[Prev][Next][Index][Thread]
Re: SCR/Triac pig controller? (fwd)
---------- Forwarded message ----------
Date: Mon, 18 May 1998 13:39:16 PDT
From: Bill the arcstarter <arcstarter-at-hotmail-dot-com>
To: haba-at-cc.hut.fi, tesla-at-pupman-dot-com
Subject: Re: SCR/Triac pig controller?
Harri wrote:
>The basic problem is that once the spark gap fires the load is like
>a short circuit. This will kill the controller.
But the extra inductive and resistive ballasting (same as needed on
variac powered pigs) will reduce this short circuit current to a
tolerable level.
I picked a particular phase-control SCR out of my Int'l Rectifier book -
the 80R1A40. This stud-mounted component will carry an average rms
current of 125 amps, with a Itsm of 1700 amps. If my understanding of
this rating is correct - this component will survive a momentary (one
half cycle?) pulse of 1700 amps.
Surely the extra ballast will prevent the current from going this high.
>A proper approach would be to detect the overload and then
>commute the tryristor/GTO/whatever off before current has got too
>large. This will need some inductance to limit current risetime.
>Current cannot rise too high before the output has been "turned off".
>Not an easy task.
How the heck do you commute an SCR off before the end of the half
cycle??? Please give me a hint! Is this technique used in the
industrial cycloconvertors (which I know are SCR-based but I can't see
how...)
(The multi-thousand horsepower electric motors at a local steel mill run
off 6-phase 9 to 21 HZ power which is produced via solid state
conversion techniques. The plant is fed by 330 KV 60Hz 3-phase...)
>Further complications would be stuff like imbalance in the
>transformer core which might saturate it if this is not handled with.
>Some other problems exist too.
Any way this can be handled w/o having an extra sense wire on the
transformer core?? :( My pig didn't come with one! :)
Thanks for any suggestions. Perhaps we should continue this discussion
off the list? I'd be interested in your comments along these lines...
-Bill
______________________________________________________
Get Your Private, Free Email at http://www.hotmail-dot-com