We have experienced collector ring standstill, or ghost or footprint, marks on our GE 7H2 collector rings. It was noticed on a regular inspection after the unit had been offline for sometime - on turning gear for one hour a day.

Since the discovery we have changed our brush rigging - holders including springs - and have continued to monitor with no appreciable progression of the marking. The unit has relatively, to the sister unit, higher vibration (seismic - collector) but does not enter the alarm range.

GE provided an engineering response which indicated the rings should be resurfaced to ultimately fix the problem and increase preventative inspection routine frequency. We have gone out for proposals to have the rings resurfaced and, as expected, it is a rather costly. That being said the ultimate conclusion could be ugly - e.g. ring fire. Keep in mind we have no empirical data, e.g. vibration at the ring face, to point to but we think the symptoms speak for themselves. We did take some shaft run out data and it is slightly (one thou) out of spec - the accuracy of the setup could be argued so I would not call it definitive.

We feel it will not get better but we may be able to control it with increased maintenance. The one concern if it continues the damage to the ring(s) surface gets past the point of being able to correct it through resurfacing, or skimming, we may be faced with ring replacement.

I would like to hear of any experiences with these issues, outcomes, solutions, etc.

 

GJRS... please provide information on slip-ring material, brush composition, environmental conditions, current-density, and if both the positive and negative rings have similar images!

Regards, Phil

 

I've seen this before many times - probably the most nagging problem we have had on our 3 units that has never been completely understood.

The footprints appear as though current was applied while the rotor was stopped, but the footprints actually develop on line.

You can sometimes see the microscopic sparking if you catch it in the act.

Some things to check are:

* collector ring vibration
* proper spring tension
* proper brush material
* contamination of brushes with organics
* carbon buildup on brush boxes that cause friction
* brush angle of attack
* oil contamination
* oil vapor contamination
* organic vapors on turbine floor - paints, solvents, BBQ'ing
* condition of doghouse filters if forced cooled
* condition of door gaskets

If you hook up a clamp on DC current meter with an output to a high speed recorder you might see that the brushes turn on and off during the revolution like semiconductors. You may also see unequal sharing of current between the brushes. Sometimes it might help to scrape the surface of the brushes. Burnishing the ring with a glass fiber burnishing rod from Morgan AMT may help

Excessive brush vibration can increase the potential for footprinting, but I've seen just as many footrprinting issues with very low vibrations.

More spring tension will decrease the potential for footprinting but will also result in more mechanical heating of the ring, as well as faster brush consumption. So you need to be careful.

There are Surface Rounding Brushes (SRB) available that have a thin wafer of abrasive material embedded in the brush that might get rid of the footprints. We don't have a lot of experience with these, but it is worth a try. We have perhaps had a dozen experiences with footprinting over the years, and you can generally hang on for a month or two, but they almost always have progressed to the point of needing to shut down and grind.

Cutsforth Electrical Products could help you with this. Morgan AM&T and Cutsforth both supply SRB brushes.