2 phase 208 to 3 phase 230

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In a wye system, V N to A must be 120 degrees away fro V N to B. However V A to B must be 60 degrees away from V A to C.

The latter is also true for a delta system. V A to B must be 60 degrees away from V A to C.

I believe that as you have drawn things, V A to B is 60 degrees away from V C to A, which is the same thing as saying that V A to B is 120 degrees away from V A to C.

I must admit, however, that it took me a whole day of pondering on this one, and I'm still a bit confused.

In any case, I believe that the open wye to open delta approach would be a good one for the OP; and if the diagram is wrong things could be fixed by swapping the connection on one transformer. Just check the voltages before you connect anything.

-Jon
 
winnie said:
In a wye system, V N to A must be 120 degrees away fro V N to B. However V A to B must be 60 degrees away from V A to C.

The latter is also true for a delta system. V A to B must be 60 degrees away from V A to C.

I believe that as you have drawn things, V A to B is 60 degrees away from V C to A, which is the same thing as saying that V A to B is 120 degrees away from V A to C.

I must admit, however, that it took me a whole day of pondering on this one, and I'm still a bit confused.

In any case, I believe that the open wye to open delta approach would be a good one for the OP; and if the diagram is wrong things could be fixed by swapping the connection on one transformer. Just check the voltages before you connect anything.

-Jon
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I must have been typing the response to my own post while you made your post... yeah, I finally caught what I highlighted in red text.
 
What about Open-Wye to 4-Wire Wye?
OpenWyeto4Wire.jpg
 
LarryFine said:
Doesn't that make C phase a higher voltage to N?
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No.
Let the top transformer be at 0 degrees and the bottom at -120 degrees and let the secondaries be 120/240 transformers:

The easy ones first: follow n to a to get 120<0 and n to b to get 120<-120

Now follow n to the center-tap of the top transformer, through the lower half and back down through the lower half of the bottom (note they are in the reverse direction): -120<0 - 120<-120 = 120<120

It might be worth noting that each transformer must be sized to carry 2/3 of the balanced load (i.e. the capacity is 75% of the total bank rating).

Add: I am using "<" for an angle indicator
 
mivey said:
Now follow n to the center-tap of the top transformer, through the lower half and back down through the lower half of the bottom (note they are in the reverse direction): -120<0 - 120<-120 = 120<120
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So you're saying there is a net 120v because the two windings buck?
 
LarryFine said:
So you're saying there is a net 120v because the two windings buck?
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I'm saying the line-neutral potential across the two coils is Vc-Vn = 120<120.

This is like what happens with the line-line voltage of a 120/208 in that is is just a voltage rise (or drop) across two coils: Va=120<0, Vc=120<120, Va-Vc = 120<0 - 120<120 = 207.8<-30

I hesitate to give an example of the center-tapped delta as we know that might wake the dead.:D Oh, who cares...with a neutral reference: Va=120<0, Vb=120<180, Va-Vb = 120<0 - 120<180 = 240<0
 
mivey said:
I'm saying the line-neutral potential across the two coils is Vc-Vn = 120<120.

This is like what happens with the line-line voltage of a 120/208 in that is is just a voltage rise (or drop) across two coils: Va=120<0, Vc=120<120, Va-Vc = 120<0 - 120<120 = 207.8<-30

I hesitate to give an example of the center-tapped delta as we know that might wake the dead.:D Oh, who cares...with a neutral reference: Va=120<0, Vb=120<180, Va-Vb = 120<0 - 120<180 = 240<0
Click to expand...

Hmmm... I think you got off track there. Van = 120V@0?, Vbn = 120V@-120?, and Vc2n = 120V@-240?... You've managed to derive the third phase, but not the desired configuration. Your output is 208Y/120 3? 4W. The OP scenario requires a 240/120 3? 4W power source.
 
Smart $ said:
Hmmm... I think you got off track there. Van = 120V@0?, Vbn = 120V@-120?, and Vc2n = 120V@-240?... You've managed to derive the third phase, but not the desired configuration. Your output is 208Y/120 3? 4W. The OP scenario requires a 240/120 3? 4W power source.
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True enough. You could still boost to the desired voltage. I wonder if there might be a transformer with tap settings that could get you close enough without having to boost separately?

It is just interesting in that most people only think of open-wye open delta. The open-wye to 4-wire is a way to derive the three phase from open wye without having to have the rotary converter mentioned by the OP
 
LarryFine said:
He knows, and mentioned it in that post. It's pretty cool. I also like the openY/openD idea. That's what the POCO does.
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The openY/openD is certainly the most common, but POCOs also use the openY/4-wire. I had never heard of it until recently and thought it was a neat set-up.
 
Open-wye, open-delta

Open-wye, open-delta

Thanks guys,

You have indeed identified a possible solution using transformers and no RotoPhase Converter. A beautiful diagram, with voltage vectors, can be found at:

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http://www.eng-tips.com/viewthread.cfm?qid=157326

Another link to the Electric Power Distribution Handbook can be found at:

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http://books.google.com/books?id=6c...g=PA186&dq=open+wye+to+open+delta+transformer

The section on "Features and drawbacks" includes this:
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Voltage unbalance - "May have high secondary voltage unbalance."

​
Primary gound current - "Creates high primary-side current unbalance. Even with balanced loading. High currents are forced into the primary neutral."

I think I should go talk to our local power company, to the distribution engineer.

Thanks guys,
Maynard
 
Maynard said:
The section on "Features and drawbacks" includes this:Voltage unbalance - "May have high secondary voltage unbalance."
Primary gound current - "Creates high primary-side current unbalance. Even with balanced loading. High currents are forced into the primary neutral."
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They also should have mentioned that the secondary C phase (as used in that diagram) is a high-leg 208v just as with a utility-supplied open-Delta service.

To me, "may have high secondary voltage unbalance" is too vague, and may not convey the risk to equipment that exists with an uninformed electrician.
 
This transformer arrangement is easy for the utility as they are using their normal L-N primaries, it is more difficult for us as our transformer primaries are normally connected L-L. Part of the problem of using this transformer connection for the OP is that it will require transformers with 120-120/240V connections. If the OP can tolerate a corner grounded secondary it can be made using a more common 480x240-120/240V transformer wired in reverse.

Remember it is not the open-delta connection that gives you the "high-leg" to ground, it is the center-tapped delta that does it. It is completely possible to have a corner-grounded open-delta.
 
Maynard said:
I think I should go talk to our local power company, to the distribution engineer.
Click to expand...

According to your OP the motor uses 1.65kW.

So we are talking about, say two 1.5kva 120:120x240 xfmrs vs. running a new service...:confused:

If you're concerned about voltage regulation, I believe you can up the kVA to get better regulation.

As for primary side current unbalance, we're talking what? ...13 amps on all three ccc's...:confused:
 
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jim dungar said:
Remember it is not the open-delta connection that gives you the "high-leg" to ground, it is the center-tapped delta that does it. It is completely possible to have a corner-grounded open-delta.
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Absolutely, and the phase common to both secondaries would be the one to ground.
 
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