Re: MOT leakage

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

Original poster: "robert heidlebaugh" <rheidlebaugh-at-desertgate-dot-com> 

Borislav: The microwave transforrmer is made to supply a lot of power in a
small space usualy 1 amp at 2100 volts.  This makes it  noisy, If you use
plastic or aluminum to mount it this will reduce the noise. Steel will make
noise. Some list members mount the transformer in an oil filled can. This
reduces the noise but may be messy. You may find the 1N4007 a little low in
current so a 3 amp diode may be a better choice to withstand your surge
current in a 400 ma supply. Your diode is rated at 1 amp. That is only twice
your expected load.
    Robert   H
-- 


 > From: "Tesla list" <tesla-at-pupman-dot-com>
 > Date: Mon, 08 Nov 2004 08:10:28 -0700
 > To: tesla-at-pupman-dot-com
 > Subject: MOT leakage
 > Resent-From: tesla-at-pupman-dot-com
 > Resent-Date: Mon, 8 Nov 2004 08:13:35 -0700
 >
 > Original poster: "Borislav Trifonov" <bdt-at-shaw.ca>
 >
 > I assume subscribers to this list are familiar with microwave oven
 > transformers, and that is why I ask my question here, even though I'm not
 > constructing a tesla coil.
 >
 > I'm working with plasma and I need a DC supply at about 3 kV and 400
 > mA.   In order to save money I got a large MOT with 2100 VAC secondary. The
 > thing was quite noisy and heating was a problem, so I rebuilt the core.  I
 > grinded out the weldings and took apart the laminations.  The secondary had
 > one end grounded to the core since insulation between the inner part of the
 > winding and the core was minimal.  I stuffed extra insulation (mica) so I
 > could safely unground that end of the winding (I'm bridge rectifying it
 > with chains of 8x the usual 1N4007s into a CLCRC filter).  I put the core
 > laminations back together with nail polish, as I have no shellac.
 >
 > Now the transformer is very quiet, except when I put it in the chassis (3u
 > format, steel, earthed) the chassis resonates very loudly.  At first I
 > thought the problem was mechanical vibration transmission, but
 > foam/rubber/springs didn't help, and I even made a pneumatic isolator with
 > a pump -- nothing.  Turns out the hum was being induced into the chassis
 > electromagnetically.  All my tests were with an unloaded secondary as the
 > load has not yet been built.  The noise is so loud that I cannot leave it
 > as it is.
 >
 > I have two questions:
 > Will the induced hum get worse/better/same when the transformer is loaded?
 > What are my best magnetic shielding options?  Mu-metal foils are expensive,
 > hard to find, and I'm guessing too thin and will saturate and be
 > ineffective; a silicon steel enclosure will buzz defeating the purpose of
 > shielding and present a cooling problem.  I'm going to go crazy trying to
 > figure out how to get rid of the noise.
 >
 >