Detection of rotor eccentricity in wound rotor induction machines using pole-specific search coils

Condition monitoring is a standard method for scheduling maintenance and ensuring that catastrophic failures do not occur. It is used in many applications where there is high capital cost or the location is remote. Condition monitoring is increasingly used in renewable energy as stated in [1]-[4] and these mostly address the rotating equipment and in particular the electrical machinery. In terms of faults in rotating machines then there are several detection methods as addressed in [1]-[4]. The faults were listed in [5] where 41 % are bearing faults, 37 % are stator faults, 10 % are rotor faults and 12 % are others. In this digest the monitoring of the non-centering of the rotor in the stator bore is addressed (called rotor eccentricity); this can indicate bearing faults or miss-fitting. Vibration monitoring is popular and the vibration signature can be complex. For overheating problems then thermal monitoring can be used. Another popular method is to use line current monitoring where signature current sidebands are monitored [6]. However, these are relevant to cage induction machines and in renewable energy, particularly wind turbines, wound rotor induction generators are used and these do not produce the same sideband currents as shown in [7]. A chart showing the relevant airgap flux waves for cage and wound rotor machines are shown in Fig. 1. The vital difference that negates cage monitoring systems being applied to wound rotor machines is the absence of the rotor cage that can also have, for a 4 pole machine, 2 and 6 pole MMF waves induced into the cage. There have been several attempts to use search coils to assess for rotor eccentricity [8][9] though these do not seem to have been developed. In this digest pole-specific search windings are put into the stator slots. Since the example machine is a 4 pole machine then 6 pole and 2 pole search windings are put into a machine at the top of a slot. This is in the area where a wedge is often fitted.