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[TCML] Re: Pros and cons
As concerns control and overall regulation,there's no question about it
,SSTCs are superior.Regarding the issues peak power vs. average power,
there are essentially 3 different variants of SSTCs.
First,SSTC with continiouos wave excitatation.The peak power is close
to RMS input power,and you're not interested in that.
Second,DRSSTC which has much higher peak power than input power,and superb
possibilities of control.You're definitely interested in that.But DRSSTC are
the most complicated tesla coil topologies and more sensitive to tuning etc.
Third,is the version nicknamed OLTC,and here an ordinary spark gap is replaced
by IGBT.Mode of operation is the same as classical SGTC and peak power
just as well.Still you have great possibilty of control,and you're definitely
interested in that too.Since you are targeting frequencies around 25 khz,which
is rather low for tesla transformers,and this version benefits from lower
resonant frequencies ,I'd recommend serious consideration of it.
Tesla circuit has the topology of a series resonant converter,and as such
it has some advantages and some disadvantages.
>From the practical point the main disadvantage is a very low
surge impendance of a primary circuit due to lower voltage IGBTs are rated for.
Regarding IGBT application,during turn off,the IGBT reacts relatively slow
(becouse of the charge stored in its' junction),and if the voltage across IGBT
is still high the turn off loss will be high.To force the current to 0 before
turning off the device is the way to eliminate these loses.
Zero current switching which can be accomplished operating the circuit below the
resonance frequency,where the current through IGBT reverses before the branch is
turned off,requires the connection an antiparallel power diode to capture
reverse current.This way switching off losses are considerably reduced
(normally,in a real world there are no zero-loss power diodes and very action
of commutation dissipates some additional energy too).
On the other hand ,IGBT switching loss at turn on ,in the converter operated
below resonant frequency,can't be reduced due to the fact that IGBT are hard
switched on.Fortunatelly,at switching frequency 100 Hz these losses are
comparatively low.Thus,total silicon loss is somewhat higher than merely a
product of IGBT on state forward voltage drop and average current through it.
Conduction state IGBT losses are mostly the function of the average current
rather then rms current.High peak currents often found in tesla primary circuit
aren't so important.This fact makes IGBT an excellent device for tesla coil
applications.And as concerns losses in the winding of primary coil,primary
in condenser,and due to perhaps inductive coupling to surrounding objects,these
are hardly calculable.They are function of rms current,frequency,geometry
and differ from case to case.Unfortunatelly they are significant ,and sometimes
even make dominant part of total dissipation..Overall dissipation can be
determined only by measurements.So,"how efficient tesla coil can be?" is not
easy question to answer to.People have measured transfer efficiency of
energy from primary to secondary to be as high as 90% on some SGTC.
Wether this can be achieved by described IGBT coil,or even surpassed,largely
depends on engineering and constructing partameters.
I don't know if anybody ever measured something like that on such coil ,becouse
there are only few of them and they old what,10 years perhaps?
SGTC are over 100 years old.
Thinking of efficiency I would say there is general agreement that
DRSSTCs are the most efficient version among all tesla coil versions invented
up-to-date.
Dex
From: <uhvsystems@xxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Thursday, November 26, 2009 2:41 PM
Subject: [TCML] Re: Pros and cons
> Hello,
> > > Decision making about building tesla coil in 5-10 kw power range,what
> > > would be a better choice:
> > I) Standard coil design with a rotary spark gap
> > OR
> > II) High peak power solid state coil > > ?
> > Keen to know advantages and disadvantages from
> > practical,experimental,economical and other aspects of coiling.
>
>Have you built either before? Or worked with high power RF? (e.g. if you've
>built a 20kW RF Power Amp, then you're in a different situation than if all
>you've done is hook up a flashlight bulb and a D-cell battery)
>The peak power on a spark gap coil is higher than the SS. SS coils have a
>lot of flexibility for controlling the behavior (e.g. if you want to have
>your coil sing songs, SS is the way to go). If you're interested in
>studying >spark propagation, an SS coil might be the better way.
I'm not that much of a musical type to make tesla coil which can sing songs
:-)
Well,my company manufactures high voltage gear.
Beside standard LI tests some products are desirable to test to
bidirectional OSIV .
For preliminary testings and in the conjucion with modified Marx generator
we have,I think that tesla coil
might serve good for generating fast OSIV and return OSIV transients.
At least 800 kV,25 kHz output is required,and aproximate calculation goes
like 5 kW at about 50 impulses per second
or 10 kw at 100 impulses per second should be enough for the purposes.
Basically,we are exploring possibilities tesla HF generators offer here and
are open to suggestions.
How efficient tesla coils can be?
If the peak power on spark gap coil is higher than on SSTC,can you recommend
best solutions for controling the spark gap coil behavior?
Regards,
Fez Zaev
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