Thermal analysis of high-speed induction machines

Abstract

A solid-rotor induction motor is suitable for rotation speeds 20 000-200 000 rpm in which the rotor surface speed exceeds 150 m/s. These machines need effective cooling because of their high power densities, and a proper model is needed for thermal analyses. The main aim of the research was to develop a thermal model for the solid-rotor induction machine. Another objective was to estimate the maximum power of the motor construction at different rotation speeds. Friction and gas-flow losses form a significant part of the total losses in high-speed electric machines. According to the measurement results presented in this thesis, these losses can be predicted by analytical equations. In addition, surface roughness caused by the stator slot openings does not significantly increase the friction losses in the air gap. A thermal-network model for the high-speed induction motor construction has been developed. The model is valid at different rotation speeds and cooling conditions of the machine. This results from the implementation of the friction and gas-flow losses, as well as from the convection heat-transfer coefficients to the model. The calculated temperature rises of the stator winding were within ±10°C the measured ones in the two high-speed motors tested. The thermal model was used to estimate the maximum power of the high-speed induction motor construction. For this reason, eight motors running at 50 000, 100 000, 150 000 and 200 000 rpm were analysed. The maximum power decreases with the rotation speed to the power of 2.1-2.2. The maximum power is obtained at the slip which gives the highest electrical efficiency of the machine. The power depends greatly on the coolant flow rate blown through the air gap. The utilisation factor and efficiency of the motor decreases when the rotor surface speed increases.