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This paper presents a new method for observing the rotor temperature of high-power squirrel cage induction machines using measurable variables. The method is based on the fact that the rotor resistance depends on the actual rotor temperature. The main problem is to separate the changes in the rotor resistance due to temperature and skin effect. By comparing the input impedance with a known circle diagram measured during commissioning, it is possible to calculate changes in the rotor temperature. Further analyses also make it possible to obtain the absolute rotor temperature at any time. Results of procedure testing are demonstrated through computer simulations and evaluations of data recorded on a 75-kW test machine.