Re: [TCML] Greg's input on inductance

[DC Cox <resonance@xxxxxxxxxxxx>]

In an ideal coil Terry is absolutely correct.  Ideals do not consider
thermal changes.

Unfortunately we do not have ideal coils.  In reality there will be a change
in freq enough so that zero crossover trigger point will dynamically change.
If you hook up your scope you can see the cross over point changing as the
component values change.  Let a solid state coil run for 3 minutes and
carefully log the zero point and you will see what I mean.

The more current in, the more power, and more heat.  Due to thermal
properties the cap value will change slightly, the inductance will change
slightly.  All these changes will change the res freq of the system, and
even this small shift will not affect a classic coil.

This small shift will definitely affect a solid state coil as it affect the
zero crossing point.  As the res freq is constantly changing then the zero
point crossing is also dynamically changing.  This can NOT be adjusted
dynamically with passives.  Unfortunately, you have to monitor the tank
circuit.  Prediktor circuit does NOT monitor the tank circuit --- only the
driver circuit.

We must also consider the drive circuit is a square wave and the tank
circuit is definitely not a square wave. This also leads to problems that
the prediktor circuit does not address.  You can not do it with passives.
You will get it close, but as component values shift a change will occur.

In an ideal coil it will work.  In the real world it will not work as the
component values have shifted slightly after 30-60 seconds of operation.
Now the zero point crossing will be different.
Active components must be used to control it, not passives as in the
prediktor circuit.

As soon as the temp changes a small amount the zero cross point will
change.

Solid state triggers require much more precision than normal classic coil
circuits.  Bad timing leads to tremendous heat buildup in the IGBT
components especially if the zero cross is not actively tracked.

If you do a nodal analysis and enter all the variables in you will see what
I mean.

Dr. Resonance




On Thu, Jun 25, 2009 at 8:44 AM, Lau, Gary <Gary.Lau@xxxxxx> wrote:

> DC, please consider the simple observation that Terry pointed out.  If one
> measures the inductance of a primary coil with a garden-variety multimeter,
> the measurement is made at a very low current at roughly 1KHz.  Contrast
> this with how the primary coil is used in operation, at ~100KHz and hundreds
> of Amps.  But the inductance value read by the meter is perfectly consistent
> with the value that the operating coil is running with.  So I would conclude
> that if there is a shift in inductance due to current or frequency, it's not
> at all significant.
>
> I must admit that I've lost sight of how the question of whether inductance
> is current-dependant, relates to the feasibility of Finn's circuit.  The
> issue at hand is whether Finn's circuit is able to provide a reliable timing
> signal to control power IGBT's over a range of operating conditions.  Finn
> accomplished this by using a small, LOW-POWER inductor to shift the phase of
> the primary current sensor.  Since the turn-on delay of the IGBT's is
> constant, regardless of operating frequency or current, the circuit deriving
> the control signal needs only provide a fixed timing advancement.  It's
> possible that if one were to shift the resonant frequency, one would need to
> re-tweak some pots to dial in a new timing value, but I've not thought about
> this too carefully.  It appears to be a plausible and elegant solution, to
> me.  The only thing that needs work IMHO is the spelling of "Predikter".
>
> Regards, Gary Lau
> MA, USA
>
> > -----Original Message-----
> > From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> > Behalf Of DC Cox
> > Sent: Thursday, June 25, 2009 10:54 AM
> > To: Tesla Coil Mailing List
> > Subject: [TCML] Greg's input on inductance
> >
> > It appears even seasoned members of the list are getting confused with
> the
> > fundamental physics of electrical circuits.
> >
> > Greg's quote: An interesting line of reasoning! Unfortunately, it is
> > impossible to independently 'change either one' without changing the
> other;
> > they will always move together, in a fixed ratio that defines the
> > inductance.
> >
> >
> > Consider what would happen if you could set just the Amperes value to
> zero.
> >
> >
> > Do you really believe it would yield infinite inductance?
> >
> > Now consider what he said ---  if you set the AMP value to Zero what do
> you
> > really have.....STATIC... you have a potential difference but no current
> which
> > means no movement and no induced voltage so...inductance does not even
> come
> > into play until there is an EMF built up some how through a changing
> field
> > and if you have a TON voltage but you have nothing to push then you have
> > static potential energy
> >
> > You can change either one independently through either constant voltage
> > control or constant current control...however no one does it in Tesla
> coil
> > work or I have never seen it done.
> >
> > Dr. Resonance
> > _______________________________________________
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