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Repulsion Motor Commutator Grounding/Isolation

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ransonjd:
I'm in the process of restoring a 1923 Hobart 212 meat grinder. It's powered by a 110V, 60Hz, single phase, 1/4 HP AC repulsion motor which is built directly into the case of the grinder. The motor has two brushes in the back which are shorted via a copper alloy frame. The shorting frame has a direct, metal-to-metal connection to the case of the grinder. I had the motor megged, and it's in good shape despite 100 year old insulation.

I would like to ground the casing of the grinder. I'm mildly concerned about the commutator's connection to the casing. Since the armature has no direct connection to AC mains, and the brushes just short the windings via the casing, nominally, no current should flow off the casing if I ground it. However, because this is an induction machine, I worry that may be woefully optimistic. So I'm considering two options.

1. Just ground the casing, throw it on a GFCI outlet, and see what happens.

2. Ground the case. Put a piece of Nomex or fish paper behind the grounding frame and replace the metal fasteners holding it in place with PEEK.

The first option is the easiest, but I feel like the 2nd option is correct. A 1946 Hobart A-200 mixer that I restored was built this way originally, with fish paper and insulating sleeves for the fasteners.

Does anyone have any input?

Zero999:
I don't see a problem. The current loop between the brushes is only connected to ground at one point and it's well isolated from the mains by the air gap and winding insulation so no current will flow to earth.

Go with option 1. If the GFCI trips, it'll probably be leakage from the stator, rather than the commutator.

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