Re: low-cost 1800-amp heating source
- From: Winfield Hill <hill@xxxxxxxxxxx>
- Date: Wed, 01 Aug 2007 08:11:04 -0700
MooseFET wrote:
Tony Williams wrote:
John Larkin wrote:
If the scr/triac conduction isn't perfectly symmetric, you can
wind up with net DC into the primary, which can get weird. And
sometimes an scr won't stay triggered when driving an inductive
load, ie the transformer leakage inductance. There are hazards,
that's all.
Perhaps no need to use a semiconductor. This could be
an opportunity to investigate the design of the old GE
theatre light dimmer, based on the saturable reactor.
There should be no net DC component from that, and
relatively soft waveform modulation.
La
ACin-----------/////////---------+
========= |
DC Control---////////------------- DC Control
========= |
ACout----------/////////---------+
Lb
The saturable reactor can be wound on E-I laminations or
on two toroids stacked together, with the control winding
wound on the stacked assembly.
Effective inductance control is just a matter of a variable
DC constant current source driving the control winding.
Just take two transformers and wire the primaries in series and the
secondaries in series bucking as the control. It worked fine some
40 years ago to control a light bulb so a bigger one should control
a heater quite nicely. DC in the secondaries makes the cores
saturate.
If you want to be a purist about it, you need to place a large
capacitor across the control winding. When you feed DC into
the control some AC currents flow in the control windings.
The capacitor keeps this out of the control circuit.
One transformer with two secondaries, wired
in series bucking, wouldn't work, I suppose,
since the dc fields would cancel.
.
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