Hot chassis on a three phase surface grinder?

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I finally got a VFD and wired up my Republic surface grinder, and as part of my check out procedure I tested for the presence of AC voltage with one of those non-contact AC sniffers. To my surprise, the entire chassis of the grinder was hot, but I can't find a DC leakage path with a multimeter, so I'm scratching my head.

I have split 220 (L1 + L2) going to the VFD through a double throw switch. The 3 phase is wired directly from the VFD to the motor. The Republic uses a lot of plastic bits to insulate the electrical boxes/conduit/etc from the chassis, so that the original 3 phase power cord was grounded to the junction box and the motor connection box, but NOT the chassis.

I'm using an extension of the house drier circuit to power all the 220 machines in my garage (always one at a time). I have an identical setup with VFD on my Nichols mill, but it doesn't have the same problem. Neither machine is grounded because the drier circuit has a separate issue that needs to be fixed. Please don't tell me it's unsafe to run this way. I know it is, but it's an old house.

I can't find a current path from the 3 phase wiring to chassis using a continuity test or even low and high (10 meg) resistance test. Yet the sniffer tells me 90V+ AC is present. I even tried hand turning the spindle but that made no difference. If it was a problem with dirt/contamination, I would think the high resistance test would pick it up. The motor is sealed, and the motor junction box is clean. The grinder and VFD otherwise work flawlessly. I'm tempted to ground the machine to the neutral lead, but that wouldn't explain what's going on.

Suggestions?

-Dave
 
The problem with non-contact probes is they can pickup noise from the environment. I would be suspicions of 90V on a 220V circuit. VFD output will have the switching frequency modulated on the AC, the probe may be picking up this modulation.

If you have tested it and the resistance to ground is above 1MΩ at 250V then I would suspect noise.

As a final test use a DVM on the chassis.
 
I sometimes test stuff like this by connecting a light bulb (60 watt?) in series with what I want to test. In this case I guess between the machine frame and ground (neutral?), then measure the voltage across the light bulb. That puts a load on the circuit and gives you a true reading of the voltage.
 
There can be capacitive leakage through the motor to the chassis which won't show up as a DC path.
You should add a good earth ground to the machine for safety.
 
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You are using a very high impedance multimeter and you are likely reading a phantom voltage - a voltage that is there is a high impedance circuit but disappears as soon as you introduce a load. I see that often in lighting circuits where one conductor runs in parallel with another. Try solidly grounding the machine directly to the grounding bus of your service panel and fix whatever is going on with your dryer ckt.
 
I measured 32VAC between the chassis and ground. The lower limit of the sniffer is supposed to be 90VAC, so it must be some sort of capacitive signal. (or it's just a cheap unit!) I crossed my fingers, solidly grounded the machine, and the issue disappeared, so I'm leaving it that way!

One other odd thing: at one point I touched one of the hot 120VAC screws on my master switch and got hit by the voltage (It's happened many times over the years, especially when working on HV CRT circuits, so I'm a one-hand-in-the-pocket type person most of the time), but this didn't feel like 120VAC (which I've always thought felt like oatmeal entering you fingers!); it felt much weaker, maybe due to high skin resistance or something- I certainly didn't want to try it again, however!

Thanks for the suggestions!
 
What is wrong with the ground on the dryer?

FIX THAT FIRST...PERIOD

is there a water pipe near?

VFD is a switch mode device.

This means the a.c. power is simply rectified into dc then chopped back into high frequency a.c. to create the variable frequency output.

This is done with inductors that generate magnetic fields that leak energy.

GFI breakers do not like such things.

All safety grounds on the machine and VFD need to be sound and motor ground important.

Safety ground to dryer ground or water pipe.

