Re: Homebrew Variacs? (fwd)
---------- Forwarded message ----------
Date: Wed, 15 Jul 1998 19:30:48 -0500
Subject: Re: Homebrew Variacs? (fwd)
On Tue, 14 Jul 1998 22:17:48 -0600 (MDT) Tesla List <tesla-at-pupman-dot-com>
>---------- Forwarded message ----------
>Date: Tue, 14 Jul 1998 18:53:56 -0700
>From: Stefan Bishay <stefan00-at-gte-dot-net>
>Subject: Homebrew Variacs?
Anything is possible, but it's going to take a steel core of some kind
to contain the magnetic flux and operate properly at 60 Hz power. To
make a winding with enough wiper contacts for smooth operation would be
very difficult indeed, the brush assembly would have to commutate with
little arcing under a loaded condition. There would need to many
contacts with very little voltage between the steps.
There is hope though. I was faced with the exact situation 3 years
ago, no big variac and needed power control. A sacrifice was made in the
design, not to have continuous power control under load, but instead have
a "switched position" design.
It involves a steel core with #10 solid plastic wire wound in a toroid
design and a 28 position switch that is wired to various taps on the
winding. Think of it as a multiple tapped autotransformer. It works
great (still) and has handled over 10 kW of power for lengthy on times.
There is not enough time or space for full detail, but here are a few
design quirks. The steel core was wound on a circular form for a toroid
core. It was wound with 1000 feet of antenna guy wire! Not the normal
silicone steel strap it should be. Over the core was wound approx. 250
turns # 10 wire tapped every so often from beginning to end. The total
winding inductance is 8 mH
The switch was the hard part. It consisted of 29 contacts made from
3/8" bolt heads in a semicircular arc on a phenolic plate. The
wiper/brush was made from 3 starter motor brushes soldered together and
mounted to a spring loaded arm/shaft with suitable bearings.
This setup works and works well to this date. Power does have to be
shut off during switching. Actual switching of the power is handled by a
125 amp contactor.
Several years ago another toroid was built weighing in at 125 lb! It
uses 4 individual rolls of 1" x 3/32" steel strapping then bundling all 4
together. It is wound with about 200' of #14 3 conductor Romex (house
wiring) left in its plastic sheath. It too has multiple taps and no load
idle current is 2 amps -at- 245 volts. Both these designs can be used as
auto transformers or variable inductive ballasts.
These were also very cheap to build using salvaged wire.
>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
>fashion, and machine up some kind of rotor/brush assembly. Would this
>work? Any special considerations? What kind of current will 10 AWG
>handle at 240v in that application?
>Stefan Bishay - KC7SJC
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