[Milsurplus] ART-13 dynamotor woes

[Meir WF2U]

Folks,

 

I have an interesting but aggravating problem with a DY-17A/ART-13 Eicor
manufactured dynamotor. The dyno, by the way is a late WW2 production, it is
in pristine cosmetic condition inside and out and it was never used before I
got it recently. After 3 weeks of running my ART-13 with it for a couple of
hours a week, in the middle of a  CW net it started to smell like hot
insulation. I shut it down and troubleshooting indicated that there is a
short to ground somewhere in the field coils. The remaining coil portion is
still able to spin the dyno at a much slower than normal speed, and the
increased current is heating it up fast. I isolated both HV and LV rotor
windings, they're OK. No other shorts to ground found anywhere in the unit.

I'm scratching my head, because the field coils are fed from the primary 28
VDC, so the insulation is not breaking down from high voltage. If there was
a manufacturing fault in the field coil installation, the short would have
existed from the get-go. How does a fixed field coil insulation break down
in a few hours of operation? Nothing is grinding, nothing is loose inside
the dynamotor. It spins completely freely as it should (by the way, there is
a label inside the end bells which says: "Do not lubricate!"). 

Should I venture and attempt to take the whole dynamotor apart and remove
the field coils to see if I can find the short?

 

Incidentally, my other dynamotor, the older DY-17/ART-13, after replacing
the HV brush caps (the newer Eicor has no brush caps in the dynamotor
itself) is working fine. It looks beat-up on the outside, and the dynamotor
itself looks like it was used a lot, but at least so far, so good.

 

Any thoughts?

 

73, Meir WF2U

Landrum, SC