delta transformer question

[JohnME]

Well, I am rather embarrassed to even ask this but here goes.


I have a delta-delta transformer, it is a 45 KVA GE 480 to 240 3 phase.


There is X1 H1 X2 H2 X3 H3 terminals and thats all. I have a strap that goes to the case down in the bottom area of the enclosure, I assume that this is where my XO is in this situation? I just added a lug to the strap and will attach the nuetral here.


Every transformer I ever hooked up has a spot marked XO, thank you all for the help, I feel better asking then hooking up and guessing.

[roger]

John, your transformer does not have a XO because it is strictly 480 to 240, (Delta to Delta) you will not have 120 anywhere in this configuration that would utilize a neutral.

You should have an EGC run with the primary conductors that would be bonded to the enclosure.

You would have to ground one phase of the secondary if you were looking for a corner grounded Delta supply on the load side of the transformer.

Note; do not try to get 120 by connecting to the bonding strap.

Roger

[JohnME]

I should have said that this is for a 240 volt system- temp power for a sand blasting machine and nothing more.

I do not know if he needs a neutral connection or not- hence my question about where to get it, which would be from XO normally- I really dont deal with many transformers as you can tell, so I thank you for your help.

So, if for some odd reason he would need a neutral for a 3 phase setup I cannot get one from this transformer I take it. I hope he doesnt need one!

I do have an EGC run with the feed :)

[roger]

John, you've got it, and you are right in your assumption that he will never be able to get 120v from this transformer

Personally, I would bond one secondary phase to the enclosure so that another fault would open the secondary OCPD.

Roger

[boboelectric]

Did anyone ever use a corner grounded xfmr, and why?

[JohnME]

Thanks Roger for all your help.

[360Youth]

We bought a step up for a motor control box that was supposed to have neutral connection and did not. The seller (re-conditioned parts) tried to tell me it should work and would not believe me when I told him it wouldn't. Everybody say it with now..."Well, all you gotta do is..." Famous last words. :grin: Needless to say they took it back.

[QUOTE=JohnME]I really dont deal with many transformers as you can tell, so I thank you for your help. [QUOTE]

I have learned here people want to help. I used to say, and still do, "I'd rather know twice than not know once."

[JohnME]

Ya I really hope he doesnt need a neutral, the last thing I need is to hunt for another transformer! At least this one was free from the pile at the shop.

[roger]

John, your welcome and I know I probably haven't cleared up all your questions.


Did anyone ever use a corner grounded xfmr, and why?

They are still numerous in industial settings and were more common in the past along with ungrounded systems.

We do some work at a textile mill that has an ungrounded Delta service even today.

Roger

[coulter]

Corner grounded 240? Never seen one. Never heard on one. Probably someone has done one somewhere - but I don't know why.

High leg 240 Delta - yes. Corner grounded 480D - yes

carl

[K2500]

Did one awhile back. It was a maint. shop for a industrial facility, 480D service, but about half the EQ was 240 3Ø, it was cheaper than replacing the EQ.

[weressl]

Well, I am rather embarrassed to even ask this but here goes.


I have a delta-delta transformer, it is a 45 KVA GE 480 to 240 3 phase.


There is X1 H1 X2 H2 X3 H3 terminals and thats all. I have a strap that goes to the case down in the bottom area of the enclosure, I assume that this is where my XO is in this situation? I just added a lug to the strap and will attach the nuetral here.


Every transformer I ever hooked up has a spot marked XO, thank you all for the help, I feel better asking then hooking up and guessing.

I would not hook up a transformer that does not have a nameplate with a wiring diagram on it. Call the manufacturer for help and have them furnish you with a diagram based on the model and catalog number of the unit.

This is ELECTRICITY folks, not LEGO!

[dnem]

John, you've got it, and you are right in your assumption that he will never be able to get 120v from this transformer

Personally, I would bond one secondary phase to the enclosure so that another fault would open the secondary OCPD.

Roger

Yep, that "floating delta" isn't the safest option. . When you wiggy a phase to ground you get zero. . The careless among us will assume the power is off.

Plus the floating delta requires ground detectors [250.21 last paragraph]. . It's easier to ground a phase and tape it white.

