Re: MOT current limiting shunts

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Original poster: Mike Poulton <mpoulton-at-mtptech-dot-com> 

Mike Poulton Wrote:
 > Original poster: John <fireba8104-at-yahoo-dot-com>


 > Hello all,

 > A question for the ages, at least in this hobby. Is there any formula for
 > finding L of current limiting shunts in Mots ,or any other transformer for
 > that matter? Now, to clear up a bit of confession, are the shunts
 > equivalent to a inductor in parallel with a winding, as the name shunt
 > implies, or are they equivalent to series inductance, as what would be
 > desirable for a neon xformer in order to limit voltage after an arc is
 > started? Finally, after I've killed your love for the written word, to what
 > winding is ohm's law applied?

The shunts are so named because they shunt magnetic flux away from its
normal path within the core. Instead of traversing the entire core and
passing through both the primary and secondary windings, some flux
circulates only through the primary winding. This increases the
leakage inductance of the transformer and effectively adds inductance
in series with the primary. This is exactly the same method used to
current limit a neon sign transformer, and it serves the exact same
purpose here. I don't believe there is a reasonable way to calculate
this, but it can be measured easily. Short the secondary and put an L
meter across the primary.

The second question confuses me a bit. To what winding is ohm's law
applied? I suppose it depends on what you are trying to calculate.
Ohm's law applies to both windings. They each have a finite
resistance. To determine the apparent ohmic resistance of the
transformer as seen from the primary (there is probably a better term
for this), first measure the resistance of each winding. Then divide
the secondary resistance by the turns ratio of the transformer. Add
the primary resistance to this. The result is the apparent resistance
as seen from the primary.  This resistance will appear in series with
the leakage inductance in series with the primary.

-------------------------------------------------
Mike Poulton
MTP Technologies
mpoulton-at-mtptech-dot-com
KC0LLX (70cm AM ATV, 33cm/12cm FM ATV, Omaha, NE)