transformer draw

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ceb

ceb

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raeford,nc
I have a 30kva 3ph transformer when I ordered it I requested a 208 to 480 step up transformer. When it arrived it was a 480 primary to 208/120 secondary. the instructions clearly state it is suitable for reverse connection.
My question is will my load cal. stay the same 83 amp on the 208 primary and 36 amp on secondary?
 
ceb said:
My question is will my load cal. stay the same 83 amp on the 208 primary and 36 amp on secondary?
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Yes.

But, there are several important application considerations which MUST be addressed when reverse connecting transformers. Personally I have been involved in more reverse connected applications rather than "special" order step-up ones.
 
What Else?

What Else?

jim dungar said:
Yes.

But, there are several important application considerations which MUST be addressed when reverse connecting transformers. Personally I have been involved in more reverse connected applications rather than "special" order step-up ones.
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Jim, other than the inrush current, what are the considerations you mention?
 
Personally I have been involved in more reverse connected applications rather than "special" order step-up ones.
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Jim:

My statement above
Get the right transformer for the application future electricians will thank you.
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Is based upon my my experience matching yours...I get calls a few times a year to check out STRANGE voltages, and the reverse transformer is the culprit. Electricians baffled, managemnet freaking out.
 
When a transformer is reverse connected, you have the following problems:

1) Taps on the 'primary side' to adjust core saturation and get correct output voltage. If your supply voltage is higher than 208/120, this may cause problems.

2) Minor inrush current differences cause by the positioning of the windings.

3) _Listing_ issues.

(IMHO both 2 and 3 are taken care of when the instructions say suitable for reverse connection.)

4) (And IMHO this is the most critical). _Grounding_ of the secondary. A transformer is a separately derived system. If you don't make an electrical connection between the secondary and ground, then you will have an _ungrounded_ electrical system, with the various requirements, benefits, and problems of such. There are several ways to ground a delta secondary, each with their own benefits and issues. IMHO if this is a new installation, you should put a wye secondary on the transformer.

-Jon
 
winnie said:
When a transformer is reverse connected, you have the following problems:

1) Taps on the 'primary side' to adjust core saturation and get correct output voltage. If your supply voltage is higher than 208/120, this may cause problems.

2) Minor inrush current differences cause by the positioning of the windings.

3) _Listing_ issues.

(IMHO both 2 and 3 are taken care of when the instructions say suitable for reverse connection.)

4) (And IMHO this is the most critical). _Grounding_ of the secondary. A transformer is a separately derived system. If you don't make an electrical connection between the secondary and ground, then you will have an _ungrounded_ electrical system, with the various requirements, benefits, and problems of such. There are several ways to ground a delta secondary, each with their own benefits and issues. IMHO if this is a new installation, you should put a wye secondary on the transformer.

-Jon
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Thanks for the edification Winnie. Now do we think the disproportionate inrush current is a result of lower leakage inductance since the LV winding is next to the core?
 
necnotevenclose said:
A little help please. Why the need for a 480V transformer? What is it going to feed?
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The only thing this transformer will feed is a over head bridge crane with a total amp draw of around 20 amps.
 
Out of curiosity are you planing to run this crane ungrounded with fault indicators or are you going corner grounded delta?
 
iwire said:
Out of curiosity are you planing to run this crane ungrounded with fault indicators or are you going corner grounded delta?
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Bottom line is that this crane was thrown in at the last minute. The owner was able to by this crane completely refurbished had it delivered and told me by the way we need this crane hooked up. As far as this transformer is concerned I am walking not in a dark area but a very dim one. By Acme's instructions for reverse connection I am to bring my 208 line voltage into the secondary side h1,h2,h3 making this my primary, then the 480 primary x1,x2,x3 will now be my secondary. But I am unclear on the grounding I know the frame has a grounding lug for the frame but as far as xo is concerned and grounding this crane it is fuzzy. My plan is to feed the transformer from the dist. panel with a cb and feed the buss bars with a fuseable disconnect. I would appreciate some solid ideas and or suggestions
 
ceb said:
I would appreciate some solid ideas and or suggestions
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Make sure you have a solid neutral from the supply to X-0 on the 208v side, or your secondary voltage will fluctuate.
 
LarryFine said:
Make sure you have a solid neutral from the supply to X-0 on the 208v side, or your secondary voltage will fluctuate.
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Larry when reverse feeding this transformer XO should remain isolated. (unconnected to anything)

You would not use the neutral from the 208 supply at all.
 
I am an industrial electrician, We use all 3P 480V equipment, unless the hoist manufacturer says differently.
All of our equipment is grounded to the building grounding system, but all 3 legs of the 480 are floating.
If the transformer says it can be reverse fed, it is effectivly a step up transformer.
I am assuming that this is a delta to delta transformer.
 
Psychojohn said:
All of our equipment is grounded to the building grounding system, but all 3 legs of the 480 are floating.
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That would mean you are running ungrounded delta.

That is fine as long as you have a ground fault indicting means and you actually repair each ground fault as it happens.
 
All of our machines use ungrounded delta, our substations us ground fault indicators. Our hoists use 480V wye center grounded.
Though, there are several machines supplied with isalation 3P transformers for sections of the machine that are not grounded or supplied with indicators.
 
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