Getting the right buck boost transformer

Shaun @ Tru6 · 07-29-2015, 10:30 AM

I've got 208V single phase in the building. I'm getting a 230V compressor. Specs are 5hp, 28 amp draw, 60 amp breaker needed.

Have found several buck boost transformers that seem to be the same with the exception of the KVA rating, and a big change in price. For example, a .25 KVA is half the price of a .75 KVA transformer.

Which one do I need? And how do you calculate what I need?

1990C4S · 07-29-2015, 12:36 PM

Motor is 230 single phase, yes?

I suspect your machine will run fine on 208...and draw about 33 amps full load. The 60 amp breaker will still work.

But otherwise you need a 7.5 KVA step up transformer.

V * I/1000 = KVA

rick-l · 07-30-2015, 07:16 AM

Have you tried this?
Buck and Boost Transformer Calculator - Schneider Electric United States

Shaun @ Tru6 · 07-30-2015, 11:29 AM

Thanks C4S and Rick. This is really frustrating. As above, I found a used GE .75 KVA B-B transformer that the guy used it for exactly what I want to do.

GE 9T51B0129 Buck Boost Transformer .750kVA, 1-Phase 120/240V 16/32V

Have called around to different electrical supply houses and used Rick's calculator and am getting conflicting info.

I just talked with the resident B-B expert at Platt and we worked out the math and he said I needed this unit, a .75KVA transformer:

https://www.platt.com/platt-electric-supply/Buck-Boost-Transformers-120-X-240-12-24/Acme/T-1-81052/product.aspx?zpid=1332

When I told him I was calculating out to 7.5, he said look at the catalog page, here it is.

Thoughts?

1990C4S · 07-30-2015, 11:47 AM

That's 7.2 kVA, which is about what I said...

I still think you probably don't need it.

Shaun @ Tru6 · 07-30-2015, 12:25 PM

Agreed, but what is puzzling is the name of these transformers. Note the 750VA in the title. They are all like that.

Acme T-1-81052 750VA, 1P, 120x240V, 12/24, Buck-Boost Transformer

1990C4S · 12:45 PM

The Craigslist one you posted is too small from what I can see....

You need a T-1-81052. The Platt web page part # referencing 750VA is wrong, the .pdf it links to, and the image you posted above, are right.

Shaun @ Tru6 · 07-30-2015, 01:13 PM

this is what is killing me. If you google that product number, it comes as 750VA or .75KVA everywhere.

But here's an interesting page. note the Boost Voltage and the Catalog Description and then way down below, the Power Rating is .75KVA and then the VA rating is 750.

This is why I think the Graig's List GE above is usable.

ACME T181052 Acme, Buck Boost Transformer, 12/24V 750va - Crescent Electric Supply Company

Man I feel like I'm beating a dead horse here, SORRY.

1990C4S · 07-31-2015, 04:31 AM

As a boost transformer the one on Craigs will give you 230v 23 Amps that's the 5KVA number. That is slightly too small for you. Put a fan on it, don't run it continuously and it might work.

From the web:

Ignore the VA ratings in the part number and look at the single phase current ratings.

And (I repeat) I still think it WILL work on 208 volts....which is what you should try. Worst case you slip a transformer in-between if it draws too much current.

Shaun @ Tru6 · 11:53 AM

OK, thank you. I just don't want to burn out the motor on a brand new compressor that is supposed to be the last compressor I will ever buy.

Just ordered the ACME T181052, it will be here on Monday.

1990C4S · 07-31-2015, 12:04 PM

Coward!

You won't fry the motor, but you might trip the breaker. I would use it 'as' is and worst case change the pulley ratio slightly...

But you rich guys in Boston have money to burn.

Good luck with the compressor.

Shaun @ Tru6 · 07-31-2015, 03:15 PM

My friends are my only riches, each is a gem of a their own kind. One of which is coming over on Sunday to look at my two available slots in the breaker box already wired for 208V machinery, one a silk screen dryer (80A) that, to my eye, leaves the panel as single phase but is 3 phase at the machine. Other (30A) leaves the panel and ends at a standard clothing dryer outlet for the exposure unit. I can get a 60A breaker easily enough if I don't use the 80A. That one I want to use for a powder coating oven I'll build next week taking the control panel and heating elements out of the dryer and using them for the oven.

