Why is my grinder shocking the hell out of me?

Mecha

Mecha

Titanium Bladesmith
Knifemaker / Craftsman / Service Provider
Joined
Dec 27, 2013
Messages
10,005
Hello all,

A few weeks ago my grinder started shocking me every time I touch any part of it. At first it was a few little shocks, and I thought it was static electricity. Now I'm getting walloped so hard my whole arm went numb and I'm afraid to touch the damn thing, as the shocks are getting worse and worse.

The wiring connections are all still clean and solid, and all of the wires are intact with no breaches in the insulation.

It's a KBAC speed controller set to 120 hz double-speed, running a Toshiba motor, on a 110 volt plug.

I suspect the motor has an internal short that's going through the housing and electrifying the entire shebang. Could that be the problem?

lUESJEx.jpg
 
I experienced that once in my Dad's shop after he had passed away and I was there taking care of the place.
Conditions :
The shop was a nice cinder block shed but was fed by a two lead wire stretched from the house to the shop. When one was done working they unplugged the wire from the patio wall out let. The wire had a two prong plug attached to it with no wide blade on the cord. (you could plug it in two ways)
I was wearing leather soled shoes.
I was standing on dampish concrete (the shop had no space below the floor like a crawl space).
I was dressing the wheel with an iron handled dresser and each time I put the dresser on the work rest BANG I'd get a jolt.

My body was conducting current and grounded into the concrete. Good thing it wasn't more wet or my feet weren't wet.
 
The shop is an old and run down, with a concrete floor right on top of the dirt, but so far the wiring has been fine for years. The machine seemed to shock me worse when my feet were wet, but it even gave a nasty zap when wearing neoprene boots and leather gloves to change the belt, or when sitting on a stool with my feet not even on the ground. I put a copper wire from the grinder to the floor to ground it when I thought it was static electricity or something, to no effect.
 
Firstly, it appears green cable from VFD to motor is the same stuff as what supplies 120V to the VFD?
What makes me suspect its just a chunk of 3 conductor extension cord what was also used to supply mainspower to VFD.
Properly connected, The motor to VFD needs 4 conductors.
Is the motorframe solidly connected to an equipment grounding conductor ? (Being a chunk of extensioncord, its safe bet motorframe is not grounded. )

But being ungrounded is not a shockhazard of itself. Grounding just prevents people getting zapped when a fault condition occurs.
Had you tried plugging into different receptacles to see if problem exists on other circuits. (Haha, I know that means getting shocked more..Or, get a voltmeter...)

Assuming its not static charge...
I suspect the motor or wiring maybe internally shorted to frame or wiring to motor shorted to frame is what energised the grinder and its barstool.

Another scenario exists.
If the receptacle or electrical service to the building is incorrectly wired, this can create shock hazards from appliances also. Has any electrical modifications occured to the premises lately ?

You really need to get a meter and understanding at least basic household wiring to properly diagnose.
 
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Thanks guys

The entire grinder seems to be electrified, so I don't know where I would put a multimeter's probes or what I would be testing in this scenario.

It just started shocking me over the last few weeks after a lot of heavy use with no problems before, and getting worse. What's even more strange is that the longer I run it, the stronger the shock is if I touch something metal on the grinder. When I first plug it in, I don't get shocked at all.

Yes the green wire is 14 gauge wire from a heavy-duty extension cord. There is no visible mis-connection in the wires or any visible short. The building has not been modified lately, except that the meters got changed to digital electrical meters.

I suspect the motor is shorted internally and it's spreading through the frame and entire grinder and trying to find somewhere to go or working back through the grounding wire. I was wondering if anyone has had this happen to them before.
 
Has weather been getting hotter and dryer there. When you get zapped is it a quick zap and nothing? I’m betting it’s static. If you motor is shorting out to the grinder then yes your grinder become charged. But if your wearing shoes it should not zap you. If its static then the grinder is not zapping you, you are zapping the grinder. As you grind your blade is insulated from the grinder and you build up a positive charge. The longer you grind the higher the charge. This is the same way a vandagraph generator works. And it can generate sparks that shoot off of it. This can be solved by wearing an anti static leash which is atatched to a ground sorce or back to the grinder. My grinder zaps me to and it can be quite a shock. I have had zaps jump over 1/4” to the grinder. Static is not a joke it can easily reach thousands of volts and up. Not uncommon to get up into the 10,000v+ range.
 
JTknives said:
Has weather been getting hotter and dryer there. When you get zapped is it a quick zap and nothing? I’m betting it’s static. If you motor is shorting out to the grinder then yes your grinder become charged. But if your wearing shoes it should not zap you. If its static then the grinder is not zapping you, you are zapping the grinder. As you grind your blade is insulated from the grinder and you build up a positive charge. The longer you grind the higher the charge. This is the same way a vandagraph generator works. And it can generate sparks that shoot off of it. This can be solved by wearing an anti static leash which is atatched to a ground sorce or back to the grinder. My grinder zaps me to and it can be quite a shock. I have had zaps jump over 1/4” to the grinder. Static is not a joke it can easily reach thousands of volts and up. Not uncommon to get up into the 10,000v+ range.
Click to expand...

The shock is instant but I jerk my hand back instantly. But the shock happens again immediately if I try to touch it again, even with the vfd on standby, until I unplug the machine. This has never happened before and I've had this grinder for years. It feels like it has some amps behind it, too! :p

It's getting to where I'm scared to touch it when it's plugged in.

