Wye and delta transformers

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Louis Hamptonsnopro

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Hi, first timer.
I got a question about transformers. 240v 3 phase with a high leg coming in. Need 480 v 3phase out. They sold me a transformer which is primary 480 delta and secondary 208Y/120. Will this work. My 480 is a 3phase load with no need for a neutral
 
Theoretically it will work. However; one thing concerns me which is the 208 part. If you feed a transformer that is wound to take 480 in and give you 208 with 240v you will have higher than normal voltage on your hi side. You most likely will see voltages highter than 480 on your secondary side. Which may or may not be acceptable depending on your end use.

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Welcome. The new primary side is not 240V but 208V so backfeeding the xfmr with 240V will give higher than 480V on the secondary. 553V if I figured correctly. You could adjust the secondary taps down to 432V tho that's still going to result in higher than 480V on the secondary.
 
That's not the right transformer for the job, and I don't think there's a good way to make it work. Your transformer has 6 windings. Three of them are 480V windings, and the other three are 120V windings. What you really need are three 480V windings and three 277V windings. Even though you don't need a neutral, you're better off with a Y secondary for bonding/grounding purposes.

The idea of using it backwards with 240V on a 208/120V Y primary and with a delta secondary will cause some problems. You'll get 553V out, and you won't have any good way to bond/ground it unless you're OK with corner grounding.
 
ActionDave said:
I agree with others, a transformer with a 240V low side is what you want, and if you don't want to do a corner ground then you want a wye on the 480V side.

What are you hooking up?
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Either you use a 480/277Y on the secondary, or you will be required to add Ground Fault monitoring on the 480V side and that's not going to be cheap...

Also, whatever transformer you use must be specifically listed by the manufacturer to be suitable for step-up (reverse feed) use. Many are, but not all and the NEC now requires this.
 
Jraef said:
...
Also, whatever transformer you use must be specifically listed by the manufacturer to be suitable for step-up (reverse feed) use. Many are, but not all and the NEC now requires this.
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450.10(B) stops a bit short of requiring the product to be listed for reverse feed...it only requires that the manufacturer have instructions for that application.
 
Jraef said:
...Also, whatever transformer you use must be specifically listed by the manufacturer to be suitable for step-up (reverse feed) use. Many are, but not all and the NEC now requires this.
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I've noticed that the manufacturers are branding a regular old delta/wye step down transformer as a gooder, better step up transformer even though I bet they are the same as they were. We have always ordered a wye secondary for step up applications.
 
To add, the high leg wouldnt matter since the primary side is 240V delta; the neutral's tap position is irrelevant. You also have 240V L-L all around with a high leg delta. To use the xfmr you have, back/reverse fed, it would need a 208/120Y primary side.

a 240D/480Y, or its reverse that is mfg listed to be reverse fed, is the xfmr you need. Here's a previous thread on a very similar install:

http://forums.mikeholt.com/showthread.php?t=86202
 
don_resqcapt19 said:
450.10(B) stops a bit short of requiring the product to be listed for reverse feed...it only requires that the manufacturer have instructions for that application.
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I know, that's why I said listed by the manufacturer not listed by a third party or anything like that.

And yes it's true, most mfrs just added the verbiage to their literature. Still, the code says it has to be suitable.
 
don_resqcapt19 said:
450.10(B) stops a bit short of requiring the product to be listed for reverse feed...it only requires that the manufacturer have instructions for that application.
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It's 450.11(B).
(B) Source Marking. A transformer shall be permitted to
be supplied at the marked secondary voltage, provided that
the installation is in accordance with the manufacturer’s
instructions.
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Jraef said:
I know, that's why I said listed by the manufacturer not listed by a third party or anything like that.

And yes it's true, most mfrs just added the verbiage to their literature. Still, the code says it has to be suitable.
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In other words, identified for the purpose.

Identified (as applied to equipment). Recognizable as
suitable for the specific purpose, function, use, environment,
application, and so forth, where described in a particular
Code requirement
Click to expand...

​
Click to expand...
 
biggest issue is OP has a 240 volt supply and a transformer with a 208 volt rating on the low side. He will get more then 480 if he uses that unit.

