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I have an all metal fully enclosed 120VDC motor with the standard red and black wires. The motor will be mounted using screws on a cast iron metal lathe. The motor is powered via a DC speed controller in a grounded metal housing also mounted to the lathe using screws.

For safety, I drilled and tapped the motor's end bell and fixed a ground cable connected to the controller's metal housing.

Motor end bell grounded

Is this a good idea, a bad one or an unnecessary one? Also, I usually test my projects using a GCFI and I wonder if there is any possibility of inducing a current in the ground wire which can trip it. It shouldn't, as the ground is an incomplete circuit, but I thought I should ask anyway.

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Is this a good idea, a bad one or an unnecessary one?

It's likely that there will be some path through the case of the motor to the lathe to the controller anyway — so adding the ground wire will just make that more guaranteed and predictable, which is good.

Two general good reasons to ground a motor:

  • General electrical safety as with any device having an exposed metal chassis — redirecting any short-to-chassis to definitely go to earth rather than someone touching it. For this purpose, the ground wire (and other ground connections all the way back to the outlet) should be able to safely carry short-circuit current from the motor controller (for the short period until the circuit breaker opens) — it may be advisable to connect it specifically to an intended ground terminal on or in the speed controller rather than just to some point on the housing.

  • Shielding against RFI generated by the motor. (Another common practice, at least for small DC motors, is to add capacitors between the case and each motor terminal. I can't say whether this is a good idea in this high-power application or what capacitor specifications you would want.)

Also, I usually test my projects using a GCFI and I wonder if there is any possibility of inducing a current in the ground wire which can trip it. It shouldn't, as the ground is an incomplete circuit, but I thought I should ask anyway.

The concern here would not be inductive but rather capacitive coupling, between one line and ground. This could trip a GFCI but if there was enough to do that then it would be enough that you would want to do something to reduce it anyway.

(And yes, I did just mention adding capacitors above — smaller ones.)

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I was going to post this as a comment to @KevinReid's excellent response, but didn't want to hijack all the comment space.

I work with 180vDC motors regularly, and they (and their controllers) always have some sort of grounding. Grounding the motor case is usually sufficient, as what it is mounted into is typically metal and grounded, but extra won't hurt. For larger motors, it is more commonly seen.

120vDC? 90 and 180vDC are common in industry. 120vAC rectified to DC is about 170vDC, so I assume your controller is dropping this appropriately, otherwise the motor would be over-driven by 170/120 = 42% and live a short life.

The insulation between the brushes and the case, and the windings and case, should be in the mega-ohms range. In fact there is a tester just for this purpose, called a "megger", which measures this isolation. It does this by injecting a high voltage into the wires and measuring how much current flows to ground.

It doesn't happen very often, but if a coil or brush did fail short to ground, the ground path should be able to handle >=150% of rated motor current. If the motor is fused, this will prevent further damage to the motor. But a shorted winding needs re-wound, which is fairly intensive.

You could try a GFCI, but I think this is asking for trouble. As long as the whole lathe is grounded and fused properly, a motor failure shouldn't hurt the operator regardless of whether a GFCI is used.

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