Thermal behaviour of induction motors under different speeds

Recently different electronic speed controllers have been proposed and used for changing the speed of induction motors (IM) under different working conditions. As is known, the thermal condition of IM significantly depends on its speed of operation. The common thermal circuit of IM, which consists of a system of heat sources interconnected by thermal conductivities, can also be used for studying IM thermal behaviour under different operating speeds. This circuit can be simplified to a system of three main heat sources (stator and rotor windings and a stator magnetic circuit) interconnected by three generalised thermal resistances. The value of these thermal resistances and their dependency on speed was investigated by new methods of testing proposed in this study. Using these methods, the thermal dependency on speed may be obtained with high accuracy so that the difference between the test results and calculation does not exceed 3-5%. Two totally enclosed, fan-cooled induction motors were tested by these methods.