# Control txfrmr w/corner grounded delta



## KenD (Feb 16, 2011)

I had a service call today because of a motor starter that would not engage. I found that the control transformer for the coil had no output. It's a typical GE type QB control transformer. The supply voltage is 240v (Corner grounded delta system!) and the output is .10 kVA 120v. 

First question, is .10 kVA enough to run a coil? What is typical draw on a magnetic coil?

Second question, Shouldn't this txfrmr be rated for use on a corner grounded delta system? Does such an animal exist or does it really matter?


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## erics37 (May 7, 2009)

KenD said:


> I had a service call today because of a motor starter that would not engage. I found that the control transformer for the coil had no output. It's a typical GE type QB control transformer. The supply voltage is 240v (Corner grounded delta system!) and the output is .10 kVA 120v.
> 
> First question, is .10 kVA enough to run a coil? What is typical draw on a magnetic coil?
> 
> Second question, Shouldn't this txfrmr be rated for use on a corner grounded delta system? Does such an animal exist or does it really matter?


Did you check the fuses on the control transformer primary and secondary?

As far as the transformer being beefy enough to run the coil; the starter should have a label on it somewhere that says what kind of current the coil draws at 120V.

As far as being rated for use on a corner-grounded system... I don't know the answer to that. If the transformer is rated for 240V and that's what the supply voltage is, then there shouldn't be a problem... assuming the grounded phase isn't being used for the transformer I suppose.


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## micromind (Aug 11, 2007)

In my experience, a 100VA control xfmr will usually operate a size 0 or size 1 coil. To be sure, you'd need to look at the inrush of the coil, and the O/L capacity of the xfmr. 

A control xfmr can supply far greater inrush current than a normal power xfmr can. This is the reason a control xfmr is much heavier than a power xfmr of the same rating. The more iron in the core, the lower the impedance; and thus less voltage dip during inrush. 

The xfmr doesn't care what type of system its connected to. It only cares about voltage. It doesn't matter if the voltage is line-to-line or line-to-neutral. If a 240 volt xfmr has 240 volts across its primary, it's happy.

Rob


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## JRaef (Mar 23, 2009)

erics37 said:


> Did you check the fuses on the control transformer primary and secondary?
> 
> As far as that transformer being beefy enough to run the coil; the starter should have a label on it somewhere that says what kind of current the coil draws at 120V.
> 
> As far as being rated for use on a corner-grounded system... I don't know the answer to that. If the transformer is rated for 240V and that's what the supply voltage is, then there shouldn't be a problem... assuming the grounded phase isn't being used for the transformer I suppose.


I don't know if starters state coil draw on their nameplates, but certainly the mfrs catalog will. You need to look at both the pull-in and sealed VA. 

As to is it big enough? Depends on the size of the starter, you didn't say. The QB transformers though are NOT Control Power Transformers, they are General Purpose transformers. There is a difference. CPTs are designed specifically to be capable of delivery high short term overloads without saturating, because they are intended to pull in coils which have high inrush to seal-in ratios. GP transformers are not made for that. Typically f you have a 100VA CPT, that's good for up to a NEMA size 3 starter (from most mfrs) but if it's a GP transformer, maybe only Size 1.

Corner grounded delta makes no difference to the transformer. But if someone hooked it up wrong, it certainly wouldn't work. I've seen where some non-electrician hooked one up wrong because he ASS-U-Me-d that 240V means 120V line to ground, and on a 240V red leg delta he may have seen that work before. But on a corner grounded delta it will not. You need to look carefully at everything there.

But first, as eic37 said, *check the fuses!*


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## varmit (Apr 19, 2009)

If the transformer is rated for 230 volt, it makes no difference about the "high leg". You will still have 230 volts between any two phases. You only would need to be aware NOT to connect any 120 volt loads to the "high leg". A .10 kva transformer would be good for a sizable starter, assuming that the total of all loads do not overload the transformer. (0.1 kva @ 120 vac = 100 va = 0.83 amp = 90 watts.)


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## erics37 (May 7, 2009)

JRaef said:


> I don't know if starters state coil draw on their nameplates, but certainly the mfrs catalog will. You need to look at both the pull-in and sealed VA.


:thumbsup:

I believe I've seen it on some but I could certainly be thinking of something else. Maybe I was just looking at the contact ratings or something :laughing:


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## KenD (Feb 16, 2011)

Thanks everyone for the reply's. I don't get the opportunity to do much motor control work. It seemed simple to me that 230v is 230v. But I know that distribution panels have to be rated for Corner grounded systems, I wasn't sure about tx's

@varmit:This is not a Delta "high leg" it's "corner grounded delta" = 240v to ground. 

@JRaef:Yes, this isn't an actual control type transformer and this is a size 0 starter, also all the data is missing. I'm guessing they used a General Purpose type because it's mounted remotely and was just simpler, it's also pretty old. I did do some searching and found some data about coil amperages and I'm suspecting that this transformer may be undersized at .100kVA and I think it should be around .250 for the in-rush. 

I'm thinking a .500kVA GP transformer should have been used or a proper Control type transformer in an enclosure. Thanks again guys, I'll do some homework before I go back for the repair.


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## HARRY304E (Sep 15, 2010)

KenD said:


> Thanks everyone for the reply's. I don't get the opportunity to do much motor control work. It seemed simple to me that 230v is 230v. But I know that distribution panels have to be rated for Corner grounded systems, I wasn't sure about tx's
> 
> @varmit:This is not a Delta "high leg" it's "corner grounded delta" = 240v to ground.
> 
> ...


 Welcome to the forum..:thumbsup:


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## JRaef (Mar 23, 2009)

Yeah, I just looked at a data sheet on a Siemens NEMA starter, they use the same coils up through NEMA size 2 and although the sealed power is only 25VA the inrush is 218VA, a bit more than I thought. So that 100VA GP xfmr is probably saturating. But 250VA (.250 kVA) would probably be fine if there is nothing else running from it.

But for that same starter, the field modification kit to add a CPT uses a 45VA CPT. That's what I meant by the transformer designs being very different. That's also why CPTs are more expensive that GPs when compared VA to VA capacity.


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## varmit (Apr 19, 2009)

OOPS, I didn't read what I thought: Corner ground, not high leg. However, It would make no difference to transformer operation either way as long as phase to phase voltage was the same.


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## miller_elex (Jan 25, 2008)

Maybe the coil isn't seated properly on the contactor.

Sometimes they wiggle out from the years of vibration.

You've got to get a meter and see where your problem is.

It could be in the OL's, could be a stop contact that won't make...

Some 100VA transformers have a fuse inside a plastic end-cap that you can't see, and can't be replaced... the whole xfmr has to be replaced when the fuse blows. Those kind come mounted to a 4square cover.

EDIT: nevermind, I'm a reh-tard


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