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Re: MOTs
Original Poster: Esondrmn-at-aol-dot-com
Hi Ed, All
Hey, now I got the chance (as midi Tesla expert) to help one of the maxi
coilers out there.
First of all, judging from the size of the core and the wire used, I have a
similar unit.
However, mine came from a small home microwave. It puts out 1.8kv and has a
short circuit secondary current of 800mA. This is over its rated limit. The
microwave it came from had a total VA of 1450W. But you will have to subtract
the (filament, fan motor, electronics, losses) VA. Taking a wild guess I would
say about 250W-300W.
This leaves you with 1150W for the MOT, which sounds okay to me for a home
oven.
My xformer doesnīt have any labeling, either. (seems to be common on MOTs). My
1.65kVA Gardner type (which are up to now, the only ones I would even think
about trying in a TC or other hv high power experiment) are clearly labeled.
These are from larger home ovens.
The magnetic shunts are usually located in between the primary and secondary
windings. It is a pretty small package (my stack on the non Gardner type is
about 5 mm thick, i.e 0.20") and is usually wrapped with kraft papper (making
them even harder to find).
I wouldnīt worry about the filament windings. I always leave them alone unless
they have shorted internally (very rare). I wouldnīt even go to the trouble of
disconnecting them. The sharp ends might invite an arc over from the hv
winding (esp. if you are seriesing the Mots).
Disconnecting the innermost HV wire is another thing. It depends on what you
want to do with the MOT. (in oil/ out of oil operation) Do you want to series
more of them, etc? For example if you want to series two and ground the
"center tap" between MOT #1 and #2, you donīt have to disconnect it. You can
ground both cores that way. You will have to connect the primaries
antiparallel. If you do this no MOT insulation sees more than its rated
voltage above ground.
Most MOTs have the innermost wire come out, so that you CAN disconnect it. A
few have the wire connected directly to the core at winding start. You
obviously canīt disconnect these.
Seriesing on most MOTS poses insulation problems. Very few will withstand this
(using more than the two described above) for a longer period of time.
My Gardner type was one of the most rugged design I have come across. However,
in TC usage you will have to use a safety gap for each and every MOT. A single
safety gap across the string of MOTs wonīt protect all the xformers (similar
problem that H&R xformers seem to have). A 4 MOT Gardner Jacobīs Ladder is a
very neat arrangment. Anything under 1/8" copper tubing will melt after a few
minutes of operation. Running my Tesla coil on the same arrangement made the
sparks about three times as long (w/o further tuning) as the same coil setup
running on a 200W OBIT.
Forget about taking the core apart. You COULD mill off the weld beads (at the
expense of a mill bit, because the welded steel is literally heat treated).
But even milling off the weld beads wonīt help you much, because the MOTS are
usually plunged into a tank with insulation varnish. This stuff really goos up
the laminations and makes it impossible to seperate them w/o damaging at least
some of them.
I wouldnīt advise just taking a DMM (on 20A AC scale) and connecting it across
the hv secondary (like you CAN do on NSTīs) to measure short circuit current.
To find out the (aprox) output voltage: Connect it to a variac on the primary
side and use your DMM on the secondary (highest AC voltage setting). Slowly
ramp up your variac until you get the highest (safe) reading possible on your
DMM. Then disconnect the DMM from the secondary and connect it to the primary
side. Now you can calculate the voltage ratio.
If you have a HV probe, you can of course measure it directly.
After you know the turns ratio, you can measure the primary current (using
your variac, too) with the secondary shorted. This will let you aproximate how
much secondary current is flowing. Like I mentioned above I would guess your
MOT has a 1-1.3kVA rating. Take note of the winding temperature. Usually the
primary will get hotter, sooner (My Gardner type stay cold after a ten minute
run on Jakeīs Ladder. They are the only ones that will do this). Anything
above 75°C is too high. You can start to limit the primary current until your
windings stay under the 75°C danger mark. That way you can aproximate the
output VA.
I wanted to build up a 13.8kV-at-700mA psu for my 8" TC. I gave up on this idea
for various reasons, tho.
You can look at it on:
http://www.geocities-dot-com/CapeCanaveral/Launchpad/4902/
in the folder TeslaCoils.
You can also have a look at
http://www.geocities-dot-com/CapeCanaveral/Launchpad/4902/MicrowaveStuff
to see how dangerous microwaves or MOTS really are. Treat a MOT as you would
treat a pole pig. There isnīt much difference !!
(For size comparison the Jacobbīs Ladder is about 15" long.)
More pictures to come......
hope I could be of help,
coiler and MOT greets from germany
where (sob, whimper) you canīt get hold of a small pole pig,
Reinhard
I salvaged my first microwave oven transformer today. It is out of a
Whirlpool microwave, home unit, not commercial but a large oven. Not sure of
the rated wattage. The transformer says made by Korea Transformer Co. The
primary wire is .053" in dia. which would be about # 15 or #16 awg. The
secondary is about .015" in dia. which would be #26 or #27 awg. I understand
I should remove the ground connection from the H.V. secondary to the core and
isolate it for use. What do I do with the filament transformer windings
located between the primary and secondary? Just cut the ends off and forget
about it? I don't see how I could get it apart to get them out, the core is
welded together.
The core is an E core measuring 3.25" high, 2.18" wide and 3.75" long. It
weighs a little over 8 lbs. I can't see any shunts in the core but they
could
be there. What would the MOT experts out there expect this transformer to be
rated at? How do you find out if it is shunted - try the jacobs ladder test
and see if the lights go out?
Ed Sonderman