Re: Concerning Peter's DC Drive
From: Greg Leyh[SMTP:lod-at-pacbell-dot-net]
Sent: Friday, July 18, 1997 1:04 PM
To: Tesla List
Subject: Re: Concerning Peter's DC Drive
Larry Robertson wrote:
> I also have been thinking for a while that a DC drive would
> be not only more efficient, but would also make the rotary gap
> completely uncritical as to speed, up to maximum power delivery.
Yes, indeed. Very useful for certain applications.
> As I was contemplating on how to get the small resonance cap.
> recharged, I thought that we use a pulse discharge already,
> so why not pulse charge it as well. If the large cap is big
> enough compared to the small cap., having them in series from
> the point of view of the primary coil should not change the
> resonant frequency much. For instance, if the small cap is
> 0.02 uf and the storage cap is 1 uf, when viewed in series
> the capacitance is 0.0196, not much of a difference.
An important point to remember however:
If a capacitor is charged through a resistor, then the
resistor dissapates as much energy as the capacitor receives.
A spark gap is, unfortunately, a resistive element, and therefore
its use in the charging path would result in a 50% reduction in
coil efficiency, right off the top!
The DC method can be more efficient, provided that a _reactive_
element is used to control the charging current to the primary
capacitor bank, instead of a resistive element.
The most common method is to employ a reactor, which is simply an
inductor that is optimized for handling reactive power.
(BTW, the energy storage cap and the charging switch can both be
eliminated, by the use of full-wave rectified 3-phase power.)
My coil uses a 3-phase DC power supply followed by a resonant charger
for providing the prime power to the primary circuit. The PS consists
of three oil-filled HV xfmrs (one per phase), followed by a 3-phase
full wave rectifier set consisting of six rectifier stacks arranged
in the standard way.
The resonant charger is a 2.5H, 10A, 28kV reactor in series with the
output of the rectifier stack, which gives the charging ckt a ringing
frequency of about 150 Hz (Cpri = 0.495uF). Cpri charges to twice the
DC voltage in one half-cycle of the 150Hz, allowing gap speeds of up to
The current pulse during charging is limited to Vdc/[sqrt[Lreact/Cpri]],
so no other current-limiting chokes or resistors are required. In fact,
the LV windings of the xfmrs can be directly connected to the 3-phase mains
when power is applied; either in the star configuration (by contactor A)
for low power operation, or in the delta configuration (by contactor B)
for full power operation.
A DC system is more complicated than a single-phase AC system, requiring
HV diode stacks, polyphase xfmrs and switchgear, and charging reactors, but
the DC method also offers a few advantages:
a) The ability to use polyphase power, for better load balancing at higher
average power levels
b) Improved power factor, due to resonant charging
c) Rotary gap speed does not need to be synchronized to the mains, allowing
the gap speed to be variable for output power control
d) Coil output voltage is more constant, since the primary charging voltage
does not cross thru zero twice each 60Hz cycle. This improves the arc
length somewhat, due to the finite ion lifetimes in the output streamers.