Both of these were professionally installed. When I called around to 5 places, no one wanted to wire in a buck-boost transformer, 2 didn't know what they are.

I may be coward on this one but remember, I did change a live 100A breaker at the box outside connected to the pole.

Thanks for your help, compressor was built yesterday, shipped today.

1990C4S · 08-01-2015, 07:20 AM

Coward and dumb ain't the same thing.

You should have said 'transformer' and left off the rest. Of course if they don't know what that is you likely don't want them working for you.

Post pics.

Of the fire...

billybek · billybek's Garage

Most 230 vac motors will be just fine on 208 VAC.

There should be a data plaque on the motor that will tell you the allowable min max voltages that is permissible.

As a percentage the voltage difference is minimal. The difference in amp draw at full load should be less than 3 amps.

Who is the compressor manufacturer?

Shaun @ Tru6 · 08-02-2015, 11:48 AM

Hey Bill!

It's a Dewalt, 5hp, It was shipped on Friday, hope to get it tomorrow or Tuesday, then get it up 2 flights of stairs. Can't wait for for that.

Aircompressorsdirect.com (I bought it from) lists it at 230.

Home Depot, Northern Tool, Amazon...everyone else lists it at 240V. Model numbers cross check and it's the only 80 gallon compressor Dewalt makes, so I'm assuming it's 240V.

When I google running 230V or 240V on 208V, I get a lot of ominous threads:

230v motor on 208v? - The Garage Journal Board

Running 230V compressor on 208V system used to power servo motors - Electric motors, generators & controls engineering - Eng-Tips

208V feeding a 240V motor? - ECN Electrical Forums

billybek · billybek's Garage

Quote:

Originally Posted by Shaun 84 Targa

Hey Bill!

It's a Dewalt, 5hp, It was shipped on Friday, hope to get it tomorrow or Tuesday, then get it up 2 flights of stairs. Can't wait for for that.

Aircompressorsdirect.com (I bought it from) lists it at 230.

Home Depot, Northern Tool, Amazon...everyone else lists it at 240V. Model numbers cross check and it's the only 80 gallon compressor Dewalt makes, so I'm assuming it's 240V.

When I google running 230V or 240V on 208V, I get a lot of ominous threads:

230v motor on 208v? - The Garage Journal Board

Running 230V compressor on 208V system used to power servo motors - Electric motors, generators & controls engineering - Eng-Tips

208V feeding a 240V motor? - ECN Electrical Forums

Hi Shaun,

You most likely will be putting the compressor through its paces so it might be good idea to nail the voltage but have a look at the motor tag when it arrives. Many of the 230 vac single phase motors can handle that bit of voltage difference and the permissible range will be on the tag.

Back before that pesky occupational heath and safety legislation came into play one of my employers dropped off a multi voltage pressure washer to me and instructed me to run it off 460 vac line to ground. 277 vac!

I was dumb enough to do that at the time but I was also very careful not to be standing in puddles of water while using it!

Shaun @ Tru6 · 08-04-2015, 04:05 PM

Call 911 tomorrow if you don't hear from me by the afternoon.

1990C4S · 08-05-2015, 04:15 AM

Dibs on any inventory.

schwarz633 · schwarz633's Garage

Connect like this and I think you'll be good. I probably would have chosen the 1KVA (T111683) just to have a little headroom.

Shaun @ Tru6 · 08-05-2015, 06:28 AM

Thanks! Compressor is getting here tomorrow. ACD.com says 230V and 28 amp draw. Others say 240V and 22 amp. I don't know if it's possible if ACD.com has Dewalt make these to 230V, which makes no sense to me, but they did they these were built to order at Dewalt. Anyway, I've got enough on my plate (like tumbling and plating) to wait until it gets here and read the plate and documentation and go from there.