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Re: Trannies, how do I rewind them?
-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Thursday, September 24, 1998 6:37 AM
Subject: Microwave Trannies, how do I rewind them
>Original Poster: MILLS MATTHEW <MISMXM-at-corporate.santos-dot-com.au>
>
>
>Hi I have looking at the many posts related to using microwave
>transformers and other transformers and there was someone or more than
>one person who said that you could rewind them. I have a number of
these
>including a 2000va one from a commercial oven and was wondering how one
>goes about rewinding them. Actually for that matter, how do you take
the
>windings off a neon as detailed in the archives? My problem is that
>these tranformers all have laminated windings consisting of either
>interleaved E shapes or an I and an E shape. Either way they are
bondend
>together tightly making it impossible to remove or rewind any windings.
> Many times these laminations ar seam/spot welded along a number of
sides
>to keep everything together.
>So, do I hacksaw one side of the figure of 8 off? and then rewind the
>core minding out for the sharp edges and hoping that the remaining
>laminations stay together? and then once rewound weld the sawn off
piece
>back on?
>
>Maybe in the southern Hemisphere our transformers are made differently
>than those in the USA and europe.
>
>Hoping for a helpful reply :)
>
>Matthew Mills
>EMAIL: matthew.mills-at-santos-dot-com.au
>Phone (08) 8364 0249
>
Hi Matthew
Sorry for taking so long to reply, been VERY busy lately.
I made a new sec. for a MOT a few months ago. The one
that I used had the core composed of an E and an I shape,
not interleaved. First I removed the mounting plate. Some
MOTs have it. Some don't. The mounting plate was tack welded
in about four spots, I just nicked the welds with an angle grinder and
then knocked the plate off with a large hammer. Then I removed the
I piece from the E piece. They are welded together along the seam
on the outside only, and the welds have very little penetration. I
ground
the welds off with the angle grinder. That was the easy part.
The next step was to remove the windings. The primary winding was
next to the I part so it had to come off to get to the secondary. This
proved to
be a tad difficult as the cardboard like insulation material seemed to
be
glued to the core. I made a tool out of an old butter knife (US Navy
issue,
very tough) by cutting off the end and grinding an edge on it sort of
like
a putty knife, just narrower and able to stand up to a hammer. I drove
this thing into
the insulation all the way around the core in the center, as well as on
the outside where the
insulation was stuck to the inside and the outside of the top and bottom
bars of the E.
(I don't know if this is normal for a MOT, or if the glued on effect was
due to overheating
as this transformer had a shorted secondary. Not shorted enough to trip
a circuit breaker,
but it would get very hot very fast.) After the primary was removed, I
took out the shunts
(easy, just drive them out with a brass drift and a large hammer) and
then the sec. The insulation
did not survive the trip on either coil.
Next I measured the wire gauges of the windings, and determined the
number of turns.
The primary has approx. 132 turns of 13 AWG wire. With 120 volts input
that works out
to about 1.1 turns per volt. The secondary was 26 AWG wire, by measuring
the cross section
it should have had somewhere around 4400 turns which works out close
enough to 2.1 kV.
I tried to unwind the secondary but it was too damaged, several wires
were welded together,
and so I just had to go from the calcs.
I wanted 10000 volts at 100 mA, and determined that 11000 turns of 34
AWG would do the trick.
I had obtained a 2 LB spool of 32 AWG for cheap (surplus) so that is
what I used.
I could find no suitable bobbin, so I used a section of a plastic bottle
(HDPE) that isopropyl (rubbing)
alcohol had come in. With a little help from a heat gun, it was easily
reformed to fit snugly over the core.
I then made a wood block with the exact dimensions of the core, and a
fixture to wind on. I slipped the primary
on one end, then five pieces of 5 mill mylar cut to fit and glued
together with polyurethane varnish, then five more
mylar pieces on the other end, also glued in between with polyurethane
varnish. Then I put a piece of 1/8"
masking tape (like body shops use to paint pinstripes) on each end near
the mylar. Then I wound the first layer.
Then I coated the first layer with polyurethane varnish. After letting
the varnish dry till tacky, I wrapped the layer
with a strip of 2 mil Kraft paper saturated with polyurethane varnish.
The next layer was wound on top of that
without letting the paper dry. Then the same thing over and over again
until I had run out of wire. The only
deviation from the routine was to leave out the masking tape about every
third to fourth layer due to the fact
that it was just a little thicker than the wire and I needed to keep
things even.
After the winding was complete, I wrapped the new secondary with a layer
of fiberglass cloth electrical tape
(3M SCOTCH 27), made the connection to the output terminal,(I used the
original one from the MOT), another layer
of the fiberglass tape, coated the tape with polyurethane varnish an set
it aside to cure for a couple of days.
It took about a week to wind the darn thing, with all the dry time
between layers and all.
I ended up with around 8500 turns (calculated, I did not count). Also I
left out the shunts to give more
room for winding so external current limiting is required. The
transformer puts out just under 8 kV and will
deliver 120 mA all day without getting warm. However less than a week
after finishing I was given
a 5 KVA distribution transformer, so I started building a larger Tesla
coil and have only used the homemade transformer
once, just to see if it would work after all that trouble. Now it sits
on a shelf waiting for the day that I have nothing
to do but play.
I hope you can find some information here that helps.
later,
deano