Transformer Question

I was looking at the commercial isolation transformers and they are very
costly, so I decided to build my own. After all, all they are is a
transformer with a power cord on the primary and an outlet (and fuse) on
the secondary. And I already have an enclosure to put it in. 

I'm looking at a bare transformer to use as an 120v isolation
Transformer. (120v in, 120v out). The transformer primary is 480 / 240.
The secondary is 240 / 120. 

This is for single phase 60 cycle AC. (U.S. power).

Will it work if I connect the 240 lugs on the primary to 120 volts, and
use the 240 lugs on the secondary to obtain 120 volts. 
Electrically, this makes sense, but I am not 100% sure, so I thought I'd
ask.

Also, this Transformer is rated at 750 va.

Using the calculator chart on
http://www.rapidtables.com/calc/electric/va-to-amps-calculator.htm
750 va should give me 6.25 A output. (Which should be enough amperage
for anything I need to test on my bench).

However, since this transformer was intended to be used at 240 / 480 on
the primary, will it still give me 750 va (6.25 A) on the secondary if I
run it on 120v?

Thanks

> 
> Will it work if I connect the 240 lugs on the primary to 120 volts, and
> use the 240 lugs on the secondary to obtain 120 volts. 
>
> Also, this Transformer is rated at 750 va.
> 
>
yes it should work but the VA rating will be 1/2.
The windings are rated for current so you can pull the rated current.
But since you are at 1/2 the rated voltage, the VA will be 1/2

m

On Thu, 02 Feb 2017 13:43:39 -0600, boomer#6877250 wrote:

> I was looking at the commercial isolation transformers and they are very
> costly, so I decided to build my own. After all, all they are is a
> transformer with a power cord on the primary and an outlet (and fuse) on
> the secondary. And I already have an enclosure to put it in.
> 
> I'm looking at a bare transformer to use as an 120v isolation
> Transformer. (120v in, 120v out). The transformer primary is 480 / 240.
> The secondary is 240 / 120.
> 
> This is for single phase 60 cycle AC. (U.S. power).
> 
> Will it work if I connect the 240 lugs on the primary to 120 volts, and
> use the 240 lugs on the secondary to obtain 120 volts.
> Electrically, this makes sense, but I am not 100% sure, so I thought I'd
> ask.
> 
> Also, this Transformer is rated at 750 va.
> 
> Using the calculator chart on
> http://www.rapidtables.com/calc/electric/va-to-amps-calculator.htm 750
> va should give me 6.25 A output. (Which should be enough amperage for
> anything I need to test on my bench).
> 
> However, since this transformer was intended to be used at 240 / 480 on
> the primary, will it still give me 750 va (6.25 A) on the secondary if I
> run it on 120v?
> 
> Thanks

The VA rating on a transformer is derived from the maximum voltage times 
the maximum current.

Running it on half the voltage won't change the maximum current -- so the 
actual "VA" that you'll achieve is half of the transformer's rating.

Other than that, and possibly more voltage drop than you'd really like, 
it should work.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!

On Thu, 02 Feb 2017 13:43:39 -0600, boomer#6877250 wrote:


> 
> Will it work if I connect the 240 lugs on the primary to 120 volts, and
> use the 240 lugs on the secondary to obtain 120 volts.
> Electrically, this makes sense, but I am not 100% sure, so I thought I'd
> ask.
>

You do need to keep the tally of the losses.  
Off the cuff, increase the secondary turn count by at least 3-5%.
If you plan to run the transformer at the max rate, then by 7-10 %.


> Also, this Transformer is rated at 750 va.
> 

Basically, the VA of the transformer is the max voltage 
you possibly have at the secondary for any type of load, 
times the max current you can possibly have in the secondary 
on a permanent basis for any type of load.  

On Thursday, February 2, 2017 at 11:47:09 AM UTC-8, boomer#...@none.com wrote:
> I was looking at the commercial isolation transformers and they are very
> costly, so I decided to build my own. 

Lots of 'commercial isolation transformers' are intended for RF blocking, and have
fancy shielding specifications and pretty cases.   If you just want a transformer, have you checked
out your local electrical suppliers (Graybar Electric in my neighborhood)? 
They don't have online catalogs, but they can get you just about anything, in
a plain steel box.

> this transformer was intended to be used at 240 / 480 on 
>the primary, will it still give me 750 va (6.25 A) on the secondary ...

That would work, but it's a bigger transformer than you need.

Tim Wescott wrote:

> Other than that, and possibly more voltage drop than you'd really like,
> it should work.

In the case of such a big transformer the drop may be negligible,
but if you use two potted 380V->12V 1.2W Hahn transformers with
connected secondaries and power the combo from 230V, the transformed
voltage would be in the 90V ballpark.

	Best regards, Piotr

On Friday, 3 February 2017 21:58:31 UTC, Judges1318  wrote:

> Basically, the VA of the transformer is the max voltage 
> you possibly have at the secondary for any type of load, 
> times the max current you can possibly have in the secondary 
> on a permanent basis for any type of load.

I always thought it was the nominal V & I. And the rating is on the basis of a certain level of MTTF, one can certainly exceed it when less MTTF is acceptable.


NT

On Thursday, February 2, 2017 at 2:47:09 PM UTC-5, boomer#...@none.com wrote:
> I was looking at the commercial isolation transformers and they are very
> costly, so I decided to build my own. 
> 
> Thanks

If you really  want to spend very little, get a couple of microwave ovens and use two transformers to make one isolation transformer.

                                  Dan

On Friday, February 3, 2017 at 4:59:37 PM UTC-8, dca...@krl.org wrote:
> On Thursday, February 2, 2017 at 2:47:09 PM UTC-5, boomer#...@none.com wrote:
> > I was looking at the commercial isolation transformers and they are very
> > costly, so I decided to build my own. 

> If you really  want to spend very little, get a couple of microwave ovens and use two transformers to make one isolation transformer.

That might be disappointing; microwave transformers never operate without a full load, 
so the possibility exists that a light load on the composite would saturate one, and get very lossy.

Magnetic parts of a transformer have to be sized according to the MINIMUM load current.

dca...@krl.org wrote:

> 
> If you really  want to spend very little, get a couple of microwave ovens
> and use two transformers to make one isolation transformer.
> 
>
   
** Completely crazy idea that cannot work.                    

Too many reasons to bother going into here, but it is both stupid and dangerous.



.....   Phil