Re: (ET) E20 motor teardown to find the ground fault (opinions needed)

[Chris Zach <cz alembic crystel com>]

The problem is the E15 and E20 have *two* fields. One is the normal 
field, which is many windings of small gauge wire.

The second one is the compensation field which is in series with the 
armature. *THAT* is the one that is shorted. It's a smaller number of 
turns of large gauge wire.

If I wire around the compensation field the problem goes away. However 
from what I have read as the armature current goes up under load the 
compensation field also goes up which slows the tractor down and 
increases torque. This is why you can't do field weakening in reverse, 
technically under load the compensating field weakens already under load.

Isolating the motor could be tricky: It's the thing that drives the 
tractor so I am guessing the mounts take a fair bit of stress and would 
break plastic bolts.

On 12/2/2019 8:03 PM, Steve Gaarder wrote:
> Here's an idea, which may or may not be useful: get (or build) an 
> isolated DC-DC converter, and power the field with that.  Then the gound 
> fault will hardly matter.
>
> steve
>
> On Mon, 2 Dec 2019, Chris Zach wrote:
>
> > Ok, so I figured the motor was not going to fix itself and took it 
> > apart today. Removed the huge front pulley with my 3 fingered pulley 
> > puller, then pressed out the armature and set it next to the E15 
> > armature.
> >
> > Note: It's a bigger armature, no doubt.
> >
> > I then unscrewed the field screws on the motor (hint: Find the *exact* 
> > size screwdriver, clean the slots of all crud that might keep the 
> > screwdriver from fully engaging, put penetrating oil in, and use a 
> > wrench on it while putting pressure down with your shoulder. These 
> > heads could strip) and started thinking about how to get the field 
> > wires out of the plastic housing (so I could thread them into the 
> > motor and pull the windings out) when I figured I should test the 
> > motor again with my sparker.
> >
> > No spark at all, no ground fault. Hm.
> >
> > Put the screws in on one field side: No ground fault. Did the other, 
> > ground fault.
> >
> > So it looks like the fault is between one of the field halves and the 
> > motor body. If the screws were not ferrous that could fix the fault 
> > but I don't know if nylon screws are made that big and if they would 
> > hold under the motor's heat range. Debating if it's worth it to pull 
> > out the field, I could wind up breaking more of the insulation and 
> > making it worse. I know that the short is to the metal field support 
> > and not to the motor body because it will spark on the inside of the 
> > motor on the side that's shorted.
> >
> > I might be stuck here running with a ground faulted motor, or I could 
> > bypass the whole compensating field and just run with the series one. 
> > Which would be a better option?
> >
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