The motor is recent, though, only 6 months old.
 
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Do you see a spark jump from you to the steel surface? If you do see a visible spark then it’s static.
 
OK, I am assuming this happens when the motor is not running but the VFD is turned on - correct?
I am also assuming that you are not particularly qualified to do electrical work (or you wouldn't be asking us).

Turn the power on to the grinder, but not the VFD in RUN/ON.
Take a multimeter and connect one lead to a grounded object like a water pipe or a metal stake in the ground.
Read the voltage from the ground to the grinder frame or motor housing.
If that shows voltage, unplug the grinder immediately and do not plug it in again until a qualified person figures out what is wrong with the wiring of the VFD and motor.
Next, check all three leads on the outlet that the grinder is plugged into. The hot lead ( right side smaller slot) should read voltage to the ground pipe, but the other two ( wide slot and ground) should not show any voltage. If the socket is showing voltage on any but the hot slot, turn the power to the building off and get someone qualified to find the problem.
 
JTknives said:
Do you see a spark jump from you to the steel surface? If you do see a visible spark then it’s static.
Click to expand...

No visible spark, not a static-like zap. It's like getting electrocuted a little bit.
 
Stacy E. Apelt - Bladesmith said:
OK, I am assuming this happens when the motor is not running but the VFD is turned on - correct?
I am also assuming that you are not particularly qualified to do electrical work (or you wouldn't be asking us).

Turn the power on to the grinder, but not the VFD in RUN/ON.
Take a multimeter and connect one lead to a grounded object like a water pipe or a metal stake in the ground.
Read the voltage from the ground to the grinder frame or motor housing.
If that shows voltage, unplug the grinder immediately and do not plug it in again until a qualified person figures out what is wrong with the wiring of the VFD and motor.
Next, check all three leads on the outlet that the grinder is plugged into. The hot lead ( right side smaller slot) should read voltage to the ground pipe, but the other two ( wide slot and ground) should not show any voltage. If the socket is showing voltage on any but the hot slot, turn the power to the building off and get someone qualified to find the problem.
Click to expand...

Thanks Stacy! Will-do, and I'll report back.
 
+1 to everything Stacy said but also get a socket tester like this and plug in your outlet to see if the wiring is off.

For static when the humidity is low it is a bear. I touch the spine of the blade to the grinder and even if I see the spark I don't feel it.

https://www.homedepot.com/p/Power-Gear-3-Wire-Receptacle-Tester-50542/206212329?cm_mmc=Shopping|G|Base|D27E|27-11_TOOLS_&_ACCESSORIES|NA|PLA|71700000034238984|58700003943782712|92700031954542185&gclid=CjwKCAjwrqnYBRB-EiwAthnBFnaK5l2nnUb055DIvzAQsUDxvyGhTQ0y0QJ_eOgUT8nsX9QzJL8yDBoC1fsQAvD_BwE&dclid=CJ3ikfCsptsCFQd3YgodQB4JLg

power-gear-voltage-tester-50542-64_1000.jpg
 
71bMt4B7GyL._SX679_.jpg

Get an outlet tester for a quick "dummy proof" test of your shop outlets. It will quickly tell you if the outlet is missing a ground, if something is reversed, etc...

Then you want to get a multimeter and learn some basic diagnostics/troubleshooting with it. For current intents and purposes, you just need continuity and/or AC voltage.
https://www.amazon.com/Neoteck-Mult...440068&sr=1-5&keywords=autoranging+multimeter

Something like the above should be fine, and not break the bank. You'll want to make sure you have continuity from your ground prong to your motor housing (or wherever else you're getting zapped), but that you DON'T have continuity between your hot prong and your housing). Same with your VFD hots and ground wires. Just switch the meter to continuity test, and touch the prongs together. You should hear a beeeeeeeeep. Now touch one prong to your ground plug, and one to your motor housing. It should beep as well. The beep tells you there's continuity, or the ability for electricity to flow from one to the other.

As Stacy said, you can also check for voltage by reading from any grounded connection to whatever is shocking you.

NOTE: You need to put your meter probe on a CLEAN part of whatever you are trying to test. They won't read through paint or rust very well, if at all, unless you're pressing hard with a sharp probe. The head of most screws will likely have continuity to ground, but not always.
 
Also, blow any metal or carbon fiber dust out of your motor and grinder, outlets, etc...

I was once getting zapped pretty good on my metal topped workbench that my grinder was sitting on. Not only did I find out the ground was bad on the outlet, but I had been grinding a lot of carbon fiber, and the dust was getting continuity from my outlet to my workbench.
 
I have a multimeter and know some basics. Really basic. :D
 
Mecha said:
I have a multimeter and know some basics. Really basic. :D
Click to expand...
Well, quit bugging us a few hours :D, & spend a few online learning. There are dozens of websites with good basic electrical info. But be leery of some guys Youtube ”how I did it” channel.
There is plenty of good info there, but just as many sincere sounding Shadetree Electro’tards posting dangerous stuff also.
 
Lieblad said:
Well, quit bugging us a few hours :D, & spend a few online learning. There are dozens of websites with good basic electrical info. But be leery of some guys Youtube ”how I did it” channel.
There is plenty of good info there, but just as many sincere sounding Shadetree Electro’tards posting dangerous stuff also.
Click to expand...

So I'm not supposed to put my tongue on the wire to see if it has power? It works for a 9-volt battery! :confused:
 
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