How to convert 2 phase to 3 phase 1

[eeprom]

Hello,
I am trying to understand a common rural utility transformer circuit, wherein two transformers are used to convert 2 primary phases of 7200V into 3 phase 480V.

We have two incoming phases (A and C) and two single phase transformers. On the primary side, the transformers are wired in series; the connection point between the two transformers is tied to neutral, phases A and C are on the other two connections. On the secondary we somehow end up with 3 phase 480V. I know that this circuit is common, but I don't understand it. I've drawn it out a few times and my calculations results in a very unbalanced voltage on the secondary.

Can anyone refer me to a book or some calculations which can mathematically describe how this works?

thanks

 

[eeprom]

Eureka! I got it. I had the polarity on the secondary of T2 backwards. Thanks for all of your help.

 

[hughkaza]

Is it a must that neutral line on the primary being connected?
I am wondering what secondary voltage will be if the neutral line is not connected?

 

[waross]

At no load each transformer will develop 416 Volts for a total of 832 Volts single phase across both transformers.
In simple theory the voltages across each transformer secondary will be inversely proportional to the load on each individual transformer.
In practice the voltage swings will be somewhat limited by partial saturation of the transformer with the lightest load. The losses due to partial saturation will appear as an increased load on the primary of the lighter loaded transformer and will limit the voltage swings with badly unbalanced loads.
Don't do this at home. There is no useful application for this arrangement but it is well to be aware of the characteristics in the event that you have to trouble shoot an open delta bank with a lost primary neutral connection.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

Hugh
Without the neutral connection, the center primary connection point will assume a voltage that is approximately midway between C and A. The two secondary windings will be in phase, and Vbc becomes zero. Similar to dropping a primary phase on a floated wye/delta. Kinda rough on the connected three phase motors that are single phased.

 

[PHovnanian]

The discussion up to this point illustrates one reason why this connection is often discouraged. Imagine a lineman trying to debug one of these in the middle of the night.