Re: [TCML] Pushing a Variac beyond rating

[Yurtle Turtle <yurtle_t@xxxxxxxxx>]

I^2R; not I^R.



----- Original Message ----
From: Ralph W5JGV <sparks@xxxxxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Fri, May 20, 2011 7:05:18 PM
Subject: RE: [TCML] Pushing a Variac beyond rating

> I'm wondering how Variacs fail and if it's Ok to overload them for short
> periods.  I'm also wondering if keeping it in transformer oil would help.


In my experience, the weak point on a Variac is the brush contact assembly.
Short time overloads *may* work out, due to thermal inertia.

Since all of the output current of a Variac must flow through the small
contact area between the carbon brush and its point of contact on one of the
turns of the winding, a serious overload will usually burn or melt the wire
under the brush.  Once this happens, it is difficult, if not impossible, to
repair.

To prevent arcing from occurring at the brush-wire contact point during
normal operation, the brush is designed to be wide enough to momentarily
bridge (short) two adjacent turns of the transformer winding.  Normally,
this does not cause a problem, as the brush material (carbon) is designed to
have enough resistance to prevent damaging current to flow in the shorted
turns.  This necessary extra resistance causes some heating of the
brush-wire contact area during normal operation. An overload can quickly
cause the heat generated by the extra current (remember I^R ?) to overheat
the brush and the wire, going even so far as to cause the brush to glow
red-hot and melt the wire.

Running the Variac under oil would probably cause problems, since the oil
would be an insulator between the brush and the wire, probably causing even
more heat to be generated at the point of contact.

Get a bigger Variac.

73,

Ralph  W5JGV - WD2XSH/7

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

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