Transformer Question

[mattsilkwood]

This is a purely hypothetical question.
If you have a single phase 120/240 system can you take two 240-120/240 transformers and make a three phase system?

I have a drawing that shows this, it is for transmission voltage down to low voltage but logic tells me this will also work on low voltage to low voltage, it just wouldn't be very efficient.

 

[ron]

No. A single phase 240/120 xfmr has two ungrounded conductors at 180 deg apart. 3 Phase need to be 120 deg out of phase.

 

[skeshesh]

Strange. Can you post the connections shown on the drawing?

 

[mivey]

...If you have a single phase 120/240 system can you take two 240-120/240 transformers and make a three phase system?

No. You would need a phase shifter as transformers alone would not do it.

I have a drawing that shows this, it is for transmission voltage down to low voltage but logic tells me this will also work on low voltage to low voltage, it just wouldn't be very efficient.

You do not have a drawing that shows that (assuming the drawing is legitimate).

I suspect you have a drawing that shows taking two single phases shifted by 90 degrees (called quadrature-phase or two-phase) and using transformers with special taps to get three-phase. Look up Scott-T transformer and see if that is what you have.

 

[mattsilkwood]

This is the drawing, http://www.jea.com/about/pub/downloads/contractor/OH-2-Transformers.pdf. It is the second drawing, labeled open wye-open delta.

Good point about the phase angle, I didn't think about that.

 

[mivey]

Definitely not single phase. You have two phases on the primary with a 120? displacement using two single-phase transformers to create a three phase set of voltages on the secondary.

 

[LarryFine]

It is the second drawing, labeled open wye-open delta.

That is a common configuration, and, while it uses only two primary conductors, it also uses the neutral, so it really is derived from a 3-phase supply, not single-phase.

 

[mattsilkwood]

Definitely not single phase. You have two phases on the primary with a 120? displacement using two single-phase transformers to create a three phase set of voltages on the secondary.

The key to this working is the phase angle, correct?
I think I've got this figured out, let me draw it up and I'll post it so you guys can check my math.

 

[mattsilkwood]

Is this right?

 

[LarryFine]

Is this right?

The wiring is correct, but the numbers are wrong. The primaries are whatever matches the POCO's delivery.

For the secondaries, you should have:

L1-N = 120v
L2-N = 120v
L3-N = 208v
L1-L2 = 240v
L1-L3 = 240v
L2-L3 = 240v

It behaves as a 1ph 120/240v supply superimposed over a 3ph 240v Delta. The reason for the 208v is purely because of which terminal is grounded.

 

[mattsilkwood]

The wiring is correct, but the numbers are wrong. The primaries are whatever matches the POCO's delivery.

For the secondaries, you should have:

L1-N = 120v
L2-N = 120v
L3-N = 208v
L1-L2 = 240v
L1-L3 = 240v
L2-L3 = 240v

It behaves as a 1ph 120/240v supply superimposed over a 3ph 240v Delta. The reason for the 208v is purely because of which terminal is grounded.

I guess I should have phrased my question better.

Are these numbers what one would expect to see if the primary was a typical 120/240 resi service, 180 deg apart.
Of course the transformers would be 240-120/240.
I think my math is right but I'm not a hundred percent sure.

 

[LarryFine]

Are these numbers what one would expect to see if the primary was a typical 120/240 resi service, 180 deg apart.
Of course the transformers would be 240-120/240.

Aha. Now, I grok.

First of all, it's impossible to derive 3ph from 1ph with just transformers.

Second, the primary of one transformer should be swapped.

Third, if you do the second one:

L1-N = 60v
L2-N = 60v
L3-N = 180v
L1-L2 = 120v
L1-L3 = 240v
L2-L3 = 120v


Having said that, if you supplied your primaries from two of three phases, you'd have a perfect high-leg open Delta.

 

[kwired]

I guess I should have phrased my question better.

Are these numbers what one would expect to see if the primary was a typical 120/240 resi service, 180 deg apart.
Of course the transformers would be 240-120/240.
I think my math is right but I'm not a hundred percent sure.

If that is what you have for a primary you do not have three phase. All you have is a single phase winding with a center tap in the same winding for your supply.

To get three phase you will need phase conversion equipment - capacitors, inductors, usually need to be carefully designed for the load served and still are not a perfect three phase supply as created by generator. Or solid state equipment that will create three phases from a single phase input - as seen with some variable frequency drives or other power inverters.

 

[steve66]

Is this right?

Do a search for "open delta" and you should be able to find something that explains it.

