Toasted Contactor Coil Question

[itsmoked]

I was called on to check out a strange machine to see if it was even vaguely operational. The machine was a Hexapod, a goofy looking thing that has a resemblance to a chest-high toadstool. It has 6 linear actuators that support a 3 foot diameter table that can twist raise and lower to provide multi-axis machining when mounted under a knee-mill. Anyway with little to go on other than the machine requiring 240 3ph @ 30A, I started checking things.

It has 6 Yaskawa drives in it and is very cleanly wired. The locals plugged it into a 3-phase power cord to show me that it didn't work... With voltmeter in hand I started checking around. The output of the 500VA control transformer was a nice clean... 62V!?? Further checking showed the control transformer was being supplied only 122V instead of 240V. Further checks showed instead of 240-240-240 I was seeing only 122-122-211 on the three phases. Working yet further up the power chain I unplugged the power cord and investigated it. It had more of the same 122-122-221V stuff in a bizarre pattern. I unplugged the 5 contact three phase with neutral and ground plugs apart on both ends of the cord to understand it. Turns out they had no neutral and had hooked the ground to one random phase pin and used the neutral pin for one of the phases. MAn..

Adding insult to the whole sordid affair it turned out they only had 208 3ph in the facility.

Anyway, to my question. The machine had only two buttons E-Stop and ON. Pressing ON activated a small 24Vdc pilot relay that was to heat up the main contactor, a 7.5HP @ 240V AB IEC looking unit with a 120V 60Hz coil. The AB was non-operative and proved to have a an open coil. This machine had never been put into service though it is probably 20 years old. Do you guys think feeding it 62Vac could've toasted it since it never would've sealed? I'm suspecting it but haven't actually seen it happen.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Yes. Low voltage, -> insufficient force to close, -> heating of the coil, -> eventual winding insulation failure, -> ending in open circuit. Happens a lot.

Were the 122-122-211 readings Phase to Ground? Because that sounds like a 240/120V 3 phase 4 wire "high-leg*" delta system, not a 208Y120 3 phase 4 wire system. In that system, phases A and C are connected by a winding that is split in two, with the center tap grounded to form a neutral, so you get 120V (nominal) from A to N (G) and C to N, but there is no direct reference to ground on phase B, so it measures as 208V, +- based on stray capacitance. I have seen instances where electricians inadvertently connect it incorrectly (it's all relative anyway) so the "high" leg is C, or sometimes that is the standard that the utility uses, often because it is a legacy. The problem is, when people do NOT know it even exists (which happens a lot), then they fail to understand the ramifications of connecting the control power transformer to the wrong points, resulting in control circuit failures a-plenty.

* AKA "red-leg" delta, "stinger Leg" delta, several other colloquial terms.

"Will work for (the memory of) salami"

 

[itsmoked]

Hi Jeff. Thanks for the confirmation.

The 122-122-211V were between the "three phases". Only it turned out one of the phases was actually wired to earth ground. I went in thinking open delta, "where's the stinger" when I got there. But after yanking the outlet face plate and measuring the real three phases it was confirmed to be 208V 3Φ.

I've seen lots of "I didn't know there was a high leg" issues. Ha!

I'm going to install a new contactor and see if it seals on the 104V instead of the 62 it was getting. If it doesn't seal immediately I'll E-stop it and come up with some plan B to prove the machine can function. I already tried straight jumpering of the contactor but it has so many auxiliary contacts that it was a bust as other lock-outs and safeties where still not satisfied.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Standard coil voltage specs are supposed to be 80% pull-in, 70% drop out. So 104V on a 120V coil should work.

"Will work for (the memory of) salami"

 

[Skogsgurra]

I would put a standard coil there and let the pilot relay connect the coil directly to your incoming grid. That way, you won't have to worry about marginal voltages or voltage drop in the control transformer.

And, the good boy you are, you also make a note in the documentation what you did and why.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Thanks for those percentages Jeff. Nice to know. I'll dial down my reflexes one notch on the test.

Gunnar, people in these here parts don't take too kindly to 200+ volt control signals. The 500VA control transformer running just a measly contactor should actually provide a higher than 120V expected voltage anyway. I think I'll not try modifying the contactor's drive right-off. Plus I'd then need some sort of documentation to be able to alter it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mikekilroy]

Wow. the G&L Variax hexapod 6 axis table.... it was all the rage at IMTS (International Machine Tool Show) - about 20 years ago! People come from all over the world for this show every 2 yrs in Chicago.... That year I recall this G&L table in a CNC was the highlight. Like 2500 vendors, and this was IT. What memories!

https://www.youtube.com/watch?v=7TowJZQi-qY

https://www.youtube.com/watch?v=7TowJZQi-qY

It never took off - too complex. But some little folks still make some little ones like:

https://www.youtube.com/watch?v=B2egFi3N3wE

https://www.youtube.com/watch?v=ClNAkz8PEPk

A little history in case your customer has not shared with you how it is supposed to work.

http://www.KilroyWasHere<dot>com

 

[itsmoked]

Thanks Mike!

This one looks just like the Newport one you show above except it's much, MUCH larger. I was told this particular unit was only used at trade shows and never run under a mill. The company tanked before making a profitable business out of it. I could see it thrashing the CNC code people to get it sync'd and happy with any random knee mills. Other than that it really looks well designed. It's entirely covered with a large bellows skirt so it could have chips and fluids poring all over it with no issues. A bit of a shame really. These days you could probably find lots of G-code generators that would spit out the right stuff.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[SceneryDriver]

I had something similar happen to me a few years ago on a freight elevator. We lost a 480v phase from the utility. That phase happened to be one of the two phases the control cabinet's control transformer was wired across. The transformer was now outputting something around 60-ish volts instead of the correct 120v; hot primary phase to dead primary phase (something close-ish to ground (maybe)) meant the trafo didn't have the correct primary voltage.

That low control voltage cooked the coil on the main contactor ahead of the VFD. The low voltage wasn't enough to pull in the contactor and allow it to seal in the magnetic circuit. It sat there and buzzed and melted all the plastic around the coil and gave off that most distinctive "something-electrical-burned" smell. The windings eventually shorted and tripped the control voltage circuit breaker in the panel. I actually finally found the bad coil by smell $150 and three days later, we had a new contactor coil and all was well.

The panel now has a phase-monitoring relay installed that looks for nasty stuff on the 480v incoming power, and opens the 120v control power circuit if things don't look right. The elevator will stop, but at least it won't burn up any components. The freight elevator is VERY important to what we do, and without it, there's a lot of people standing around with hands in pockets. Unplanned outages are to be avoided.

Bottom line is, low voltage is a killer for AC relays and contactors. Tell the yahoos that tried wiring the machine they should be slapped. "If at first you don't succeed, electricity is not for you."


SceneryDriver