Re: [TCML] Re:Pros and cons

[Steve Ward <steve.ward@xxxxxxxxx>]

Fez,

It sounds to me like you know what you are doing, to the point that it will
work.  the IGBT you have should be fine for the modulator approach you are
going for because the long turn-off tail is mitigated by turning off the
IGBT switch while the primary current is free-wheeling through the IGBT
anti-parallel diode (giving a huge window for zero-current turn off).  Those
IGBTs would NOT be very suitable for DRSSTC operation, where an H-bridge of
IGBTs must switch at the coils resonant frequency (its basically a big AC
drive to excite the primary system).

155J is an *impressive* energy.  Something on that size would have the
capacity of producing sparks in the 20 foot range (if optimized for spark
production), though i realize that is not your goal.

Steve Ward

On Sun, Nov 29, 2009 at 12:17 PM, <uhvsystems@xxxxxxxx> wrote:

>
> Well,it appears that for our purposes 3rd variant is the most adequate
> then.Having less complexity than DRSSTC but still offering enough of the
> control.Moreover,I checked files of the power electronics department to
> reveal some CM 600HB-90H modules are kept in stock.Rated 4500V,900A they are
> big IGBT bricks (maybe even too big).I'm not that much into power
> electronics to be sure if the module is suitable for tesla coil service I
> have on mind.Will investigate in their catalogue to find out more
> details.Generally such devices are quite robust,hot-spot and current
> crowding problems are nonexistent,and the need for external snubber circuits
> is minimal.If we manage to make it work in tesla coil service that would be
> great.With primary tank capacity 20 uF charged up to 4400V, and with tesla
> coil efficiency say about 80%, 155 joules could be supplied to tesla coil
> secondary circuit.The estimate  is sufficient to cover upper testing limits
> we are prepared to go to.
> I must say few words about the efficiency and where is its importance for
> us.
> The importance is mainly in the point that efficiency determines primary
> energy needed to charge secondary circuit to predetermined voltage.From the
> point of technical operation and reliability it is also important since
> nobody wants to overheat or blow up things due to increased dissipation.
> >From the point of chasing some high efficiency figures for the sake of
> economical power utilization,well...that is totally irrelevant to us.
>
> Regards,
>
> Fez Zaev
>
> >
> Dex Dexter wrote:
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 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
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla