O/H Open Delta Unique Configuration

[BPrescott]

At a neighboring utility we discovered a very unique connection on a three phase O/H bank. The coneection was open wye-open delta one two single bushing pots. The secondary had 480v L-L with one leg split to get 240v. The oddity came where the one leg was slipt was get 240, the utility also claimed that configuration would give them 120v. They called it an "A-frame" configuration. Has anyone heard of this and possible have a wiring diagram or vector diagram to explain where the 120V comes from?

Thanks!

 

[waross]

There is an unusual configuration to derive three phase wye equivalent from two phases of a wye primary. It will supply 120/208 volts if two 120:240V transformers are used.
It has been discussed in this forum. If I have time later to locate it I will.
respectfully

 

[stevenal]

It takes three phasors to draw an A, therefore three cans are needed. 480 three phase is served open delta. Connect the third winding from center tap to center tap of the first two to create the cross bar of the A. This winding closes the delta for 240 three phase. Center tap this third winding for 120 single phase loads.

 

[BPrescott]

For the cross-bar of the "A", one end of the winding is connected to the center tap of a 480 bank, but the other side of the winding is only brought out to connect to load, it does not connect to any other windings. Though I'm told that if you were able to place a volt meter between the open connection on the cross-bar and the center tap on the second 480 pot that it would read zero volts. So in theory that connection could be closed.

 

[waross]

Sorry, my bad. It takes three transformers to generate three phase wye from two primary phases.
In the instance at hand, are you sure that the "A frame" connection is not another name for a four wire center-tapped delta connection?
This connection is most common with 120:240 volt transformers which give 240 volt 3 phase delta and 120:240 volts from the center tapped transformer.
respectfully

 

[BPrescott]

Well no, this connection would actually have, 480v, 240v and 120v. There is a open delta at 480v per leg. and then the "upper" part of the A is a 240v delta circuit with the cross-bar of the A being made by the 120/240v transformer. Though the cross-bar is only connected to one of the center taps on one 480 pot. The other end of the cross-bar is not connected physically to the center tap of the other 480 v pot, but has a voltage difference of zero between the center tap and that end of the cross-bar.

If anyone knows a way to post a diagram I have this drawn out.

Thanks!

 

[stevenal]

Maybe the broken delta is used so that a smaller crossbar transformer may be used to feed single phase loads only without sharing the 240 three phase load.

 

[waross]

This sounds like a misunderstanding. Originaly two transformers were stated. Sounds like a 480 volt open delta.
This configuration will give 240 volts three phase and 120:240 volts single phase if 120:240V transformers are used. This is a common connection.
While it is possible to add a third transformer in crossbar configuration to construct an "Open delta" inside an "Open Delta" it is not a common connection, and iot requires three transformers, not two as originally stated.
respectfully

 

[BPrescott]

I must appologize. I did miss speak in my original statement. This bank does have three (3) transformers. They are all single single bushing on the primary. Two are 480v on the low side and the third is 240v on the low side.

Thanks!

 

[waross]

It sounds lioke they are combining three systems in one. you have a 120V:240V single phase system, a 240 volt three phase delta system and a 480 volt three phase open delta.
You will have to see the connections to determine if the 240 volt three phase system is open delta, closed delta, or broken delta.
Two 240:240 volt transformers may be used as an open delta autotransformer bank to derive 480 volts 3 phase from a 240 volt 3 phase syetm. This sounds similar, (not the same, but similar).
respectfully