MEP-804A random overspeed shutdown

[micochico]

I am helping a friend troubleshoot a recently purchased MEP-804A that randomly shuts down for an overspeed trip. Sometimes it trips almost immediately after startup and other times will run for 5+ hours. Sometimes it will actually trip immediately when cranking before the engine starts. I have never witnessed the generator actually speeding up when the trip occurs. It will even trip with the load contact open and the trip doesn't even seem related to actual slight increases in frequency that occur when the electrical load is reduced. The generator seems to do an excellent job regulating frequency and voltage with varying loads.

I have confirmed the mag pickup to be ~2.4V while cranking. I believe it increase to about 9.8V with the genset running at 60 hz. I ran the genset up to 66Hz by adjusting the freq trimpot on the woodward governor controller and it did not overspeed trip. I adjusted the overspeed trimpot down on the woodward governor and it did not trip until I had gone several turns downs. When it finally did trip the generator would not crank or restart (overspeed as soon as cranking was attempted). I adjusted the overspeed trimpot back up a couple turns and the generator would start and run but still randomly overspeed trips. TM makes it sound like overspeed trimpot should be about 1Hz per turn.

My only thought is the governor controller is bad or the overspeed trimpot is broken. Can anyone advise anything else to check or additional testing to do to the mag pickup which I have concluded (maybe incorrectly) is the only speed reference for the overspeed trip circuit?

Thank you for any assistance

Tom

 

[Guyfang]

Get a chair, with a nice seat on it. Start the set up, and let it run. Flip up the S7 Battleshort switch. Let the set run. Dont need a load. Watch it. Will the set at some time show an over speed fault on the A2 malfunction indicator? Will the set keep running when this happens? If the set should go into over speed, that's the reason you are sitting in the nice chair. To be there, and turn off the set, should it happen.

 

[micochico]

Thank you for the reply. Based on my interpreation of the TM 9-6115-643-24 section 1-12.1.7 the battle short switch will NOT override the engine overspeed circuit. Can anyone confirm this is true? It is definitely the overspeed circuit tripping and the A2 panel indicator illuminates overspeed when the genset shuts down. As soon as the light comes on the set shuts down or vice versa as soons as the set shuts down the light is on. Should the light come on first before the set shuts down or will the battle short let the light come on and the set keep running?

It is stranger to me that it sometimes trips overspeed when first trying to crank when the engine is definitely not overspeed.

I feel like the Mag pickup side of the woodward controller must be working because the governor does a good job of keeping the genset at the set frequency and I don't know how it would be able to do that if it was getting an inconsistent or bad signal from the MPU.

I assumed my MPU voltage of 2.4 VAC while cranking is ok because I seek the spec of 2-3V listed. Does the range mean any value between 2 and 3V will work or mean some units like 2 volts and some like 3 volts and you must find what works on each particular unit? Does this need to be adjusted down by increasing the MPU air gap which would lower the voltage of the MPU signal at the governor? My assumption is that the governor actually reads the frequency of the MPU signal to determine engine speed and not the voltage. I never see the genset output Hz spike or hear the engine speed up when the unit goes over speed, it just calmly shuts down for no reason.

Thank Tom

 

[Guyfang]

Tom,

Indeed, the MPU only reads RPM. The closer to 3 volts AC that you get from the MPU, the better. If you want to pull the MPU and clean/readjust it, by all means do so. But it sounds like it works.

The reason I wanted to see if the S7 was engaged, was to see if the light came on before the fault, or after it.

Next thing I would check is K14. Attached is the test procedures. Ignore the K20 shown below. I used this to help someone else, with a different problem. Same type relay, just different wires. Anyway, test the relay, AND look for loose wires, broken connectors or any other thing wrong with the relay, and socket. COULD be just loose, never know. Are you good with schematics? Have you down loaded them? You are on line now, so maybe we can do some real time troubleshooting.