Variac - how does it work?

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mbrooke

mbrooke

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Ok, my apologies for not being job or code related, but this has me wondering in a theory sense. How does a variac achieve an infinite voltage selection without burning up? :? I just can't explain it.


I know in large power transformers you can never have two taps directly in series because as soon as any turn or set of turns are shorted together they practically become a third set of windings where large amounts of current will circulate either over heating that section to the point of damage or tripping out differential protection because energy is being "dissipated" in the transformer (heat from the shorted section). Thus, when changing taps under load power transformers have either resistors or reactors in series with the two taps simultaneously being bridge. This same concept applies to pole mounted regulators where the unit must be in the zero (neutral) position before the linemen close the bypass, and the regulator itself always shifts taps with a reactor or resistor.


However, a variac does not have either and by the looks of it the carbon brush bridges multiple turns at times when the dial is rotated. How come the shorted section does not cause the unit to draw more current or burn up?


Here is a variac being taken apart (yes I know the green wiring inside is silly:eek:):


https://www.youtube.com/watch?v=M8cJrJyR87o
 
mbrooke said:
How come the shorted section does not cause the unit to draw more current or burn up?
Click to expand...
A variac is also a transformer-a variable auto transformer. But a small one. So the problems you describe for big ones also happen in it but on a minor scale and so their effect takes a longer time to show up.
 
Sahib said:
A variac is also a transformer-a variable auto transformer. But a small one. So the problems you describe for big ones also happen in it but on a minor scale and so their effect takes a longer time to show up.
Click to expand...

I don't know how these bad boys are wound, but Id think that 1 turn equaling 1 volt on 1.5 mm enamel copper would produce at least a dozen amps of current to flow. At minimum, the transformer would draw more power when the tap is on any position besides neutral or exactly on one winding. Yet input current appears to vary in synchronous steps regardless of position.


But, I could be wrong in that regard.
 
winnie said:
Take a look at figure 4 here, and the paragraphs below it.

http://sound.whsites.net/articles/variac.htm#s1

The explanation seems reasonable, but I have not verified it.

-Jon
Click to expand...
The article is accurate, and even if the bulk resistivity of graphite were not anisotropic proper brush shape would still allow the brush to span two or more turns and still work tolerably well.

The warning about fusing the secondary rather than the primary is a critical one. I have a secondhand variac that was abused by the previous owner (essentially supplying controlled current into a short circuit) and the bottom 10% windings were partially melted and burned by brush to wire arcing.
The one thing that is not shown well in the illustration is that the windings are somewhat flattened in the area the brush sweeps over so the inter-winding gap is a fairly narrow slot, like in a motor commutator. That makes the proportional change in voltage as the brush sweeps over two coils much more linear.
I had to sand down the winding surface to produce a unit that could handle less than half the original normal current in that part of its range and felt "gritty" over that part of the rotation.
Another thing that the article did not emphasize is the design of the brush holder as an effective heat sink to keep brush temperature down.
 
winnie said:
Take a look at figure 4 here, and the paragraphs below it.

http://sound.whsites.net/articles/variac.htm#s1

The explanation seems reasonable, but I have not verified it.

-Jon
Click to expand...

K8MHZ said:
Here is a decent article about Variacs

http://www.popsci.com/diy/article/2009-10/what-variac

Ham radio operators use them to gradually bring the voltage of old, tube type radios up gradually after they have been sitting a while.
Click to expand...

iwire said:
Ham radio guys are boring nerds. ;)

Now on the other hand guitar players like Eddie Van Halen they also use variacs to save their tube amps or change how the amp behaves.

http://ultimateclassicrock.com/eddie-van-halen-reveals-his-biggest-lie/
Click to expand...




Ok, so digesting this as I read. Basically, if I understand correctly: the carbon brush itself is a resistor network? Because each coil has a volt or less potential, the current that passes through the carbon brush is small, but the current between the whole of the brush can pass in large amounts because the voltage is much greater between all those windings in series and the connected load?
 
mbrooke said:
Ok, my apologies for not being job or code related, but this has me wondering in a theory sense. How does a variac achieve an infinite voltage selection without burning up? :? I just can't explain it.


I know in large power transformers you can never have two taps directly in series because as soon as any turn or set of turns are shorted together they practically become a third set of windings where large amounts of current will circulate either over heating that section to the point of damage or tripping out differential protection because energy is being "dissipated" in the transformer (heat from the shorted section). Thus, when changing taps under load power transformers have either resistors or reactors in series with the two taps simultaneously being bridge. This same concept applies to pole mounted regulators where the unit must be in the zero (neutral) position before the linemen close the bypass, and the regulator itself always shifts taps with a reactor or resistor.


However, a variac does not have either and by the looks of it the carbon brush bridges multiple turns at times when the dial is rotated. How come the shorted section does not cause the unit to draw more current or burn up?


Here is a variac being taken apart (yes I know the green wiring inside is silly:eek:):


https://www.youtube.com/watch?v=M8cJrJyR87o
Click to expand...
First, the difference between working of a variac and transformer/regulator on load tap changer is due to change in voltage level. Second, the brush of variac shorts same number of turns at any position and so input current changes smoothly instead of in steps.
 
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