I acquired a complete MEP-006 60kW gen-set from Ft Lewis about 1-2 years ago. It had about 7hrs on a fresh rebuild when I received it. Now it's up in the 400-500hr range and I have a problem.
The set was powering a large shop (crane/heaters/air compressors/welders/etc.) and it appears that either someone put an improper load on it, or the rotating diodes on the exciter rotor failed. Three of them (two on one polarity, one on the other) shorted, burned off the top of the diode, and generally created havoc. The gen-set obviously stopped generating.
I yanked the exciter box and inspected everything while diodes were coming in the mail. All appeared well so re-installed the box.
I got the diodes and installed them. Soldered up all the connections.
I fired it up last night and could only get the set to generate voltage if I held the Start-Run-Stop switch in the start position (flashing the exciter). I stopped the set and inspected for hot parts, shorts, blown diodes. After I fired it up again, it took a couple more tries but it finally locked in and started generating. Voltage/freq/regulation was all good. I walked away from the instrument panel for 45-60 seconds inspecting other items and when I returned, it had kicked off. I immediately shut down the set and inspected the diodes again. One of them had de-soldered it's wire and a visible cut/broken/overheated wire was visible behind the aluminium mounting piece. This connected one end of the diode, the other...somewhere in the pack. It appears it shorted out against one of the other diode feeding wires.
I need to now pull the end off the head and see if the exciter rotor winding is repairable. I am hoping it is only damaged feed wires, and not the coils themselves (smells bad, but the enamel looks to be in okay condition and not burnt/crispy/non-existent. I used to work in an alternator/starter shop doing repairs, so I have some familiarity with this topic.
I saw some mention of adding a reverse snubbing diode and also a current shunt + meter on the two small wires feeding into the head to prevent the diodes from getting killed due to high back EMF after flash and also being able to early detect any future issues.
Thoughts? Suggestions? If I end up needing the exciter rotor, where am I gonna find one? I have all the documentation/manuals/etc. on how to re-wind it, but if these are available surplus without getting a whole set, that would be ideal.
As a side note, I've rebuilt the injection pump and re-built the fuel system using current racor type filters. I've also sound insulated all the doors/chassis where I could (which makes a HUGE difference). If this issue is repairable, next on the list is to finish the tandem axle trailer I am building for it and them build a quiet box for the output end of the unit (radiator end) to point noise up and muffle it with some baffles.
This unit generally gets used off-season at events where I volunteer running a power grid (480V distribution with remote transformers down to 120V circuits).
MEP-006 Shorted Rectifier Diode Repairs
