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Discussion Starter · #1 ·
Hi
I have to Feed a giant 75 hp motor through set of 4/0 wires @ 208v available at the faculty

Question is
Can I buy 480-208v dry type transformer
installed in reverse
And run smaller wires to the motor adjusted @ 480v

Tanx
 

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Hi
I have to Feed a giant 75 hp motor through set of 4/0 wires @ 208v available at the faculty

Question is
Can I buy 480-208v dry type transformer
installed in reverse
And run smaller wires to the motor adjusted @ 480v

Tanx
You are better of buying a 208 Delta 277/480Y transformer made for this exact purpose. I know Square D offers them from their catalog.

You could reverse feed a tranny, but you often end up with a floating wye delta connection, which isn't to desirable. Inrush also tend s to be greater since engineering specs design around normal conditions. But, if you had no option, its doable.
 

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Hi
I have to Feed a giant 75 hp motor through set of 4/0 wires @ 208v available at the faculty

Question is
Can I buy 480-208v dry type transformer
installed in reverse
And run smaller wires to the motor adjusted @ 480v

Tanx
Can I get a bit more info on the motor?
 

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Tamas said:
Hi I have to Feed a giant 75 hp motor through set of 4/0 wires @ 208v available at the faculty Question is Can I buy 480-208v dry type transformer installed in reverse And run smaller wires to the motor adjusted @ 480v Tanx
Only if you want to let the smoke out. It's a 208 volt motor. Were you thinking about stepping the voltage up at the source and then back down again at the motor. That could work. It'll cost you three arms and a leg to accomplish this.
 

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Only if you want to let the smoke out. It's a 208 volt motor. Were you thinking about stepping the voltage up at the source and then back down again at the motor. That could work. It'll cost you three arms and a leg to accomplish this.
He never gave the voltage of the motor, but at 75 HP I would assume he will reconnect it for 480
 

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Bad Electrician said:
He never gave the voltage of the motor, but at 75 HP I would assume he will reconnect it for 480
If he's running 4/0 at 75 horse, it's either a three mile run to the motor at 480 or the motor is 208.
 

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If he's running 4/0 at 75 horse, it's either a three mile run to the motor at 480 or the motor is 208.
I know your work on different codes than we do but at 75 Hp 208 3 phase VAC 211 amps Great White North or not the I think the physics should be the same?
 

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Unless it is a long run the transformer is going to cost more than the wire.

And I have never thought of a 75hp motor as being giant. More like medium small.
 

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I know your work on different codes than we do but at 75 Hp 208 3 phase VAC 211 amps Great White North or not the I think the physics should be the same?
Huh? Our boy says he's running 4/0 to this "giant" motor. You say he didn't mention the voltage. He gives it away with his choice of wire, that being 208. The only reason a person would use 4/0 to run a 75hp motor at 480 volt is because its miles away, because of voltage drop. It's clear the motor is wound to 208. He's confused thinking it can run at 480.


Sent from my iPad using electriciantalk.com
 

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Huh? Our boy says he's running 4/0 to this "giant" motor. You say he didn't mention the voltage. He gives it away with his choice of wire, that being 208. The only reason a person would use 4/0 to run a 75hp motor at 480 volt is because its miles away, because of voltage drop. It's clear the motor is wound to 208. He's confused thinking it can run at 480.


Sent from my iPad using electriciantalk.com
Oh and you know this how, you're Clairvoyant? :laughing::laughing:WE (you and I) and the rest of the members only know what he posted.
 

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Hi
I have to Feed a giant 75 hp motor through set of 4/0 wires @ 208v available at the faculty

Question is
Can I buy 480-208v dry type transformer
installed in reverse
And run smaller wires to the motor adjusted @ 480v

Tanx
Yes you can, is it practical that depend on many variables, cost wise it will be more expensive unless (as noted by others) there is a long cable run involved. Additionally you are adding inefficiency to the mix. Though it is minor it will add some KW when the motor is running and when sitting at no load. Lastly you need to check the transformer you are purchasing can supply the starting current, without significantly lowering the secondary voltage.
 

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This can certainly be done, provided the motor can be connected 480. 4/0s are pretty grossly oversize for 75HP @ 480, but I assume the wire is already existing.

The transformer will need to be at least a 112.5KVA (maybe 150KVA if it starts across the lines), and a basic 480∆-120/208Y will work.

If so, do not connect anything to XO. Not even the ground......especially not the ground. Also, ground one of the 480 phases. Yes, just connect one of them (usually H2) to ground. This is called a 'grounded B' system. The motor does not care about voltage to ground, only phase to phase voltage.

If you're using any type of electronic motor control, read the directions carefully; some do not allow grounded B systems, others need modifications to use that system. If grounded B is not allowed, you'll need to get a transformer with a 277/480Y on the high side, and ground the center of the Y. The low side can be either a 208Y or a 208∆.
 

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Discussion Starter · #15 ·
Hi
I have to Feed a giant 75 hp motor through set of 4/0 wires @ 208v available at the faculty

Question is
Can I buy 480-208v dry type transformer
installed in reverse
And run smaller wires to the motor adjusted @ 480v

Tanx
Also,
note that, the 75 hp motor is 175a @230v according to its nameplate
 

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Hi
I have to Feed a giant 75 hp motor through set of 4/0 wires @ 208v available at the faculty

Question is
Can I buy 480-208v dry type transformer
installed in reverse
And run smaller wires to the motor adjusted @ 480v

Tanx
Yes, you can. Recalculate your wire size! 4/0 is a bit big unless there are other factors. Rough calculation is 75 x 746 = 55,590 watts, roughly drawing 120 amps @ 480V. That's rough! Three phase makes it less, ~ 100 amps. 1/0 or 2/0 should be fine, but I know from experience engineers like to oversize! Actually sounds like 4/0 could feed the primary?

Let's be clear here. In this application you are stepping up, and the tranny is called a step up! BUT, transformers are exactly that, TRANSFORMERS. We give them a name after it has been installed. Larger ones are designed differently, but in your case, it's a transformer connected as a step up.

I have seen a case where the transformer, in this size range, had been wired incorrectly for it's application. Luckily no damage was done, as it was noticed doing simple commissioning of the cabinet it fed! :eek:

Hope that helps!

Thanks, Borgi
 
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