Generator Question - Guru advice Invited

[JLR]

I could use some help. After running for ten hours, my Onan 12KW genset (1300 hours) tripped a number of my breakers. It is a model MDJC. On my digital multimeter, it was outputting only 95 volts at the time. The only breakers it tripped were the Hatteras breakers (2 of them) for the upper level lights and recepticals and the lower level lights and recepticals. These are the 15 amp breakers with the "test " buttons below them. Sure enough, I had my genset fellow out to fix the problem and he can not duplicate the problem. Everything appears to be running fine, frequency, output voltage etc. I am quite certain that this was not an overload situation as the breakers tripped when there was virtually no load on them. Assuming that the digital meter reading was correct, is there a reason that only these two breakers would trip (with low voltage) and not, let's say, the battery charger or the fridge or the air conditioner. I am trying to find out if there is, or is not, a problem and having had the genset fellow there today with good news (it's running fine) I am uncomfortable with doing nothing. Any thoughts or help would be greatly appreciated.

[Genesis]

Those breakers are GFCIs and will trip if there is leakage between hot and ground on that circuit.

The "90 volts" thing is not good though - it may be a precursor to failure. My old Onan did that intermittently before it decided to quit entirely - once it failed "hard" and was able to be diagnosed, the fault was a shorted winding - the cost of having it rewound was not worth it, and I replaced it with a modern Kohler....

[Nonchalant1]

Where were you measuring the current at the time it read 95V? (and come to think of it, why did you happen to be measuring at that time?)

If you were measuring that circuit as opposed to measuring at the hot and common bus, maybe the genny is OK and the circuit you were measuring is the problem?

What was the electric panel meter reading? You probably have voltage meters you can watch when the genny is running, to see what the output is in 240V and both 120V sides. If you don't see the needles dip there, on the power supply side, maybe you've got moisture somewhere between the hot and common in those wires. That'll produce corrosion that can conduct between the two and could show a voltage drop past the corrosion point. It'll also trip the ground fault circuit breakers.

Doug Shuman

[67hat34c]

if you want to extend the life of your gen then I recommend you pull the armature out and have it over haluled. I would guess it is the old style with brushes. if so then the surface where the brushes touch needs to be cleaned and the mica insulators cleaned out. this is done by taking a hack saw blade and restoring the gap. The copper had a tendency to plate the top of the mica causing it to conduct electricity. this can cause frequency to be lower. the onan service manuel shows how to do this.

[JLR]

Ran the genset yesterday for 5 hours with no problems. At the time of the problem the analog gauge read 240 but the digital meter I put on showed an output of 95 volts. Genset guy said run it until it happens again (and then he can fix it) or run it and hope it doesn't happen again. I do have a 20KW kohler as a backup but that is not really the answer.