to severe thermal cycling, which negatively impacts the motor winding mechanically. Such impact results in loss of adhesion, abrasion, cracking, and wear of different components within the slot and the motor, leadingtodegradation or complete failure. Before relating the mechanical stress resulting from thermal stress to the failure of the motor, a deep investigation is carried out to understand the physical phenomena inside the slots as a reaction to heating and cooling. The first heating and cooling cycle in the life of a motor is the curing process of the impregnation material, and thereafter, many heatingcooling cycles occur during normal operation. In this article, mechanical strain on hairpin winding resulting from the curing process of the impregnation material is investigated to get an understanding of residual mechanical stresses inside a motor after curing. An impregnated specimen is exposed to thermal cycling to study the resultant mechanical stress and its related parameters. The experimental results are compared with the simulation results, and they correspond well. Different impregnation materials are checked to allow choosing the suitable combination of materials that exposes the motor to the lowest stress.