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The inverter fed induction machine is becoming one of the most important drive application due to speed variability, dynamics, and robustness. These advantages are possible by the development of high-power transistors. On the other side these fast switching components lead to steep voltage rises and high stress on the winding insulation of the machine. Faults in the winding system of a machine can cause a dramatic damage due to their fast developing behaviour. An early detection of winding faults can reduce the consequences. In this paper a method is investigated to detect stator winding faults in their early stage. The method is based on voltage pulse excitation done by inverter switching. Current response measurements and subsequent signal processing lead to a fault indicator with high precision and low dependence on the machine point of operation. Measurements on a specially designed machine are carried out to determine the sensitivity of the fault indicator to early stage faults.