Re: MOT trouble

["Area31 Research Facility" <rwstephens-at-xxxxxxxxxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

Finn,
 
Take a step back for a moment and look at the big picture.  What component are
you trying to protect with the use of this safety gap you are trying to set so
carefully.  If the answer is the cheap, disposable MOT, perhaps you don't need
to worry about the safety gap.
 
Robert W. Stephens
Director
AREA31 Research Facility
AREA31 Radio Observatory
Co-curator
Hangar31 Vintage TV Museum
<http://www.area31-dot-org>www.area31-dot-org
>
> ----- Original Message ----- 
> From: <mailto:tesla-at-pupman-dot-com>Tesla list 
> To: <mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com 
> Sent: Sunday, November 26, 2000 18:52
> Subject: MOT trouble
>
> Original poster: "Finn Hammer"
> <<mailto:f-hammer-at-post5.tele.dk>f-hammer-at-post5.tele.dk> 
>
> Gang!
>
> I have just discovered a problem with using MOT`s as transformers for
> Tesla Coils.
> This would not be so big a problem with static gap operation, but it is
> a problem with rotary driven coils.
>
> MOT`s are cheaply made transformers, engineered at the limit to meet a
> prize point, where a minimum of insulation, copper and iron is used.
> However I wanted to give them a try, now that they are cheap.
>
> The lack of copper in the primary winding reveals itself as a high idle
> current, there are just not enough turns to create the inductance needed
> for a low idle current draw. This is perhaps not a problem when the coil
> starts to draw current, but when trying to set the safety gaps, it
> becomes a nuisance.
>
> The way I set the coil up for work is this:
> I short the power controller, and turn the current limiting variac up,
> untill the current that I want to run is reached. Then I connect the
> MOT`s. However, with the heavy current draw at idle, a very large
> voltage is present across the current limiting variac, so I am unable to
> set the safety gaps. If, on the other hand, I remove the current
> limiting inductance, by turning the variac towards full stop, I have
> another problem: Although this produces the full voltage at the output
> terminals of the MOT`s, when the safety gaps fire, I also see an intense
> current flowing, causing the fuses to blow. Putting damage, although not
> irrepairaple, to a single winding of the current limiting variac.
>
> I guess the only way to get around this problem is to either knock the
> shunts out of the MOT`s and use the liberated space for additional
> primary windings, or set the gaps with the assistance of another, more
> well behaved transformer.
>
> Any ideas?
>
> Cheers, Finn Hammer
>
> (Guessing it is time to wind another porkchop)
>
>