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Re: Continued Problems (fwd)
---------- Forwarded message ----------
Date: Thu, 21 May 1998 08:50:43 +1200
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Continued Problems (fwd)
Greetings Bart, All,
> From: Barton B. Anderson [SMTP:mopar-at-uswest-dot-net]
> Sent: Monday, May 11, 1998 1:38 AM
> To: Tesla List
> Subject: Re: Continued Problems (fwd)
>
> Malcolm,
>
> Tesla List wrote:
>
> > ----------
> > From: Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
> > Sent: Sunday, May 10, 1998 3:29 PM
> > To: Tesla List
> > Subject: Re: Continued Problems (fwd)
> >
> > Hi Bill (Turbett),
> >
> > > From: fxphoto [SMTP:fxphoto-at-centuryinter-dot-net]
> > > Sent: Friday, May 08, 1998 4:09 PM
> > > To: Tesla List
> > > Subject: Re: Continued Problems (fwd)
> > >
> > <snip>
> > > I was trying to ask about Xl and was
> > > typing Xc. Restated, I was trying to find out what effect the additional
> > > inductance of the current control devices (i.e. welder, variac, chokes etc.)
> > > would have on the secondary's inductance. Upon further investigation, it
> > > looks like there would be absolutely no affect on the Xl of the secondary
> > > since these devices control the current before it reaches the primary.
> >
> > Wrong. Whatever choke you stick on one side of an ideal transformer
> > appears on the other with its value modified by the turns ratio.
>
> Just trying to understand this. I understand the tanks reactance is affected by added
> inductance and therefore the resonant frequency changes, etc... , but are you saying
> the secondary coil which has it's own reactance, would change due to an inductance
> change on the input side of the tank?
>
> Bart
Sorry for not replying sooner. I have been away for a week or so and
unsubscribed. Someone may have already answered this query.
A crude model of the transformer primary + choke while connected
to the mains is that of the X of the choke at mains frequency
connected directly to the transformer primary since the mains is
effectively a short circuit (voltage source) in series with the two.
In this situation, the transformer is transforming the choke
impedance by the turns ratio squared to appear on the secondary side.
A simple example to illustrate using a transformer with a stepup turns
ratio of 1:2:
Let Vp = 1V and the reflected load cause a current
flow of 1A in the primary (assume no magnetizing current as this is
separate from the load current and may be made infinitesimal by
using a huge primary inductance). Then, Vs = 2V and Is = 1/2 Amp.
Zp = 1 Ohm (1V/1A) and Zs = 4 Ohms (2V/0.5A) for Ns/Np of 2/1.
One can model an equivalent circuit of the primary connected directly
to the mains and a choke appearing in the secondary side modified as
above. It is this effective choke that forms a resonant circuit with
the Tesla primary cap.
To illustrate the resonant charging mechanism, assume such a
transformer (e.g. NST) and the gap set such that it fires once per
half cycle. The gap fires and the cap empties into the Tesla secondary
in a few tens to hundreds of microseconds depending on the Tesla Fr,
k and energy loss from the secondary. Compare that timescale with the
mains (8.3333mS per half cycle for 60Hz mains). The gap goes out when
the cap energy has gone. Now, the cap partially recharges as the NST
comes down off its peak voltage. As the transformer swings on through
zero volts to the peak of opposite polarity on the next half cycle,
total voltage the cap will reach by the next gap fire (assuming the
gap is set wide enough) is Vs of the NST + the voltage of the
partially recharged cap. In theory, Vcap can reach 2Vs in half a
cycle of charging. In practice it is somewhat less as recharging duty
cycle is less than 100% and there are resistive losses in the
circuit.
For the purposes of analysing Ep x BPS in the coil survey I did,
I took all static gap systems allowing Vcap to reach SQRT(2) x Vs. In
many cases it would have been more. The consequences of a gap set
wider than 2Vs are now obvious. Since I^2.R losses in the charging
resonant circuit rise with voltage (V^2/R), the system if allowed to
by the gap setting can ring up until those losses on each half cycle
equal what the transformer can deliver on each half cycle. I have
measured Q's in the vicinity of 6 for a 12kV NST and cap of 25nF.
Malcolm