Thermal analysis of TEFC induction motors

In order to manufacture smaller and more efficient electric motors an accurate prediction of the motor thermal performances at the design stage is a necessity. In addition, the thermal performance often needs to be analyzed when a motor is used in a different way to which it was originally designed for. An important example is the use of variable speed drives with induction motors designed for mains supply. These thermal requirements mean than accurate and reliable motor thermal models are required by the designers. In this paper the thermal behavior of Total Enclosed Fan Cooled (TEFC) induction motors are investigated using a commercialiy available software package called Motor-CAD((R)). This software has been used to develop thermal models for five industrial induction motors from the same series (rated powers of 4 kW - 7.5 kW - 15 kW - 30 kW - 55 kW, 4 poles, 380 V, 50 Hz). The models produced have been experimentally verified. In particular, for each motor, the results obtained using the software's default parameters have been compared with solutions obtained using tuned parameters obtained through thermal tests. The analysis and the results reported in the paper allow us to give some general guidelines useful for obtaining accurate thermal models of TEFC induction motors. In particular we give advice on developing models for use in the calculation of motor derating when changing from mains to inverter supplies.