Forums

The wiper in a variac

Started by Robert Roland May 9, 2018
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? >
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
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
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.
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?
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.
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
That is interesting. Thanks. -- RoRo