THERMAL MODELING OF HIGH-SPEED INDUCTION MACHINES

High-speed electrical machines need effective electromechanical and thermal design tools because of their mechanical construction, wide operation-speed range and high power density. In addition, the practical experience associated with the high-speed machines is still rather limited. The work presents a thermal-network model for highspeed induction machines. In the modelling, special attention is paid to cooling aspects, such as friction losses and convective heat-transfer coefficients. After the theoretical part, the report deals with the experimental results obtained by two highspeed motors. The first motor was tested up to a power of 51 kW at 100 000 rpm (water pump) and the second of 34 kW at 50 000 rpm (air compressor). Because of the direct coupling between the electrical motor and turbomachine, the losses were evaluated by calorimetric methods, no-load tests and retardation tests. The friction losses measured were 1.4 higher than was expected. On the other hand, the cooling losses were much lower than the calculated ones. The method of measurement was not accurate enough to verify the calculated electrical losses. After the loss analysis, temperature-rise tests were carried out for the motors. By correcting the friction and cooling losses according to the measured results, a satisfactory agreement between the computed and measured temperature rises was achieved. In the stator winding, the difference was +/- 15 % for both the motors. The sensitivity analysis indicated that the accuracy of the thermal model is affected more by estimates for the losses than for the convective heat-transfer coefficients.