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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:

  1. 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?

  2. 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.

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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.

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  1. Imagine we have a pendulum: You kick it at time=0, then it does oscillate until it loses the reactive energy, by means of air drag, dampening,...So to start the new cycle you kick it back again - initial condition. The supply transformer is connected to mains 50/60Hz, meanwhile the resonant frequency of TC is several tens of kHz, bigger the TC, lower is the resonant frequency. Indeed the spark gap is used as initial kick - the capacitor gets a burst charge.

  2. Sure the frequency of primary and secondary are equal, can't be differently. The quality of resonant circuit determines its major propreties.

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Tesla coils don't have to be excited by impulses from a spark gap, they can also be driven at their resonant frequency by an RF amplifier/oscillator.

Search for "transistor tesla coil" or "solid state tesla coil" for some examples.

Unless otherwise modulated, these coils don't generate the exciting buzz that a spark-driven coil does. To make up for that though, the RF can be modulated to play music.

The reason for the spark system is that it is simple and generates a large amount of RF power without using any RF components, and for large coils, it generates RF voltages larger than can be generated by regular transmitters (but at a very low duty cycle).

Early radio transmitters also used this type of excitation - a motor-driven rotary spark gap would produce a series of impulses, which "ring" the resonant circuit of the transmitter. Efficiency could in fact be reasonably high, though not as good as a transistor or valve amplifier.

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