Re:: Ballasting question

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

Original poster: "Charles Hobson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <charles.a.hobson-at-btinternet-dot-com>


Hello Richie,

Thanks for your helpful explanation on the Tesla list "ballasting Question".
I had a quick look at your web page on the subject to get the flavour of it
and it appears to answer a lot of my questions. Anyway I copied this most
excellent tutorial on ballasting  for a closer study later on.

I always felt uncomfortable with  my "steady state" approach to Tesla Coil
problems. As indicated, the steady state approach is not fully applicable
to the transient nature of Tesla coil operation. I had a go at Laplace
transforms a long time ago and appreciate their application in transient
analyses, but my memory has faded somewhat since then. Anyway, I will look
into Microsim also and see if I can get my head around it.

Chuck

----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, June 27, 2001 11:54 PM
Subject: Re: Ballasting question


> 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 ???
>
> I feel that the resonant frequency of the charging circuit should be
> chosen to complement the firing rate of the spark gap,  not the supply
> frequency.  i.e.  The ballast and tank capacitor should be chosen so that
> the capacitor is charged to peak voltage in the time between firings of
> the spark gap.  This has been the aim of the work I presented a couple of
> weeks back.
>
> Chuck,  the behaviour of the circuit can be modelled with differential
> equations.  It reduces to a series resonant circuit,  in which the
> capacitor voltage is periodically reset when the spark gap fires.
> Unfortunately, most circuit equations are "geared-up" for analysing
> resonant circuits in the steady state,  rather than this transient or
> "time-domain" analysis.
>
> I use Microsim for most of my analysis work because it does all the
> Laplace transform stuff for me.  (I'm an EE not a mathematician.)
> Microsim is really great for getting a feel for what is going on with
> resonant charging and sync rotaries.
>
> There is some info on ballasting and resonant charging on my web page too:
>
> www.staff.ncl.ac.uk/r.e.burnett/ballast.html
>
> I'm glad that there are more discussions about ballasting and res charging
> lately,  as I feel that the charging circuit is pretty important for good
> TC performance overall.
> Cheers,
>
> -Richie,
> On Tue, 26 Jun 2001, Tesla list wrote:
>
> > Original poster: "Charles Hobson by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <charles.a.hobson-at-btinternet-dot-com>
> >
> > ----- Original Message -----
> > From: Tesla list <tesla-at-pupman-dot-com>
> > To: <tesla-at-pupman-dot-com>
> > Sent: Monday, June 25, 2001 11:15 PM
> > Subject: Ballasting question
> >
> >
> > > Original poster: "Michael O. Poley by way of Terry Fritz
> > <twftesla-at-qwest-dot-net>" <mpoley-at-mindspring-dot-com>
> > >
> > > Okay, since I got such a fast response on the other question, let me
ask
> > > this...
> > >
> > > Can someone explain the theory and practice of "ballasting"?  I'm
familiar
> > > enough with the physics behind the Tesla coil, but the term has been
> > > bandied about a lot and I haven't seen anything explaining what
exactly
> > > ballasting is nor when it should be done, and when it is acceptable to
> > > overlook it.
> > >
> > > Mike
> > >
> > > Michael O. Poley
> > > mpoley-at-mindspring-dot-com
> >
> > Hi Mike,
> >
> > I hope you get some good clear answers to your question. I am looking
> > forward to them also. My understanding for ballast is limited relative
to
> > Tesla Coil application, but as I see it, the ballast in needed for
systems
> > using "pole pigs", transformers where if the secondary is shorted, "like
> > with a spark gap" , the primary see the short also. Hence, the minimum
> > inductance needed in series with the primary appears to be a function of
the
> > transformer's maximum VA rating. (You experienced guys please correct me
on
> > this if I am wrong)
> >
> > But that's not where it seems to stop. Between bangs the transformer is
> > looking at the Tesla Coil's primary tank circuit capacitor Cp. Thus,
there
> > appears to be an LC series circuit comprising Cp and L where L = ballast
> > inductance x N^2 (pole pig turns ratio). Please again you experienced
guys,
> > jump in and correct me if this is wrong. If I am right in this, then the
> > ballast inductance can be increased from its minimum value to where it
will
> > resonate with Cp at 50/60Hz. Again the transformer primary circuit will
> > behave as a short circuit at resonance. So--- some value between the
minimum
> > inductance and the resonating inductance needs to be worked out. I think
> > inductances greater than the resonant value can also be
considered.(Please
> > again you experts, help!!) There seems again here to be computer
programs
> > around to work this all out, but I would like to see and understand the
> > algorithms used.
> >
> > I hope this is helpful here or has stirred up better and more correct
> > answers to your question.
> >
> > Chuck
> >
> >
> >
> >
>
>
>
>
>
>