Transformer backwards

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trigger

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Will using a 15 kva dry transformer 480v primary 240v secondary, wired backwards, fed 240v through secondary and be able to have 480v to carry load of a single phase 480v 50amp welder?
 
I say no, A 15kva can only handle about 62a thru the secondary side and you need... what, about a hundred? Not to mention the temperature ratings.
 
jumper said:
I thought that wiring trannys backwards was not a good thing. Am I wrong?:?
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Some transformers are designed to do so. I would not be too concerned about single phase stuff provided the temp and current ratings are followed.

(talking about an enviornment where skilled workers are on site to monitor it)
 
iwire said:
You do not have to consider duty cycle at all. But you can.:)
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By considering duty cycle the apparent undersized transformer could end up being acceptable.I haven't looked into requirements for this before posting but it is worth consideration. A welder is not like most other loads.
 
kwired said:
By considering duty cycle the apparent undersized transformer could end up being acceptable.
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I don't see anything in 630 that would allow a reduction of the transformer size, all I see is about the conductor sizing.

But that was really not my point, Article 630 is entirely voluntary and we do not have to do any of the things it says.

A welder is not like most other loads.
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I agree, and also agree that looking into 630 is smart if you are wiring a welder. It could save you money. :)
 
I hadn't thought of this before. But why would back feeding be a problem? Sec coils are set up for 240, 240 going in or out should be the same load, right? Primary coils for 480, why does it matter which way it flows?
 
jmellc said:
I hadn't thought of this before. But why would back feeding be a problem? Sec coils are set up for 240, 240 going in or out should be the same load, right? Primary coils for 480, why does it matter which way it flows?
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One issue is the inrush current is higher due to the design.

But I have done it successfully.
 
A transformer is just a core with 2 sets of windings with a ratio, in this case 2:1. It is also 1:2, it doesn't know or care. So yes, 240 in, 480 out.

But that does not mean there are not issues:
  1. If the equipment connected to the 480V side requires a Y circuit, i.e. 480/277, it's unlikely that an off-the-shelf standard transformer will give you that. Most 480-240V 3 phase transformers are Delta-Delta, maybe some with a hi leg split phase on the 240V side, but rarely will you find a 240V delta to 480/277V Y transformer.
  2. It can be really dangerous for some poor future electrician who comes upon this without VERY CLEAR markings as to exactly what it is.
  3. Often times a 240V system will have trouble with the magnetizing inrush current on what now becomes the primary.

Now, all that said, a 15kVA single phase transformer will be capable of the following:
240V side, 62.5A FLC
480V side, 31.2A FLC
Your mileage may vary slightly, but less than 5% in most cases.

You cannot get 50A from this transformer, moot point.
 
Jraef said:
A transformer is just a core with 2 sets of windings with a ratio, in this case 2:1. It is also 1:2, it doesn't know or care. So yes, 240 in, 480 out.

But that does not mean there are not issues:
  1. If the equipment connected to the 480V side requires a Y circuit, i.e. 480/277, it's unlikely that an off-the-shelf standard transformer will give you that. Most 480-240V 3 phase transformers are Delta-Delta, maybe some with a hi leg split phase on the 240V side, but rarely will you find a 240V delta to 480/277V Y transformer.
  2. It can be really dangerous for some poor future electrician who comes upon this without VERY CLEAR markings as to exactly what it is.
  3. Often times a 240V system will have trouble with the magnetizing inrush current on what now becomes the primary.

Now, all that said, a 15kVA single phase transformer will be capable of the following:
240V side, 62.5A FLC
480V side, 31.2A FLC
Your mileage may vary slightly, but less than 5% in most cases.

You cannot get 50A from this transformer, moot point.
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Well with that then a 30kva just might make it happen!:)
 
trigger said:
Well with that then a 30kva just might make it happen!:)
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If the transformer supplies only the mentioned welder then depending on the duty cycle of the welder the 15 KVA could be just fine, maybe, 630 does not address transformers supplying welders but why wouldn't the concepts there apply to a transformer? Unless the 50 amp rating is what is needed after considering the duty cycle.

Most of information in 630 is all about how hard it will be loaded over a specified amount of time. Not many loads have similar load characteristics as welders, that is why they have art 630.
 
kwired said:
If the transformer supplies only the mentioned welder then depending on the duty cycle of the welder the 15 KVA could be just fine, maybe, 630 does not address transformers supplying welders but why wouldn't the concepts there apply to a transformer? Unless the 50 amp rating is what is needed after considering the duty cycle.

Most of information in 630 is all about how hard it will be loaded over a specified amount of time. Not many loads have similar load characteristics as welders, that is why they have art 630.
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The nameplate reads 50A, but does not list the duty cycle, therefore I can only assume 20% to 30% is a safe cycle rating cosidering it is an older "looking" welder.
 
kwired said:
Cannot and should not do not mean the same thing.http://forums.mikeholt.com/images/smilies/wink.gif
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OK, I'll concede that point.

Heck, I even know better! I remember the first time I learned that. I was dealing with a PoCo engineer and we needed 1250kVA but the transformer was a 1000kVA. We paid only a small fee and I though it was very generous of the PoCo to swap out that transformer so cheaply. But the engineer just showed up with an envelope, pulled out a new nameplate that said "1250kVA" and put in on the existing transformer.
 
Jraef said:
OK, I'll concede that point.

Heck, I even know better! I remember the first time I learned that. I was dealing with a PoCo engineer and we needed 1250kVA but the transformer was a 1000kVA. We paid only a small fee and I though it was very generous of the PoCo to swap out that transformer so cheaply. But the engineer just showed up with an envelope, pulled out a new nameplate that said "1250kVA" and put in on the existing transformer.
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Lmao, I bet they even put an ice-melting system in the area too- no extra charge!
 
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