Got two isolation transformers

[Cursitor Doom]

Hi all,

I've got two 240VAC/240VAC 500VA isolation transformers. Before I sell
one off I thought I'd best check whether there are any circumstances in
electronic repair when it's really handy to actually have two of them and/
or can they be hooked up together to provide 1KVA supply?
thanks,
cd.

 

[Sjouke Burry]

On 09.04.15 20:02, Cursitor Doom wrote:

Hi all,

I've got two 240VAC/240VAC 500VA isolation transformers. Before I sell
one off I thought I'd best check whether there are any circumstances in
electronic repair when it's really handy to actually have two of them and/
or can they be hooked up together to provide 1KVA supply?
thanks,
cd.

Maybe.
Maybe.
Beware.......

 

[Phil Allison]

Cursitor Doom wrote:

I've got two 240VAC/240VAC 500VA isolation transformers. Before I sell
one off I thought I'd best check whether there are any circumstances in
electronic repair when it's really handy to actually have two of them

** Where there are two technician at work?


> or can they be hooked up together to provide 1KVA supply?

** If the output voltages are *identical*, you can wire them in parallel and double the current rating.

To check this, with both trannys connected to the AC supply, link the secondary wingings at one end and measure the voltage across the remaining wires. It should be either 480V or close to 0. Reverse the connection if it is 480.

Close to 0 = about 0.5V max.

To safely parallel the pair, the primaries must be wired to the SAME AC plug - cos using two plugs means one will have exposed live pins.


.... Phil

 

[Cydrome Leader]

Phil Allison <pallison49@gmail.com> wrote:

Cursitor Doom wrote:


I've got two 240VAC/240VAC 500VA isolation transformers. Before I sell
one off I thought I'd best check whether there are any circumstances in
electronic repair when it's really handy to actually have two of them


** Where there are two technician at work?


or can they be hooked up together to provide 1KVA supply?

** If the output voltages are *identical*, you can wire them in parallel and double the current rating.

well, and the %Z must match too if you want to get real serious, otherwise one transformer may
still be doing more work as it takes more of the load.

 

[Cursitor Doom]

On Thu, 09 Apr 2015 20:15:04 -0700, Phil Allison wrote:

Cursitor Doom wrote:


I've got two 240VAC/240VAC 500VA isolation transformers. Before I sell
one off I thought I'd best check whether there are any circumstances in
electronic repair when it's really handy to actually have two of them


** Where there are two technician at work?


or can they be hooked up together to provide 1KVA supply?

** If the output voltages are *identical*, you can wire them in parallel
and double the current rating.

To check this, with both trannys connected to the AC supply, link the
secondary wingings at one end and measure the voltage across the
remaining wires. It should be either 480V or close to 0. Reverse the
connection if it is 480.

Close to 0 = about 0.5V max.

To safely parallel the pair, the primaries must be wired to the SAME AC
plug - cos using two plugs means one will have exposed live pins.


... Phil

Thanks, Phil. Best I hang on to the pair of 'em then, just in case...

 

[Phil Allison]

Cydrome Leader wrote:

or can they be hooked up together to provide 1KVA supply?

** If the output voltages are *identical*, you can wire them in parallel and double the current rating.

well, and the %Z must match too if you want to get real serious, otherwise one transformer may still be doing more work as it takes more of the load.

** Both transformers are stated to be rated at 500VA so the input and output voltages should match exactly at that load to comply with the 240:240 spec.

So the extra condition that the off load voltages also match nails the voltage regulation characteristic for resistive loads.

Ideally, the trannys should be of the same design, ie both e-core, c-core or both toroidal.



.... Phil

 

[mike]

On 4/10/2015 11:46 AM, Cydrome Leader wrote:

Phil Allison <pallison49@gmail.com> wrote:
Cursitor Doom wrote:


I've got two 240VAC/240VAC 500VA isolation transformers. Before I sell
one off I thought I'd best check whether there are any circumstances in
electronic repair when it's really handy to actually have two of them


** Where there are two technician at work?


or can they be hooked up together to provide 1KVA supply?

** If the output voltages are *identical*, you can wire them in parallel and double the current rating.

well, and the %Z must match too if you want to get real serious, otherwise one transformer may
still be doing more work as it takes more of the load.

Simple test.
Parallel the primaries.
Measure the input POWER (loss).
Parallel the secondaries.
Measure the input power again.
All that power is heat in the transformers. The increase
is due to voltage mismatch across the impedance.
Notice, I didn't say measure current. I said measure REAL power.
That should give you some idea of how long before
the smoke gets out.

Pass that test? Move on to load tests
and measure the currents in each secondary winding.
Easy way to do that is run the individual secondaries in opposite
directions thru your current probe. Result should be zero if the load
is shared equally.

 

[Phil Allison]

mike wrote:

Simple test.

** Really ???

Parallel the primaries.
Measure the input POWER (loss).
Parallel the secondaries.
Measure the input power again.
All that power is heat in the transformers. The increase
is due to voltage mismatch across the impedance.
Notice, I didn't say measure current. I said measure REAL power.
That should give you some idea of how long before
the smoke gets out.

Pass that test? Move on to load tests
and measure the currents in each secondary winding.
Easy way to do that is run the individual secondaries in opposite
directions thru your current probe. Result should be zero if the load
is shared equally.

