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RE: Variac vs. Fan speed control - some minor corrections

Subject:   RE: Variac vs. Fan speed control - some minor corrections
  Date:    Wed, 23 Apr 97 06:50:20 UT
  From:    "William Noble" <William_B_Noble-at-msn-dot-com>
    To:    "Tesla List" <tesla-at-pupman-dot-com>


I have interspersed some commentaty within your response to correct a
few 
minor errors.

[Bill]  snip 
The dimmer you have Aaron is a triac phase shift control dimmer and 
it is for NON-INDUCTIVE LOADS ONLY!!! 
[Bill]  There are special versions for inductive loads, but they aren't 
designed for "highly" inductive loads.  You can get a triac dimmer for 
fluorescent lights, and the fan motor itself is inductive compared to an 
incadescent bulb.  Generally the inductive load circuits I have looked
at (a 
few years ago) had better filtering and higher breakdown voltages on the
triac
It works by cutting off power 
at different points along the 60 cycle sine wave feeding the light.
[Bill]  no, it works by turning the power ON at a preset point along
each half 
wave of the 60 hz sine.  The triac is turned on by a pulse to the gate,
and 
turns off when the current through it reverses.  There is a device
called a 
GTO, or gate turn off (SCR or thryristor) but they cost more and so are
not 
used in low cost consumer speed controls.

This type of dimmer should NEVER be used with any inductive loads it
destroy transformers and AC induction motors! 
[Bill]  not to mention destroying the triac
There is no solid 
state substitutes for a variac that I am aware of! 
[Bill]  but there are substitutes - or at least there are solid state
circuits 
that can regenerate a sine wave with 1% or less distortion at multi
kilowatt 
power levels - however these circuits are quite expensive compared to a
variac
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. 
[Bill]  a thing called a magnetic amplifier works this way too -
building a 
high power magnetic amplifier would let you modulate your tesla output
with 
your stereo - now that would be cool.
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! 
[Bill]  another good way is a high power rheostat such as might be found
in an 
old lighting control board.  Some were carbon piles which can control
quite a 
few KW