It is a capacitor, we were finally able to shut down a similar machine and it tested and 6.8 uf. We cleaned up the leads on the "bad" one and it tested the same.
Question is, why a capacitor to ground on a shaft grounding system?
The brush assembly was not being maintained properly and there was some major arcing. This is the only brush directly on the shaft. I like the light bulb idea!
More Investigation
I may have been leading you in the wrong direction(as i was), it appears to be a shaft grounding system. The brush being directly on the shaft before the compressor, connected to the resistor?, then to ground through the motor casing.
I think I'm looking for resistor...
I am troubleshooting a 1967 GE synchronous motor used to run a reciprocating compressor. It is a 1000 HP machine. I am trying to figure out the size and purpose of the capacitor connected to the DC field. Is it there to absorb voltage fluctuations?
The capacitor has burnt up and can't be...