From: clewis@ferret.ocunix.on.ca (Chris Lewis)
Newsgroups: rec.woodworking
Subject: Re: More on 3-phase power
Date: 30 Jul 92 00:33:55 GMT
Organization: Elegant Communications Inc., Ottawa, Canada

In article <Jul.28.15.02.38.1992.11043@pilot.njin.net> ron@pilot.njin.net (Ron Natalie) writes:
>> OK, what's a SCR and a TRIAC?  What do the acronyms stand for,
>> what electrical behavior do they exhibit?

>SCR = Silicon Control Rectifier.  Imagine a diode that doesn't conduct
>at all until you put a voltage in a third pin and then it does.  Take
>two of these in paralell but hooked up with the diode part reversed
>from one another and you've got a triac (you wanted it to conduct in
>both directions once it is "fired").  It stays conducting for as long
>as there is voltge present, when that drops back to zero (120 times
>a second for household AC) it shuts off again.

[Regarding derivation of "triac", I always imagined it was somehow
derived from "diac", which is essentially two zeners connected back-to-back.
A diac only conducts when the voltage across it exceeds some value.
Either direction.  Add a lead to control the turn on characteristics
(ala a SCR trigger) and you get: "triac": "TRiggered dIAC".  Diacs
used to be used in SCR dimmer circuits as part of the trigger circuitry.]

>> Also, how are they wired into the 3-phase circuit?

>Damned if I know.  The three phase converters I've seen are either
>things that use a motor to either generate or flywheel the third phase
>or a kludge thing which blasts the third phase to start and then lets
>the motor run on one phase.

Probably use a TRIAC or SCR to do the blasting.

>> Are they rated, like most other components, to handle 220V?

>They come with a variety of voltage and current ratings.  You probably
>want to use one larger than what you want.  The voltage ratings, like
>diodes are PIV, not RMS like 117vac refers to.

When using semiconductors, particularly in AC circuitry, or something
as noisy as line power, you take something several times the peak
voltage.  110V RMS is around 160V Peak or 320 Peak-Peak.

At a very minimum, you'd choose a 400V device.  Even that would be
marginal unless you paid some attention to incoming voltage transient
filtering.  600V is better, and 1000V is best.

Fortunately, the pricing of SCRS, Triacs and other rectifier-like devices
(eg: diodes) is dependent almost exclusively on current capacity, not
voltage rating.  So, even tho the data books will list devices at 100V,
200V, 400V, 600V and 1000V, chances are that the 1000V device is (a)
not much more expensive than the others, and (b) the only ones easily available.

Doing these things properly and reliably takes quite a bit of specialized
electronics knowledge given the currents and the inductive loads you're
dealing with.  At my best, I was only able to design teensy little switching
supplies with IC-based controllers.

Any design worth doing will have *complete* instructions, including *exact*
device numbers.  Winging your own design is not for amateurs.  Mistakes
can be very expensive.

Incidentally, I seem to remember that it is possible to turn off a SCR
while the current isn't zero by shorting the gate to the cathode.  But
it has been quite a while.

>> > If you turn off the converter when the current is crossing zero, then
>> > there should not be a significant transient introduced into the supply
>> > side. 

>What the original author here ment to say is "turn on" not turn off
>probably.  The way your light dimmer works at home is that it has a
>triac that fires at some variable point after the time the line
>crosses zero.  The earlier it does, the brighter the light.

Precisely.

>However,
>triacs respond very quickly, as you get to the 50% point you are
>switching from 0 to over 100 volts rather quickly.  This will generate
>a big transient and RFI and such.  The common way to diminish this is
>to put some inductance in the line to slow this down (crack open your
>light dimmer you'll see a coil in there).

I'm just commenting that even the "very quickly" is an understatement.
They switch in microseconds.  Which leads to overshoot, and ringing
(at some frequency or frequencies determined by the R/L/C values
in the circuitry).  The RFI can contain frequencies up to several 10's
of Mhz or even higher.
-- 
Chris Lewis; clewis@ferret.ocunix.on.ca; Phone: Canada 613 832-0541
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