From: mwr@tridom.uucp (Mark Reardon)
Newsgroups: rec.woodworking
Subject: Re: More on 3-phase power
Date: 28 Jul 92 23:34:59 GMT
Reply-To: mwr@tridom.uucp (Mark Reardon)
Organization: AT&T Tridom; Marietta, Georgia

In article <1992Jul27.163432.6302@microsoft.com>, stevenj@microsoft.com (Steven Johnson) writes:
|> In article <Jul.24.14.16.48.1992.27570@pilot.njin.net>, ron@pilot.njin.net (Ron Natalie) writes:
|> > > 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.  A proper SCR or TRIAC controller would do that automatically.
|> > 
|> > Proper or not, a SCR or TRIAC can't shut off excpet at zero.
|> 
|> OK, what's a SCR and a TRIAC?  What do the acronyms stand for, and 
|> what electrical behavior do they exhibit? (I'm trying to get an 
|> understanding of why one of these is benficial.)  Also, how are
|> they wired into the 3-phase circuit?  Are they rated, like most
|> other components, to handle 220V?
|> 
|> These sound interesting, but I'm getting conflicting feedback.  Some
|> responders have said a rotary converter won't cause *any* problems
|> on the input side of the line, but this thread implies there *will* be
|> potential problems. WHat's the real story?

Sometimes I wish I had listened more closely to my old boss discussing
the use of these things.  ANYWAY, even though I am a non electrical
type I can answer some of these questions (I used to do characterization
testing for International Rectifier back in 1978 while I was in
college).

SCR - Silicon Controlled Rectifier, A diode that blocks in both
directions until current is applied to its trigger.  Then the
current flows until the current drops below a certain level (close
to zero) even if the trigger current is removed.  Also refered to
as triodes because of the similarities to diodes except three 
connections are required.

TRIAC is not an acronym but is a play on TRIode for AC.  It is done
by hooking to SCRs in parallel but with the "polarity" reversed.
The triggers are joined so that the one current triggers both.  The
packaging does not look like two devices in parallel but that is
how it is described.  I am willing to bet that the same silicon
contains both junctions.

Though I can't help on the circuit for three phase I do remember
testing these devices at voltages over 220.  We had some SCRs with
3.5 inch junctions, in what they called hockey puck packaging that
included tubes for bringing coolant close to the junction.  One of
my jobs was to raise the power until the junction exploded.  This
information was then put in the product data sheets.  Be aware that 
all the SCRs I documented were done as DC devices.  Voltage therefor
is measured differently, not RMS.

-- 
Mark

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| Mark Reardon           |  AT&T Tridom                             |
| mwr@eng.tridom.com     |  840 Franklin Court                      |
|                        |  Marietta, GA 30067                      |
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