[Prev][Next][Index][Thread]

Re: "DC drive"

DavidF wrote:

> I have always been interested in a DC coil, would you tell me more about your
> powersuply and gap and how the DC affects the design and performance of your
> coil as compared to an AC neon garden variety?


  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 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.

  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 as 
long as the DC part is powered by polyphase.

-GL