Re: A double resonance solid state Tesla coil

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

Original poster: "Jolyon Vater Cox by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>

Pardon me, but isn't having a capacitor in SERIES with the primary winding a
recipe for HUGE circulating currents which will not simply be confined only
to the primary and associated capacitor but will also flow in the DC power
supply rails, the rectifier and the mains wiring as well?

Would this not be the unfortunate consequence of making the primary part of
a series resonant tuned circuit where by definition the capacitive and
inductive reactances cancel at the resonant frequency leaving only the
resistance of the wiring to restrict the current?

If this is the case and presuming  the primary resistance is small, then
logically the circulating current will be LARGE.

What you are saying sounds suspiciously like power-factor correction where
capacitors are connected to inductive loads to eliminate the inductive
wattless current that would otherwise circulate on power distribution
networks only here the capacitors are connected in parallel with the load,
not series. This means that the inductor and capacitor form a tank circuit
and a heavy circulating current flows in the "tank" but not in the supply
wires.
The reason for this is that the dynamic impedance (i.e. the impedance "seen"
across the supply wires) of the parallel-tuned circuit at resonance is high,
in theory infinite if resistance is zero.

So,  from the point of view of reducing the effect of heavy circulating
currents in the DC rails and mains wiring wouldn't  a capacitor in parallel
with the primary be better than one connected in series?

Jolyon.

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, December 30, 2002 3:46 AM
Subject: Re: A double resonance solid state Tesla coil


 > Original poster: "Jan Wagner by way of Terry Fritz <teslalist-at-qwest-dot-net>"
<jwagner-at-cc.hut.fi>
 >
 > Hi,
 >
 > On Sat, 28 Dec 2002, Tesla list wrote:
 >  > Original poster: "jimmy hynes by way of Terry Fritz
 > <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>
 >  > I've posted a schematic and some simulated waveforms at
 >  > http://www.hot-streamer-dot-com/temp/drsstc.doc
 >
 > I hope the mosfets/igbts and diode backtabs are insulated from the
 > aluminium heatsink, right? ;)
 >
 >  > I think there's a clear advantages to having a resonant primary circuit
in
 >  > addition to the 'coupled' secondary resonance. The SSTC is efficient
for
 >  > 'continuous wave' energy transfer, but to achieve the quick secondary
 >  > voltage rise required for streamers, a very low primary inductance is
 >  > required. This results in very large magnetizing currents with low
power
 >  > factor. Soft switching loss is low in most designs, but when you are
trying
 >  > to get quick rise times, there is so much magnetising current that you
 >  > don't switch at a zero crossing anymore. The conduction loss is also
large
 >  > because of all the extra current, so most SSTC designs are stuck with a
 >  > soft rise.
 >
 > Yes... basically you can get rid of the magnetizing current going
 > "nowhere" by using a series capacitor as you did.
 >
 > The downside of making the primary capacitor and shunted inductor resonant
 > at the resonant freq of the TC secondary is that you'll get a huge current
 > circulating in the pri cap and inductor.
 >
 > I just did a model copied from your simulation model, and with a 1V
 > sine drive voltage (yup, just one volt!), it ended up with an rms current
 > of 17A. The peak was around 69.45kHz. Re-running with a 230V sine drive
 > voltage, the circulating current was as high as 2800A.
 >
 > Unloaded, the system is low-Q (i.e. high reflected TC secondary
 > load/streamers impedance). With the streamer load, impedance goes down
 > and Q goes way up. You'll have to be careful in designing the pri<->sec
 > reflected impedance, so that you still end up with a low-Q loaded circuit
 > and more current into the secondary than circulating in the primary. This
 > is just from a quick-and-dirty simulation. It may or may not be a
 > problem in a real running SSTC... these Q considerations are from a
 > "slightly" different circuit, so it just could be it isn't a problem here
 > (?)
 >
 > But in any case, you'll have to make the primary side capacitor large
 > enough (not capacitance wise, of course) for it to handle the higher
 > currents involved in the double resonant scheme. After that, i'd
 > guess it will work very well  :o)
 >
 > good luck!
 >
 > cheers,
 >    - Jan
 >
 > --
 > *************************************************
 >   high voltage at http://www.hut.fi/~jwagner/tesla
 >   Jan OH2GHR
 >
 >
 >