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Re: BIG Xfmr - current limiting

Nick,

The three MOT's in series may be severely stressing the, core to
secondary winding, insulation (depending on what the insulation looks
like).  There may be a work-around by submerging them in oil . . . or
take your chances if they are cheap.

The idea of just putting a 4KW, five volt transformer, in series with
the primaries of the MOTs, is not much of a current limiting solution. 
If it works, it would be more by accident, than by design.  A shorted
transformer is, just that: shorted.  It won't go a long way towards
limiting current, and won't provide any active current limiting.

You may be able to do something like put a power rheostat on the 5 volt
side for current adjustability, but a series variac would be a better
choice.  

Ideally, what you want for ballast, is something that will actively
regulate current. (act like a dead short when the current is light, and
increase in impedance when the current tries to exceed some value . . . 
A shorted NST for instance, or a welding transformer, or other current
regulating inductor, tungsten light bulbs, ballast tubes, etc.)

It wouldn't be all that tricky to use a simple self-biasing transistor
current limiter, on the low voltage side of a transformer.  A five volt
transformer would be pulling 800 amps, on the secondary, at 4 KW.  That
is too high for a cheap transistor current regulator.  Use a 50 volt
transformer, and it could be done with eight inexpensive transistors, a
full wave bridge, and load resistors . . .

The easiest way is to get a welding transformer, electric space heater,
or wind an adjustable inductor.

good luck
bob

Tesla List wrote:
> 
> Original Poster: NickandSim-at-aol-dot-com
> 
> Hi All,
>          I am planning a big coil. If any of this is wrong please say so.
> It is to be DC powered with 3 MOTs in series double rectified, and with 4kW
> 5v transformer with shorted secondary for current limiting    Large (3in)
> inductors between psu and tank.  A 0.04uF tank cap.
> A 2800rpm 8 electrode rsg with heatsunk electrodes. A 7mm copper pipe primary
> ,15 turn.  A 6in o/d secondary 24in tall wound with 24swg magnet wire.
> I would appreciate any feedback as It is a fairly big leap from a few hundred
> watts to
> 4.5kW.
> 
> Thanks In advance
> 
> Nick Field