3 phase transformer

[barclayd]

Not that I have seen, I have always used 120/240 volt rated panel boards with 120/208V 1? 3W supples.

Pretty darn common in city networks and apartment buildings.

Oh sure - I've fed single-phase panels from a 3-phase 208Y/120v panelboard - and, like you say, the single-phase panel was labeled 120/240.
But, I have never tried to use just part of a 3-phase transformer to provide a single-phase output. I'm still curious about what is bonded where for the neutral.
db

 

[weressl]

If he only wants single phase on the secondary then he will only need single phase on the primary.



I'm thinking the same thing. OP, are you just trying to use what you have available or is there some other reason you are asking this question?


A three-phase transformer is just three single-phase transformers inside the same box. If you don't want to use all three of them, the others will still work fine. The worst thing I could say about it would be that it is wasteful. If you're OK with that, go for it.

On the primary it would be two phases connected to a single winding.

Inefficient as heck with stray fluxes circulating in the unused legs.

Make SURE you insulate and isolate all unused leads!

 

[LarryFine]

If that one secondary winding has 208 volts measured from one end to the other, where would you get 120v?

It's not one secondary producing 208v. It's two 120v secondaries producing 208v.

Where is the neutral?

It's the grounded point where the two secondaries are tied together.

 

[barclayd]

It's the grounded point where the two secondaries are tied together.

Then how do you energize two secondary windings with only one primary winging energized?

db

 

[LarryFine]

Then how do you energize two secondary windings with only one primary winging energized?

You can't. There must be either 3 phases or two phases and the system neutral to produce 2/3 of a Y secondary (or a whole 3ph anything).

 

[barclayd]

The original question was whether he could feed the primary of the transformer with a 2-Pole breaker. I maintain that it will not work.
To get 208/120 single phase on the secondary, you need to have two secondary windings energized with their ends bonded together (As Larry says). Instead of messing with the tap wiring, and using two 2-pole breakers on the primary - Just use a 3-pole - do it the way it was designed.
If you only want to use two of the secondary windings to feed your single phase panel, go ahead. I agree with Laszlo that there will be some strange electrical stuff going on in the unused winding.
A quick look through an Acme catalog did not find ANY single phase transformers with 208/120 secondaries.
Not sure what all this does to impedance, fault current, arc flash, and other stuff like that.
db

 

[Besoeker]

If that one secondary winding has 208 volts measured from one end to the other, where would you get 120v?

I still don't think this will work real good.
Where is the neutral?
db

It's a Delta/wye transformer.
If you apply rated volts across one input winding you will get rated voltage across one secondary winding. For a wye connected secondary (see op) that would be 120V. The neutral is still the star point of the secondaries.

What you wouldn't get is the 208V, of course

 

[brian john]

If he only wants single phase on the secondary then he will only need single phase on the primary.



I'm thinking the same thing. OP, are you just trying to use what you have available or is there some other reason you are asking this question?


A three-phase transformer is just three single-phase transformers inside the same box. If you don't want to use all three of them, the others will still work fine. The worst thing I could say about it would be that it is wasteful. If you're OK with that, go for it.

Single phase 120 VAC yes, But single phase 208/120 3-wire No way.

 

[jim dungar]

The OP stated it is a delta wound primary and a wye secondary.
In reality the transformer is either a 3-winding delta-wye (typical for >15kVA) or it is a 2-winding T connection (typical for transformer <15kVA). In either case there is no neutral provision on the primary side, so it can only be wired with 2 or 3 phase conductors.

For all piratical purposes (ignoring additive vs subtractive connections and with generous rounding):
If only 2 phase conductors are brought to the primary the results are;
For a delta winding - one winding will be energized at full voltage and two windings will be energized at 50% voltage (they are connected in series) The resultant output should be 3 individual single phase 2-wire circuits of 120V L-N, 60V L-N and 60V L-N.
For a T connected windings - one winding will be energized at full voltage and the other winding at 0V. The resultant output will be single phase outputs of about 104V L-N, 104V L-N and 1 at 0V. Or one winding will be energized at 87% and half of one winding will be at 37%, the outputs could be about 75.6V, 44.4V and 0V (it depends on which primary winding is energized).

If 3 phase conductors are brought to the transformer, the output will be as designed even with one output phase conductor left 'open circuited'.

If the OP had a special transformer design of either an open-delta primary or an open wye primary, the effect of connecting only 2 phase conductors to the primary would be equivalent to having 2 single-phase transformers connected in series, the windings of an open-delta would about 50% voltage and those of the open-wye about 87%

 

[LarryFine]

I don't see what the confusion is. If he has 208v available, use a 2-pole breaker to feed that to the primary of a 208v-to-120/240v transformer. No neutral is needed on the primary.

Now, you have a separately-derived floating 120/240v center-tapped secondary. Bond the center tap and give it an appropriate electrode. No relationship to the primary supply.