Re: Variac vs. Fan speed control

Subject:  Re: Variac vs. Fan speed control
  Date:   Sun, 20 Apr 1997 02:25:15 +0500
  From:   "Alfred A. Skrocki" <alfred.skrocki-at-cybernetworking-dot-com>
    To:   Tesla List <tesla-at-pupman-dot-com>

On Wed, 16 Apr 1997 00:09:13 -0500 ADatesman-at-aol-dot-com wrote;

> I received some advice recently which didn't work at first try, but I'm
> curious what the list wisdom has to offer.  I was shopping for a Variac
> (don't know where to find a new one) when I was told to use a fan speed
> control instead - it would do the same thing.  The one I bought, however,
> I burned two neons with - when I finally thought to check it, it turns out
> that my dimmer-type switch moves immediately from "OFF" to "120 Volts".
> So it isn't surprising that I burned out the two Xformers. Obviously the
> switch I bought doesn't control the fan speed by  varying the voltage input
> to the fan.  1) How does it work?

The dimmer you have Aaron is a triac phase shift control dimmer and 
it is for NON-INDUCTIVE LOADS ONLY!!! It works by cutting off power 
at different points along the 60 cycle sine wave feeding the light.
This type of dimmer should NEVER be used with any inductive loads it
destroy transformers and AC induction motors! There is no solid 
state substitutes for a variac that I am aware of! The only way I can 
think of varying the output of a neon sign transformer would be with 
a saturable reactor. This looks like a transformer, but what you do is 
apply an adjustable DC current to what would be the primary, and you 
use the secondary as a choke in series with the neon transformer. What 
happens is this; with no DC applied to the primary the transformer 
operates substantially as if the reactor didn't exist, BUT as you 
apply more and more DC to the 'primary' winding the DC flowing 
through the windings creates a magnetic field which magnetizes 
the core and decreases it's responce to the AC traveling through the 
core and this decreases the current flow to the transformer until 
with enough DC flowing the core would be saturated and not respond to 
the AC at all! 


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