Re: DC TC

To: tesla-at-pupman-dot-com
Subject: Re: DC TC
From: Greg Leyh <lod-at-pacbell-dot-net>
Date: Sun, 16 Feb 1997 22:21:48 -0800
Subscriber: lod-at-pacbell-dot-net Mon Feb 17 10:16:48 1997
Gary Weaver wrote:

> Has anyone built a DC powered Tesla Coil other than a tube type Tesla
> Coil?

[snip]

  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 300 PPS.  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 are 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 offers a few advantages over an HV AC system:

  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.

  I wonder what Tesla would think about powering one of his coils from DC?
(remember War of the Currents?)  Perhaps it would be sufficent consolation that 
the DC part is powered by polyphase.

[snip]

> The next question is what is the best way to pulse a DC power supply
> with out a tube?  How about a DC power supply with an adjustable charge
> rate for the capacitor bank. The discharge rate threw the spark gap
> could be adjusted to find resonate frequency?  The capacitors would have
> to be sized correctly for the proper charge rate.

By 'best', do you mean cheapest or most efficient?  Spark gaps appear to be
the cheapest alternative at this time, while Modified GTO's are the most 
efficient and least maintenence-intensive option nowadays.

-GL