Re: Energy stored in primary again

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

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>
 >
 > Dear List,
 > I have been thinking about the "energy stored in primary" concept again. I
 > was wondering if the switch SW1 could withstand the high emf induced in the
 > primary when SW1 opened there would be no need for the capacitor C1 to tune
 > the primary OR the quenching switch SW2 to turn off the primary circulating
 > current after predetermined period.

The voltage over the switch would be limited only by parasitic
capacitances if C1 is not included, and the transformer has a coupling
coefficient smaller than exactly 1. Or, you always have C1, and so it's
better to use the right value. In classical induction coils, without
C1 the mechanical switch produces an intense spark that dissipates most
of the primary energy when opening. An electronic switch would be very
surely destroyed in little time. About the quenching switch, would it
really make some difference if the primary has low losses? Remember that
there is no spark gap in this configuration.

 > This is my reasoning: with the tesla coil T1 viewed as an ideal transformer
 > the secondary voltage is referred back to the primary by the turns ratio;

Ok.

 > now T1 isn't an ideal transformer as not all the magnetic flux from the
 > primary is coupled to the secondary i.e. there is a "leakage" component
 > inherent in the primary inductance which can be visualised as an inductor
 > in series with the primary of the hypothetical ideal transformer.

Ok too. K<1, high-frequency approximation.

 > the leakage reactance will be series-resonant with secondary capacitance C2
 > over the turns ratio of the ideal transformer after SW1 is opened
 >
 > Thus SW1 will "see" a voltage across its terminals
 > not equal to the power rail voltage
 > but to the voltage across the "transformed" C2 resonating with the leakage
 > reactance.
 >
 > Is my reasoning correct?

Not exactly. The problem is that the interruption of the current in the
leakage inductance, without C1 across the switch, produces a voltage
impulse (an infinitely large pulse with infinitely small duration, but
nonzero area) over the switch. A fraction of the initial energy
disappears instantaneously, before the start of the oscillations. In
practice, a parasitic C1 will limit the amplitude (in a huge value)
of the pulse and stretch it in time. A high-amplitude, high-frequency
oscillation will appear over the switch. You are trying to interrupt the
current in an inductor, and this always produces a voltage L di/dt
over it.

Antonio Carlos M. de Queiroz