Buck Boost Xfrmrs again

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Grend

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I have just found your forum from a Google search on Buck / Boost transformers. I have had experience of using them to provide 230V from 208V but now I need to use one to buck 277V to 230V.

When boosting the voltage I from say 200V to 250V I understand that the power provided by the 50V winding is supplied by the 200V winding and increases the 200V supply current by the transformer ratio, so for a 100A load the primary must pull 50/200 x 100 = 25 A so the supply current is 100+25 = 125A IE 25kVA. I am also ok that the transformer rating need only be 50 x 100 VA = 5kVA. However, when bucking the voltage (down) I get a little confused!

Taking my example above I now have a 250V supply and need to supply a 200V 100A (different) load IE 20kVA. With a supply of 250V the current should be around 20K/250 = 80A.

The 50V winding will be wired to reduce the voltage to my load by 50V ? ok, but how does it reduce the current from 100A to 80A? I guess the
200V winding is now supplying some power to the load.

Working backwards I assume the 50V winding is passing only the 80A so in this example the transformer is 4kVA. Now 4K @ 200V is 20A so it looks like the 200V winding is providing 20A of the load. However I do feel I started with the answer and worked backwards!

Does anyone know the correct way to size a buck transformer??

Graham
 
277 is line to neutral and 230 is normally line to line

277 is line to neutral and 230 is normally line to line

Before you use a buck boost to go from 277 to 230, please make sure that your equipment that is using the 230 is needing line-neutral 230.

Since the 277 comes from a 480Y/277, the common point on the buck boost will be the neutral.
 
Correct way to size a buck boost

Correct way to size a buck boost

Use the on-line tools supplied by the manufactures.

www.us.squared.com/buckboost (great tool)

Or you can verify it by looking at the diagram after you connect the units and the amp ratings on the nameplate.

So example: if you have a 500VA - 120/240 to 12/24 being used for the buck boost
the maxium current at the boost voltage is equal to the maximum current on the name plate.
500VA - 24V = 20.8amps 12V = 41.6amps

So 230 x 20.8 = 4.78kVA
 
fordaputz said:
Before you use a buck boost to go from 277 to 230, please make sure that your equipment that is using the 230 is needing line-neutral 230.

Since the 277 comes from a 480Y/277, the common point on the buck boost will be the neutral.
Click to expand...
If you supply the equipment with 230 volts, why would it care if the 230 was from a line to line supply or line to neutral supply? It sees 230 volts either way.
 
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