On Wed, 09 Feb 2005 14:20:55 -0800, Koz
wrote:



Just curoius...one of these days I have to switch an inductive load on a
forging machine somehow. Output is about 6000 amps at 3 volts. Makes
the theoretical (no loss) input about 75 amps at 240 V. The problem is
that things are so highly inductive, any digital switching would have to
be far greater than the 75 amp rating. That gets spendy. If you could
gang the darned things, the cost would be MUCH better.

Any ideas on a cheap but digitally controlled solution? In the old
days, one might use mercury relays. Regular relays will fry and lock.
Of course there are ways to suck up the initial inductive load but I'd
rather keep things simple.

Inductive loads don't have an initial current surge. The problem is
a voltage surge when you try to turn them off, unless you turn them
off at an instant when current is passing thru zero. Back-to-back
SCR's inherently do this and are suitable for switching inductive AC
loads. You can buy these as "solid state switch" modules. These
guys have modules with ratings up to 125 amps and 660 VAC:
http://www.crouzet-usa.com/catalog/gordos/gnssr.pdf