Detection of air gap eccentricity in the presence of stator inter-turn fault of inverter fed induction machines

Online monitoring of air gap eccentricity using only current and voltage sensors is still considered most difficult during mains operation. At high dynamic inverter- fed operation it then becomes extremely challenging. The only industrially accepted method so far is the usage of the vibration spectrum obtained from extensive measurements. As has already been shown, the detection and exploitation of the machines transient phase reactance offers the possibility to determine machine asymmetries with high accuracy. The detection is realized by a transient stimulation of the machine with voltage pulses by the inverter as well as by measuring the reaction of the machine current. The stimulation can be either realized by a specific switching sequence of the inverter, or by using the pulses generated by the PWM in specific operating points. In the resulting detection signal however, all effects that influence the transient phase reactances superpose. Whereas the inherent influences like spatial saturation or slotting can be identified and eliminated, the air gap eccentricity and stator winding inter turn short circuit and especially their simultaneous occurrence need additional attention and signal processing. Measurements on specially manufactured machines are presented to show both the separated occurrence of air gap eccentricity and an inter turn isolation fault in the stator windings, as well as their simultaneous incident. As both fault conditions have strong impact on the transient reactance their interaction has to be determined in all operating states to enable an accurate separation of both defects.