gcleary47

I'm a second semester, second year mainly resi guy. However in class a week or two ago we were discussing open deltas. It made me wonder that if you have three single phase transformers to create a three phase transformer, or power. And one of the transformers blows, you can still deliver the three phase power. Does this mean there is still the three sine waves? Would they still be 120 degree's out of phase? Now as i said i am a residential guy, with minumal commercial experience. I don't know know much of any thing when it comes to it, but i can't get my head out of this question. It is not to help me on a test but just to aid a curious mind. Thank you in advance to any onput i receive on this.

Greg

MDShunk

Draw it out and your qiestion might answer itself.

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Peter D

Draw a triangle and remove one of the legs. You still have 3 points to connect to. There's your open delta. Simple as that really. Oh, plus advanced calculus to explain why it works that way, but you don't need that.

InPhase277

You still get 3 phase because the ends of each coil are phase shifted in relation to each other, but the amount of available power is reduced to 57% of the full delta's capacity.

Dembones

I understand how the open delta works, but I'm having trouble with why it doesn't work in some cases. Lets say we have a motor operating on 240V delta. A phase is lost, now we have a single phasing condition, which I understand to be the worst case scenario for a three phase motor. Assuming the overloads/fuses don't open from increased current, why doesn't the motor continue to operate normally as an open delta? For clarifiaction, let's say it's B phase that dropped, it is still tied to A&C just as an open delta connection would be right? Does it come down to conductor sizing for 57% greater current?

gcleary47

thanks for the responses. I think i'm just thinking into this one too much. Time to go to class

zod68

if a motor is a delta a phase b phase and c phase to opererate , can it operate without b phase ? consider t1 t2 n t3 . do not all three wires have to connected 9-6-3/ 8-5-2/ 7-4-1 . if you lose line too t2 what happens ?

InPhase277

Yes losing a phase is called single phasing and would occur if one pole of a three pole breaker went bad, or that conductor somehow became open. But this is not the same as removing one of the transformers from a delta bank. Think about it. In single phasing, you only have two good conductors going to the load. In an open delta, you still have all three conductors.

Dembones

I know you're right, I'm not argueing theory, just wondering how the connections are physically made. Know of a good diagram? Can't find one online.

InPhase277

Like this?

Attached Images

opendelta.jpg (6.2 KB, 142 views)

Dembones

All coming back to me now thanks.

kadams

In a 240v secondary assume this is why there is still 240 v across the open transformer space something about a phase shift? Sometimes I look at the open delta and I wonder why there wouldn't be 480 v across the open leg? I know it only has 240v but it is a little counter-intuitive.

Richard Rowe

We have a 480 3ph drop in one building that reads 504 to ground on two legs and 504 across those two legs. The third leg read 0 to ground and 504 to the other legs.... is this what you are talking about?

InPhase277

This sounds like a corner grounded delta to me. A different beast altogether than an open delta.

Richard Rowe

I was thinking it is still called an open delta. I know you can get that an open delta from two trans, but not by droping a leg. Our other drops are 248 across and 121-121 and 214 to ground.

Buddha In Babylon

intereting stuff. I am studying transformers right now, and i confess it's more than i expected. So much to learn...so little time...
I can only pray i live long enough to be a good electrician with a solid understanding of this kind of thing.

InPhase277

You could have a corner grounded open delta, but that would be a rare find indeed. The other you describe is a delta high leg. You get 240 between phases, and 120 from two phases to the neutral. And 208 from one phase to the neutral. It is usually the B phase that is the "high leg". Also known as the "stinger" or "snake" or "wild leg", or several other names.

zgozvrm

See my post from another discussion:
http://www.electriciantalk.com/f30/explain-120-208v-7538/index3/#post133747