Air Gap MOT Reactor

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "S & J Young by way of Terry Fritz <twftesla-at-qwest-dot-net>" <youngs-at-konnections-dot-net>

Recent Tesla List wisdom from Ritchie and others indicates DC charging
reactors really need an air gap in them so they don't saturate as easily and
lead to power arcing in RSGs.  So I decided to sacrifice one of my salvaged
MOTs and convert it into an inductor with an air gap.  It turned out to be
much easier than I anticipated.

My MOT measured about 19 Henry before surgery.  I decided to see how easily
the weld beads joining the "E" and "I" core sections could be hacksawed
along the joint.  No sweat!  It only took a few minutes to saw down through
the welds which was fairly soft steel.  A couple of taps with a cold chisel
and it came apart cleanly.  Whole operation was about ten minutes.

I removed the shunts, primary and filament windings, leaving just the
secondary.  I then measured the inductance for various gap thicknesses
(using post-it squares as spacers).  Here are the approximate measurements
of gap thickness vs inductance in Henries:
None = 16.5 Henry
0.004 inch = 13.8
0.008 = 12.0
0.015 = 9.6
0.03 = 7.4
0.06 = 5.6
Infinity (no "I" core) = 2.5

After I retune my twin TC for a bigger MMC I just finished, I will try the
following experiment:  With no reactor air gap, I will determine the
approximate break rate at full power which causes the RSG to start power
arcing.  I will repeat with a 0.01 inch MOT air gap and see if the RSG can
go slower before it power arcs.  This would indicate the reactor can indeed
withstand more current through it without saturating.  Stay tuned for the
next exciting episode . . .

By the way, some worry about MOTs not being able to withstand high voltages
across the secondary.  I think they are good for 10 KV or so.  They have to
be insulated well enough to not break down when power turns off at the peak
AC voltage and the magnetic field rapidly collapses, generating more than
the usual 2 KV  steady state output.  I have run 9 to 10 KV DC into a MOT
charging reactor with no signs of breakdown.  Remember, the charging reactor
nearly doubles the DC supply voltage so one doesn't need more than about 10
KV DC to charge the tank cap to nearly 20 KV.

I would be interested in hearing of others trying gapped MOTs as cheap
charging reactors.

--Steve