** That process looks like the HARD way and needs test gear most do not have.



.... Phil

 

[Phil Allison]

mike wrote:

** That process looks like the HARD way and needs
test gear most do not have.



Well, the default test is to just parallel them and wait for smoke.

** No it isn't.

No smoke, you're good to go. Smoke, you're done.

** Absolutely idiotic "straw man" pile of bullshit.

I'd argue that there are a lot of reasons not to just hang
two transformers in parallel.

** Then you'd be arguing with yourself.

If I had two transformers with a nameplate that claimed
they were each 500W and isolation, I'd not even think
about paralleling them without making some measurements.

** Tell me, did you read my posts at all ??

Rhetorical question, the answer is obvious.

Advising someone who'd ask that kind of question to do it
is irresponsible

** It would be.

But has not happened here.


.... Phil

 

[mike]

On 4/10/2015 11:05 PM, Phil Allison wrote:

mike wrote:



Simple test.

** Really ???


Parallel the primaries.
Measure the input POWER (loss).
Parallel the secondaries.
Measure the input power again.
All that power is heat in the transformers. The increase
is due to voltage mismatch across the impedance.
Notice, I didn't say measure current. I said measure REAL power.
That should give you some idea of how long before
the smoke gets out.

Pass that test? Move on to load tests
and measure the currents in each secondary winding.
Easy way to do that is run the individual secondaries in opposite
directions thru your current probe. Result should be zero if the load
is shared equally.


** That process looks like the HARD way and needs test gear most do not have.

Well, the default test is to just parallel them and wait for smoke.
No smoke, you're good to go. Smoke, you're done.

... Phil

Your choice.
You can get some indication of the input power by putting
an incandescent light bulb in series with the input.

I'd argue that there are a lot of reasons not to just hang
two transformers in parallel.

If I had two transformers with a nameplate that claimed
they were each 500W and isolation, I'd not even think
about paralleling them without making some measurements.

Advising someone who'd ask that kind of question to do it
is irresponsible.

 

On Thursday, April 9, 2015 at 11:15:11 PM UTC-4, Phil Allison wrote in sci.electronics.repair:

Cursitor Doom wrote:

I've got two 240VAC/240VAC 500VA isolation transformers. Before I sell
one off I thought I'd best check whether there are any circumstances in
electronic repair when it's really handy to actually have two of them

** Where there are two technician at work?

Whoa, whoa wait a minute now, there's an idea. Isn't that what they teach you in electronics school? Always work with a helper, so if you're injured then, maybe at least SOME ONE will know to call for help.

 

[Cursitor Doom]

On Sat, 11 Apr 2015 07:51:21 -0700, mogulah wrote:

Whoa, whoa wait a minute now, there's an idea. Isn't that what they
teach you in electronics school? Always work with a helper, so if
you're injured then, maybe at least SOME ONE will know to call for help.

All too often not possible, I'm afraid.

 

[Ron D.]

Testing polarity is easy. Wire the secondaries in series. You will either get the sum or difference. Identical windings will be zero. Wire them to get the sum. You now know the phasing.

Two identical secondaries can be paralleled.

 

[Cursitor Doom]

On Sat, 11 Apr 2015 20:08:22 -0700, Ron D. wrote:

Testing polarity is easy. Wire the secondaries in series. You will
either get the sum or difference. Identical windings will be zero.
Wire them to get the sum. You now know the phasing.

Two identical secondaries can be paralleled.

You talk about primary and secondary but there are no such markings on
the trannies. I guess given the 1:1 windings ratio the sec/prim aspect
simply doesn't matter?

 

Actually in most cases it does matter. For one there will be loss, so that will be copensated. Also per a discussion over in SED a while back, there is soe other differennce between primary and secondary that has to do with the flux or something.

But it doesn't ALWAYS matter. One way to tell would be to try it both ways and see which way works better, like less current drain with no load or higher output voltage. Invariably, the wrong way won't work as well some how.

 

[Phil Allison]

Cursitor Doom wrote:

Two identical secondaries can be paralleled.

You talk about primary and secondary but there are no such markings on
the trannies. I guess given the 1:1 windings ratio the sec/prim aspect
simply doesn't matter?

** So called 1:1 transformers normally have non identical windings.

There is a primary and a secondary winding, with the secondary voltage somewhat higher than the primary under no load conditions, coming down to match primary with the rated load attached.

Wired back to front, the output voltage will be low by around 10 to 30% under load.


.... Phil

 

[Ron D.]

Typically the primary is the winding closest to the core for power applications. You can sometimes determine that physically.

 

[Cursitor Doom]

On Sun, 12 Apr 2015 17:53:08 -0700, Ron D. wrote:

Typically the primary is the winding closest to the core for power
applications. You can sometimes determine that physically.

OK, thanks for all the tips, gents. I shall carry out some tests!

 

[Cursitor Doom]

On Sun, 12 Apr 2015 17:47:21 -0700, Phil Allison wrote:

** So called 1:1 transformers normally have non identical windings.

There is a primary and a secondary winding, with the secondary voltage
somewhat higher than the primary under no load conditions, coming down
to match primary with the rated load attached.

Wired back to front, the output voltage will be low by around 10 to 30%
under load.

Well, well. You're right, Phil. Just like you said. thanks!

 

>"Well, well. You're right, Phil. Just like you said. thanks! "

And when you run them the right way they are considerably more efficient.