Re: low-cost 1800-amp heating source
- From: MooseFET <kensmith@xxxxxxxxx>
- Date: Thu, 02 Aug 2007 06:41:15 -0700
On Aug 1, 7:10 pm, Winfield <winfieldh...@xxxxxxxxx> wrote:
On Aug 1, 11:11 am, Winfield Hill wrote:
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.
I'd appreciate a response on this. The saturable reactor is
appealing to me because it's a linear way of dealing with an
ac-transformer power-control problem.
This depends on how you define "linear". The cores of the two
transformers don't start to saturate at low currents.
As you increase the control current the saturation starts to happen
near the end of each alternation. The transformer in which the AC and
the control adds, saturates first. An AC current flows in the control
winding coupling the low impedance of the saturated one over onto the
unsaturated one. This is why I suggested a capacitor across the
windings.
As you increase the control current, the saturation point moves
towards the peak of th ewave form. This means that the control gain
is still increasing in the early part of the range.
Once you pass the peak, the gain of the control signal starts to
decrease. When you get to the point where the core is saturated with
no AC, the circuit is nearly fully one and the gain is very low. As
you go above this point, you start to saturate the core that the AC is
bucking in. This takes the leakage inductance out of the picture.
The transformers will have the I^2 * R losses due to the current. For
this reason, I don't think you could go with a much smaller
transformer.
One thing that slows
me down is considering the two transformers needed to create
a saturable reactor - how big do they need to be? Thinking
about this, with no DC current, they create an open circuit,
no current flows and no power is dissipated. Alternately,
with maximum DC current the two transformers are saturated,
and simplified, look together like a length of copper wire.
So the intermediate condition provides the greatest stress.
That should be the region to evaluate.
Another issue, how much power will be involved generating
the DC-current to control say 1.0kVA? Will it be a similar
amount, say 500W dc? Ahem.
.
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- From: Winfield Hill
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