David

[dnem]

Corner grounded 240? Never seen one. Never heard on one. Probably someone has done one somewhere - but I don't know why.

Why ?
1] Only need one voltage
2] Smaller cheaper transformer
3] Don't need ground detectors

[dnem]

I would not hook up a transformer that does not have a nameplate with a wiring diagram on it. Call the manufacturer for help and have them furnish you with a diagram based on the model and catalog number of the unit.

True but we can still make some points about the situation like:
You don't connect a neutral to a delta/delta transformer.
You are better off corner grounding the delta secondary rather than leaving it floating, unless you're powering process equipment that you want/need to keep running in the event of a single fault.

[weressl]

True but we can still make some points about the situation like:
You don't connect a neutral to a delta/delta transformer.
You are better off corner grounding the delta secondary rather than leaving it floating, unless you're powering process equipment that you want/need to keep running in the event of a single fault.

Yes and no.

All your points are valid, more or less, but they all assume that the individual KNOWS what transformer he is dealing with. When we "ass""u""me" we make a donkey of ourselves and in this case it could be a dead donkey.......:grin:

[masterelect1]

We do some work at a textile mill that has an ungrounded Delta service even today.
Roger

Many facilities use the ungrounded Delta as it allows(I'm sure you know this) the plant to continue to operate even with a unintentionally grounded phase.
I've heard of an instance where two phases have had a partial grounds and because of impedence the MCB would not trip and hazardous touch/step potentials were created. Can anyone confirm this possibility or know of this happening.

thanks-John

[roger]

Laszlo, the opening post described the Delta-Delta transformer so we know what the configuration is.

The suggestions to ground a secondary phase or provide Ground Fault Detection would be the only addition to a straight forward hook up, however, if this is not understood by the OP or anyone attempting this, I would whole heartedly agree with you that they should not attempt it.

Roger

[coulter]

dnem -
My comment was that I have never seen a corner grounded 240D. Has anyone else ever seen one? If so, I'd be curious why it was speced that way. The only time I ever see 240 D is for specific loads like K2500 posted.

K2500 -
Was the one you refered to a corner grounded 240D, 240V high leg grounded D, or un-grounded 240D?

... 1] Only need one voltage
I don't know what that means. There is only one voltage - 3 phase 240V delta. Corner grounded or not, that is the only one there is.

... 2] Smaller cheaper transformer ...
I don't see that either. I've never heard of a transformer mfg making a different transformer for corner grounded D as opposed to ungrounded D.

... 3] Don't need ground detectors
That's true, But the cost of three transformered 240V pilot lights isn't much.

carl

[weressl]

Laszlo, the opening post described the Delta-Delta transformer so we know what the configuration is.

The suggestions to ground a secondary phase or provide Ground Fault Detection would be the only addition to a straight forward hook up, however, if this is not understood by the OP or anyone attempting this, I would whole heartedly agree with you that they should not attempt it.

Roger

My confusion. It never occured to me that we are talking about such basics that there is no 4th wire in a three wire system. I thought that he was questioning how to convert it into a grounded system because he was aware of the HIGHLY restricted use of ungrounded systems, or that there was no wiring diagram available on the transformer and the D/D configuration was an assumption made.

[dnem]

dnem -
My comment was that I have never seen a corner grounded 240D. Has anyone else ever seen one? If so, I'd be curious why it was speced that way. The only time I ever see 240 D is for specific loads like K2500 posted.

K2500 -
Was the one you refered to a corner grounded 240D, 240V high leg grounded D, or un-grounded 240D?

... 1] Only need one voltage

I don't know what that means. There is only one voltage - 3 phase 240V delta. Corner grounded or not, that is the only one there is.

What it means all depends on what’s being compared.
Comparing 3Ø corner grounded delta to 3Ø floating [ungrounded] delta, both have only one voltage.
Comparing 3Ø delta to 3Ø wye, delta has one voltage, wye has 2. . My comment wasn’t focused only on just different types of deltas.

... 2] Smaller cheaper transformer ...

I don't see that either. I've never heard of a transformer mfg making a different transformer for corner grounded D as opposed to ungrounded D.

