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Re: Current Limiting and MOTs
Tesla List wrote:
>
> Original Poster: RWB355-at-aol-dot-com
>
> Hi Bart and all,
>
> Bart wrote:
> " MOT's generally have magnetic shunts and are therefore self current
> limiting."
> SNIP
>
> While this is generally true, these magnetic shunts are designed to run a
> magnetron and that is the catch. The magnetron does not seem to act as a
> complete short.
>
> I have measured various MOTs (hoping to be able to use them). 8 out of 10
> different manufacturers I measured, will burn up in a complete short.
> Depending on the type, either the primary gets so hot, that it starts to
smoke
> or the secondary will get very hot in a few seconds (no smoke, tho).
>
> The Gardner variety seems to be massively overbuilt and capable of
surviving a
> complete short. These MOTS are about twice as big (core size) as a "normal"
> MOT.
> Their insulation is also nicely overrated. I was able to connect 4 of
them in
> series (with no center-tap grounding) and they did not arc over. I added a
> fifth MOT and it started to arc from the core to primary. However, the fifth
> one was NOT a Gardner. So my idea was using 2 x 4 Gardner MOTS under oil
for a
> nice compact 13.8kV 700mA PSU. I would have grounded the "center tap"
between
> MOT#4 and MOT #5, so that the MOTS would have never seen more than 4x their
> output voltage (which they survived) Unfortunately (for me) the Gardner
> variety is impossible to come by, so I gave up on this idea.
>
> Another problem with seriesed MOTīs is the fact that EACH MOT MUST have a
> seperate safety gap (even the Gardner type) across it (for Tesla usage). A
> single safety gap across all MOTs doesnīt seem to work. (Similar to the H&R
> transformers).
>
> For more info on my experiments and comments about MOTs have a look at the
> archives.
>
> Will Payne has a nice MOT psu supply idea. The only problem with his setup
> (was discussed on the List, too) is that you will need 12 MOTS to build it.
> Finding 12 MOTS of the exact same type and company (same batch would be
best)
> is next to impossible. I do NOT recommend mixing MOTS of different types
(even
> if V & A are the same). Each manufacturer builds his MOTs a little
different.
>
> MOTless greets from germany,
> Reinhard
A magnetron in no way behaves like a resistor. As far as the current
vs voltage relationship, it is much more like a large, HV zener diode
with a threshold voltage only a few percent below the voltage at which
it develops rated power. A relatively small value of leakage reactance
would give the desired current regulation for a maggie load, but
wouldn't keep the short-circuit current within safe bounds.
Ed