I have been reading about Tesla coils and their principle of operation – if I have understood correctly – after the primary is sufficiently charged, its circuit is closed (by means of a spark gap or a solid state device) which much like an LC circuit generates an oscillatory circuit.
Due to the magnetic coupling between primary and secondary coils however, there's exchange of energy towards the secondary (and backwards, after the first half of the entire cycle) – the secondary acts like an LC circuit too. Since the capacitance of the secondary is low, the energy that is transferred in its entirety from the primary after some point, generates a massive voltage which can ionize the surrounding air and cause discharges.
My questions are the following:
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Why does the primary have to be intermittently charged in order to cause resonance on the secondary? Isn't the primary always oscillating at its natural frequency? Why can't it be driven at that frequency by the supply transformer constantly?
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Is the frequency of voltage in the secondary the same as the frequency of its excitation in the primary? What determines how fast energy from the primary transfers to the secondary and vice versa?
The Wikipedia page on Tesla coils mentions:
[…] Current flows rapidly back and forth through the secondary coil between its ends […] The secondary current creates a magnetic field that induces voltage back in the primary coil, and over a number of additional cycles the energy is transferred back to the primary.
Best Answer
The secondary doesn't need to be excited intermittently.
The second, less important reason is that for large coils, the peak power required during the excitation is so high that it would be a practical impossibility to supply that power continuously from a standard wall socket.
The main reason is that the original coils were designed a long time ago, when the only available power switch was a spark gap. This needs a steady build up of voltage, storing energy in a suitable capacitor, until the voltage becomes so high that the switch breaks over, dumping most of its energy into the primary coil. This automatically means intermittent operation. The most convenient voltage source that had this 'steady build up' sort of characteristic was AC mains.
A modification of the spark gap was the 'rotary spark gap', where electrodes were moved to modulate the breakdown voltage.
With the advent of power electronics such as high voltage IGBTs, 'solid state' Tesla coils can be run continuously, but only if they are small ones. If you do the sums for even a fairly modest coil in the 1m height region, the power required to excite the secondary can run into 10s of kW, which means intermittent operation is required for domestic use. You can make a benefit out of this drawback by timing the bursts of operation from a music input, to make a musical coil.