Once again my comment wasn’t focused only on just different types of deltas. . When you take a certain size transformer enclosure and wind a specific amount of insulated copper in its windings, you’ll get more KVA out of it if you configure it as a delta. . You need a bigger enclosure and more copper for the same KVA wound wye. . That was my only point.

... 3] Don't need ground detectors

That's true, But the cost of three transformered 240V pilot lights isn't much.

carl

If because of the single voltage, cheaper transformer reasons you choose delta instead of wye, then a corner grounded will operate the same as a floating delta. . The floating gives you the option of continued operation after the first fault. . The corner grounded is safer in some ways.

Most of the delta secondarys that I see are center tapped/high leg types. . Of the remainder, I see many more corner grounded than floating. . I’ve actually only seen 2 floating delta systems during inspections. . Most of my exposure came from my brother-in-laws floating delta in his machine shop.

I’ve got a “rare bird” for you.

Have you ever seen a corner grounded system that uses the building steel as the corner grounded phase itself ? . It has 2 wires in a steel conduit [the combination of building steel and conduit functions as the 3rd phase / grounded phase]. . No fault current only equipment ground anywhere. . Each motor case has a bond wire to the building steel. . It was installed during WW2.

David

[coulter]

... When you take a certain size transformer enclosure and wind a specific amount of insulated copper in its windings, you’ll get more KVA out of it if you configure it as a delta. . You need a bigger enclosure and more copper for the same KVA wound wye. ...
240D vs 208W right? Are you sure they would have different kva? I think they will be the same.

carl

[coulter]

... . The corner grounded is safer in some ways. ...
I've never seen any evidence of this. Why would this be?

carl

[hardworkingstiff]

I’ve got a “rare bird” for you.

Have you ever seen a corner grounded system that uses the building steel as the corner grounded phase itself ? . It has 2 wires in a steel conduit [the combination of building steel and conduit functions as the 3rd phase / grounded phase]. . No fault current only equipment ground anywhere. . Each motor case has a bond wire to the building steel. . It was installed during WW2.

David

I've heard it was to save copper during the war.

[dnem]

... When you take a certain size transformer enclosure and wind a specific amount of insulated copper in its windings, you’ll get more KVA out of it if you configure it as a delta. . You need a bigger enclosure and more copper for the same KVA wound wye. ...

240D vs 208W right? Are you sure they would have different kva? I think they will be the same.

carl

No, I intended to compare same voltage to same voltage. . For example, take a 480v delta compared to a 480v wye.

On the wye, when a phase conductor carries 100a, the attached phase winding carries 100a.
On the delta, when a phase conductor carries 100a, multiple phase windings combine to contribute to the current flow on that conductor and there is no individual winding that carries the 100a. . Less current on the winding means less copper needed, smaller winding, and possibly smaller enclosure.

I might be wrong about the KVA because the 480delta individual winding voltage would be 480v at less than 100a. . While the 480wye individual winding voltage would be 277v at the full 100a. . I'm not sure how the wattage [or KVA] would compare.

David

[dnem]

... . The corner grounded is safer in some ways. ...

I've never seen any evidence of this. Why would this be?

carl

At my brother-in-laws floating delta machine shop, I've scene his employees stick their hands in places that have energized parts/terminals without using any care. . In the past they've always done so without incident because you can touch any single energized part anywhere in the building that's supplied by the floating delta and not get electrocuted even if you're standing in water.

But if a fault occures, the only change will be in the ground detection lights on the main gear up near the office. . If the employee is unaware of the change in the lights or forgets that something has changed, that same action can result in his electrocution.

Plus the issue that I mentioned before about the electrician taking a wiggy reading phase to ground and getting no voltage might wrongly assume the power to be off and start his work on the equipment. . He could be hurt/killed by electrocution or sudden equipment startup.

David

[jcopeland]

I’ve seen Corner Grounded Delta Systems used on remote pump stations and rock quarries because it only uses 3 conductors.
Saves a lot of money over long distances. The grounded phase is usually bare.

With high impedance grounds in different legs of an Ungrounded Delta System can cause inductive and capacitive voltages much higher than the supply voltage causing destruction to electronic equipment. I saw this happen on 2 300hp A/C drives.