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jeff46

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Hi Everone, i have a small lathe i've been using for the past 10 years or so and never noticed todays problem before, today I got a small electric shock when i touched the bed of the lathe and a scaffold pole I have situated next to the lathe which holds up a scaffold lean-to roof.
After this shock I tested using a wooden handled lathe tool to short the pole to the lathe bed which resulted in small sparking (almost like running a 12v wire onto earth in car).
The lathe 3 phase motor runs from a vfd single to 3 phase inverter, I checked the earths etc and connections in the motor along with the connections from inverter to motor as well as the inverter 240v feed, no problems were detected, I even tried disconnecting the earth connections but same problem.
I tested using a multi-meter set on (ac voltage 250v) from the lathe bed to the pole and it reads around 130v, also from motor casing to the pole same reading.
Thinking it may be a problem with the motor i wired a new motor using the existing motor connections and the reading from new motor case to pole was still 130v.
I then tried a spare vfd on the new motor and low a behold when running i get a voltage reading from the motor case to the pole of 130v??
Anyone any idea why this may be happening as I say I've run the lathe for 10 years or more and never noticed any problem, any help appreciated thanks.
By the way the voltage readings are only present when the inverter is switched on.
Help appreciated, Jeff
 
Okay, there's a lot to unpack here.

If you get a shock of any significance there's usually more than one fault simultaneously occurring.

Firstly there's usually an insulation fault whereby there's some kind of path between the usual current path and the chassis of the appliance. If this fault path is low resistance then sufficient current will flow to earth to trip the circuit breaker. If the fault path is high enough resistance then the circuit won't trip. Either way, big fault current or small fault current the electrical supply circuit is designed that the CPC (earth wire) is big enough to sink the fault current to ground without the chassis of the appliance rising to a dangerous voltage.

Second problem is when the CPC (earth) is either high resistance or missing entirely. In this case the fault current has nowhere to flow until someone touches the chassis and they then become the lowest resistance path to ground....ie they get a shock. When the earth is missing and there's an insulation fault the voltage of the appliance chassis rises.

So somewhere there's an insulation fault and somewhere there's a missing or poor earth. I say 'somewhere' rather than saying 'the lathe' because, whilst it's possible it is a lathe fault it's also possible that another circuit or appliance entirely has an insulation fault and missing earth and that circuit or appliance is in direct or indirect physical contact with the steel roof. This could mean the voltage of the roof rises and if the lathe has a fully functional earth you get a shock if you're in contact with any elements of them both.

I'm afraid this is heading towards you getting an electrician in who has a full range of test equipment and the experience to track down this type of fault for several reasons.

1. Your situation with the lathe VFD cancomplicate the fault finding process, VFD's produce a modulated waveform that cheaper testers/multimeters can't understand so it's possible the voltage test results you got are incorrect.

2. You need to test all the circuits in the vicinity, not just the lathe.

3. You need specialised test equipment like insulation and loop impedance testers.

4. You're almost certainly looking for more than one fault simultaneously which is beyond the scope of DIY.

I'd suggest if you state your location you might find one of the more experienced members here is local and willing.
 
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Okay, there's a lot to unpack here.

If you get a shock of any significance there's usually more than one fault simultaneously occurring.

Firstly there's usually an insulation fault whereby there's some kind of path between the usual current path and the chassis of the appliance. If this fault path is low resistance then sufficient current will flow to earth to trip the circuit breaker. If the fault path is high enough resistance then the circuit won't trip. Either way, big fault current or small fault current the electrical supply circuit is designed that the CPC (earth wire) is big enough to sink the fault current to ground without the chassis of the appliance rising to a dangerous voltage.

Second problem is when the CPC (earth) is either high resistance or missing entirely. In this case the fault current has nowhere to flow until someone touches the chassis and they then become the lowest resistance path to ground....ie they get a shock. When the earth is missing and there's an insulation fault the voltage of the appliance chassis rises.

So somewhere there's an insulation fault and somewhere there's a missing or poor earth. I say 'somewhere' rather than saying 'the lathe' because, whilst it's possible it is a lathe fault it's also possible that another circuit or appliance entirely has an insulation fault and missing earth and that circuit or appliance is in direct or indirect physical contact with the steel roof. This could mean the voltage of the roof rises and if the lathe has a fully functional earth you get a shock if you're in contact with any elements of them both.

I'm afraid this is heading towards you getting an electrician in who has a full range of test equipment and the experience to track down this type of fault for several reasons.

1. Your situation with the lathe VFD cancomplicate the fault finding process, VFD's produce a modulated waveform that cheaper testers/multimeters can't understand so it's possible the voltage test results you got are incorrect.

2. You need to test all the circuits in the vicinity, not just the lathe.

3. You need specialised test equipment like insulation and loop impedance testers.

4. You're almost certainly looking for more than one fault simultaneously which is beyond the scope of DIY.

I'd suggest if you state your location you might find one of the more experienced members here is local and willing.
Thanks for the reply mervo.. bit more technical than i thought will start to check all the other machines and circuits to seek any faults to earth.. hopefully find something before having to call for the professional help.. jeff
 
Apologies if my reply was over-technical.

The short simpler version is that there's probably more than one fault, it could be the lathe that's faulty but it could also be a fault elsewhere and be careful because your tester might give you false voltage readings at your lathe motor or anywhere else downstream of the VFD.
 
Apologies if my reply was over-technical.

