[Amps] HV transformer and Variac

[Doug Ronald]

The electrical resistivity of carbon brushes is highly anisotropic, due to
the orientation of the graphite particles during compression. Electrical
resistivity anisotropy (cross-plane/in-plane) can vary from a low of 6 for
KS primary synthetic graphite up around 14 for natural graphite (BE grade).
The variable autotransformer has its brush oriented so the axial conduction
is the lower resistance orientation. The contact resistance is also
non-linear, with resistance decreasing at higher current densities. From
what I read, this is a due to impurities in the brush material itself,
although there may be other causes I didn't investigate...

 

-Doug W6DSR


 

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Sunday, July 26, 2015 2:17 PM
To: amps at contesting.com
Subject: Re: [Amps] HV transformer and Variac

 

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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