Re: Ballasting question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Ritchie,
            I do have one point to add ;)

On 27 Jun 01, at 16:54, Tesla list wrote:

> Original poster: "R.E.Burnett by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<R.E.Burnett-at-newcastle.ac.uk>
> 
> 
> Hi Michael, Chuck, all,
> 
> I see the functions of the ballast as follows:
> 
> 1. It limits the current flow at the instant when the spark gap fires and
>    shorts out the HV supply.  Without some ballast inductance,  the arc
>    would reflect an almost dead short across the line and trip the supply
>    breaker.  The ballast inductor limits the rate at which this short 
>    circuit current can rise during the brief conduction of the spark gap.
> 
> 2. It defines the capacitor charging current.  The inductance in series
>    with the supply defines the rate of rise of the current after each
>    firing of the gap.  Without any ballast,  the charging current would
>    be infinite immediatley after each discharging of the capacitor.  Not
>    healthy for either the transformer or capacitor ;-)
> 
>    The inclusion of a ballast inductor (or resistor) defines the charging
>    profile for the tank capacitor in between bangs.
> 
> 3. The ballast limits the fault current to a safe value if a short circuit
>    occurs on the HV side for any reason.
> 
> The ballast can be either inductive or resistive,  and can be placed at
> either the low voltage (primary) side of the transformer or at the
> secondary.  The operation of resistive ballast is fairly simple.  It
> results in the classic RC exponential charging characteristic.
> 
> I posted a step-by-step description of how I percieve inductive ballasting 
> to work the other day.  The fundamental difference that I see is this:
> 
> A resistive ballast wastes energy as heat in order to limit the
> current.  Whereas,  an inductive ballast utilises this energy to charge
> the capacitor to a higher voltage. (No energy is wasted with the 
> inductor.)
> 
> I agree with Chuck that the ballast resonates with the tank capacitor at a
> particular frequency,  but I personally think we should get away from the
> fascination with resonance at the supply frequency.  (50Hz or 60Hz)
>
> I particularly dislike the "Xl cancels Xc" theory for one reason.  This
> only applies in the STEADY STATE and at ONE FREQUENCY !  Yes,  if you
> apply a pure 50Hz signal with the cap across an NST and wait a while,  you
> will get terrific voltages and currents if there is no spark gap.  But,
> this occurs due to resonance over MANY cycles,  and is not what generally
> happens in normal TC operation.
> 
> Just because Xl = - Xc,  does not imply that the charging current is
> always limited by stray resistance alone.  This is a resonant circuit in
> which the steady state current will be high,  but we typically do not let
> the resonant charging circuit ring for more than half a cycle at most,
> before depleting the cap of all its stored energy !  In my opinion
> resonance at the line frequency is a moot point since it never has chance
> to really get going ???
>

It does define a charge time for the cap which does have a bearing on 
static gap operation (lumpy vs smooth), especially if the gap is set, 
say to the peak transformer (non-capacitor loaded) voltage.

Regards,
malcolm