Temperature field of a double squirrel-cage motor during startup

During startup an electric motor can be subject to severe electrical and thermal loads. These loads, despite their relatively short durations, significantly affect motor´s lifetime and reliability. The use of the double squirrel-cage arrangement has resulted in the improvement of the startup coefficients for induction motors. However, the cage is still subject to non-uniform thermal loads and, in consequence, to overheating. These transient loads are particularly heavy during a prolonged startup, routinely encountered under tough industrial operating conditions, for example when attempting to start a motor with a locked rotor. Typical examples are starting a motor powering a longwall shearer buried under a collapsed coal face or an overloaded coal mill. A methodology is presented to compute the temperature field of a double squirrel-cage and the core of an induction motor. The procedure employs a three-dimensional model of the motor heating obtained using the control volume method. The model provides the basis for calculating the stress field within the cage. The input data to the model can be obtained from the motor windings specification sheet. The thermomechanical analyses can be performed during the design and construction phases, and may facilitate motor performance optimisation.