SCR drives - induction furnaces

From:  Bill Pollack [SMTP:arcstarter-at-hotmail-dot-com]
Sent:  Wednesday, January 21, 1998 8:22 AM
To:  tesla-at-pupman-dot-com
Subject:  SCR drives - induction furnaces

It was written:
> is currently ON). You need a flip-flop, or comlementary driver to 
> the SCR's.
>Not really short out the SCR, just put a reverse bias on it long enough 
>drop the current to zero to turn it off. And yes, there is a UJT 
>driving a flip flop to generate the trigger pulses in complementary
>fashion. There are is also a lot of circuitry to create the proper 
>pulse for this application (big spike at beginning, then a long back
>porch). Hey, if it were simple, everyone would do it.

If I understand this - someone is using one SCR (call it S1) in series
with a cap, placed across another SCR (call it S2).  By triggering S1,
you can get S2 to turn off while under load.  Is this right?

It seems to me that once both SCR's are triggered, then both SCRs
are sharing the load current (at least for an instant), wouldn't reduce
the current flowing through SCR S2 to zero, which is required to turn it

Can someone clarify this?  Is the cap charged up (to reverse-bias S2) 
before S1 is triggered, or something?  What am I missing?

Commercial induction heating furnaces typically run at 3KHz to 
30KHz, with 10KHz being the typical peak at the higher powers 
(>50KW).I assume there is an output transformer which converts line 
voltage (chopped by the SCRS) into a low voltage high current 
output, etc.

Information on the drive electronics is sketchy, but I believe they 
are probably using this switched-SCR method...

Back in the old days, there were several medium power (~20KW) 
induction heating furnaces available, based on spark gap technology. 
 I'm trying to adapt this sort of thing to tesla coil service.  
Anyone out there tried this other than myself?

I've not been able to locate any decent (technical) websites for
induction heating. Some companies that provide this equipment are:
 IHS - Induction Heating Specialists (in Ft. Worth)


-Bill (the arcstarter)
Starting arcs in Cinci, OH
Coil pics at: 

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