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Re:Triac Power Control
It was written:
>If I use only resistive ballast (heater elements) to limit my 5KW
>pig, couldn't I also use a big Triac for power control? I know
>Triacs don't like highly reactive loads (like pole xfmrs!), but it
>seems like the heater elements should smooth things out enough for a
>reasonably rugged Thyristor to survive. Of course, the resulting AC
>would be messy, but Tesla coils don't require "clean" power, do they?
> I'd use something like a 40A / 600V TO-3. I realize such a control
>won't give true variable AC voltage like a Variac, but it should
>provide adjustable input power. Has anybody tried this?
Greg,
About 6 months ago I started a thread along these lines. Search the
archives for the specifics, but here are my conclusions:
* Yes, it can be done. My employer uses SCRs to phase control power
flowing into an unballasted 60KW transformer. You will have to use
back-to-back SCRs as opposed to triacs due to a triac's poor reverse
recovery rate.
* A proper snubber circuit (look in the data books) will allow the use
of scrs with highly inductive loads w/o much problem. The key is
reducing the dv/dt across the device at shutoff...
* You will have to go to extremes to keep the HFRF out of the scr
trigger circuits or all hell will break loose!
* Remember - the failure mode for most solid state devices involves
going into a short circuit condition. This means that full unrestricted
power will be sent to your pig if your parts fuse. This being the case
-I suspect you would be wise to incorporate additional ballasting to
limit the max draw under these conditions.
* D C Cox apparently tried this years ago, and he didn't like the
failure mode aspect. Neither do I, so I didn't ever build one.
Thats all I have to say about that... :)
-Bill
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