Ken wrote:

So, what is the design method for a transformer that saturates easily?

Without getting into a bunch of formulae, etc. (which I'd probably screw
up anyway)... a couple of factors - 1) absolute minimum core to couple
the windings - i.e. magnetically "starved". 2) no gaps in the core - let
the DC current's field circulate well - such that it "interferes" with
the AC field. The core can only hold so much flux - if DC is "pushing"
the field one way -- the AC (when it opposes) is only going to "reduce"
it - not reverse it - (or not fully reverse it) so that the coupling
becomes very inefficient. If you look at most output transformers
designed for single-ended use - they have a gap in the core somewhere.
Obviously - such a gap would not be appropriate for a saturable reactor.

And that sets me pondering again whether the primary "effect" is bucking
or just reactance... Let's say for the moment that bucking is not the
primary mode - and reactance is. Then why the reversed phasing (if
bucking isn't a factor)?

Well - as I just noted - in a true saturable reactor - the DC flux
"overwhelms" the AC flux. Since the AC and DC are additive half the
time - and subtractive half the time - the control isn't going to be
symmetrical. This is overcome in "the real world" by twin reactors -
with the DC "reversed" through one (compared to the other). This way
the "offset" in one reactor is "countered" by the other --- and then
they "switch roles" when the AC reverses polarity. If you look at the
circuit here - (and again - for discussion sake totally ignore bucking)
- the AC is "reversed" all the time at one end - or the other of the
primary -- as the two coils are phase reversed.

Back to saturable reactor theory - when the DC control winding drives
the core into saturation - the reactance in the AC winding drops
dramatically. That being the case with this circuit - then the two
windings would 1) loose coupling so bucking is no longer a factor - and
2) have virtually no reactance in series with the bulb. Then by 1/2 the
AC "reactance winding" reversed - both halves would contribute their
part to the overall source impedance - providing better symmetry.

Now I'm not so sure that pure reactance doesn't play a larger role than
originally thought... That perhaps control is indeed more reactance -
and "bucking" is just a happy "bonus" to the equation...

without taking some measurements (esp. being able to Un-reverse phase
the two windings) - it's hard to guess...

best regards...
--
randy guttery

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