Re: pigs and other novice questions

Travis Tabbal <bigboss-at-xmission-dot-com> wrote:

>SCRs usually don't like inductive loads. Most SCRs I've seen would fry 
>attempting to drive a transformer. Some coilers have tried to use light 
>dimmers as a cheap variac, those use SCRs to control the output, and most 
>of those units fail. There are some that will work, but I think they use 
>something else to control the AC wave.

Actually you can use SCRs with inductive loads (like transformers).  You 
have to place a snubber ( a resistor in series with a cap) across the main 
terminals of the SCR.  This snubber reduces the change in voltage per time 
the SCR sees at the current zero crossing.

I have a homemade SCR controller which uses two back to back SCRs, with (I 
think) a 100 ohm 1 uf snubber across the SCR pair.  With that I can control 
42 amps at 240 volts into a 0.42 power factor load (arc welder).

The Motorola semiconductor site has much info on SCR snubber theory.

But - all that being said - I don't think an SCR controller would help with 
a PIG.  Once the SCR turns on - you get NO current limiting until the zero 
crossing.  I think you'd still need a ballasting inductor in series with the 
pig w/ any form of SCR or lamp dimmer arrangement.


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