O.T. Three Phase by Single Phase Transformers

MasterBlaster

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Join Date
Aug 2005
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Solano County, CA
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Gents:

When the primary of a 480v 3ph by 240v 1ph transformer is connected across two legs of a three-phase source, is it not true that the 240v secondary is actually only 120 degrees in phase separation, not 180 degrees like we find in household 240v power?

I've Googled this and find no definitive answer (that I'm capable of understanding, anyway!).

Thanks,

Bill
 
Gents:

When the primary of a 480v 3ph by 240v 1ph transformer is connected across two legs of a three-phase source, is it not true that the 240v secondary is actually only 120 degrees in phase separation, not 180 degrees like we find in household 240v power?

I've Googled this and find no definitive answer (that I'm capable of understanding, anyway!).

Thanks,

Bill
I'm trying to envision such a device, and coming up empty.
Unless you're talking about an auto-regulating transformer, like what we generically called a "Topaz" (back in the old days.)
 
Yes. Each phase is 60 degrees out from each other. The power coming out of the transformer will follow this phasing. There are other types of power supplies that I believe, correct me if I'm wrong so that I'll know, will compensate for this, switched mode being one of them. I have not encountered issues due to this
 
I think you need to specify your primary connections ie
are Phase A and B connected primary windings A and B
or are phase A and B connected to only one winding ie A.

Dan Bentler
 
I think you need to specify your primary connections ie
are Phase A and B connected primary windings A and B
or are phase A and B connected to only one winding ie A.

Dan Bentler
Dan:

Primary is as you first stated, across two of the three phases of the source. Envision even a small control transformer in a typical motor control bucket.

Bill
 
MB - your question is a little difficult to answer. If you measure each leg of the 480 primary referenced to ground, each leg is 277V and 120 deg off phase from one leg to the next. What happens on the secondary depends on your wiring, but usually you tie one leg to ground so you end up with a single phase. There is no phase separation to talk about.

In your home, you are correct. Usually you get 240 by tying together 2 phases of 120 that are 180 deg out of phase to each other.
 
When the primary of a 480v 3ph by 240v 1ph transformer is connected across two legs of a three-phase source,
Bill,
Where did you find a transformer with a 3-phase primary and a 1-phase secondary? Those are kind of rare now days. There most common configuration that I remember was three single-phase secondary 240 volt circuits. Often this was created by the power utility using 3 single-phase pole transformers, wired together to a 3-phase line to produce 3 separate 240/120 volt home supplies. The resulting three single-phase 240 volt circuits had the same "degrees separation" between poles as always (120 degrees I think), otherwise this set-up would not have worked as household power supplies.

EDIT: Hmmm, reminds me of some of the questions on the local city high-voltage Master Electrican license test. An electrical utility question is about the only place you find this type of scenario these days.
 
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Lancie:

Your bring out a good point about the wording of my question. I should have said 480v primary and 240v secondary. But the fact remains that you don't get 480v from one leg to ground in most commercial systems. You get 480v across two phases, and those two phases are 120° apart. So, is the secondary voltage not also 120° apart, as opposed to 180°?

If one uses such a transformer to power only 120v, single phase loads, no issue. But if used to run a 240v load, is there a derate for the fact that the phases or not the normal 180° apart?

Sorry for the original poor wording.

Bill
 
So, is the secondary voltage not also 120° apart, as opposed to 180°?
Yes, exactly, and it has always been so! What reason do you have that made you think it could be 180?

Most common 480 3-phase transforers now have 208 volt 3-phase "Y" secondaries. A true 480 3-phase Delta-configuration to 240 3-phase Y-configuration is very rare. Usually you see a Delta-Delta setup, with 480 3-phase delta primary and 240 volt 3-phase delta secondary. I never used those due to no hard ground point on the secondary side, although many plants will use nothing else - asking for trouble in my book!
 
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No reason to think otherwise, Lancie. But I've seen 240v single phase motors connected to such transformers and they seem to run fine (maybe the motor was rated at 33% more horsepower than actually required). Maybe I'm just over-thinking this. Perhaps retirement allows my mind too much time to wonder.

Thanks for taking the time to make me at least ask a coherent question!

Bill
 
Other readers can correct me if i'm wrong, but i believe the phase shift is of no importance if you connect 2 legs of the 480V service to a 120/240V single phase transformer.

The reasoning behind this is everything is single phase since you are only using 2 legs of the 3 phase service. The 120/240 is also single phase, however you just have a center taped transformer enabling you to split the potential voltage in half.

Not sure if this answers your question, but you may want to read up on the difference between a 240/120 split phase connection and how its not a "2 phase" setup.
 
Other readers can correct me if i'm wrong, but i believe the phase shift is of no importance if you connect 2 legs of the 480V service to a 120/240V single phase transformer.

The reasoning behind this is everything is single phase since you are only using 2 legs of the 3 phase service. The 120/240 is also single phase, however you just have a center taped transformer enabling you to split the potential voltage in half.

Not sure if this answers your question, but you may want to read up on the difference between a 240/120 split phase connection and how its not a "2 phase" setup.
I understand that part perfectly, and there is absolutely no issue with 120v single phase loads. My concern was having a 240v single phase motor running across the secondary, where the phases are not 180° out of sync, but 120° instead.

Thanks for your input.

Bill
 
Having given this some thought
1. You have only one secondary winding.
2. How can you have two voltages in only one winding?
3. The secondary voltage will be the net vector sum of the magnetic field.
4. Given this I believe if you put a scope across secondary you will get a single sine wave.

Dan Bentler
 
Having given this some thought
1. You have only one secondary winding.
2. How can you have two voltages in only one winding?
3. The secondary voltage will be the net vector sum of the magnetic field.
4. Given this I believe if you put a scope across secondary you will get a single sine wave.

Dan Bentler
That's an excellent point, Dan. All we have is one winding with a center-tap (for the 120v loads). That explains why the 240v loads work exactly as expected. Too simple!

Thank you very much.

Bill
 

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