Buck Boost Xfrmr

Sparky Joe

Anyone sized a buck boost transformer lately? We had one last Friday, the machine(a t-shirt heater) draws 13.5 amps at 230V according to nameplate, building has 208V. I tried to remember from school and we looked at an Ugly's, but finally resorted to a chart for a .75 kVA.

Anyone know what math is involved in figuring this out?

mdshunk

I prefer Acme transformer as my brand for buck-boost transformers. They have an excellent calculator/product selector on their site: http://www.acmepowerdist.com/product...ide/index.aspx

Sparky Joe

Wow, those are all 8 to 10 times larger than what we bought, hope it works, and furthermore hoped we saved some money in buying what we did

And the one we bought was 23 pounds not 110

mdshunk

For single phase, you'll be using one very heavy transformer. For 3 phase, you'd use either 2 or 3 very light transformers. I calculate you'd need about a 5 KVA. That's got a little weight to it.

The formula for single phase is KVA=(output volts * secondary amps)/1000

In your case, 3.2, and the next common size is 5.


The 3 phase calc is necessarily much more involved.

Sparky Joe

A 5 kVA was what I figured while looking at them in the supply house too, but now figuring it's a 30 volt difference and would be connected to a 20amp breaker, I get .6 kVA and the next one to match was a .75. Just my guess though

Maybe I'll go post it in Mike Holt's seeing as neither my Dad or my teacher are responding to e-mails

Sparky Joe

I know what kVA is, just a fancy way of saying watts, or kilo watts rather

That's what I was thinking(as noted above) you only figure the kVA of the boost, but by that logic would it be a negative kVA for buck?

Sparky Joe

Now MD or Pete(the code Guru) would we now also need some sort of over current device after the transformer? something to do with the tap rules? Whereas the thing was originally cord and plug connected.

mdshunk

The OCPD for the primary of a .75 KVA buck-boost will be 20 amps. That will work to protect the secondary as well. The primary and the secondary are interconnected on a buck boost.

Sparky Joe

Here's what I got from Holt's forum. seemed to look right to me;

Boosting 208 using a 240:32V buck boost.
Have 208V available so boost is 208/240*32 = 27.73V
( 240:32 => 208:27.7 )

Total boost is 208 + 27.73 = 235.73V

Total load is 13.5A x 230V = 3105VA (given)

Current in secondary coil is 3105/235.73 = 13.17A.

Buck/Boost VA = 13.17 * 27.73 = 365.2VA

Current in primary coil is 365.2/208 = 1.76A

The currents add in this case for primary 13.17 + 1.76 = 14.93A

14.93A x 208V = 3105VA (input VA = load VA)

The kVA of the transformer is where it says 365.2VA (which is 0.3652 kVA for those slow on their math)