Re: Homebrew Variacs? (fwd)

---------- Forwarded message ----------
Date: Wed, 15 Jul 1998 10:49:01 -0700
From: Jim Lux <James.P.Lux-at-jpl.nasa.gov>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Homebrew Variacs? (fwd)

Tesla List wrote:
> ---------- Forwarded message ----------
> Date: Tue, 14 Jul 1998 18:53:56 -0700
> From: Stefan Bishay <stefan00-at-gte-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Homebrew Variacs?
> Is it possible? I was thinking take a piece of 12" thickwall PVC pipe,
> about 6" long, wrap #10 enamel coated wire around it in the standard variac
> fashion, and  machine up some kind of rotor/brush assembly. Would this
> work? Any special considerations? What kind of current will 10 AWG wire
> handle at 240v in that application?
> Stefan Bishay - KC7SJC
> Seattle, WA
> ICQ 2141646

Without an iron core, it's not going to work very well as a transformer.
And, to work as a transformer, you probably want a closed magnetic
circuit. Now you begin to see why variacs are round, not only does it
give you a useful way to move the brushes (bearings, etc.) but it is the
cheapest way to close the magnetic core.

When variacs are converted to variable choke (ballast inductor) duty,
you cut the core to interrupt the magnetic circuit.So, if you just want
to make a variable iron core choke, why don't you just wind the coil on
a piece of straight pipe, and then move an iron core in and out of the
coil to vary the inductance. You could even hook the core on a lead
screw which would give you a knob to turn to move the core.  No hassling
with brushes, and you can leave the insulation on the wire, etc.

As to current carrying capacity of #10 AWG (they call this ampacity in
the Electrical Code), a useful number to work with is 700 circular
mils/Amp, #10 is 10400 circular mils, so 14 Amps.  If the wire has high
temp insulation (and you don't wind it on a low temp PVC form), you
could probably go to 500 cmil/amp, for 20 Amps, or even more. It is
mostly determined by how hot you want the wire to get.