Re: [TCML] saturable reactors

["J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>]

Hi Neal!  (answers inline)

--- Neal Namowicz <neal@xxxxxxxxxxxxxxxx> wrote:
> 
> I see them use low voltage DC to saturate, but could
> low voltage AC be used? If not, (I'm just curious)
> why not?
> 

Because the control flux is Amps x Turns.  An AC
control voltage would be subject to both the
inductance and DC resistance of the control winding,
making the current much lower unless the voltage were
quite a bit higher.  So DC means a much lower control
voltage because the current is limited only by the DC
resistance of the wire.

> Can the existing secondary on a mot be used for the
> control voltage, rather than removing it and
> replacing it with many fewer turns of a heavier
> gauge wire? 

Yes indeed.  In fact, more turns is probably better,
for the reason explained above.

> Why doesn't voltage created by the primary windings
> transfer through and fry the low control voltage
> supply?

Ordinary saturable reactors have a control winding on
the middle leg, and power windings on the outer legs. 
If the outer windings are connected correctly, the
flux generated by the application of power to these
outer windings cancels out in the middle leg, hence
not inducing any voltage there.  In a multi-MOT-based
saturable reactor design, two MOTs (or a multiple of
two) can be connected in series on both the high- and
low-voltage sides such that the high-voltage windings
are out-of-phase and the induced voltages cancel,
allowing these high-voltage windings to function as
control windings.  If you do this, the MOTs must be
approximately identical so ensure that their voltages
cancel fully and don't "fry" your control.  Here are a
few schematics that illustrate this:
http://silicon-arcana.com/MMSR.gif

Another MOT-based approach would be to cut away the
low-voltage windings of the MOT and create two new
ones on the outer legs, thus permitting a single MOT
to be used as a small saturable reactor.

The multi-MOT approach avoids modifying the MOTs at
all, but requires care in selecting them.

> One more, please, re: mots for ballasting-  I've
> come across one mot that's much bigger than any of
> my others. If one mot used as a ballast is not
> enough to bring the current down enough, could I use
> two? (I want to stay safely under 20 amp draw) If
> so, do I parallel them, put them in series, or does
> it really make a difference? 
> 

Parallel = more power, series = less.

Cheers,
Aaron, N7OE

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