Your 1st problem with the diagram you drew is the primary voltages: You have to start with 2 lines of a 3 phase source. So the two 120V signals would actually be 120 degrees apart. That means you have 208V between L1 and L2 on the primary.

Power companies often use this setup to provide a 3 phase service without actually running all three phase wires.

Steve

 

[LarryFine]

Your 1st problem with the diagram you drew is the primary voltages: You have to start with 2 lines of a 3 phase source. So the two 120V signals would actually be 120 degrees apart. That means you have 208V between L1 and L2 on the primary.

He's using 120/240 1ph as a source for this hypothetical example, not 120/208v 1ph, and also using 1:1 transformers.

The important thing here is that there is no timing difference in the two transformers, so it's impossible to attain a 3ph output.

 

[mattsilkwood]

If that is what you have for a primary you do not have three phase. All you have is a single phase winding with a center tap in the same winding for your supply.

To get three phase you will need phase conversion equipment - capacitors, inductors, usually need to be carefully designed for the load served and still are not a perfect three phase supply as created by generator. Or solid state equipment that will create three phases from a single phase input - as seen with some variable frequency drives or other power inverters.

Do a search for "open delta" and you should be able to find something that explains it.

Your 1st problem with the diagram you drew is the primary voltages: You have to start with 2 lines of a 3 phase source. So the two 120V signals would actually be 120 degrees apart. That means you have 208V between L1 and L2 on the primary.

Power companies often use this setup to provide a 3 phase service without actually running all three phase wires.

Steve

I understand that this will not create three phase power, I understand why. This is purely an exercise in theory. Transformers are not something I'm a wizard at and I'm trying to learn a little more about them. That's all.

He's using 120/240 1ph as a source for this hypothetical example, not 120/208v 1ph, and also using 1:1 transformers.

The important thing here is that there is no timing difference in the two transformers, so it's impossible to attain a 3ph output.

Thanks for the input Larry, I looked that over after you posted the correct voltages and I see where I messed up. I was quartering where I should have halfed.

This should be correct.

 

[kwired]

I understand that this will not create three phase power, I understand why. This is purely an exercise in theory. Transformers are not something I'm a wizard at and I'm trying to learn a little more about them. That's all.


Thanks for the input Larry, I looked that over after you posted the correct voltages and I see where I messed up. I was quartering where I should have halfed.

This should be correct.

That drawing will work if the input is 2 phases and neutral of a wye system maybe 120/208 but it can't be 120/240. You can not get 120/240 combinations from a wye system. If the input is from a delta supply all of your supply connections are on the same winding and you only have single phase input.

Your output voltages are not possible either.

If both transformers are same voltage output then the line to line voltages will all be the same (L1-L2, L2-L3, L1-L3). The voltage to N from L1 and L2 will be 1/2 the line to line voltage. The voltage to N from L3 will be the L1/L2-N voltage times 1.73.

So if line to line voltage is 240, L1/L2-N will be 120 and L3-N will be 208.

 

[mattsilkwood]

Your output voltages are not possible either.

I disagree. Not only are they possible but I think that they are exactly what you would get.

If both transformers are same voltage output then the line to line voltages will all be the same (L1-L2, L2-L3, L1-L3). The voltage to N from L1 and L2 will be 1/2 the line to line voltage. The voltage to N from L3 will be the L1/L2-N voltage times 1.73.

So if line to line voltage is 240, L1/L2-N will be 120 and L3-N will be 208.

That doesn't apply here because we are not talking about a three phase system.

 

[kwired]

I disagree. Not only are they possible but I think that they are exactly what you would get.

That doesn't apply here because we are not talking about a three phase system.

Well we once was talking about three phase. How did it become single phase?

The drawing is right if the input voltage is single phase 120/240. I don't see what purpose the drawing would serve other than the different voltages available. That can be easily be done with a single core and coil with multiple taps though. The out put will not be three phases just four points with different potentials between them.

 

[mivey]

Second, the primary of one transformer should be swapped.

Third, if you do the second one:

L1-N = 60v
L2-N = 60v
L3-N = 180v
L1-L2 = 120v
L1-L3 = 240v
L2-L3 = 120v

With the drawing as shown, I get:
L1-N = 60v
L2-N = 60v
L3-N = 60v
L1-L2 = 120v
L1-L3 = 0v
L2-L3 = 120v

Swapping the primary on the second transformer I get:
L1-N = 60v
L2-N = 60v
L3-N = 180v
L1-L2 = 120v
L1-L3 = 0v
L2-L3 = 120v