three phase mootr starter, only two overload heaters?

Bill D · Jul 18, 2016

I have a Furnas motor/starter on a piece of older equipment I bought. Probably 1970's or maybe even 60's. It is three phase but only has two overloads. The wiring diagram also shows it this way. The previous owner, a high school shop, added a separate box with another contactor. I have not puzzled out all the wiring but I think they added it to switch control voltage for the third phase which is not protected. But it does not add a third heater circuit.
Just wondering if any one has seen similar?
I will probably gut the wiring and the second contactor and switch to single phase. They also changed the switch and it's location so this may be part of that change.
Bill D.

atomarc · Jul 18, 2016

Protecting only two legs was done for a bazillion years...years ago, and worked fine. I wouldn't change a bunch of stuff just to protect that third leg unless you're going to do other updating as well.

Stuart

rbent · Jul 18, 2016

One of my testing machines is a 3ph machine with only two overloads per motor, 1957 vintage I think.

wippin' boy · Jul 18, 2016

"will probably gut the wiring "

replace it if your in there anyway.
you can get modern used on eBay for a pittance

Modelman · Jul 18, 2016

atomarc said:
Protecting only two legs was done for a bazillion years...years ago, and worked fine. I wouldn't change a bunch of stuff just to protect that third leg unless you're going to do other updating as well.

Stuart

Since all the electrons have to go through one of the heaters to even get to the third leg, it's protected. That being said, it's no longer code. So if it's just a matter of wiring it up to get it to run, do it. If it needs more work than that, replace the started with something more modern.

Dennis

atomarc · Jul 18, 2016

Modelman said:
Since all the electrons have to go through one of the heaters to even get to the third leg, it's protected. That being said, it's no longer code. So if it's just a matter of wiring it up to get it to run, do it. If it needs more work than that, replace the started with something more modern.

Dennis

"It's no longer code"?? Is this correct, the NEC no longer requires that all 3 legs have overload protection. New one on me, but then again, I'm retired.

Stuart

Bill D · Jul 18, 2016

I figured they added the third leg protection in the 90's. I know school shops here had to add extra drop out protection for single phase 120 stuff. Simple contactor control for the bench outlets so they had to push a button to restart if the power went out.
I can see a problem if the unprotected leg faulted to ground there would be no overload to sense the fault. Of course the branch circuit breaker should trip pretty fast on that.
I guess I will remove half the wiring and use it as a single phase control. I may even add an extra stop switch or replace the factory mount position. This is a belt/disk sander so it needs a switch you can reach from two sides. I do not need a switch that can be locked off. And I prefer a rubber cord to flex conduit hardwired into a box.
Bill D.

Modelman · Jul 18, 2016

atomarc said:
"It's no longer code"?? Is this correct, the NEC no longer requires that all 3 legs have overload protection. New one on me, but then again, I'm retired.

Stuart

No, what I said is all three legs DO have overload protection... It's a motor starter, it's purpose is to protect the motor from overloads, and any overload has to go through one heater or the other. What it doesn't provide is short circuit protection on the third leg, but that wasn't it's purpose; the upstream breaker would have to catch that. Sometime long ago the NEC changed, and now the motor starter also provides short circuit protection on all three legs.

Dennis

Heavey Metal · Jul 18, 2016

Corner grounded delta

Hook it up to delta and it could cost you a motor cause it won't trip with the right combo of phase out prolems.

gadget73 · Jul 18, 2016

all I can offer is that the wye setup at work all has 3 heaters. Its mid 1980s vintage.

I was always taught that the overloads are for phase out problems, the breakers are for over-current and short circuit problems. I know I do have one motor at work that doesn't have overloads, and its roasted twice from loss of phase, never popped a breaker. Its out of service now until it gets re-done with overloads.

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three phase mootr starter, only two overload heaters?

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