A flux probe test determines the existence of rotor winding shorted turns by measuring the magnetic flux produced by each coil in a generator's rotor.

The amount of magnetic flux produced by a coil is the product of the field current and the number of active turns in the coil (amp-turns or magnetomotive force [MMF]). If a shorted turn is present, the number of active turns in a coil is reduced and the resulting decrease in magnetic flux produced by the coil can be determined by analysis of the flux probe waveform.

The annotated Analysis Graph below shows an analysis graph from a rotor with 6 coils/pole and a short in Coil 5-Pole A (Coil 5A). The rotor cross-section shows that each coil has two slots, with one slot leading and the other slot lagging the pole face in terms of the rotation direction. Raw flux probe data (red) shows numbered peaks corresponding to each coil slot displayed in the cross-section. The flux density curve (dotted blue) shows the magnetic field strength around the rotor. The flux density zero-crossing (FDZC) position is marked with a vertical green line. The coil aligned with the FDZC position has the best sensitivity for true turn shorts and the least sensitivity false turn short indications. False turn shorts are peak height differences that are not related to true shorts. These are normally modulation artifacts that grow larger as the FDZC position moves away from the affected coil.

In order to get the best sensitivity for detecting shorted turns in all rotor coils, a series of waveforms is recorded as the load on the unit is varied. Real (MWS) and reactive (MVARS) load changes will change the FDZC position of the waveform. An ideal test set would have waveforms with FDZC positions aligned with each coil in the rotor. For a real test, a waveform is analyzed that has the closest FDZC position to each rotor coil.

Since it is important to record data over a wide range of loads, flux probe testing during startups or shutdowns is ideal. The Generatortech software automatically records optimized waveforms for each rotor as load on the generator is changing, so the equipment can be setup before a startup or shutdown and be left to record the necessary data. The startup or shutdown can be performed as is normally done by the plant. If testing cannot be done during a startup or shutdown, then testing is done with the widest range of real power permitted. If possible, reactive load changes at minimum and maxim real loads can increase detection sensitivity in the smallest and largest rotor coils.

The animations below show a test in a rotor with 6 coils/pole and a short in Coil 5-Pole A (Coil 5A). The seven waveforms were recorded during a startup that reached full load. The FDZC position moves to the right as the load changes from no-load to full-load The FDZC position is determined by the Rotor Power Angle, which is the angular separation between the north pole of the rotor and the north pole of the rotating stator field. The rotor power angle changes with both input torque changes (Real Power changes) and the rotor magnetic field strength (determined by Field Current, which can be altered through Reactive Power changes).

This rotor had a single turn short in Coil 5-Pole A (Coil 5A), which was not well detected at higher loads, however, the Coil 5A turn short was very well detected at low loads (see Lead Slot Overlay Graph animation. The Trend Curve shows the calculated turn short indication for Coil 5A was maximized when the FDZC position was near alignment with the #5 coil slots.

Overview of Shorted Turn Theory & Flux Probe Testing

Shorted Turns: Cause & Effect