Why is my grinder shocking the hell out of me?

Wall outlet wiring is all fine.

With VFD in standby mode the grinder frame read 11 volts with a good ground, and none while running.

Thanks for the tips, everyone. I'll have to start digging deeper.
 
i see a wet spot on the floor, try the multimeter between the water spot on the floor and the body of the grinder to see if there is voltage.
 
How many volts does the frame read if the grinder is running?

Have you taken the motor pecker head cover off and made a visual inspection?
 
With the grinder running, there reads either nothing or a flashing gobbledygook of numbers. This matches what's coming off of the hot lug going into the motor from the VFD.

Similarly, with the VFD at standby, I'm getting stable voltages off of the grinder at up to 18 volts, which matches what I get at the hot lug going into the motor from the VFD.

So it seems that the motor has some sort of internal short that's feeding power through the motor housing and into the grinder frame.

From the concrete floor to the frame, there is almost no measurable voltage.

I guess that's what I get for winning a motor auction on Ebay for 99 cents, even though it was supposedly brand-new.
 
What kind of meter do you have? Most cheaper meters will prefer sinusoidal waves when reading AC voltage, but unfortunately, a VFD puts out something closer to square wave, so unless you have a meter with a LP filter, you probably won't get much more than gobbledygook. In this case, A continuity check would be more telling.
Check continuity from your hot leads to your motor housing (U, then V, then W) and also check continuity to ground.

Could be as simple as a fractured wire insulation, or a loose wire nut, though I'm surprised the VFD isn't faulting out. Are you sure all of your ground wires are connected? Is there continuity from ground to your motor housing? Is it just the motor housing that's giving you a shock? Or is the VFD housing? Or the grinder frame itself?
 
Stupid question: There's no cracks or splits in your cable where it contacts your grinder table is there? How about where it goes into the pecker head on the motor? Maybe when the outer sheath of your cable was stripped off, you cut into one of the leads? You might try cutting the cable off at the pecker head and carefully re-stripping and re-connecting everything. Strip about 3/4" off of each cable and twist a red or yellow wire nut on each connection. You shouldn't see any copper when the nut is twisted tight. If you do, cut a little bit off and re-twist. Wrap some table from the end of the nut to about 1 or 2 inches down the wire. This isn't really necessary on a properly installed wire nut, but it doesn't hurt either.
 
knife to a gunfight said:
What kind of meter do you have? Most cheaper meters will prefer sinusoidal waves when reading AC voltage, but unfortunately, a VFD puts out something closer to square wave, so unless you have a meter with a LP filter, you probably won't get much more than gobbledygook. In this case, A continuity check would be more telling.
Check continuity from your hot leads to your motor housing (U, then V, then W) and also check continuity to ground.

Could be as simple as a fractured wire insulation, or a loose wire nut, though I'm surprised the VFD isn't faulting out. Are you sure all of your ground wires are connected? Is there continuity from ground to your motor housing? Is it just the motor housing that's giving you a shock? Or is the VFD housing? Or the grinder frame itself?
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The multimeter is just a cheapo from Harbor Freight, as my normal one seems to be broken. I assumed gobbledygook was due to the square-wave output from the VFD, being familiar with IGBT welding machines.

I triple-checked the wires and connections, they're all clean and secure with no breaks or gaps, and being paranoid about electricity I always make sure they're carefully connected. The entire grinder, every metal part including the base and the motor housing itself, all read the same voltage that's coming out of the hot wire from the VFD and deliver a shock. Since I can't find a single loose connection or wire fault, it seems it could only be within the motor itself. :[
 
Is there a ground wire going to your main? I have seen electric motors induce voltage into the frame but usaly a good ground connection solves that.
 
Hmm... Like you say, could be something internal. If you're feeling a little salty, you might disassemble the motor and look for some black spots on the inside of the housing or somewhere on the windings. Might even just be one of the lead wires rubbing somewhere. A little liquid tape may be enough to get to squared away. Next option would be to isolate the motor from the table with a sheet of rubber or micarta, and just try not to touch the motor anymore. ;)

That said, if something is arcing inside the motor frame, that motor may not be long for this world....

