# 208 in 456 out



## Southeast Power (Jan 18, 2009)

We had to setup some 480 to test a MCC we built. The customer wanted to keep the cost tight. I came up with using a 6kva delta/Wye 480/ 208 3 phase encapsulated transformer form Automation Direct ( about $450 Shipped) 

All went well until we checked the output voltage. The 480 taps, we had a choice of three, put out about 452 to 462 volts phase to phase with no load.
We changed it to the #3 taps and got only 415 volts. We did have it clockwise.:smile:
My concern is this: Can the VFDs be programmed using a lower 450 voltage? 

Much thanks for any constructive responses.


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## Cow (Jan 16, 2008)

Are you backfeeding the trans or is it an actual step up transformer?

I'm trying to figure out if you're wye on the primary or secondary?

I would not hesitate to program at 450v, I would take a peek at the nameplate though, but I would think it's fine.

If anything, I'd imagine it would show an undervolt fault, but at 450v, I think that's still high enough that it won't care.


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## Incognito (Apr 14, 2019)

What’s the primary voltage? Is it actually 208?


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## splatz (May 23, 2015)

https://cdn.automationdirect.com/static/specs/acme3phxfmrs.pdf



> 2. Can Acme Transformers be reverse connected?
> ACME dry-type distribution transformers can be reverse connected without a loss of kVA rating, but there are
> certain limitations. Transformers rated 3kVA and larger can be reverse connected without any adverse effects or
> loss in kVA capacity. The reason for this limitation in kVA size is, the turns ratio is the same as the voltage ratio.
> ...


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## splatz (May 23, 2015)

*Turns Ratio Compensation*

I did not know this...

https://www.reynoldsonline.com/ASSETS/DOCUMENTS/ITEMS/EN/ACMET2527031_Spec.pdf



> 7. Can Acme Transformers be reverse connected?
> ACME dry-type distribution transformers can be reverse
> connected without a loss of kVA rating, but there are
> certain limitations. Transformers rated 1 kVA and larger
> ...


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## Southeast Power (Jan 18, 2009)

I studied up a bit and new about the 3kva limitation when wiring it backwards. 
I wasn’t prepared for the voltage loss but I suspect someone here might be able to mathematically demonstrate why.

As for the questions.
Yes, it was fed Wye, output delta

The input voltage was a solid 208.

I chose a 6kva as it was twice the minimum recommended size to connect backward and was the maximum I wanted to put on a 30 amp breaker to get ~15 amps out.


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## micromind (Aug 11, 2007)

The VFD will very likely do fine with 450 volts but it cannot output more than its input so the motor will max out at 450 volts. This shouldn't be a problem unless it's loaded to the gills........lol. 

If the VFD is being fed will anything other than a solidly grounded Y, you'll need to cut or disable the surge capacitors, MOVs or whatever they're using, on the input side. Some VFDs have jumpers you move, others you need to actually snip the leads.


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## CoolWill (Jan 5, 2019)

Southeast Power said:


> I studied up a bit and new about the 3kva limitation when wiring it backwards.
> I wasn’t prepared for the voltage loss but I suspect someone here might be able to mathematically demonstrate why.


208 in, 460 out is roughly a 1:2.2 ratio. So going the other way, 480 in, would result in about 218 out. As it's loaded toward its rating, the voltage will come down to 208.


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## frenchelectrican (Mar 15, 2007)

Splatz just hit the jackpot there were a turns ratio compensation on that and yes I have heard and see it once a while espcally with smaller transformer useally under 3KVA that where it will get ya if you are not aware of that. 

I think you should be ok on 450 volt source once the motor is not loaded to max setting. 

the volt/Hz ratio is not too bad it kinda close to convental 460 volt motor setting. 

If you think it is too low you can use B/B to boost it up little more if you have to.


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## Southeast Power (Jan 18, 2009)

micromind said:


> The VFD will very likely do fine with 450 volts but it cannot output more than its input so the motor will max out at 450 volts. This shouldn't be a problem unless it's loaded to the gills........lol.
> 
> If the VFD is being fed will anything other than a solidly grounded Y, you'll need to cut or disable the surge capacitors, MOVs or whatever they're using, on the input side. Some VFDs have jumpers you move, others you need to actually snip the leads.


That ground reference is also a concern. They want to set up the drives before they ship out to a site. If they disable the surge components in the shop, can we assume they can simply be restored to working order after programing and before shipping out?


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## Cow (Jan 16, 2008)

We were in a similar situation a while back. Another guy in our shop had backfed a trans to get 480v for testing. After it was brought up that we weren't getting a grounded wye output, we replaced the trans with a proper step up trans to get a wye. 

Less likely to make a mistake during testing this way. Seemed worth it to me.


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## splatz (May 23, 2015)

More thorough whitepaper: 

http://apps.geindustrial.com/publibrary/checkout/Transformer2?TNR=White Papers|Transformer2|generic


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