Take the time to fix your safety ground as your life depends on it.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 
Sounds like you are experencing commom mode current issues which are simple to corrrect. Run a ground wire from the ground point in motor conduit box directly to the PE terminal on the drive. you also need to run a ground wire from the PE terminal to the ground point in the feed box on the machine. Improper ground paths causes several issues with drives it is the most common rreason DRI's read incorrectly. Also read the manual on grounding on ungrounded and high resistance gorunded power sources. the manufacture may require that you remove a jumper or clip a wire in the drive on these power sources.
 
tq60 is correct, you may not have a good neutral connection from your dryer outlet. With most homes the neutral is supposed to be grounded at the power pole but it may not be. In that case you should provide the ground yourself to a cold water pipe, and then, add ground wires to all your machinery.
Also you should have the power company come out and check that neutral issue
mark
 
Couple things to check.
First is the motor's and other components are electrically isolated from power when they are not running so a motor winding shorted to ground or a wire on the load side of a contactor shorted to ground could energize the chassis and not present if the machine was tested with the machine powered off and the test was done from the plug to the chassis.

Second. Grounding.
Assuming that the machine is 3 phase, how are you creating the 3 phase power?
If you are running an RPC (rotary phase converter) did you pass the neutral/ground through to the chassis of the equipment? If not you need to do so.
The neutral of a 220 3 line cord is also ground back at the panel. It's called a neutral ground bond and should only be done in a breaker box. So as far as the comments about grounding the chassis to a water pipe or something like that..... DON"T. It creates ground loops and can run the entire ground for your facility through the machine connection to power if there was ever a problem with the ground bond in the building.
If you are running a VFD that has 220 in are you running a ground in from the power and then connecting a ground into the machine off that ground? Again, if not, you need to.

Side note.
If you are wanting to ground machines with a separate ground for electrical noise reduction or anything of that nature, it can be done but has to be done right. A simple wire to a water pipe isn't really enough. You should build a ground system. A grounding system would consist of a copper bar in the area that the equipment needing grounding is. Drive several 8 foot ground bars in the ground and either braze (never solder) or exothermic weld number 2 or larger wire between them and then run the number 2 or larger wire to the ground bar. A second number 2 or larger wire has to be run to the main breaker panel and connected to the ground buss bar in the panel. That second wire to the panel is VERY important. It eliminates the possiblilty of the machines acting as a path to ground for the rest of the building wiring via the connection to power. Once you have that installed, run a number 6 minimum to each machines chassis for grounding it. You still need to run a neutral return from the machines to the power feed but the machines will be grounded properly. Also, if you are in a metal building, you will need to run a ground to the structure frame from the ground bar. Never solder any of this. high compression lugs, exothermic welds (cadweld) or brazing should be used for innerconnection of wires. Other than in the breaker panel,,, put electrical rated paste on the wire and then it goes under a screw on the ground buss bar in the panel.

As mentioned before. DMM meters have a very high impedance input. Measuring from a chassis of a machine to some other ground very well may show voltage when very little actually exists. And it have no ability to indicate where the voltage actually is being sourced from. If the device under test is properly grounded and has no voltage present on it and you test it to an object that has a floating ground with some voltage potential on it you will get a reading, but the device you are testing is acting as a path to ground and the device with the floating ground is the source of the power that you are reading and there is no way to know that based on one test with a simple DMM. Unless you have a known ground you are testing from make two tests, one to the device under test and one to another grounded device to verify the ground you are testing with is actually ground.
Someone mentioned the lightbulb test, and this is a good test but shouldn't be the only test conducted. Take a light socket and connect two wires. Connect one wire to a known ground and the other to the machine under test. Then run the machine. The light should NOT be lit at all. This should not be your only manner of testing as it will show high leakage current but not a low leakage current. If the bulb lights brightly, You have a dead short to the chassis. If the bulb glows dim it's not a dead short, but is still a problem and needs to be addressed. The reason that it's not the only test to do is if there is a low current leakage it may not light the bulb but still presents a shock hazard. Shock hazards on a machine with spinning parts present more than just an electrocution hazard. If you get buzzed, you have a tendency to pull away quickly, and that can cause you to get into moving parts. So then you can use a DMM for the final test. Measure from the chassis of the machine to a known ground. There should be 0 volts indicated. Anything above 10 volts is cause for concern.
Those non-contact voltage testers look for an electrical field. A running motor will create a similar electrical field and may impose that field on anything it's attached to including the chassis it's bolted to. They are great for testing wires for the presents of voltage as a quick safety test. Not much else. You really should not rely on them for safety testing for ground leakage on a chassis of a piece of equipment as they can give you inaccurate readings.
 
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