Re: MOT Primary Current Question

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Justin <rocketfuel@xxxxxxxxxxxxx>

Greetings!

I have a rudimentary question that's been nagging me every since that
pole pig discussion of last week.

Experience and my gut tell me that an open secondary will draw very
little real current on the primary side.  What I can't figure out is
*why* that's true because from one POV that primary should be a low
resistance short circuit and current should be through the roof!

Is it the impedence of the coil + core that keeps things from going
bananas?  Some sort of magneticlaly induced EMF that opposes the current
flow?  My gut also tells me that if I simply make a coil out of wire
and stick it in my wall outlet the current would definitely be there.

What gives?


Justin




On Mon, Jun 12, 2006 at 09:03:31AM -0600, Tesla list wrote:
> 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.
>