More about MOT currents

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Original poster: norman@xxxxxxxxxxxxxxxxxxxxxxxxx 

My thanks to all who took the time to answer my question about MOT currents.
Several suggestions were made which led me to make the following
measurements:

SMALL  MOT:
I removed the secondary coil, clamped the transformer back together again and
measured the primary current.  From 0 volts up to 75 volts the primary
impedance measured 25 ohms (primary voltage/primary current).  At 120 volts the
impedance dropped to 12 ohms and the current was increasing very rapidly with
voltage.   Thus the high primary current (no secondary) was due to the core
saturating.  They are clearly pushing these transformers to the limit!  (The
MOT is 2.75" x 3.25" x 3.75".  The primary  is 120 volts and the secondary is
about 1800 volts.  The primary wire is aluminum and it came from a 1000W
oven.)

LARGE MOT:
>From 0 volts to 60 volts the primary impedance was 120 ohms.  At 80 volts the
impedance dropped to 108 ohms,  at 120 volts the impedance was 32 ohms, and at
125 volts it was 26 ohms. (The MOT is 5.25" x 4.5" x 2.25" and the wire is
copper.)   Thus all MOT are not created equal!

NEW QUESTIONS.
If  I draw current from the secondary, calculating the primary current for an
ideal transformer is straight forward.   I(pri) = I(sec)V(sec)/V(pri).  If  my
MOT draws 10 Amps with no secondary current will the total primary current for
my MOT with secondary current be I(pri) + 10 Amps.

I assume that the problem of the core saturating cannot be solved by using a
power factor correcting capacitor in parallel with the primary because the
primary current will remain 10 Amps even though the line current will be
reduced.  Also the nonlinear inductance due to core saturation should generate
harmonics in the primary current.  Does anyone have a feel for how seriously
core saturation and the resulting harmonics reduce the effectiveness of the
capacitor in reducing the line current?

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