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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