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Re: MOT - PSU



Hi "jkooi", all,

Sorry, but somehow I overlooked this post yesterday.
Comments below.

>Original Poster: jkooi-at-wish-dot-net
>Hi Reinhard, all
>I'm building a TC with MOT's also, but still can't seem to fix my
>problem of blowing my 16A fuses When I measure it I see that
>it is taking 40-50Amps from my wall socket before blowing my
>fuse :-((

>I've got 2 MOT's (different ones, since identical ones are hard
>to find) connected the primary's of the MOT's in parallel, and
>the secondary's in series. It doesn't matter if I switch
>connections of the primary, still the same. When I take one of
>either of the MOT's , it works fine, but together.........POW.
>Each one of the MOT's alone takes a normal 6.6Amps, and
>it's own limiting works just fine when it is on it's own.

First of all, you MUST recognize that the shunts in a MOT DO
NOT act as true limiters in a full short. A magnetron NEVER
(unless it is bad) presents a full short, so the manufacturer
can skimp on shunt design (a few cents times a few 10
thousand MOTs equals $$$). You MUST externally limit the
current in TC usage. If you donīt, you WILL fry your xformers
sooner or later.

It may sound stupid, but are you SURE that you are seriesing
the secondaries (per my described method)? It MUST make
a difference how you phase (primary connections) the two
MOTs, because in one case the two outputs will add up and
in the other case the will negate (and cause your fuse to blow,
esp. if they are of different voltages).

Could you please, describe EXACTLY, what you are doing
(+ a schematic)? Does your fuse already blow, as long as
the two outer MOT connections are still open? Or do you
measure the high primary current with the outputs shorted
or across a spark gap (which would be bad for your DMM)?

The reason your 16A outlet can supply 40-50 amps for a
short period of time is due to the time lag of your c.b. It
is designed for 16A continuous usage. Short term (pulse)
overload isnīt supposed to pop your breaker right away,
otherwise you would be resetting your c.b. every time you
plug an inductive load at or near the 16A borderline into
your wall outlet. Usually, anything more (for standard c.b.īs
ONLY) than 3-5x their rating will pop the breaker instantly.
In my shop (and coil supply mains), I have a special breaker
called a "K" type, which allows current surges of up to 20x
the normal rating (being 3ph 25A) before popping instantly.
Usually these are used in the industry and / or for very heavy
inductive loads. They are also quite a bit more pricey than
their normal "B" (2-3x rating) or "C" (3-5x rating) counterparts.

Coiler greets from Germany,
Reinhard