Two-Frequency Heat Run - A Method of Examination for Three-Phase Induction Motors

The system equations for a three-phase induction motor fed at two frequencies are derived. The system of equations for the steady-state quantities, characterizing the average steady- state operating point, and for the "deviations" which determine the superposed multi- frequency oscillations, are established. The scope and the mutual dependenceof the systems of equations do not permit a general analytical solution; they have to be solved by numerical iteration. Considering the results obtained by numerical calculation , the operational behaviour of the three-phase induction motor fed at two frequencies is discussed. A heat run performed with the induction motor by the two-frequency method and the results obtained are compared with the results of a conventional heat run in which the motor shaft is mechanically loaded. It is found that, provided the voltage and frequency of the back-up system are judiciously chosen, the noinal temperature rise in the induction motor can be accurately determined by the two-frequency method.