Case Study - 34 MW generator, was not generating sufficient reactive power. Checked at FSNL, PMG current is 12 A (typically 4A). Checked all points, found main rotor IR = 0 MÎ©.
Rotor threaded out & DC voltage drop test conducted on rotor (DC to winding, checked drop between winding & ground). Rotor coils = 12. Large drop occurred between 2nd & 3rd coils, +ve to –ve changeover occurred at the 6th coil. Boroscope inspection showed melted copper between 2nd & 3rd coils under the turbine end retaining ring.
Next, passed 250 A DC current through the rotor body & checked drop between winding & body. Null point was detected between the 2nd & 3rd coils about 700 mm for the turbine end retaining ring. We have now started the repair activity.
Other observations: Both bearings have significant pitting marks on them, forming an axial band across the bearing lower halves. Rotor winding resistance has gone down from 0.235 Î© to 0.116 Î©. Bearing pedestal insulation is healthy.
1. Do we have a single or double rotor earth fault? Can we expect a fault at the 6th coil also?
2. Have the coils been shorted too? Or is the lower resistance due to the earth fault?
3. Why did the bearings get damaged by shaft current? As these are white metal bearings with no prox. probes, how does one detect this problem in advance? I have seen high amplitude, high frequency peaks in case of anti-friction bearings. Would these also show up for plain bearings?
4. A shaft grounding brush was provided & was found healthy. Why did it not take care of the shaft current? Could the earth fault be responsible?
5. Could electrical signature analysis have some early indication of this problem? Has anyone experience in monitoring generator rotors with it? We have an EMPATH unit that could be tried I guess. It did not pick up anything in the stator current & voltage when we checked two months back.
I would appreciate your views on this matter.
AdityaThis message has been edited. Last edited by: Aditya,
Good to hear from you again!
I'm not an expert in the DC motor field area, but do know of one and have requested him to answer your post. I do have some questions.
1. What is the generator exciter supply?
2. Are the bearings split babit with pressurized oil flow?
3. What is a white metal bearing?
4. Where was the pigtail end of the shaft grounding brush attached and was the brush on the exterior of the motor?
I have used the PdMA RMS inrush/startup current test on a wound rotor but no other MCSA tests. I see the Empath manual doesn't give a lot of help either.
Hopefully we'll hear from our expert soon,
1. The generator exciter is in two parts, a PMG whose output is fed to an AVR; which then supplies the field of the main exciter. The armature output is rectified by diodes & fed to the rotor winding.
2. Yes, the bearings are of split babbit type with pressurized flow.
3. A white metal bearing is the same as a babbit bearing (local term over here).
4. The brush was on the exterior of the generator, close to the turbine end bearing. The pigtail was screwed to the generator base.
We opened the retaining rings today. The coils have shifted axially, overhang spacers are damaged & some turns of the coils have melted & fused. A full rewind is required.
I have yet to see any generator fault picked by ESA, but that was with the stator current. Will start collecting rotor data fron now.
I'm looking forward to hearing from your expert too. I got some recommendations to install air-gap search coils, would like to hear others experience with this technology.
Read your post with interest. We have a lot of large rotating DC equipment in our plant. Following the messages it sounds like a complete rewind is in order. To your question # 3 we only insulate one bearing pedestal usually on the commutator end. This is to keep eddy currents from traveling through the bearings as you already know. The shaft grounding brush is usually located on the shaft with the ungrounded bearing pedestal. I have seen pitted bearings in house usually caused by pedestals insulation becoming compromised by dirt. The ground you had in the machine may have contributed to shaft currents that were in excess of the capacity of the grounding brush. Going forward I would suggest at a minimum quarterly megger checks and annual pole drop tests on the fields. I would also continue to use the new technology which is in it's infancy for DC machines.
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