2 Phase 4wire

[SG-1]

Is there a book or a web site i can more information on this??

Try http://www.3phasepower.org/2phasesystems.htm

 

[gravy]

2-phase, 5-wire service

2-phase, 5-wire service

The result of all this is that I am going to specify a Hammond Solutions custom built Scott T transformer, 150KVA, 240V, 2-phase, 5-wire input and a 120/208V, 3-phase, 4 wire output in order to covert the entire OLD 2-phase, 5-wire service to a new 3-ph service. I confirmed with Hammond that they can build this in about 4-weeks to these specs. I know how to calculate the secondary current of the 120/208V, 3-phase secondary ....but I am uncertain how to calculate the input primary current on the 2-phase,5-wire side of this Scott T configuration for the four phase conductors. This sounds like a PE exam question, but does anyone know how this would be done so I can make sure the incoming primary fused disconnect switch and conductors are sized appropriately?

 

[LarryFine]

I bet the people making the transformer can tell you.

Added: The normal method of converting secondary load at its kva to the primary load at the same kva should work.

 

[lakee911]

??? You should have 170 volts phase-phase if you have 120 volts to the common on a 2-phase.

On 2-phase it is:
V12 = sqrt(V1^2 +V2^2)

Not if you use two of the 4 phases that are 180 degrees apart (120 + 120 = 240). If you use 2 phases that are only 90 degrees apart then yes, 170v.

I'm not sure that I follow ... looking through my notes, I see that it is a 4W240/120 open delta. They have one 25KVA transformer and one 50KVA transformer w/ a center a tap.

See attached PDF ... option III we are talking about here.

 

[mivey]

I'm not sure that I follow ... looking through my notes, I see that it is a 4W240/120 open delta. They have one 25KVA transformer and one 50KVA transformer w/ a center a tap.

See attached PDF ... option III we are talking about here.

Then you do not have what is traditionally called 2-phase.

 

[mivey]

The result of all this is that I am going to specify a Hammond Solutions custom built Scott T transformer, 150KVA, 240V, 2-phase, 5-wire input and a 120/208V, 3-phase, 4 wire output in order to covert the entire OLD 2-phase, 5-wire service to a new 3-ph service. I confirmed with Hammond that they can build this in about 4-weeks to these specs. I know how to calculate the secondary current of the 120/208V, 3-phase secondary ....but I am uncertain how to calculate the input primary current on the 2-phase,5-wire side of this Scott T configuration for the four phase conductors. This sounds like a PE exam question, but does anyone know how this would be done so I can make sure the incoming primary fused disconnect switch and conductors are sized appropriately?

The 3-phase side has line currents of I3 = VA / (sqrt(3) x V3_line-line) amps.

Assuming a balanced load, the kVA will be equally split in the 2-phase side and the line current will be I2 = VA / (2 x V2_line-line).

VA = I2 x 2 x V2_line-line = I3 x sqrt(3) x V3_line-line

I3/I2 = 3-phase to 2-phase current ratio = (2 x V2_line-line) / (sqrt(3) x V3_line-line) = 1.1547 x V2/V3

So, for equal line-line voltages on the 2 & 3 phase sides, the 3-phase line currents will be 15.47% higher than the 2-phase line currents.

Similarly, I2/I3 = (sqrt(3) x V3_line-line) / (2 x V2_line-line) = 0.866 x V3/V2

For your case, V2 = 240 volts and V3 = 207.85 = 120 x sqrt(3). So, to find the 2-phase current given the 3-phase current you have:
I2 = I3 x sqrt(3) x V3 / (2 x V2) = I3 x sqrt(3) x 120 x sqrt(3) / (2 x 240) = I3 x 0.75

To find the 3-phase current given the 2-phase current you have:
I3 = I2 x 2 x V2 / (sqrt(3) x V3) = I2 x 2 x 240 / (sqrt(3) x 120 x sqrt(3))= I2 x 1.33333

I bet the people making the transformer can tell you.

Added: The normal method of converting secondary load at its kva to the primary load at the same kva should work.

Yes. As usual, energy is conserved and for balanced loads at unity power factor it is just simple math. No engineering degree required, so all you electricians knock yourself out. :grin:

For unbalanced loads, you would need to know the load in the main (100%) winding and the teaser (86.6%) winding and consider that the current through the teaser winding splits equally between the two halves of the main winding. This requires that you keep up with the angles and requires vector math except in a simple unity power-factor case.

The teaser winding is easy in that the current in the 3-phase current side 2/sqrt(3) or 1.1547 times the 2-phase current. The more complicated part is when the current in teaser coil on the 3-phase side divides into the main winding on the 3-phase side.

In the simple case of unity power factor, the current through 1/2 of the main winding on the 3-phase side will be the hypotenuse of a triangle with the legs made of:
1) 100% of the current in the main (Im)
2) 50% of the current from the teaser (50% of It). BTW, this has a net zero mmf in the whole winding.
The current in the line connected to the main winding is sqrt(Im^2 + It^2).

 

[lakee911]

Then you do not have what is traditionally called 2-phase.

I'm confused as to how ... two phases are present on the lines, why don't I have that? So, what's the diagram of two phase?

 

[mivey]

I'm confused as to how ... two phases are present on the lines, why don't I have that? So, what's the diagram of two phase?

In a strict technical sense, you do have two phase, but it is not the same as what has been historically labeled as "two-phase" in our industry.

The historic two-phase has two voltage waveforms with phase angles that differ by 90 degrees. There are plenty of diagrams on this site if you search for two phase or 2 phase.

Let me know if you have trouble finding info.