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Induction motors have nowadays been more frequently used with frequency converters. The use of frequency converters affects the behavior of the machine and makes fault diagnosis more complicated. One of the widely used diagnostic methods is motor current signature analysis (MCSA). This method is based on measurement of sidebands in the stator current spectrum. Those sidebands are usually located close to the main supply frequency. Frequency converter causes supply frequency to slightly vary in time and, as a result, some additional harmonics in the current spectrum are induced and sidebands are reduced. Those harmonics can be easily misinterpreted as the sidebands caused by the rotor faults. Although the rotor faults in the case of frequency converter supply are not so often, precaution is still necessary. In this paper the experimental results of fault diagnosis carried out using standard supply and using frequency converter have been compared and presented. All tests were performed on 22 kW, 380 V, 1470 r/min induction motor. The current spectra are given for the motor with two broken bars with both types of supply.