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The wiper in a variac

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?

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RoRo
Reply to
Robert Roland
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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
Reply to
Phil Hobbs

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

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

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.

Reply to
George Herold

That's been my experience too. Spans three or four turns with a graphite wiper.

Reply to
default

Check out:

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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.

Reply to
default

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.

Reply to
whit3rd

That is interesting. Thanks.

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RoRo
Reply to
Robert Roland

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