RE: [TCML] fused Variac

[<sparks@xxxxxxxxxxxxxx>]

> Seems that a quick fuse would be a wrong application in this
> manner due
> to turn on surge...

IIRC, the fuse in in the output line.  It is extremely important that
the contact brush be protected, as excessive current through the brush
can easily burn the wire that the brush is in contact with. I use
fast-blow fuses in my Variacs.

> BTW what gauge of wire is used on the Variac
> core??  that will be the determining factor in amperage capacity.

Sort of - the real limit is the contact area of the brush.  Larger
Variacs have bigger brushes. The wire can handle a lot more current than
the brush can without overheating.

I also recall a query about the volts-per-turn ratio in these
transformers.  It is usually kept rather low.  The reason for this is
that as the contact brush is a make-before-break device. This means that
as the brush passes between the winding turns, it will short circuit
adjacent turns. If the voltage difference between turns is too great,
sparking and overheating will happen.

Most carbon based brushes will have a contact potential (the voltage
drop between the copper winding and the carbon brush) of between 0.8 and
1.0 volts.  Some grades of brushes going as high as 3.0 volts.

Note that the exact voltage drop will depend of the amount of pressure
between the brush and the copper winding.  The spring that presses the
brush against the winding is critical to the proper operation of the
brush.  A weak spring may cause brush and wire overheating even when the
transformer is operated well below the nameplate ratings.

As long as the voltage between turns is less than the contact potential,
relatively little short circuit current will flow between turns when the
brush bridges two turns of the winding.  For this reason, most variable
transformers tend to have windings with between 1.0 - 1.6 volts per
turn.

73,

Ralph   W5JGV - WD2XSH/7

_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla