[Amps] HV transformer and Variac

[Manfred Mornhinweg]

About the carbon tap of a VARIAC: I also have wondered about the loss caused by 
the nearly shorted turn or turns. And I have a slightly more elaborate theory 
about what happens there:

It happens that carbon (or maybe the metal-carbon transition) has a nonlinear 
conduction curve. When measuring around in electric motors using carbon brushes, 
I hve found that typically there is about 1 to 2 volt of drop between the brush 
connection and the rotor of the motor, no more and no less, pretty much 
independent of the current density. So it seems that the carbon brush acts to 
change its contact resistance to roughly stabilize the voltage drop to 1-2V.

If we now consider the Variac's carbon wiper, probably the voltage of the single 
shorted turn (there doesn't need to be be more than 1 turn shorted at any time) 
falls below this critical "carbon voltage", so that the additional current that 
flows through that one turn, and the wiper, is pretty low, and the loss ends up 
being even lower than the few watts mentioned in this thread. Probably less than 
one watt!

In my Variac, a cheap Chinese 500W one that has been working well for at least 
fifteen years now, the wiper stays almost complete cold, and it's not large. It 
  always contacts one or two wires, never three, so it "shorts" just one turn, 
and the loss in the wiper (at no load) seems to be way below 1W, judging from 
the little warming.

Can anyone confirm whether such a carbon brush has sort of a barrier voltage, 
below which it almost stops conducting? Or is my theory totally off?

Manfred



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