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Re: Triac transformer drivers or something like that...
Bert Pool wrote:
>Many of us sorely wish triacs worked for transformer control, but
>they perform very poorly in this function. I believe it is because
>the waveform coming out of a triac at partial power setting is no
>longer a pure sinewave. I'm sure some of the degreed engineer types
>here will give a more learned escription of the exact mechanism of
>why triacs don't work with transformers or brushless motors.
Bert is correct.
Triacs will never be able to handle highly inductive loads due to their
inability to "turn off" at the end of the cycle. Triacs are basically
back to back SCRs located on the same semiconductor die. The charge
carriers from the conducting side drift over into the nonconducting side
- turning it on despite any gate signals. The commutation dv/dt across
the triac will force the other scr into conduction immediately.
I've never tried using a cheapo lamp dimmer to control an inductive
load. The lamp dimmers are designed to hangle the large (x10) inrush
currents of a tungsten lamp, but, being consumer devices, are desgned as
cheaply as possible. As suggested elsewhere - you might be able to get
away with this if you place a snubber across the terminals of the
dimmer. Construct this snubber out of a 100 ohm resistor in series wtih
a 0.22 uf 400 volt capacitor. If this doesn't let it work then you are
SOL...
That being said - I have a (homemade of course) back-to-back SCR dimmer
powering a highly inductive load (a welder). Unlike most folk on this
list - I'm actually using the welder for (gasp!) welding! :) This
dimmer allows me to control the welding power on the fly (TIG). The
power factor is awful - 5000 watts delivered while 10000 VA are drawn...
After giving it much thought - I doubt that it'd be possible to make a
pig control on the same premise w/o using at least some sort of
inductive limiting. Pigs are just too "ideal" and very closely resemble
a short circuit once the gap fires...
-Bill (the arcstarter)
Degreed engineer type,
Starting arcs in Cinci, OH
http://www.geocities-dot-com/CapeCanaveral/Hangar/6160
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