Reverse wired tranny inrush

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chris kennedy

chris kennedy

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Location
Miami Fla.
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60 yr old tool twisting electrician
I'm looking for a resource that explains why inrush current is higher in a reverse wired transformer. Application would be a 480 delta primary 208y secondary reverse wired.

Thank you.
 
http://www.mgmtransformer.com/faq.html
Why do I need a bigger breaker when reverse feeding a transformer? Typically the output winding is wound first and is therefore closest to the core. When used as exciting winding a higher inrush current results. In most cases the inrush current is 10 to 12 times the full load current for 1/10 of a second. When the transformer is reverse fed the inrush current can be up to 16 times greater. In this case a bigger breaker with a higher AIC rating must be used to keep the transformer online.
 
chris kennedy said:
Nice link, I'll get back to that. Why would a higher AIC rating be required?
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Must be a typo.

The AIC rating of a breaker has nothing to due with its normal trip curve.
In fact if it did, you would actually want a breaker with a lower AIC. One way to get a high AIC is to have the breaker begin to open at a lower current level.
 
ron said:
http://www.mgmtransformer.com/faq.html
Why do I need a bigger breaker when reverse feeding a transformer? Typically the output winding is wound first and is therefore closest to the core. When used as exciting winding a higher inrush current results. In most cases the inrush current is 10 to 12 times the full load current for 1/10 of a second. When the transformer is reverse fed the inrush current can be up to 16 times greater. In this case a bigger breaker with a higher AIC rating must be used to keep the transformer online.
Click to expand...

Good link, I always knew they had a higher inrush, but did not know why. It is interesting that the manufacture calls for a higher AIC rating, but does not explain why.
 
What I am looking for is something I can print and take to the shop tomorrow.

Here are the details of the actual install in question.

I have a 150kva 480delta 208y/120 transformer that will be wired as a step up. This will be fed from MDP with a Siemens JXD6 400/3 breaker. The owner of my company is looking to me to verify that his value engineering of this install will work. While I was trying to explain inrush and core saturation he looked baffled. I don't feel inrush will be a problem with this install, just want to give him some paper from a reputable source explaining this.

Again, I thank you.
 
I have installed reversed transformers and never had a problem so you guys are saying to be on the safe side if you are installing a reversed transformer you should increase the breaker and the wire??
 
100214-2115 EST

Chris:

Consider a 1 to 1 transformer. If both windings are comparable, on the same core element, for example on the center of an EI lamination, and the windings are side by side, then which is considered primary makes no difference in virtually any aspect. Both primary and secondary have the same VA rating.

Now change one coil to have twice as many turns. Both coils still have the same VA ratings because they occupy the same physical space. Both coils have the same magnetic flux coupling. If you double the number of turns, then you need 1/2 the current to produce the same flux as the coil whose turns were unchanged. If with 200 turns the maximum inrush current is 100 A, then with 100 turns it will take 200 A to produce the same flux density conditions as the 200 turns and 100 A.

.
 
gar said:
Consider a 1 to 1 transformer.
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Speakiung of which, isn't it true that a 1:1 transformer wouldn't have exactly the same number of windings on both coils, and, likewise, a 240-120v unit, for example, isn't exactly 2:1?
 
100214-2158 EST

Larry:

That is true for power transformers because an attempt is made to provide the defined ratio at full load or near it and therefore a drop is assigned to compensate for the internal impedance of the transformer.

For Chris's purposes of an explanation it is simpler to assume the actual turns ratio is the same as the transformer input output ratio.

Because of this difference in the real transformer the reverse operation will have somewhat high inrush than predicted based on the transformer voltage ratio.

.
 
Why would a higher AIC rating be required?
Click to expand...
I imagined that since they were talking about the primary breaker on the low voltage side, its AIC would be relatively higher because it was seeing utility contribution direct than if the low voltage side was on the secondary which would have the impedance of the XFMR too.
 
ron said:
I imagined that since they were talking about the primary breaker on the low voltage side, its AIC would be relatively higher because it was seeing utility contribution direct than if the low voltage side was on the secondary which would have the impedance of the XFMR too.
Click to expand...
Either side will have an impedance.

Add: gar's #10 matches my thoughts.

Add 2:
Ron,
Nevermind, I think I mis-read your post.
 
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gar said:
Larry:

That is true for power transformers because an attempt is made to provide the defined ratio at full load or near it and therefore a drop is assigned to compensate for the internal impedance of the transformer.
Click to expand...
Wouldn't that mean that a reversed transformer will have a lower output voltage than expected?
 
chris kennedy said:
What I am looking for is something I can print and take to the shop tomorrow.

Here are the details of the actual install in question.

I have a 150kva 480delta 208y/120 transformer that will be wired as a step up. This will be fed from MDP with a Siemens JXD6 400/3 breaker. The owner of my company is looking to me to verify that his value engineering of this install will work. While I was trying to explain inrush and core saturation he looked baffled. I don't feel inrush will be a problem with this install, just want to give him some paper from a reputable source explaining this.

Again, I thank you.
Click to expand...

Chris, are you going to corner ground the delta? And with the wye primary, XO would have to be insulated.
 
ibew501ed said:
Good question..
I understand corner grounded transformers are NOT the preferred method for new work is this correct
Click to expand...

IMO it is more of a personal choice than a written in stone fact.

My understanding is that engineers do not like corner grounded delta 480 as it stress's the insulation much more than 480Y would.

I am not sure that they have any issue with corner grounded 240 Delta.
 
100215-0846 EST

Larry:

You are correct the ratio would be slightly less than 2 to 1, maybe by 3 %, and therefore in the reverse direction the new secondary voltage would be slightly less than double the primary, and the new primary inrush slightly less than predicted based on a 2 to 1 turns ratio. So slightly under double the inrush current.

Somebody else proofreading is always better than doing it yourself. Thanks.

.
 
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