Isolation transformer question

[Fester Bestertester]

I have 2 isolation transformers, 115:115 and 115:230, same make, both 1.0KVA.
These will be powered via Variac.

I’m in the USA so will use mostly 115v output but occasionally need 230v.

When I need 230, obviously I’ll use the 230v trans. But when I need only
115v is there any advantage in using the 115:115 transformer rather than the
115:230 with Variac at half turn?

Thanks.

 

[Tom Biasi]

On 7/18/2016 5:46 PM, Fester Bestertester wrote:

I have 2 isolation transformers, 115:115 and 115:230, same make, both 1.0KVA.
These will be powered via Variac.

I’m in the USA so will use mostly 115v output but occasionally need 230v.

When I need 230, obviously I’ll use the 230v trans. But when I need only
115v is there any advantage in using the 115:115 transformer rather than the
115:230 with Variac at half turn?

Thanks.

Other than small efficiency issues I can't think of any.

 

[Phil Allison]

Fester Bestertester wrote:

I have 2 isolation transformers, 115:115 and 115:230, same make, both 1.0KVA.
These will be powered via Variac.

I’m in the USA so will use mostly 115v output but occasionally need 230v.

When I need 230, obviously I’ll use the 230v trans. But when I need only
115v is there any advantage in using the 115:115 transformer rather than the
115:230 with Variac at half turn?

** Yes - you can get double power from the 115:115 one.

The 115V tranny is rated at 8.7 amps output while the 230V one is rated at half that.

The current rating does NOT go up at reduced input voltage.


..... Phil

 

[Tim Wescott]

On Mon, 18 Jul 2016 14:46:32 -0700, Fester Bestertester wrote:

I have 2 isolation transformers, 115:115 and 115:230, same make, both
1.0KVA.
These will be powered via Variac.

I’m in the USA so will use mostly 115v output but occasionally need
230v.

When I need 230, obviously I’ll use the 230v trans. But when I need only
115v is there any advantage in using the 115:115 transformer rather than
the 115:230 with Variac at half turn?

1.0KVA means the _maximum_ primary current times the _maximum_ primary
voltage equals 1KVA. To get 1KVA at 115V from your 1:2 transformer,
you'll run things at half the rated voltage and twice the rated current.

IF you're running no more than 400KVA or so then it's probably no
problem. If I really felt I needed to I might try it up to 600KVA or so,
but only if I monitored the temperature of the bits to make sure that
nothing's getting too hot.

--

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

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

 

>"Other than small efficiency issues I can't think of any. "

Except making more heat and burning up the wiper. Ever have a variac apart ? Once I saw the build it was "I am not using that thing unless I really need to".

Hmmm, I wonder now, are those things supposed to be cleaned every once in a while like guns n shit ?

 

[John Larkin]

On Mon, 18 Jul 2016 14:46:32 -0700, Fester Bestertester
<avril@whitehouse.usa> wrote:

I have 2 isolation transformers, 115:115 and 115:230, same make, both 1.0KVA.
These will be powered via Variac.

I’m in the USA so will use mostly 115v output but occasionally need 230v.

When I need 230, obviously I’ll use the 230v trans. But when I need only
115v is there any advantage in using the 115:115 transformer rather than the
115:230 with Variac at half turn?

Thanks.

The output impedance will be much lower if the variac is near 100% and
the transformer doesn't step up.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Fester Bestertester]

On 19 Jul 2016, jurb6006@gmail.com wrote
(in article<3e916f42-e3fb-4368-b754-1a82ae34e676@googlegroups.com&gt:

"Other than small efficiency issues I can't think of any. "

Except making more heat and burning up the wiper. Ever have a variac apart ?
Once I saw the build it was "I am not using that thing unless I really need
to".

Hmmm, I wonder now, are those things supposed to be cleaned every once in a
while like guns n shit ?

I did clean both brush tracks. (Many don’t realize that theres two brushes:
one rides on the coil; the other connects the “disc” that holds the main
brush to the connector terminals.) Much better continuity linearity with
those tracks cleaned.

 

[Fester Bestertester]

On 19 Jul 2016, John Larkin wrote:

The output impedance will be much lower if the variac is near 100% and
the transformer doesn't step up.

How would this display in everyday use? More output current? Or...?

 

[John Larkin]

On Wed, 20 Jul 2016 10:04:04 -0700, Fester Bestertester
<avril@whitehouse.usa> wrote:

On 19 Jul 2016, John Larkin wrote:

The output impedance will be much lower if the variac is near 100% and
the transformer doesn't step up.

How would this display in everyday use? More output current? Or...?

Maybe more current, but certainly less voltage droop with increasing
load current. If you are willing to crank the variac to tune the
voltage under load, that may not matter.

Variacs have least loss, and lowest impedance, near 100% output
voltage.

If you use a 2:1 step-up transformer to make 120 volts at 1 amp, the
variac has to drive the primary at 60 volts and 2 amps.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Cydrome Leader]

jurb6006@gmail.com wrote:

"Other than small efficiency issues I can't think of any. "

Except making more heat and burning up the wiper. Ever have a variac apart ? Once I saw the build it was "I am not using that thing unless I really need to".

Hmmm, I wonder now, are those things supposed to be cleaned every once in a while like guns n shit ?

I had a lovely variac (really more like a "Staco Energy") fire. Evercise
those wipers by turning the knob from time to time, as they're sort of
self cleaning, but even that's not enough some times like in my case.

I smelled something awful, entered the room full of horrible phenolic
smoke and saw the thing on fire. Half flames, half glowing wire and brush
assembly.

 

"If you use a 2:1 step-up transformer to make 120 volts at 1 amp, the
variac has to drive the primary at 60 volts and 2 amps. "

Now that someone had boiled this thread back to the top I see you fucked up there buddy. That would be 1:2 not 2:1.

I am sure you know that and I am not accusing you of stupidity, it is just one of those errors. And there is no edit button. I thought a correction was warranted.

Anyway, if possible the variac should run off the transformer, not the other way around. In fact I would consider using the 1:1 in series with a variac to get to 220 provided you don't really need the isolation and you don't have to go below 110.

I am pretty damn good at working on hot stuff. Walk into wirk ad work on a hot chassis TV all wet from the rain outside and on a cement floor. It takes some care, and knowing, and you can't expect everyone to understand it well enough and think it out well enough to be safe working on hot. In fact electricians who can do it are highly sought.

So err on the side of safety and don't zap yourself. (not you, you're probably alright when it comes to this shit) But people need to be able to identify the parts of what they take apart. See that AC cord ? Where the wires from it goes is the dangerous part when it comes to a grounding issue. And when you see those big fat wires that feel all rubbery, that is high voltage.. Also, if it has any tubes in it, just keep one hand in your pocket, and even that doesn't work. Last year I made a mistake, basically because I hadn't worked on tube equipment for so long. Even the one hand thing didn't work, it just burned a hole in my one hand.

You can work without isolation, but IIRC OSHA requires the area to be cordoned off and danger signs planted around it. That because there are times in industrial settings where you have no choice. They also still sell wood ladders...

There is alot of equipment to which this doesn't apply - but the dim bulb tester is more useful for some things. Like audio amps and TVs for example.