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Re: CDI vs. Inductive Ignition Drivers and the "ideal transformer"



Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <teslalist-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

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

 > I was musing over merits of electronic drivers for ignition coils used as
 > HV transformers in TC service,
 > trying to figure out whether capacitive discharge ignition (CDI) or
 > inductive ignition drivers were better in a circuit where the function of
 > the coil is to charge the primary cap of a spark-gap TC.
 >
 > This led me to wonder if the coupled magnetic field in the ignition coil
 > could be used for energy storage
 > or was this just a "gearbox" which would refer a "transformed" value of the
 > capacitor back to the LT side; if the latter were true surely energy could
 > not be stored in the coupled inductance  but only in the uncoupled flux
 > i.e. the in "leakage inductance" that would appear to be in series with the
 > otherwise "ideal" transformer.

In the CDI system, the initial energy is in a capacitor, with nothing in
the inductors. In the inductive system, the initial energy is all in the
primary inductor, with nothing elsewhere (except for a little fraction
in the primary capacitor, that is in parallel with the primary coil
while it is being "charged"). The mutual inductance only has effect
during the energy transfer to the secondary side. The CDI system
operates
as a regular Tesla coil. The inductive system as an induction coil.
In both cases, the coupling is usually too high for optimum energy
transfer. But high coupling produces a big pulse at the secondary coil
immediately, that is the objective of these systems.

 > Thus, it would appear that less energy can be stored in the magnetic field
 > of a transformer with a capacitor connected across the secondary than one
 > without. Is this true, and if so what are its implications in the use of
 > inductive ignition drivers versus capacitive ignition drivers in TC service?

I don't see any effect of the capacitive loading of the secondary in
the initial energy or in where it is.

 > Finally, is it true that in the hypothetical ideal transformer there can be
 > no magnetic field since all the magnetic flux added by the current in
 > primary must be subtracted by the magnetic flux from the current in the
 > secondary if power in is equal to power out, according to Lenz's Law?

Yes. An ideal transformer is a static device, without any energy
storage.

 > And is this reason why ideal transformers don't exist in reality?

An ideal transformer is a transformer with k=1 (impossible in practice)
and L*di/dt and M*di/dt products at both sides much higher than the
voltages across the windings. You can built a transformer that operates
as an ideal transformer with little error, but the real ideal is a
limit (L->infinity or di/dt->infinity, k->1) that can't be reached in
practice.
A real transformer, considering only the essential, can be modeled
as an ideal transformer with an inductor across one of the windings
(that models the finite inductance) and another inductor in series
with one of the terminals (that models k<1).

Antonio Carlos M. de Queiroz