3 phase open delta voltages - 2 single phase transformers

chris kennedy said:
There is only one primary???
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One per transformer, of which there are two.

As I said, if it was drawn correctly, it would be a high-leg open delta.

Unless it was drawn that way intentionally.

Example:
(note the high-leg conductor is missing from the far-right secondary terminal)

1739514151105.jpeg

Here's another example:

1739514404456.jpeg
 
The main difference between the two examples is that the second one only needs two primary lines because the primaries use the neutral.
 
designer82 said:
I'm not following this question.

So you get 240v 3 phase on the secondary of this configuration then?
(no such thing as 208v available?
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If it is an open delta constructed with 120/240V transformers, then you should have 240V L-L three phase, two 120V L-N legs suitable for normal 120V loads, and a 208V L-N leg that really should not be used.

(The 208V L-N leg loads the transformer in inefficient ways, and is also a less stable voltage than the legs directly supplied by coils. On top of this, to use it you need a breaker rated for 208V L-N; in a common 240V panel most single pole breakers are only rated for 120V to ground. The 'open jaw' 240V is also less stable, and should not be used for single phase 240V loads.)
 
ptonsparky said:
I know I've used the high leg for single phase water heater. Yes, proper breaker.
Still an issue? 400 amp service. IDK if it was a closed or open.
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I don't believe there is any prohibition if the load is rated for 208V L-N and the proper breaker is used.

I'll stand by 'should not be used', but if you dot your 'i's and cross your 't's, you can do lots of non-standard things.
 
ptonsparky said:
I know I've used the high leg for single phase water heater. Yes, proper breaker.
Still an issue? 400 amp service. IDK if it was a closed or open.
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I wouldn't say the voltage of high leg is all that unstable, just that current between it and either the neutral or the other open end of the delta needs to pass through the common corner of the delta and the transformer bank simply can not handle same load as a full delta bank and there is no way it can handle fully balanced loading like a full delta would. This is also why the "stinger pot" is often smaller size than the 120/240 pot, it was only intended to handle the amount of load needed by the "third phase"
 
ptonsparky said:
I know I've used the high leg for single phase water heater. Yes, proper breaker.
Still an issue? 400 amp service. IDK if it was a closed or open.
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Was your water heater wired the extremely rare L-N (208V) or the more common L-L (240V)?

There are many myths about these wild/high leg open-delta systems, probably to scare people away from them. In many rural parts of the country, fed by overhead systems, these were and still are very common.

The big problem with the 208V L-N connection is overloading the transformer which leads to the voltage regulation issues previously mentioned. A properly sized transformer bank could be created for this loading, but almost never is.
 
jim dungar said:
Was your water heater wired the extremely rare L-N (208V) or the more common L-L (240V)?

There are many myths about these wild/high leg open-delta systems, probably to scare people away from them. In many rural parts of the country, fed by overhead systems, these were very common.

The big problem with the 208V L-N connection is overloading the transformer which leads to the viltage regulation issues previously mentioned.
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Until you have severely overloaded it the voltage is much more stable than the derived third phase when using most electromechanical phase converters though
 
ptonsparky said:
WH Load was 240v. I've never used the hi leg by itself. (Well, other than that one and only time I needed temporary power at a rooftop unit.)
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But if you connected it from high leg to the other open end of delta, the current passes through entirety of both transformer coils, lessening the remaining capacity between A and C, where if you had a full delta it only loads the A to B coil.
 
kwired said:
But if you connected it from high leg to the other open end of delta, the current passes through entirety of both transformer coils, lessening the remaining capacity between A and C, where if you had a full delta it only loads the A to B coil.
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Connections between any two Lines (phases) are intended connections The big issue is the overloading of 1/2 of a winding, as found when using the neutral and the high leg.
 
jim dungar said:
Connections between any two Lines (phases) are intended connections The big issue is the overloading of 1/2 of a winding, as found when using the neutral and the high leg.
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That's kind of what I was trying to say there. If you connect a 240 volt load to both open ends of the delta what is your circuit path through the source? You should be loading both coils.

Now go full delta (close the open end) and connect same load - isn't the current path going to be through the now closed end of the delta?

A load from high leg to neutral is going to load half the A to C winding of open delta. High leg to neutral of full delta will still have same load on A to C winding but has two ways to get there and exactly how much goes each way will vary with conditions.
 
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