600v -> 480v Transformers (Can you Buck that low?)

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glick

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We need to use a 480v Motor on a 600v source.
The motor is a 20A maximum, so we need a ~17kVA XFMR.
(480*20*1.73 = 16.6)
It also needs to be a NEMA 3 enclosure due to rain exposure.

It seems that all of the transformers this size, go from 15kVA to 30kVA.
The 15kVAs are ~100lbs, and the 30kVA are ~300lbs! They are also quite large in dimension.

Can you use a smaller Buck Boost transformer for 600 -> 480?
It seems that no one has a Buck Boost that does that ratio.

I know that Buck Boosts can be much smaller in size, and cheaper.

Thank you!
 
You don't need 17kVA worth of buck/boost transformer (the magic of buck/boost)...I don't have time to calc right now...

You should be able to use three 480:120V transformers.
 
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glick said:
We need to use a 480v Motor on a 600v source.
The motor is a 20A maximum, so we need a ~17kVA XFMR.
(480*20*1.73 = 16.6)
It also needs to be a NEMA 3 enclosure due to rain exposure.

It seems that all of the transformers this size, go from 15kVA to 30kVA.
The 15kVAs are ~100lbs, and the 30kVA are ~300lbs! They are also quite large in dimension.

Can you use a smaller Buck Boost transformer for 600 -> 480?
It seems that no one has a Buck Boost that does that ratio.

I know that Buck Boosts can be much smaller in size, and cheaper.

Thank you!
Click to expand...

Yes, most transformer manufacturers have schematics for creating an auto transformer to go between 600 and 480V. You can use two single phase 480-120V units connected in open delta. For sizing: Load kVA/8.6 = kVA of each single phase unit, so in your case 17/8.6 = 2kVA each. I would always size my motor transformer to at least be equal to my motor HP (provides better starting).

Remember a buck-boost is an autotransformer so NEC2005 450.4(B) applies which has an FPN that refers you to 210.9 and 215.11.
 
XFORMER SIZE, voltage drop

XFORMER SIZE, voltage drop

I have run into a few pump motors the last few weeks. I have 240V 15hp pump, 3 phase, 200A. I need to be able to size the XFORMER, for this motor, plus a 2000sq ft house will be fed off this XFORMER, 200A, plus one more 100A panel for lighting their workshop/garage.
When I calc the load for the XFORMER, will it be 240*200*1.73? That gives me the FULL LOAD FOR THE XFORMER,right? The house is 1 phase, so the would be 200*240. The extra lights would be 100*240.

The problem I'm having is getting the voltage drop. The lighting is literally 400' away from the XFORMER, so now I'm getting into a voltage drop/flicker issue. When I calc the flicker, don't I need to know the XFORMER impedance? That's what I don't know. Also, if I have the NEMA motor code for the 15hp pump, how can I use that to figure the voltage drop/flicker?

Thanks guys,
 
jim dungar said:
Yes, most transformer manufacturers have schematics for creating an auto transformer to go between 600 and 480V. You can use two single phase 480-120V units connected in open delta. For sizing: Load kVA/8.6 = kVA of each single phase unit, so in your case 17/8.6 = 2kVA each. I would always size my motor transformer to at least be equal to my motor HP (provides better starting).
Remember a buck-boost is an autotransformer so NEC2005 450.4(B) applies which has an FPN that refers you to 210.9 and 215.11.
Click to expand...
Can you tell me where you got the 8.6 number from?
I cannot seem to find any references online on how to properly size an open delta system.

Thank you!
 
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