The short simpler version is that there's probably more than one fault, it could be the lathe that's faulty but it could also be a fault elsewhere and be careful because your tester might give you false voltage readings at your lathe motor or anywhere else downstream of the VFD.
Hi malvo again thanks for your input.. thinking about what I've already tried virtually tried new motor on new vfd but same result a voltage from motor casing?? Also it appears like when motor is running it somehow makes the earth casing on the motor live all quite weird but will do some further trials running various motors from both vfd that i have.. also plugging in from different power points.
Just a quick question a normal 3 phase motor when running from a vfd should have no reading at all from casing to earth? Sorry for further questions. jeff
 
A motor should not be “leaking” any electricity from its windings to its case if it is in good condition.

I presume if you switch the power off to the lathe but leave it plugged in, the fault goes away and you measure no voltage from the roof steelwork to the lathe chassis.

This really needs someone with an earth loop impedance tester and the skills required to use it properly
However I am going to give best guess from what we know.

Lathe is a 3phase motor converted to single phase using a variable frequency drive.
Frame of lathe is becoming live with the voltage being high enough to cause a shock.

Many vfd have circuits inside them that leak a small amount to earth, this would not usually be a problem as the earth connection will hold the voltage down to near zero volts.
The problem is likely to be a poor earth connection to the lathe, this is allowing the case voltage to rise when the vfd is leaking power to earth (lathe bed) if the bed was well earthed, the voltage could not rise high enough to feel or measure.

Note, information above is best guess from limited info available and for your understanding of how a fault could give rise to the symptoms listed.
There are many assumptions made to come up with it and this really needs looking at by a professional.

Please isolate machine and wait for a proper test to be done,

although previously you have just felt a shock, a change in circumstances could lead to the next one being fatal.
Rain, humidity, wet hands, different footwear, any of these could make a difference and change a survivable shock into a deadly one.
 
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...... it appears like when motor is running it somehow makes the earth casing on the motor live .....
Yes, this is possibly one of the faults you have but the solid earth wire connection would usually prevent the motor body from having a voltage that could give you a shock. The current would usually run through the easiest path ie the earth wire and not someone who's touching it and the motor casing should always test 0V to a proper earth.

This earth problem is the most important one to fix.

VFD's make fault finding complicated. Old motors and VFD's don't always play nicely together because they don't handle the electrical waveform from the VFD well. It's a very complex area to explain but using a VFD can cause old motor windings to burn out prematurely and also can cause circulating currents in the rotor which causes bearing failure. Added to this VFD's actually use the earth in a functional capacity as well as for protection against shocks. They have internal filter arrangements that cause small currents to flow in the earth as part of their normal operation. This makes fault finding more complicated because when there's an earth current you need to establish if is a functional current or a fault current.

In laymens terms these kinds of faults are notorious for running even experienced electricians around in circles. You have to be absolutely methodical and start with the basics like testing the incoming supply, testing the circuits and the earthing system then once you've ruled all that out you go looking for more complex faults. Obviously you need a range of suitable test equipment and equally important you need an understanding of the entire supply system and the operation of the VFD.

In your particular case there's a good chance you might get a shock from something whilst doing live tests. There's a good chance your tester is going to give you misleading test results. There's a good chance the only way you'll even half-solve the problems by expensive trial and error swapping out of components. Honestly, it's going to be safer and cheaper in the long run to get a good sparky in even if just for an hour to point you in the right direction.
 
A motor should not be “leaking” any electricity from its windings to its case if it is in good condition.

I presume if you switch the power off to the lathe but leave it plugged in, the fault goes away and you measure no voltage from the roof steelwork to the lathe chassis.

This really needs someone with an earth loop impedance tester and the skills required to use it properly
However I am going to give best guess from what we know.

Lathe is a 3phase motor converted to single phase using a variable frequency drive.
Frame of lathe is becoming live with the voltage being high enough to cause a shock.

Many vfd have circuits inside them that leak a small amount to earth, this would not usually be a problem as the earth connection will hold the voltage down to near zero volts.
The problem is likely to be a poor earth connection to the lathe, this is allowing the case voltage to rise when the vfd is leaking power to earth (lathe bed) if the bed was well earthed, the voltage could not rise high enough to feel or measure.

Note, information above is best guess from limited info available and for your understanding of how a fault could give rise to the symptoms listed.
There are many assumptions made to come up with it and this really needs looking at by a professional.

Please isolate machine and wait for a proper test to be done,

although previously you have just felt a shock, a change in circumstances could lead to the next one being fatal.
Rain, humidity, wet hands, different footwear, any of these could make a difference and change a survivable shock into a deadly one.
James thank-you for the info, when I find out the problem I will publish on here, I do have a friend who is suitably qualified to lend me a hand when he's free..
By the way I have noticed on my newer vfd there is no earth connection from the drive to the motor just the 3 motor connections, the single phase feed to the vdf has an earth connection, there is an earth on my older vfd which I did test when it's running and as you suggested it was showing a low voltage??.
To answer your first assumption yes the problem is only there when the vfd is running (either of them). Jeff
 
Apart from thinking that you've an earth missing somewhere and/or a floating Neutral, I'm curious about the scaff pole.... what's it doing/near/situation? The symptoms could just as easily be going FROM there to the lathe....

I once chased my tail for hours and hours on a rural / small-holding style property trying to find why the internal conductive parts of the house were live and sparking in a really weird way, turned out to be an incorrectly fitted electric fence energiser up in a field that was back-feeding into the TT system!
 
Apart from thinking that you've an earth missing somewhere and/or a floating Neutral, I'm curious about the scaff pole.... what's it doing/near/situation? The symptoms could just as easily be going FROM there to the lathe....

I once chased my tail for hours and hours on a rural / small-holding style property trying to find why the internal conductive parts of the house were live and sparking in a really weird way, turned out to be an incorrectly fitted electric fence energiser up in a field that was back-feeding into the TT system!
Hi. The scaffold pole holds the roof up.. the leanto type canopy is made from scaffold with a steel sheet roof..
 

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