Re: Saturable reactors?

On Mon, 07 Dec 1998 19:44:09 -0700 in the Tesla List
Bill the arcstarter <arcstarter-at-hotmail-dot-com> wrote:

> I've been spending some quality time reading through a fine old
>book of mine: Magnetic Amplifier Engineering, by Attura, (c) 1959

I've been searching for books on saturable reactors or magnetic
amplifiers as they are also sometimes called! You were very lucky
to come across one! Any chance of getting you to share the design
equations with the group?

>Anyway - this book describes the basics of a little-known-today
>component called the saturable reactor.  Looks like if I had one I
>could build a quite-unkillable pig controller out of it!

Except for the distortion it would introduce on the sine wave it
should work admirably. I have read about saturable reactors in
various books on transformer design but never found any that gave
any insight into their actual design. I have made several small
saturable reactors over the years but they required a LOT of trial
and error and none were large enough to control a neon sign
transformer let alone a pole pig!

>Here's my question - I have access to several tape-wound
>transformer cores of the 220v 20amp variety (medical grade
>isolation xfmrs).  Does anyone have a clue if this core material
>would have the required "square" B-H curve required to make a
>decent mag amplifier?  I'd love to try this sometime, and am
>thinking this might be the time!  :)  It was also suggested that
>variac cores might be suitable for this - comments?

The "square" B-H curve you refer to is for an almost ideal
saturable reactor. You can use any decent transformer iron to make
a functional saturable reactor!

>I'm also wondering if there'd be room to wind control windings
>around the shunts of a NST.  This would create another form of
>saturable reactor - similar to the large ones used in the TIG
>welders of the 1970's and before...

Even if there was enough space it won't work! The primary and
secondary are of the wrong dimensions and orientation to function
as a saturable reactor. In a saturable reactor you need two A.C.
windings having an identical number of turns but each wound in an
opposite direction and these are placed on the two outside legs of
an E and I core, the D.C. or control winding is wound on the center
leg. The two A.C. windings have to be wound oppositely so their
fields will cancel in the center leg. To illustrate it's action try
and visualize the A.C. windings as L2 and L3 and connected in series and
the D.C winding as L1. Now assume that there is no
magnetic flux in the core; that is, the core is in an unsaturated
condition. At this time the inductance of L2 and L3 is large and,
when an alternating current is applied to the windings, a large
value of magnetic flux is produced in the core. This high value of
flux results in a high inductive reactance of the two A.C.
windings. In effect, this high inductive reactance prevents the
alternating current from appearing in the load; hence, there is a
relatively small amount of A.C. power delivered. Once again assume
that the iron core is in an unsaturated condition. When a direct
current is applied to the control winding, L1, the steady current
through this winding produces magnetic flux in the core. By
increasing the amount of direct current through this winding, the
amount of flux in the core is made to increase until a point is
reached where the amount of flux is at it's maximum. This point is
called the magnetic saturation point of the core and any further
increase in current will not produce any more flux. An alternating
current now applied to the circuit will flow through the A.C.
windings and the load. The reason for this is: since the core is
saturated with flux produced by the direct current in the control
winding, it cannot accept any more flux; hence, the inductive
reactance of the A.C. windings is low. Thus there is a large amount
of power delivered to the load when the core is saturated. In
between these two extremes you can vary the inductive reactance of
the A.C. windings by changing the amount of D.C. applied to the
control winding. As you can see the windings that are in a neon
sign transformer are totally wrong for them to function as those in
a saturable reactor.
                       Alfred A. Skrocki
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