Lathe power disconnect options

[David C. Roseman]

Roger and William, as I say, I'm not an EE, but I have looked into these issues. I'm just passing along wisdom from folks who are and know these things.

Putting a do-it-yourself DPDT switch in the 220v input line is an easy and tempting way to kill all power to the lathe (or other equipment) in an emergency. There are 220v DPDT magnetic switches, even "paddle" or "knee" switches, available online for under $15. I know of turners who have done this. When a VFD is involved, though, we should be aware of the other considerations.

If one is used, I suggest thinking of it only as a substitute for "pulling the plug", and making sure anyone who uses the lathe knows why. If under the circumstances you would pull the plug, use the switch. So, if the lathe is actually operating, that means only in an emergency, never merely as a convenient way to shut down the lathe. But even then, be sure to wait two or three minutes before flipping the switch back on. Absent an emergency, I would use the switch only after the lathe has been shut down normally and you want to disconnect from the line power to avoid damage from power surges. In the latter case, though, a DPDT switch only disconnects the two hot legs, not the ground leg. So a lightning ground strike might conceivably reach the lathe anyway. And even with respect to the hot legs, the disconnect is only the gap between the switch contacts. Pulling the plug from the receptacle would still seem to be the best protection.

[John K Jordan]

DPDT switch in the 220v input line ...
If one is used, I suggest thinking of it only as a substitute for "pulling the plug",...

That's how I use mine - turn off the lathe with the control switch, then throw the switch when leaving the shop. I'd like to replace it with a 3-pole single-throw switch to include the ground as you mentioned but I've never found one - maybe a 3-pole contactor instead. BTW, I you should only need a DPST switch but the double throw doesn't hurt anything.

[David C. Roseman]

[snip]
BTW, I you should only need a DPST switch but the double throw doesn't hurt anything.

Good point, John. I should have said DPST, otherwise the throws would have to come tied together, as on a typical 220v circuit breaker. In fact, the inexpensive magnetic paddle switches I mentioned are DPST.

[Glenn C Roberts]

John, Can you use a 3 phase DPST?

[John K Jordan]

John, Can you use a 3 phase DPST?

I've never had one in my hands but I don't see why that wouldn't work! Duh - I had searched for "3-pole" but never thought to search for "3-phase power switch" - now I see lots of options. Thanks!!

Due to the contact separation issue mentioned earlier it still might not help with a local lightning strike but then there's not much that will other than unplugging. Should be perfect for moderate power line surges though. Fortunately, all the power lines on our farm are underground.

JKJ

[Bruce Schoenleber]

Would a near ground strike (lightning) still charge the ground wire sufficiently to fry a VFR? I too have below ground utilities but still unplug my lathe for just that fear. I would prefer the 3-pole switch solution. My paranoia stops short of wrapping my shop in lead in case of a Solar Mass Ejection, how cavalier is that?

[John K Jordan]

Would a near ground strike (lightning) still charge the ground wire sufficiently to fry a VFR?

That's a good question. I was told that the underground wiring helps. Maybe someone who lives on the top of a hill will volunteer to put up some lightning rods and test this!

My power is 600' underground to a 7200v transformer near the house, then split about 75' underground to the house on the top of the hill, 250' to the shop, and maybe 2000' to a shed, barn, and electric gate. We've had several "good" lightning strikes following trees to the ground but I've not yet had anything fry. One time the 7200v underground feed went out with a pinhole in the insulation - the utility guy said it may have been from lightning but he didn't know. That blew the fuse on the pole at the property line as did once in the past when termites got in under the transformer - when the fuse blows it sounds like a cannon going off!

I did have a VFD go bad once but it was a gradual failure. I don't think it was from a power surge but it was during those years when I left the lathe plugged in all the time.

JKJ

[JohnC Lucas]

I haven't read this whole thread (it's quite long) There seem to be 2 issues here. One is disconnecting the lathe from the power source (unplugging) and the other is a remote switch. The switch that turns the lathe on so it spins I will call the power switch. Those are usually hooked into the VFD and are low power. The 3 phase simply goes from the VFD to the motor. You never need to switch that, it's done internally in the VFD. The power switch is hooked directly into the VFD and simply tells the VFD to turn the 3 phase on or off. Very easy to put a remote switch in to turn the lathe off. The other switch is I will call the main switch. This is the switch that applies the 220 single phase to the inverter. Like John I turn the circuit breaker off each night. On my powermatic if you leave the main power on the VFD stays on or has power. I turn the circuit breaker off for two reasons. One is to disconnect the power to the VFD and the other is to try and protect it from lightening strikes and power surges. I've been doing this more than 12 years now and my lathe still runs perfectly. (Powermatic 3520A) I also did this on my Nova 3000 that I added a VFD to. No problems.
I hope I didn't miss some important issue in the other threads. I know John K knows all of this already.

[Bruce Schoenleber]

What caused me to ask is that there is always (or should be...) a 5-6' copper ground stake near the main power panel. That is where I would expect to pick up the surge from a strike.

[Dick Strauss]

Just so that we are all clear here...the emergency stop button interrupts a 12-24V (maybe 0.01-0.1A) DC signal between the VFD and itself. If the VFD doesn't see the signal on its inputs, it shuts off power to the motor. You can wire another switch in series with the e-stop where both have to be on for power to the motor. You can also wire one switch in parallel with the existing e-stop whereby the motor will be receiving power if either or both switch(es) is/are on...

For protection from surges and strikes you want to interrupt power between the incoming wall power and all conductors (including the ground) to the VFD. A three phase disconnect will do the job but unplugging is better because lightning can jump a gap in a switch! The more distance the electricity has to jump, the better protection you have.

[Olaf Vogel]

Hey Brice,

IMO it problem is a lot simpler than you think.

Ideally what you have is:

One central power panel that distributes to all your machines (Including the VFD's)
At the end of the day, hit the main panel and disconnect everything. That insulates against power surges etc.

One circuit breaker for each major machine. This is required, by law, in industry. Commonly known as a disconnect switch.
I use these for 3 phase: https://www.ebay.com/itm/CHNT-Triple...4AAOSwYIhWlMD8

There are single phase versions.

Whether your shop is single or three phase - its all the same idea.

To shut down your lathe, hit stop or emergency.
Wait about 30 sec, disconnect it - either via the disconnect switch or the main power.

I have 6 VFDs in the shop, from 1 hp up to 10 hp.
Thats been my setup for 5 years now - installed by an industrial electrician. Never had an issue.

And the shop is in a rural area where brownouts are common.
Plus I have a steel roof, which must be lightning friendly....

[Olaf Vogel]

I've never had one in my hands but I don't see why that wouldn't work! Duh - I had searched for "3-pole" but never thought to search for "3-phase power switch" - now I see lots of options. Thanks!!

JKJ

John

I think you have two options:
https://www.ebay.com/p/Never-GE-Heav...ole/1600001876

These are huge, heavy, expensive and the fuses are a one shot deal.
The best of 1920's technology, wickedly overpriced.

or the European solution
https://www.ebay.com/itm/CHNT-Triple...4AAOSwYIhWlMD8

Get a 4"x4" distribution box from Home Depot and cut a square for the switch

Cheap, easy to reset, small.


Guess which ones I use?

[Don Bunce]

Olaf, Those breakers are only rated at 2 amps. I would think that you would need something like 20 amps.

[Olaf Vogel]

Olaf, Those breakers are only rated at 2 amps. I would think that you would need something like 20 amps.

Sure, I meant that type. They go from 2A up
https://www.ebay.com/itm/CHNT-Triple...QAAOSwfZ1WbBjk

Pick the size you want