RE: MOT Primary Current Question

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Mark Dunn" <mdunn@xxxxxxxxxxxx>


Norman:

I typically find Zprim ~ 100 ohms (secondary open) on MOT's. With the
secondary shorted Zprim ~ 5 Ohms(note MOT will saturate at 120 VAC).
These values are with the shunts in - you did not mention the shunts so
I assume you have not removed them.  If you take out shunts, secondary
open values don't change significantly, but shorted Z goes down.

It is possible that you have a short in the secondary that is bypassing
a small section of the secondary coil.  This would provide a partially
shorted secondary for the primary to "see" during the open circuit test
and still provide a secondary voltage output.

At 120 VAC input and 1800 VAC output you have 15:1 ratio which is not
unheard of but typically low for MOT's.  They are usually around 18:1.

You might try applying low voltage(12 VAC) to the secondary windings and
measuring Z with primary open and shorted.  Your secondary values may
shed more light.

For conclusive proof, knock out the shunts and re-test.  Then cut the I
core loose from the E core and remove the secondary.  Test the primary
alone with the I core clamped to the E core.  If this reveals that the
secondary is bad, you can use the unit as an inductive ballast for your
good MOT's.  Gap the E and I core to get the Inductance that you need.

Mark


>Original poster: norman@xxxxxxxxxxxxxxxxxxxxxxxxx

>I measured the primary current of a small 120 VAC MOT.  To my surprise
the primary current >was 10 Amps with the secondary open!  The MOT is
about 4" x 3" x 3" and came from a 1000
>Watt oven.  Is the MOT bad, or do these transformers just have a low
primary inductance?  >The primary wire is aluminum and the secondary
voltage was measured by me at about 1800
>Volts.  Also, I also measured a much larger transformer from (I think)
an older oven with >a copper primary and it drew 6 Amps.  The small
transformer was very beat up and it is
>possible that there is a short in the primary.  If you guys out there
think that the
>primary must be shorted then I will rewind it, but if the transformer
is acting normally
>then rewinding it will not reduce the current.