dry transformer

[alberto]

Hi good afternoon,
I have a big question i know you can help me.

there is a factory that has a substation of 300 KVA, 13200-220/127, but there are 4 molding machines whit a voltage of 480 V, they are thinking to install an used dry transformer of 112.5 KVA, primary side 480 V, secondary side 208/120 V, conection delta-star.
My question is:

is correct to use this transformer no matter they feed for the secondary side and the load for secondary side?

i think it is not correct because i will install a dry transformer 208-480/277 delta-star

 

[bob]

Re: dry transformer

Alberto
You question is a little confusing to me. You say you have a 300 kva 120/208 volt, I think. Is this correct. What is the load on the transformer in KVA?
I assume the molding are a new addition and you need 480 volts 3 phase.
You say " they are thinking to install a used dry transformer of 112.5 KVA, primary side 480 V, secondary side 208/120 V, connection delta-star."
Where are they going to get the 480 volts for the primary? Are they thinking of feeding the transformer 208 primary to get the 480 volts on the secondary?

"I think it is not correct because i will install a dry transformer 208-480/277 delta-star"

If the molding machines do not need a neutral connection (277 volts) then you could feed the machines with 3 phases at 480 volts and an equipment grounding conductor.

You need to verify that the 300 kva transformer has the extra capacity for the new load.

[ September 14, 2005, 11:14 PM: Message edited by: bob ]

 

[alberto]

Re: dry transformer

they are thinking to feed the transformer for the secondary side, is that correct?
no matter if the primary side is 480 volts and there is no 480 volts in the factory

 

[infinity]

Re: dry transformer

Sounds like he wants to feed a 480-120/208 transformer 'backwards' to get 480 on the secondary.

Where did you get 220/127? Is that an actual voltage reading of what should be a 120/208v secondary?

 

[ka6chp]

Re: dry transformer

To the gentleman who isn't familiar with 220Y/127v.. that is the standard voltage in Mexico. Hope that helps. Jim Green

 

[hardworkingstiff]

Re: dry transformer

Originally posted by ka6chp:
To the gentleman who isn't familiar with 220Y/127v.. that is the standard voltage in Mexico. Hope that helps. Jim Green

Some manufacturers are offering that here in the states for use at marinas to help with low voltage from a 120/208 (low on the 240 the boat is expecting).

 

[jtester]

Re: dry transformer

From my experience, if you backfeed a delta wye transformer there are a couple of issues. First, you must float the wye neutral to avoid nuisance tripping and/or overloads. Secondly, if the 480 side serves premises wiring, not just an individual piece of equipment, it should be grounded. This will require a corner grounded system and the usual restrictions on voltage ratings, overcurrent device limitations, etc.

I often backfeed a transformer for an individual machine, but usually opt for a 208 delta to 480 grd wye if I am serving a bunch of equipment.

Jim T

 

[tom baker]

Re: dry transformer

This question comes up freqently. A 480X120/208 is a Detla-Wye transformer. If connected 120/208x480 then its a Wye/Delta. You would have an ungrounded secondary and would want to run a corner ground connection.
However I maintain unless the transformer is listed for 120/208x480 it would be a violation of 110.3(B).
And square D makes a step up transformer, its more expensive than the step down.
It would work but would be confusing and the next person would wonder who did this?
Use the correct transfromer so its a grounded neutral secondary. If you don't understand the difference between a delta and wye transformer, then don't install a delta, they are more dangerous than a Wye?
Wye (sorry) is that?
Voltage to ground on 480/277 wye is 277
Voltage to ground on 480 delta is 480.

 

[alberto]

Re: dry transformer

I agree with you tom.

I understand the difference between delta and wye connections.

Thanks a lot everybody, i appreciate your help.

 

[jtester]

Re: dry transformer

Tom

I've read your opinion on these before, and am intrigued. I have had supply houses tell me that their manufacturers instruct them to have the contractor reverse feed these transformers at smaller kva. I don't know why smaller kva would be ok and larger units not ok, but that is what I've been told. I have never contacted a manufacturer directly, and now plan to do so. Have you or any other members received specific feedback from a manufacturer?

Jim T

 

[jim dungar]

Re: dry transformer

Tom,

UL lists transformers as having primaries and secondaries OR high voltage and low voltage. Follow the nameplate and the manufacturer's literature.

Square D general purpose transformers now only say high and low voltage connections.

If we had to follow only the nameplate then every buck-boost installation would be a problem.

 

[oliver100]

Re: dry transformer

Every transformer can be used in any direction. There is nothing wrong to connect delta-wye-wye-delta. However, in this particular case 13.2/220/208/480 V the end voltage will be above 500 volts. Which would be a problem for the equipment

 

[tom baker]

Re: dry transformer

Every transformer can be used in any direction
Yes true, but if the transformer is not listed for the application it would be a violation. And why would Sd d make a step up and step down transformer?

 

[oliver100]

Re: dry transformer

And why would Sd d make a step up and step down transformer?

It is a very good question. Probably SQ D would give us some information. Does anybody do that (step-down and step-up transformares and what is the difference)? The name plate?

 

[sandsnow]

Re: dry transformer

There have been a few posts regarding backfeeding a standard 480 delta - 208Y/120 transformers.
I ran across this the other day. The manufacturer evidently has had a lot of questions about this and put a sticker on their XFMR's on this very subject. I'm not a photographer by trade ( I know, hard to tell

 

[iwire]

Re: dry transformer

Originally posted by tom baker:
Every transformer can be used in any direction
Yes true, but if the transformer is not listed for the application it would be a violation. And why would Sd d make a step up and step down transformer?

From Square D. FAQs (Thanks Roger )

Question

Does back-feeding a low voltage dry type distribution transformer 15kVA or larger void the transformer`s UL label?

Answer

Square D does not label our low voltage dry type distribution transformer`s terminals as primary or secondary. They are labeled as high voltage or low voltage. In the event that the line-to-line voltage of the designed primary is identical to that of the designed secondary, then the designed primary is marked high voltage (HV) and the designed secondary is marked low voltage (LV.) This does not prevent the transformer from being backfed. Square D does not recommend backfeeding transformers.

Question

Can Transformers be backfed (used in reverse)?

Answer

1. Backfeeding is not recommended for general purpose transformers smaller than 3kVA, and is not allowed for any Industrial Control Transformers of all sizes, because windings are compensated. Backfeeding will result in lower than expected output voltage.

2. Backfeeding causes very high excitation inrush, making coordination to breakers or fuses difficult without exceeding the limits set by the National Electrical Code. Avoid backfeeding wherever possible for this reason.

3. If a Delta-Wye transformer is to be backfed so that the Wye side is the input, do not connect the neutral terminal to the primary system neutral, nor should the neutral terminal be connected to ground.

4. If the transformer has taps, when backfed the taps do not help compensate for poor (other than nominal) source voltage to provide the appropriate magnetic flux levels in the core as they are designed to do.

 

[sandsnow]

Re: dry transformer

Interesting info from Sq D. that they don't mark transformers as primary/secondary
The UL guide info states that transformers are marked with Primary Voltage and Secondary Voltage.
Category XQNX

 

[jcormack]

Re: dry transformer

Instead of corner grounding the deta - why not use a zig-zag to derive a neutral? ? ?