Lathe electrically "hot" due to VFD? WTF?

I have a VFD, mounted on my Clausing 6913 lathe.
The VFD is mounted on an aluminum plate, which in turn is bolted to a bracket, which is bolted to the lathe, with various metal parts and
metal screws.
The VFD has a ground lug, which I connected to the incoming cable.
I am pretty sure, though I will triple check tonight, that I have a continuous ground path to my home panel and to the buried ground rod outside.
I always thought that there was no problem with the VFD/lathe etc, but that is only because I always wore rubber soled shoes (crocs, boots etc).
Yesterday I ran it without shoes and I was unpleasantly "zapped" when I touched the VFD or the metal lathe itself.
So my question is WTF. Even if the motor "leaks" current due to coltage spikes, shouldn't the grounding take care of it?
My second question is what would you suggest doing. Just giving up on the VFD and using a phase converter, or try to replace the 3 HP motor? Anyone knows if the motors on 6900 series lathes are 182 frame?
In any case, my plan for tonight is
1) Check that I do have a continuous ground path from home ground rod to the VFD 2) Measure the voltage of that leakage 3) Add one more single conductor green cable, from the VFD ground lug, bolted to the body of the lathe.
Then I can think about what to do with this.
i
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On 10/27/2010 10:25 AM, Ignoramus15569 wrote:

Definitely check or establish continuity between the VFD and lathe frame to the ground wire. It should be there already, though.
Depending on how old the house is and who did the work, someone may have cheated somewhere along the line and connected ground to neutral. This is not uncommon in old two-wire systems that have been wired by someone who knows the inspectors will check with a tester without looking further (Lookie -- the green lights go on!). Depending on how the rest of the house is wired and running you can have some significant voltage on neutral, leading to mild shocks.
In large industrial installations its not uncommon to have different grounds in different sections of a building be tens of volts different -- I had a problem with this in a former workplace, where two such sections came together in one part of the factory floor; plugging diagnostic computers into one section and the system to be diagnosed in the other lead to the magic smoke coming out. I can't imagine that happening in a normal residential setting, though.
If you can figure out a way to check the potential between the ground pin and the floor it may be educational, although I'm not sure what you could do with it.
--

Tim Wescott
Wescott Design Services
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Had a house in MD, garage on one side of a stream house on the other side.
"Ground" for the house was about 80 volts different from "Ground" at the garage.
Dave
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Tim, I will reply to other fine points that you made when I gather more data, but yes, we had a very long dry spell with a very minor rain a couple of days ago.
I will check, very meticulously, whether I have a continuous ground, first.
Ultimately, though, the question is, is that the motor that is leaking, and if so, isn't that the primary issue?
i
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On 10/27/2010 01:45 PM, Ignoramus15569 wrote:

Well, whatever's causing the problem is the primary issue.
Do you only get zapped with the lathe turning?
With the VFD on no matter what?
Any time at all?
The further you get down that list and still say 'yes' the less likely it is a motor problem, and the more likely it's a really odd ground problem.
--

Tim Wescott
Wescott Design Services
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Yes.
No
Only when the motor is running.

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On Oct 27, 4:45pm, Ignoramus15569 <ignoramus15...@NOSPAM. 15569.invalid> wrote:

I'd suspect bypass cap current into neutral also. Can you run the VFD with the motor disconnected to check?
jsw
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On 10/27/2010 03:45 PM, Ignoramus15569 wrote:

No. All motors "leak" as they have a lot of wire in close proximity to the frame. The VFD makes this much worse, as instead of 60 HZ on the windings, you have extremely fast sharp-edged PWM pulses at 400 V. The wire to frame effectively creates a capacitor, and the faster the dv/dt, the more current flows through the capacitor.
If you only got zapped when the motor was running, then the ground of the lathe has to be open. If you got zapped ANY time you touched it, then it sounds like the house ground might be going out.
Jon
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"Ignoramus15569" wrote in message
Tim, I will reply to other fine points that you made when I gather more data, but yes, we had a very long dry spell with a very minor rain a couple of days ago.
I will check, very meticulously, whether I have a continuous ground, first.
Ultimately, though, the question is, is that the motor that is leaking, and if so, isn't that the primary issue?
i
Make sure that ground and neutral at the plug are the same. Years ago had a problem with a disk drive failing to spin up properly, and they had left off the jumper at the pole between ground and neutral. Was 12 V between.
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A minor addition to all the discussion about Iggy's ground problem.
When we lived in Washington state, about 12 years ago, the basement of the house was finished with a suspended ceiling. It was very tight to the floor joists above, so the panels were a bear to remove.
The electrical code, in WA, required two heavy wires to go from the breaker panel to ground. They used the water pipes, as they were soldered copper pipes all the way to where the line entered the house, then 100+ feet to the meter.
Sure enough, there were two heavy wires going from the panel up into the ceiling. And there were two heavy copper wires going from the nearest cold water pipe, about 40 feet away in another room, going up into the ceiling.
For some reason, I had to remove one of the ceiling panels and discovered there was only a single copper ground wire up there. Some smart electrician had tricked an inspector into believing both wires were installed properly!
I just put the panel back in place.
Paul
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wrote:

