MOT trouble

["Finn Hammer" <f-hammer-at-xxxxxxxxxxxxx> (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

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)