open delta 3 phase voltages - 2 single phase transformers

[Designer_82]

What would be the 3 phase secondary voltage from this configuration?
Would it be 208v or 240v 3phase?

Thanks

IMG 3398 — Postimages

postimg.cc

 

[JJ_Roy]

Probably some very bad voltage. There are lots of things wrong with the drawing including xfmr on the right appears to have only one secondary connection and the "X" bushing nomenclature doesn't look right. But the drawing is probably trying to go for a 3ph 240V open-delta maybe?

This is a good reference for xfmr connections but it doesn't have everything.
https://www.eaton.com/content/dam/e...es-and-connections-information-tc202001en.pdf

The old GE Distribution Transformer Manual handbook is probably the best if you can find one!!!!

 

[jraef]

That looks to be a transformer configured for an open delta 240/120 3 phase 4 wire “high leg” system, where one pot is center tapped to provide 120/240 single phase for lights and outlets, then the other pot is straight 240, with the open delta providing 240V 3 phase for motors. Generally in this configuration, the 120 capacity is limited to 5% of the total kVA rating.

There would technically be 208V from one leg of the 240V-only pot to ground, which is required to be the B phase when it enters the service panel and marked with orange, but that 208V should not be used. So to avoid potential confusion and equipment destruction in the field by people who don’t understand, they don’t sell a single pole breaker rated for straight 240V (which is what that would require). Most panelboard mfrs have a limitation to not allow 1 pole breakers on B phase, or force you to have to use a 480V rated single pole breaker if you want to try.

So to summarize, you get 120V L-N from A or B phase, (typically limited to 5% of the kVA of the bank), 240V single phase from any 2 phases, and 240V 3 phase anywhere.

You also have an “artifact” voltage of 208V single phase from B to N, but that really shouldn’t be used. People do, but it’s risky for someone who comes along later who doesn’t understand and if they tap into it thinking it is 120V because it is L-N, they fry something.

 

[JJ_Roy]

It might be a "high leg" as jraef pointed out..... Especially with the four secondary conductors. But I'd probably ask what the intent is.

 

[waross]

Where did you get that POS drawing.
The numbering is wrong according to the transformer terminal locations.
Going by standard terminals locations:
120/240 Volt single phase. One line, a voltage to no-where.
If you add the missing jumper from X0-T2 to either of the outside terminals of T1, you will have an open delta, 120/240, High Leg 240 Volt three phase.
Trust me. I used to have a couple of these open delta banks on my system.

 

[7anoter4]

If there are 2 single phase transformers 240/120 you'll get only 120 V between X1,X2,Xo ,if instead of H1 and H2 connected in the middle [short-circuit!] you'll put H3.

In order to get "high point" you need 3 transformers, but for only 104 V. For 208 you need 240 V secondary [not primary!]

 

[waross]

In order to get "high point" you need 3 transformers,

You have to get out more.
Adding a second transformer to a single phase 120/240 Volt supply to get "120/240 Volt, high leg" three phase is the most common source of 120/240 Volt high leg power in many areas.
Typically an area will be fed 120/240 Volt single phase from a large, 50 KVA, 75 KVA, 100 KVA or 150 KVA single phase transformer.
One user requests a three phase service.
One, small transformer is added to develop open delta, high leg, 120/240 Volt three phase power.
Thee added transformer may be 5 KVA or 10 KVA.
It is often the smallest standard size transformer used by the utility.

Anecdote alert;
I spent years in a country where wye/delta transformer banks were common.
120/240 Volt high leg was common.
One blown primary fuse on a wye/delta bank was common.
With one primary fuse blown, the load was carried by only two transformers acting in open delta.
(More wye/delta anecdotes available on request. eg; the effect of elections on wye/delta transformer banks, and refrigerators.)

 

[waross]

Generally in this configuration, the 120 capacity is limited to 5% of the total kVA rating.

There are a couple of variations of thee open delta 120/240.
The most common is a 120/240 Volt single phase supply,with a teaser to supply a small amount of three phase.
The maximum 120 Volt capacity is the normal 120 Volt capacity of a single phase, 120/240 Volt transformer.
The maximum 3 phase capacity is based on the current rating of the teaser transformer.
The actual current through the 120/240 Volt transformer will be the single phase current plus the three phase current at their respective power factors.

The other variation leads us into anecdotes.
In a country where wye/delta banks were common,
1. It was also common that any electrician, engineer or contractor bringing in an order for transformers would receive a kick-back from the supplier.
As a result transformer banks tended to be sized at 200%, 300% or more of the actual maximum load.
The worst that I encountered was a 150 KVA bank serving a maximum load of 17 KVA.
There was never a danger of overloading when the bank went open delta.
2. Distribution circuits in the country were commonly switched in the field by opening single phase drop out fuses.
Operating with one primary phase open occasionally resulted in a failed transformer, but more often in a blown fuse.
3. Almost all of the wye/delta banks in the country had one primary fuse blown and were running open delta.
A common factory installation was a wye/delta bank with one transformer supplying 120/240 Volt single phase loads, and all three (or two) supplying three phase loads.
The oversized banks had more than enough capacity to support the load on open delta.

Anecdote #1
And what did elections have to do with this?
When the government changed, the top people in all of the government departments were replaced with favourites of the incoming politicians.
The new electrical Czar would notice the open fused cutouts all over the country and issue orders for them to be repaired.
Over the next months, the crews would be replacing fuses with larger fuses until transformers started blowing instead of fuses.
Eventually, they would go back to one blown fuse and everything running on open delta.
Anecdote #2
The utility did not do hot line work.
Weekend outages were frequent for line maintenance.
When the power was restored it was restored one phase at a time.
Typically the power would have been off long enough that all refrigerators, freezers and A/Cs would be ready to start.
If there was a wye/delta bank with all of its fuses on the circuit, when the power came on the first phase, the other two phases would get about 50% voltage on a back-feed.
The compressors would start to turn but would stall against building head pressure.
When full voltage arrived, most of the compressors would remain stalled.
The internal over-temperature switch would save most of them but it was common for one or two compressors on the circuit to fail.
I added a lot of under-voltage relays to a lot of refrigerators.

Back to the original question:
Trying to get 120/240 Volt wild leg three phase but can't because the wiring is wrong.
There is a jumper missing.