The VFD on my 120v Jet 1642 lathe burnt up and I would like to replace it with a 240v VFD. (they are smaller, cheaper, and more dependable than 120v) The only problem is that the tach requires 120v. I can put it on a separate 120v circuit, but that seems so inelegant. I would like to run a hot and a ground to it. I am aware it is illegal, but millions of appliances are wired like that and no one ever gets hurt.
I ran a 120v circuit to it, but it draws less than the 10ma that is the minimum on my meter, so I don't know how much it uses but it can't be much. I then disconnected the neutral and attached that wire from the tach securely to the lathe. The lathe is on plastic pads, so I wired it to my drill press. I measured 121v and the the tach lit up normally. I disconnected the line to the drill press and put it on my leg and took my shoe off and put it on the floor. I felt a very vague tingle, but the tach did not light up; it measured 100v.
I reconnected the drill press and also connected it to my leg. There was no voltage and no tingle.
Finally, it occurred t me that the drill press was grounded through it's power cord rather than through the floor. (duh) So I connected the lathe to a chuck on the ground. The tach didn't light up.
I drew 4 conclusions from this:
1) The less than 10ma passed by the tach isn't particularly danger, even if worst came to worst.
2) The concrete floor isn't a good ground; at least not when it is dry.
3) If I wanted to cover all the bases, I could wire the lathe to the drill press (or something on a separate circuit, if the drill press isn't)
4) I have to get my lathe fixed because I obviously have too much time on my hands.
It seems to me that using the ground as a neutral is quite safe. I would be getting less than 10ma, probably much less.
I could connect the lathe to the drill press to avoid the issue, but that would create a ground loop. Would a ground loop under these circumstances matter?
Any comments (other than that I am a fool) would be welcome.
I did wear rubber soled shoes and gloves for all the testing.