RE: Ballasting Basics

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Dean by way of Terry Fritz <twftesla-at-uswest-dot-net>" <deano-at-corridor-dot-net>

Hi Ryan,

>
>
> I have some basic ballasting questions.  First of all, am I to understand
> that it takes 10kva worth of microwave oven transformers in order to
> ballast a circuit to 10kva?  Since it is said that your ballast will only
> pull as much current as it will pull in it's normal environment,
> that seems
> logical.  If all that is true, why not just turn the MOT's around and use
> THEM to power the entire coil?
>
First, you could power the coil with MOTs. The voltage is low, though, so
you have some special problems to consider. You need a large cap to get the
bang size up, gap losses will be high, and quenching will be much harder to
achieve. These problems can be solved, however, and several people on this
list have done some work in that area. There are ways to get higher voltage
out than just two MOTs "back to back" will give. Some folks have
experimented with stringing several in series. A problematic approach, but
solvable. Another thing that can be done is to rectify the output using a
level shifter or voltage doubler design which works OK, but is also
problematic in its own right. If you are going to use DC, then you need some
way to limit the current on the output side of the High voltage power
supply. This may not be so much of a problem with a sync rotary and the
level shifter because the caps will act as current limiters, but if you have
storage or smoothing or filter capacitors (whatever you want to call them)
when the gap fires, they will be drained as the gap will not go out until
the energy in the caps is gone. So now you need to use a resistor or an
inductor to limit the current out of the DC supply. One way around that
problem is to use a rotary gap that alternately charges and then discharges
the tank cap, that is the gap fires between the storage cap and the tank cap
and the tank cap voltage rises to equilibrium and the gap goes out. then
another contact comes into play and the tank cap is discharged through the
primary coil. Another way that has been used with DC is a rotary gap that
fires several sets of contacts alternating the voltage across the tank
circuit in a configuration called H-bridge. This circuit has the problem of
more difficult to construct, but gives the advantage of giving twice
(effectively) the voltage across the tank circuit as the DC power supply
puts out. All of these methods work well with a rotary gap of some kind, but
a static gap is another ball of wax. You are back to current limiting, and a
static gap needs a different kind of limiting than a rotary does, perhaps
not in theory, but definitely in practice.

When using a MOT or a pair of MOTs in series to ballast a pig, the thing to
keep in mind is that with a Tesla coil you are not looking at a sine wave.
The current is drawn in spurts, and the amplitude of that current may be
quite high, but the average is more in a reasonable range. The MOT is
designed to work into a doubler circuit which makes the current pulses
shorter in duration but larger in magnitude than a simple rectifier. The
shunts in the MOT work in this mode of operation to give a non linear (NST
like) characteristic. IF however, you place a Jacobs ladder on the output of
the pig, The waveform becomes sinuous. (more so, anyway) The arc "looks"
very much like a resistor to the transformer, and a more linear impedance is
reflected back to the primary, which causes the current through the MOTs to
be more continuous rather than a series of short bursts. This will cause the
MOTs to overheat and self destruct in short order as they were not designed
for this type of duty.

Now in the case of a static gap, as long as it is not power arcing, you will
be OK. A single string of two MOTs should be fine based on the size of your
tank cap, at least as well as I can remember. However at this point I would
like to repeat that my experience with a pig and a static gap was that it
gave erratic operation when using inductive ballast. I needed to have at
least some extra resistance in the circuit to smooth things out and get
stable operation. At the time I did not have a variac that was large enough
to handle the pig, so it was all or nothing.