He will let smoke out if it if he connects the X0 to the neutral of the supply circuit - if anything because the high leg will put 208 volts on a winding rated for 120.
 
kwired said:
....
He will let smoke out if it if he connects the X0 to the neutral of the supply circuit - if anything because the high leg will put 208 volts on a winding rated for 120.
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I wouldn't do it and I advise not doing it, but I wouldn't bet on smoke being let out if someone does. The reason is, I doubt 208V is enough voltage to break down the winding's insulation just by itself. I don't know for a fact what the winding insulation voltage rating is or required to be, but I imagine it is much like most power and lighting wire being rated 600V.
 
Smart $ said:
I wouldn't do it and I advise not doing it, but I wouldn't bet on smoke being let out if someone does. The reason is, I doubt 208V is enough voltage to break down the winding's insulation just by itself. I don't know for a fact what the winding insulation voltage rating is or required to be, but I imagine it is much like most power and lighting wire being rated 600V.
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The voltage would not break down the insulation, but putting 208V on a winding intended for 120 is almost certainly going to overdrive the core into saturation leading to excessive idling current that will toast the winding.
There are two potential problems here:
The first, as mentioned, is driving a transformer rated for 208 with 240 instead.
The second, more serious but only mentioned in passing, is connecting the X0 (now primary wye point) to anything, let alone a high leg source neutral, when the acting secondary is a delta.
 
GoldDigger said:
The voltage would not break down the insulation, but putting 208V on a winding intended for 120 is almost certainly going to overdrive the core into saturation leading to excessive idling current that will toast the winding.
There are two potential problems here:
The first, as mentioned, is driving a transformer rated for 208 with 240 instead.
The second, more serious but only mentioned in passing, is connecting the X0 (now primary wye point) to anything, let alone a high leg source neutral, when the acting secondary is a delta.
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That is what I was getting at - connecting the XO. It should be fine to connect high leg otherwise but still will be having too high of an output on the 480 side. Will overdrive the core but should take a long time to cause damage as you are only driving it at about 115% its rating where if you connected the XO you overdrive that one leg at about 173%.

This all before considering the potential circulating currents you might have if you connected XO when supplying it with 208/120.
 
GoldDigger said:
The voltage would not break down the insulation, but putting 208V on a winding intended for 120 is almost certainly going to overdrive the core into saturation leading to excessive idling current that will toast the winding.
...
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Won't the idling current times the voltage have to exceed the winding kVA rating to "toast" the winding. That's definitely excessive idling current... :D
 
Smart $ said:
Won't the idling current times the voltage have to exceed the winding kVA rating to "toast" the winding. That's definitely excessive idling current... :D
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Yes, it would, and given a difference in inductance of a factor of ten or more when the core is fully saturated, I suspect that it is possible.
And, of course, at full load the higher idling current would be adding (in quadrature) to the load current.
Also, the circulating currents if you connect the input wye point can easily be several times the full load current.

Both effects are bad.
If you apply, for example, 208 volts to one 120v wye winding the delta secondary will look a lot like a short circuit and so the resistive component of the idling current will be far greater than even the increased magnetizing current.
The delta winding will, in effect be trying to have L to L voltages of 480, 480, and 600. That triangle does not add up!
 
Smart $ said:
Won't the idling current times the voltage have to exceed the winding kVA rating to "toast" the winding. That's definitely excessive idling current... :D
Click to expand...

No, it doesn't need to be that high. The kVA rating is for power transfer through the transformer, not power dissipation in the transformer. Transformers are pretty efficient, so they don't have to dissipate very much heat compared to the power flowing through them. Core saturation will greatly decrease the efficiency and raise the power dissipation enough to fry the windings - even if the input power is way below the transformer's kVA rating.
 
mpoulton said:
No, it doesn't need to be that high. The kVA rating is for power transfer through the transformer, not power dissipation in the transformer. Transformers are pretty efficient, so they don't have to dissipate very much heat compared to the power flowing through them. Core saturation will greatly decrease the efficiency and raise the power dissipation enough to fry the windings - even if the input power is way below the transformer's kVA rating.
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The other way to look at it is that the power dissipation in the transformer is a function only of the currents in primary and secondary windings.
The voltage that produces those currents is nice to know, but is irrelevant to calculating the power dissipation.
The heating will be the sum of the heat from both windings, but even with the secondary open it will not take more than a 40% excess of current in the primary to drive the transformer to its full heat limit.

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