I found one when I helped a friend move into a rented house. The panel was at the front of the house and had a 3#8 stove cable going into the wall. The kitchen at the back of the house had an identical 3#8 cable coming out of the wall to connect to the stove. When I hooked up the stove and re-inserted the fuse carrier, one of the fuses popped. The replacement fuse cured the problem but we soon noticed an odd smell. Turns out, the replacement fuse was higher rated and further investigation revealed that when the kitchen had been moved to the back of the house, the stove cable had been extended with 3#14 cable and cross wired so that line 1 was connected to the neutral. There was enough current to blow the 40 A fuse but not the 60A which heated the #14 wire running through the attic Gerry :-)} London, Canada
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Bill & All, Never connect safety ground and neutral anywhere but the building entry point. It is unsafe, against code everywhere and it creates ground loops. Safety ground must be non-current carrying. If you find this practice corrects a fault, you have another problem find that and stick to code. Steve

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I have looked at everything very carefully and found a place where ground was not connected properly. Thanks to all. This lathe runs very well now, everything pertaining to variable speed works extremely well, VFD is working, etc. It cost me more to fix than I hoped (VS stuff), but the result is very nice. The last thing remaining is to make stops for the taper attachment.
i
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On 10/27/2010 08:29 PM, Ignoramus15569 wrote:

Glad you found it -- it's really unpleasant when the electrical stuff starts biting.
--

Tim Wescott
Wescott Design Services
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I agree. So now I have the best of all worlds, VFD for running from single phase, and hydraulic variable speed drive for preserving mechanical advantage.
i
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wrote:

Indeed. I believe I mentioned working on a misbehaving Large machine..and after laying on the concrete floor in a pool of sweat..reached up and grabbed the side of the machine to pull myself up.
And having the shop owner kick my arm loose as I was doing the Spastic Trout On The Dock dance......
Gunner
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wrote:

That's NOT the approved Method of Faulty Ground Tracing as mentioned in the Sparky manual, sir. But it found your problem for you, dinnit? (ouch!)
-- Small opportunities are often the beginning of great enterprises. -- Demosthenes
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Larry Jaques wrote:

I guess you learned by experience that before working on any machine it is prudent to check the grounding of the machine, but preferably not by your method. I find a number of machines especially in small shops that are not properly grounded. Some people should not do their own wiring, especially three phase systems. Grounding is the most important part of wiring, that is why the NEC devotes a whole chapter to it.
John
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John wrote:

You can't trust Electrical Contractors, either. I recently found a 208 V pipe bender that was reporting low voltage on the VFD drive. Brand new install after a freind's company moved. The contractor had wired the thing for 120 volts at one end, and 208 at the outlet. The business assured me that it had to be wired right, but it wasn't. The outlet had the right NEC color codes, but the ass had connected the red conductor to neutral in the three phase breaker box. He had used a single pole breaker, as well.
Then they wanted to wait a week before fixing their mess while the company was waiting for parts to make shipments. After a heated convesation, someone showed up an hour later but without the proper breaker. That lead to another heated call, and the owner bring over the proper breaker.
Another problem is a machine that was properly grounded, but the wire has failed from vibration or some idiot hitting the conduit with a forklift. This doesn't just happen to machine tools. I know of one commercial fire alarm in a school that used the old 10A loop current that had a wire short to a piece of conduit and people were getting shocked with about 60 volts, DC. A connection in the conduit was corroded, so it was no longer grounded, so it was a double fault condition that could have killed someone. Cheap assed people think I'm crazy to run a ground conductor inside of EMT.
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On Wed, 27 Oct 2010 12:25:22 -0500, Ignoramus15569

Just a data point for you. I just got my large mill power supply running. I get 21 volts D.C. between neutral and ground. I attached a ground rod to this main panel. I've never seen this large a value. If I touched neutral while standing bare foot on steel, I'm sure it would tickle.
Anyway, I'd suspect neutral is touching your frame someplace.
Karl
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