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FW: Which DC Drive?



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From:  Steve Young [SMTP:youngs-at-konnections-dot-com]
Sent:  Monday, June 08, 1998 11:34 PM
To:  Tesla List
Subject:  Re: Which DC Drive?

  Greg Leyh replied:

> If you are using a shunted xfmr such as an NST or MOT, then a
> charging reactor is not required.  The reactor is used to limit
> and define the charging current in a 'hard' DC supply, whereas
> the MOT's mag shunt will define the max current in your system.
> 
> I don't know if the BIL rating is sufficient on MOT's to put
> them in series; will they operate with one output term at gnd?
> -- 
> 
> 
> -GL
> www.lod-dot-org
 

Greg,

Thanks for the reply.  Yes, my DC supply will be self-current limiting,
thanks to MOT mag shunts.  My idea for also using a charging reactor (e.g.
made with secondary of one or two additional MOTs) was to prevent the spark
gap from completely discharging the supply output filter cap on every bang,
requiring the supply to completely recharge it.  I thought with a reactor,
the supply output cap would supply roughly only enough energy to charge up
the TC primary cap.  This assumes use of a RSG and a long enough time
constant (reactor & supply cap) that the RSG would stop conducting before
all the supply cap energy was dissapated across the gap.  The idea was to
improve efficiency and reduce spark gap electrode erosion. 

But on further thought (inspired by your post), voltage doublers don't
really need an output cap as long as they are supplying a capacitive load,
such as the TC primary cap.  Eliminating the supply output cap would also
eliminate any need for a charging reactor.  I will give this a try, but all
comments on this approach are welcomed!      

(The DC supply itself uses two MOTs tied together and grounded at their
cores, acting the same as a NST.  Each MOT "side" has its own 2-diode
voltage doubler.  So MOT voltage breakdown is not a problem.  See below.)  

             MOT     C        D                               C pri
             |||0---||----|--|>|--|--- + about 5 KV ------|---||-----|
     -------0|||0         |       |                       |          |  
            0|||0    |-|>|-      --- C (don't need?)      |          |  
     -------0|||0    |  D        ---                      |        L 0
             |||0----|------------|---- Gnd               o      pri 0
                     |                                    o gap      0
             |||0----|------------|                       o          0 
     -------0|||0    |           ---                      |          | 
            0|||0    -|<|-       --- C (don't need?)      |          |
     -------0|||0      D |        |                       |          |
             |||0---||----|--|<|--|--- - about 5 KV ------|----------|
             MOT     C        D

Suppose I knocked out the MOT magnetic shunts, or had some other "hard" DC
supply.  Again, what are guidelines for sizing a current-limiting charging
reactor?

Thanks in advance for comments,

Steve