Two single phase transformers connected in a bank
Two single phase transformers connected in a bank
(OP)
So here is an interesting one. I have a customer that has a two single-phase transformers banked together, connected to three-phase primary service.
Configuration is similar to below, except X1/X2/X3 go from left to right.

Couple questions on this configuration. Is it possible to connect this configuration to form Delta-Wye and achieve 120V to neutral on all three phases?
For some odd reason I cannot locate two single phase transformer bank configuration thats connected in a Delta-Wye configuration.
Why can't we connect X2 (neutral) on secondary of both transformers together?
The configuration above give us 240V between all three phases, but only 120V between b-n and c-n, but what about a-n?
Need some help with this one. Thanks in advance.
Jack
Configuration is similar to below, except X1/X2/X3 go from left to right.

Couple questions on this configuration. Is it possible to connect this configuration to form Delta-Wye and achieve 120V to neutral on all three phases?
For some odd reason I cannot locate two single phase transformer bank configuration thats connected in a Delta-Wye configuration.
Why can't we connect X2 (neutral) on secondary of both transformers together?
The configuration above give us 240V between all three phases, but only 120V between b-n and c-n, but what about a-n?
Need some help with this one. Thanks in advance.
Jack






RE: Two single phase transformers connected in a bank
With just two transformers you can't have three branches that all meet at a wye point and have an equal phase separation.
On both triangles, erase the A-C and c-a lines, there's no transformer winding there; then some of the rest of it will become clearer.
RE: Two single phase transformers connected in a bank
I don't really care about having a balanced Wye, since we wouldn't have a 3 phase 120V service connecting to anything.
What would happen if I connected both neutrals together?
RE: Two single phase transformers connected in a bank
This connection was used to develop 120:240 Volts single phase for lighting and other single phase loads.
The addition of a "Teaser" transformer develops 240 volts three phase for three phase loads.
Typically the transformer connected for 120:240 Volts may be much larger than the second transfoemer.
A hint on the voltage at a-n
Make 120 Volts equal to one unit.
You have a right triangle, n-c-a.
n-c = 1 unit
c-a + 2 units
Solve for a-n
This connection is called an "Open Delta".
This connection is more often used with two primary phases and a neutral.
On long rural lines it saves the cost of installing insulators and conductors for the third phase.
There is a connection that I have heard called the "Winnipeg" connection which will produce 120:208 Volts three phase power from two phases and a neutral but it requires three transformers.
Bill
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"Why not the best?"
Jimmy Carter
RE: Two single phase transformers connected in a bank
I suppose it would be okay if I only had 3 phase breakers/3 phase loads and no 1 PH (1 pole) breakers on the A phase?
How do I prevent someone plugging in a 1 PH breaker and connecting it to neutral?
Put a warning label on the panel? This seems like it would be a code violation to me.
RE: Two single phase transformers connected in a bank
RE: Two single phase transformers connected in a bank
One service will be 120:240 Volts single phase.
The second service will be 240 Volts three phase.
OR
You may use a three phase panel and don't connect any loads between the neutral and the "High Leg" or the "Wild Leg". (208 Volts to neutral.)
Follow up on jghrist's post.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Two single phase transformers connected in a bank
Jack