ETA: You might also pull the screws out that's holding the pecker head to the frame and make sure somebody didn't drive an extra long screw into the motor frame.... Just thinking out loud.
 
It appears the cord from the vfd to the motor is the same as from the wall to the vfd. The cord from the vfd to the motor MUST be a 4 wire conductor.
 
Is there a ground from motor to VFD?
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As mentioned above, There should be a grounding conductor but I suspect its absent. Appliance should never been put into service without. None the less, If the appliance is not grounded, thats not a root cause for electric shock anyway.
Grounding only makes an appliance safer in the event its chassis becomes energised due to some kind of fault condition.

Firstly, it needs to be determined why the appliance frame is energised.
 
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Sorry I haven't been able to reply. Been in a remote location.

The VFD has three inputs for wires in from the wall, and three outputs for wires out to the motor. The motor has only three connection lugs (220v 3 phase, two hots and a ground, just like my welding machines), they are all connected securely. I don't understand where the fourth wire is supposed to be. Would that just be from the motor frame to the VFD frame?

Just adding that I used the grinder a lot with no problems and then it only recently started to shock. Since there are no breaches in the wires and connections, it seems likely that the moving parts (motor internals) are the problem.

Thanks for your input everyone (no pun intended). At this point it just seems that the damn motor is feeding power to the grinder frame and through the grinder. I'm just going to get a different motor.
 
Mecha said:
motor has only three connection lugs (220v 3 phase, two hots and a ground, just like my welding machines), they are all connected securely. I don't understand where the fourth wire is supposed to be. Would that just be from the motor frame to the VFD frame?
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I hope you meant to write ”3 hots and a ground” !!

I dont remember ever seeing ”Ground” on a motor being a wire lead. Its always made as a connection somewhere to the motors metallic frame.

Using only 3 wires to your motor, I can envision a couple incorrect wiring scenarios what will have the motor ’run’, but create shock hazard on regular threephase. But surprised if thru a VFD it could even run run or at least display some kind of faultcode.

You might again study your VFD instruction manual knowing your motor needs 3 HOTS & A GROUND !
 
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Lieblad said:
I hope you meant to write ”3 hots and a ground” !!

I dont remember ever seeing ”Ground” on a motor being a wire lead. Its always made as a connection somewhere to the motors metallic frame.

Using only 3 wires to your motor, I can envision a couple incorrect wiring scenarios what could create shock hazard on regular threephase, But surprised if thru a VFD could run it without at least some kind of faultcode.

You might again study your VFD instruction manual knowing your motor needs 3 HOTS & A GROUND !
Click to expand...

Thanks Liebald, I'll go through it again. The motor I have only has three inputs, just like my 220v welders. It is a three-phase motor.
 
That is a KBAC VFD correct?
You should be able to follow the diagrams in this PDF to make sure everything is wired correctly:
https://beaumontmetalworks.com/wp-content/uploads/2017/05/VS-setup.pdf

3 phase motors will have 3 hot connections, one for each phase. Electric motors, whether single phase or 3 phase will rarely have a "Ground wire", or if they do, its typically just a pig tail going to a lug in the motor frame. In your case, there's a ground lug on both bottom corners of the kbac vfd, they're normally painted green. One side will be for your single phase ground going back to your wall outlet, and the other is to run from your VFD to a ground lug in your motor. Many times you'll find a green screw inside the peckerhead of the motor, but any screw that has continuity with the motor frame would work. There should be at least one or two screws holding the peckerhead on, so if there isn't a dedicated ground screw/lug, you can use one of those.

Now, as has already been pointed out, lack of ground isn't necessarily your issue here, as the motor in a normal state shouldn't have an energized chassis.

Can you take a picture of the wiring in your peckerhead and the nameplate on your motor? Is this an inverter rated motor? I suppose if it's not inverter rated (or has at least an F class insulation), maybe the insulation has been cooked off inside the motor somewhere, causing a short. Just thinking out loud.
 
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