Wiring 3 phase switch for 2 phase power

Eeeek. I've done a lot of sketchy things in the past, but I would really really try to avoid doing that - wiring a machine for 240 with one leg always hot (unswitched). Aside from the code violation, I don't think I could sleep at night knowing that the motor had 120 between the windings and the frame, and that an insulation breakdown could cause trouble.

I would only do that for temporary testing, or if I had to run it that way long term for whatever reason, I see that it could be unplugged when not in use.

Jim

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Thans Don,

No reverse button....

I appreciate your help

Too Funny, I like this group, beers on me! Alan

I have an ingersol-rand 5 hp 3ph air compressor that I rebuilt for a customer (and he aint getting it until he pays me..) that only has two pair of contacts for the power. When it came to me, the third leg was constant hot. And its factory original, which is completely weird.

Gunner

That rifle hanging on the wall of the working-class flat or labourer's cottage is the symbol of democracy. It is our job to see that it stays there. - George Orwell

Maybe it was specially designed to run on grounded leg delta service? My experience is that manufacturers tend to leave the specifics of contactors and wiring to the local electricians, so machines will come missing the last link that hooks up to the local service.

Jim

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I think it may have been legal at one time to switch only two leads of a three 3 phase motor. My 60s vintage IR compressor has a Square D pressure switch nameplate rated

I know that it used to be acceptable to put overload heaters on only two poles of a motor starter, and this is no longer the case.

Ned Simmons

From the 2002 NEC

Article 430.84 The controller shall not be required to open all conductors to the motor. Exception: Where the controller serves also as a disconnection means, it shall open all ungrounded conductors to the motor as provided in 430.111

---A controller that does not also serve as a disconnecting means must open only as many motor circuit conductors as may be necessary to stop the motor, that is, one conductor for a DC or single-phase motor circuit, two conductors for a 3-phase motor circuit, and three conductors for a 2-phase motor circuit.---

Nate

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I'm sure I've read that section, but that never registered.

Ned Simmons

Well ... the heaters sense the current to the motor, and the third can be no worse than the sum of the two being monitored, so it should be possible to detect any kind of fault circuit (the most common being that of power applied to only two phases, which would be feeding a stalled motor, and thus be sufficiently over-current to trip the circuit fairly quickly anyway.) This is intended to protect the motor, not anything else, as there should be a breaker or a fuse in each hot anyway to protect everything else.

This means that my earlier circuit to reverse a 220V single-phase motor with a single three-pole double-throw center off switch would be legal, as long as there was a disconnect provided (which in my case would have been the twist-lock power connector near the lathe).

However, I agree with others that the idea of keeping one side of the motor live does not feel right anyway.

Thanks, DoN.

It could be called that, but by convention a 180 degree phase shift is considered only a periodic polarity reversal.

Gary

Hey Nate,

And as just a follow-up to that, we used MG sets that were operated Star-Delta. These were effectively 6 lead motors, at 575 to 600 volts AC producing 0 to 240 volts DC, or sometimes PWM DC. The AC end of the MG's had three "hot" leads at all times, and the motor is started and run by switching the other three leads at the controller. That way, a whole contactor, valued at from $50 to $250 (depending on size) was "saved". Only problem is, these MG's sit for at least 50% of the time in a shut-down state, but the 600 volts is still a potential on the windings, and these MG's would blow the AC ends at a much higher rate than the same models in a more conventional "all motor leads open" type.

Take care.

Brian Lawson, Bothwell, Ontario.

By electricians.

Jim

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But then again, that was then. This OTOH would be 2004 so to be absolutely certain that code book should be inspected.

Jim

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Okay, that was false economy in action... But I don't see new equipment built that way anymore, we've learned better.

(Though I catch pool men "handymanning" their own electric and single-pole switching 240V pump motors all the time because they /don't/ know any better. When I get to a complex where the controls are screwed up, they complain about blowing out new motors, and I learn the Pool Man has worked on the wiring, they come /all/ the way apart for a full inspection...)

An example - modern hydraulic elevator machines with open "submersible" pump motors in the oil tank that are run with 208V Star-Delta starters, and they have two contactors in the controller. (Even if you don't have access to the equipment room, the second "clunk!" is quite audible from outside.) One for total shutdown, the other for the Delta switch when up to speed.

They do have a second contactor involved, but the complexity of the starter must be outweighed by the longevity of the motor. Especially since the hydraulic oil might get traces of moisture in it. That and if the motor shorts out there's 50 or so gallons of oil there.

(There is also a sticker on the top of the tank to not use water based hydraulic fluids. Well, Duh....) ;-P

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A fault to ground in a leg with no heater would not trip the overload if the feed was Y connected.

That's the primary purpose of the overload, but since the overcurrent protection in a motor branch circuit can be much higher than the ampacity of the conductors, the overloads appear to provide some protection for the conductors as well.

And most industrial practice would agree. I've seen hundreds, if not thousands, of installed motor starters and can't recall any where all the hots were not switched, even on old installations with two heaters on three phase starters.

Ned Simmons

The 2002 version is still current.

Ned Simmons

Thanks for the heads-up. I do know that things change in code like that over time.

Jim

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