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

Gap Configurations and DC Excitation Questions...

Fellow Coilers,

While doing some thinking about how a rotary works in conjunction with 
the tank cap, pig, and ballast, some thoughts/questions came to mind:

1. It would seem that if one had large enough (>100 mH) secondary RF 
chokes, capable of withstanding the large RF voltage seen on the primary 
cap, it should be possible to run with EITHER the gap or the capacitor 
across the input power source, particularly with the BIL of a pig. Safety 
gaps could still be used for added protection. Has anyone tried this 
configuration recently, and is there ANY advantage over the preferred 
configuration (i.e., gap shunting the supply side) that would motivate 
its usage? In particular, would the gap be any easier to extinguish? Is 
this a blind alley?

2. A while back, there was a description of a large coil which used a 
large DC source coupled with a comparatively large energy storage 
capacitor. A variable "charging" inductor was connected from this source 
to the classical spark-gap/tank combination. When the gap fired, the 
inductance of the charging inductor limited the rate of rise of current 
from the large supply capacitor into the gap, allowing the gap to be 
extinguished. The DC source and charging inductor rapidly recharged the 
tank cap (at a rate that could be significantly greater than that 
available at 50/60 Hz). This approach looks like it had a number of 
significant advantages:

  - Maximum energy could be controlled via adjustment of the maximum DC  
    voltage and the amount of "charging" inductance

 ** Independence from 60/50 Hz supply and rotary speed interactions

  - ability to "tune" the system via rotary speed and charging           
    inductance settings.

  - elimination of the "dead-time" around the 50/60 Hz zero crossings in 
    AC excited systems.

  - improved power factor on the input power side to the pig

  - Elimination of the primary ballast, since the secondary charging     
    choke and rotary should limit "short circuit" current

  - Relatively inexpensive banked PFC, Filter, oil or other caps could 
    be used for the energy storage cap since it would not be seeing high 
    RF currents

The obvious downside is added complexity and the need to be CNSIDERABLY 
more careful in discharging the caps!. 

However, the ability to set break-rates independent of phasing to the 
50/60 Hz supply, at to operate with consistent/controlled pulsing at 
rates significantly higher than with AC excitation, may make it 
worthwhile. 

Any thoughts from those with more coiling experience?? Thanks in advance, 
and safe coilin' to ya!


-- Bert --