The wiper in a variac

R

Robert Roland

Guest
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?
--
RoRo
 
On 05/09/18 13:37, Robert Roland wrote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?
Click to expand...
I'm not a big variac guy, but in the couple I've seen the wiper is made
of graphite, and spans a couple of turns. The series resistance is
small enough not to matter much for the main output, but large enough
not to cause serious loss due to the shorted turn due to the low voltage
drop between turns.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Wed, 9 May 2018 14:47:53 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 05/09/18 13:37, Robert Roland wrote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?

I'm not a big variac guy, but in the couple I've seen the wiper is made
of graphite, and spans a couple of turns. The series resistance is
small enough not to matter much for the main output, but large enough
not to cause serious loss due to the shorted turn due to the low voltage
drop between turns.

Cheers

Phil Hobbs
Click to expand...

I think there's some anisotropic conduction thing going on too.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics
 
On Wednesday, May 9, 2018 at 1:37:19 PM UTC-4, Robert Roland wrote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?
--
RoRo
Click to expand...

The small one I have here, (1.75 A) has a wiper that spans at least
three turns at a time, but there's ~1000 (?) turns on the thing.
(Staco type 171) I couldn't find a good image.

George H.
 
On Thu, 10 May 2018 05:58:46 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Wednesday, May 9, 2018 at 1:37:19 PM UTC-4, Robert Roland wrote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?
--
RoRo

The small one I have here, (1.75 A) has a wiper that spans at least
three turns at a time, but there's ~1000 (?) turns on the thing.
(Staco type 171) I couldn't find a good image.

George H.
Click to expand...

That's been my experience too. Spans three or four turns with a
graphite wiper.
 
On Wed, 09 May 2018 19:37:14 +0200, Robert Roland <fake@ddress.no>
wrote:

I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.

If the wiper is too wide, it will bridge two contact points, causing
one turn of the transformer to be directly shorted. Although it is not
a lot of voltage, it is also not a long piece of copper, so the short
circuit current, I expect, would be considerable.

Obviously, getting the wiper the perfect with, and also the wire
spacing equally perfect is not a practical approach.

How to they do it in practice? Is there some kind of snap action that
makes the wiper click from one turn to the next?
Click to expand...

Check out:
https://www.beckwithelectric.com/docs/tech-papers/underload.pdf

This is a description of a tap-changing high power distribution
regulator. Basically a Variac with a servo motor doing the switching.
They cover shorted turns when the device is between taps.
 
On Wednesday, May 9, 2018 at 10:37:19 AM UTC-7, Robert Roland wrote:
I have been thinking about the way a variac works. One thing that
puzzles me, is the wiper design:

If the wiper is too narrow, the output voltage will cut out when the
wiper is between two turns of the coil.
Click to expand...

Yep, the wiper gets hot, too. Sometimes that kills the
variac... usually not, though. It's just not energy
efficient because of the shorted turn, it might be amusing
to look at a FLIR image of a variac in operation.
 
On Fri, 11 May 2018 04:46:44 -0400, default <default@defaulter.net>
wrote:

Check out:
https://www.beckwithelectric.com/docs/tech-papers/underload.pdf
Click to expand...

That is interesting. Thanks.
--
RoRo
 
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