VFD wired in, reading 30v to ground

Hi all,

I'm wiring in a TECO FM100 VFD to my new-to-me drill press, and need to ask some advice. After completing any power wiring, i like to check with a multimeter to male sure none of the machine surfaces are hot. When i did this, i got a reading of ~30v between the drill press table and ground. This happens only when the spinddle is running. I got the same reading (30v) between any 2 points on the drill press table or column, which doesn't make much sense to me. Similarly, checking 2 bare points on the VFD electrical box (wall mounted) gives the same reading. I decided it best to stop, disconnect and ask for advice before going any further.

I'm assuming i have a problem with the wiring either between vfd and machine, or in the machine itself. But, I've checked and don't have continuity between any of the hot leads and ground. How else would i have voltage?

The VFD is wired to single phase 220v, then to the three power wires for the motor and a ground to the casting. But, between the motor and those three leads there is a large 5-position drum switch which was originally used for speed changes on the multi-speed motor. I know it's not recommended to have a switch between motor and VFD, but I'm not planning to use it. The wiring to the drum switch is complicated, and i was hoping not to have to get into that if possible as i don't have a wiring diagram. Could that be somehow part of the problem?

The TECO VFD was working fine on another machine.

Thanks in advance for any advice you can offer.

Lee

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I suspect the meter is picking up an interference signal if you get a voltage between 2 point on the table.

Try touching ONE probe to the table, and, with the wires draped the same way as before, touch the second probe to the first probe, and NOT the table. If you still get the reading, it is radio interference.

Try again the same way, but twist the meter leads together fairly tightly. You will probably read much less voltage. If not, the meter is directly receiving the signal. Move the meter and it should change.

What kind of meter? If it is a cheapie, many of them do that.

Heh, that's good to know. But, in the meantime i took a look inside the drum switch and the (16!) leads have a bunch of bare wire showing in many areas - looks like a rat was chewing on them! I don't think i can safely re-energize this until i replace all of those wires, which requires a partial teardown. At this point, I'd prefer to just bypass the switch altogether and wire directly to the motor, but without a wiring diagram i think a 16-lead motor might get the better of me...

Sigh...

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yes, best to NOT have switches on VFD outputs.

Thanks for all the input. I bypassed the drum switch completely, and i still have 30-ish volts to ground. Twisting the wires doesn't affect the reading. Yes, it's a cheapie multimeter.

A friend suggested it might be vfd-related voltage arcing through the motor bearings to the shaft/chassis, which is apparently fairly common on bigger motors and can trash the bearings fairly quickly. I've ordered a couple of motor brushes and housings - will try attaching one to the shaft to provide a ground path and hopefully protect the motor bearings.

Lee

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If you get a reading between two pints on a piece of metal, either there is a LOT of current through the metal, in which case it would be pretty hot, OR there is an issue with the meter.

Try a good meter. There very likely is nothing wrong with the wiring, if you have not left any big issues out of your description.

I'll see if i can scrounge a fluke from a friend.

Thanks

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I would think that is just common mode noise created by the VFD. I suspect you do not have good solid connections of your machine frame to ground, your motor frame to ground, your ground wire from the motor is not terminating on the VFD ground terminal (it should by the way), something like that or a combination of some or all of the above.

I would agree, but he is getting the same reading with the meter leads both touching the same piece of metal. It looks and smells like a meter problem. It COULD be a CM voltage, but there is probably also a meter problem.

I recall having one version of a Simpson 260, one of the modern ones but still an ANALOG meter, that I could not measure a DC voltage with. I was wanting to measure the boost DC of a PFC front end, and the silly meter read double the voltage. I borrowed a different 260, not the same sub-model, and it read fine, as did Fluke meters (but I was reading so many voltages I needed the 260).

Something about that ANALOG meter would not read the DC with a high frequency AC on it. And I was using the analog meter because I figured it would NOT be bothered. Instead it was worse. But the cheap Beckman digitals were even worse, they gave no reading at all, just garbage.

JST said:

I would agree, but he is getting the same reading with the meter leads both touching the same piece of metal. It looks and smells like a meter problem. It COULD be a CM voltage, but there is probably also a meter problem.

I recall having one version of a Simpson 260, one of the modern ones but still an ANALOG meter, that I could not measure a DC voltage with. I was wanting to measure the boost DC of a PFC front end, and the silly meter read double the voltage. I borrowed a different 260, not the same sub-model, and it read fine, as did Fluke meters (but I was reading so many voltages I needed the 260).

Something about that ANALOG meter would not read the DC with a high frequency AC on it. And I was using the analog meter because I figured it would NOT be bothered. Instead it was worse. But the cheap Beckman digitals were even worse, they gave no reading at all, just garbage.

Click to expand...

Ah, I missed that he got the same reading by touching any two points on the same machine. I agree then, meter error.

Might be EMI/RFI coming off of the VFD or unshielded cable going to the motor being picked up by the meter leads as an antenna, in which case if he does twist them and it makes a difference, would lend credence to it.

leeko said:

A friend suggested it might be vfd-related voltage arcing through the motor bearings to the shaft/chassis, which is apparently fairly common on bigger motors and can trash the bearings fairly quickly. I've ordered a couple of motor brushes and housings - will try attaching one to the shaft to provide a ground path and hopefully protect the motor bearings.

Click to expand...

That shaft voltage is typically only about 1.5V at most, often even much less than that. It takes a very specialized meter to see it. It also would not do what you are describing, it only shows up between the motor shaft and ground.

Quick update: the weird readings went away with a decent meter, as had been suggested. Thanks everyone for the advice,

Lee

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