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Thread: Old Lathe Electrical Question

  1. #1
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    Old Lathe Electrical Question

    Hello,

    I purchased an old Oliver lathe a number of years ago, and it came with a DC motor and a controller/converter that lets me adjust the speed. While using it recently, I experienced a slight shock when running the lathe and touching the tailstock screw handle with my right arm. While running, i connected a volt meter to the chromed handle and the outlet box and got a reading of 55 volts. This number drops to 5 volts if I stop the rotation and drops to 0 volts if I turn off the power to the controller/converter. Is this a grounding issue? A few photos have been included.

    Thanks,
    Steve

    IMG_4597.jpgIMG_4596.jpgIMG_4595.jpgIMG_4594.jpg

  2. #2
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    55 volts is a lot. Sending that to a ground rod should trip the breaker shouldn't it? Is that 55 volts AC or DC?
    Last edited by Richard Coers; 05-15-2022 at 6:04 PM.

  3. #3
    After we had a lightning strike on a jobsite I was working on, I got a 50 volt reading to ground on the doorknob of the iron workers trailer. The trailer was properly grounded but nothing tripped. (long story)

    It's possible your lathe has an open leg or at least a leg that has more resistance in it than the ground. Check all the wiring connections and the wiring itself. Look for anything that's loose or any wire that may have a nick in it.
    “Travel is fatal to prejudice, bigotry, and narrow-mindedness..." - Mark Twain

  4. #4
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    Steve, What is the primary voltage of your DC motor controller? If it is 120 volts, it could be a result of the rectifier creating a neutral - ground short. Neutral AC and negative DC are not synonymous.

    Screen Shot 2022-05-15 at 8.33.17 PM.png
    Last edited by Maurice Mcmurry; 05-15-2022 at 9:39 PM.
    Best Regards, Maurice

  5. #5
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    The DC motor controller is plugged into a 240 volt outlet. The 58 volt readout was measured with the meter in AC mode. Thanks to everyone who has responded!
    Last edited by Steve Kirincich; 05-16-2022 at 7:14 AM.

  6. #6
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    Does the machine have only the DC motor?
    Best Regards, Maurice

  7. #7
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    The DC motor is the only motor and gets powered by a rectifier.

  8. #8
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    I checked the wiring and then checked with the volt meter. Perhaps magic but I now do not get any strange voltage readings.

  9. #9
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    It is a quirky mystery that I just like to think about. Is any dust collection connected? Were any other appliances running when you got shocked? I had some clients who had electrical problems that occurred when the electric cloths dryer was running. Turns out the neutral wire of the direct burial service entrance had turned into dust, the running dryer, completing the circuit, exposed the problem.
    Last edited by Maurice Mcmurry; 05-16-2022 at 9:23 PM.
    Best Regards, Maurice

  10. #10
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    I am fairly certain that no other appliances were running at the time, but since the problem mysteriously disappeared, I am assuming that it will rear its ugly head eventually.

    Thanks!

  11. #11
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    Did you ever call your utility company? They could certify the lines up to your house and probably verify the connection at the house and might just include viability of the major ground rod and connection by the meter.

  12. #12
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    If the lathe frame is truly "grounded" then there is "essentially" no chance of getting a shock. The fact that you were shocked suggests to me that the ground (green) wire to your lathe isn't or wasn't connected or the wire connecting the ground wire to the breaker box is open (not connected) somewhere.

    If it was me, I'd ohm out the frame of the lathe to the ground pin of the receptacle and then to the ground bus bar inside of the breaker box.

  13. #13
    Quote Originally Posted by Brice Rogers View Post
    If the lathe frame is truly "grounded" then there is "essentially" no chance of getting a shock.
    Go tell that to the iron worker foreman after he got shocked grabbing the doorknob of his grounded office trailer.
    “Travel is fatal to prejudice, bigotry, and narrow-mindedness..." - Mark Twain

  14. #14
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    Julie Moriarty, this anecdote doesn't make sense. If there was a ground fault somewhere, then yes, this could happen. And if there was some other issue, maybe. But if the foreman was standing on a ground (zero volts) and touched the knob (grounded, zero volts) then no shock. But because he did get a shock, then there was some fault condition, broken ground, mis-wiring, etc.

    (BTW, I'm a degreed electrical engineer with 50 years of experience. I've gotten my share of shocks, but it was always my screw-up, lack of attention, or something not grounded).

  15. #15
    Quote Originally Posted by Brice Rogers View Post
    Julie Moriarty, this anecdote doesn't make sense. If there was a ground fault somewhere, then yes, this could happen. And if there was some other issue, maybe. But if the foreman was standing on a ground (zero volts) and touched the knob (grounded, zero volts) then no shock. But because he did get a shock, then there was some fault condition, broken ground, mis-wiring, etc.

    (BTW, I'm a degreed electrical engineer with 50 years of experience. I've gotten my share of shocks, but it was always my screw-up, lack of attention, or something not grounded).
    I realize it doesn't make sense but here's what happened...

    After testing the voltage from the ironworker's metal stair railing to their trailer door knob (the two were not mechanically bonded) and finding 50v, I immediately thought the trailer must not be grounded. So I walked around the trailer looking for evidence of grounding. I found two locations. One bonded the shell of the trailer to a ground rod. The other bonded the electrical panel to a second ground rod. Granted, the trailer bonding did not necessarily mean the entire frame of the trailer was also bonded. The door was on aluminum hinges but if it was live, so was everything bonded to the doorknob. The metal stairs were resting on wet ground. (I could never determine how voltage leaked over to make it to the doorknob because the focus was get the job up and running again.)

    But it gets better. Inside the building, which was mostly a concrete shell at that point, temporary lights were either very bright, dim or out completely. The man lift (skip) had 480v going to it, all legs read 277v, but would not operate. One guy said he plugged in his radio and it fried. I went back to our trailer to report to our general foreman. As I was approaching the trailer I saw the door fly open, heard a scream and saw a smoking telephone fly out the door. The GF said he was talking on the phone when it started smoking.

    I traced the problem to a temporary 120/208 3ph transformer. It had lost a leg. The only thing I could figure is we had a lightning strike overnight which caused the damage (there was a big thunderstorm overnight). I've seen lightning strikes do strange things.

    We replaced the transformer and things were back to normal. But the skip had to have the 120v controller replaced as it was fried. Maybe the lightning strike. Maybe over-voltage from backfeed caused by the lost leg.

    And we had to get a new phone.
    “Travel is fatal to prejudice, bigotry, and narrow-mindedness..." - Mark Twain

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