To understand how a Tesla coil works you need to understand a couple of points about its components.
1) Inductors:
The Tesla coil's primary and secondary coils are both inductors in
electrical terms. When the current flowing through an inductor changes,
it will create an opposing or reverse voltage.
A Wikipedia Article for all the theory
2) Spark Gaps:
A sparking plug in a car is a simple spark gap, its break down voltage
depending on the electrode gap. Once a spark gap conducts it has the
ability to carry on so long as a reasonable current is flowing (hot
ionized air in the gap).
3) Capacitor:
A good analogy for a capacitor is to regard it as a sponge that is
placed on some spilt water and left to slowly soak it up. If you leave
it a minute then give it a very quick, hard squeeze, a large amount of
water is released all at once, this is one minutes worth of slow
soaking-up released in a mere fraction of a second.
In a Tesla coil the so called soaking-up stage lasts for a few
milliseconds, but the squeezing-out can be a thousand times quicker,
lasting for a few
micro or millionths of a second.
Wikipedia Article
4) Resonance:
The property of resonance is fundamental to the operation of all Tesla coils.
A good analogy is the garden swing. If it is left to swing on its own it
does so at its resonant frequency, only slowing down due to friction
and gravity.
If you stand behind the swing and push it just as it swings away from
you each time, it will get higher with each subsequent push. This is
because you are adding power at, and only at, the correct time-point in
the swings cycle.
You are therefore adding momentum at the same time interval as the
swings resonant frequency, this means the push you gave it, is in
resonance with the swing.
Resonance does not increase the overall amount of energy, it only facilitates its transfer.
So
if you're looking for tesla related, so-called 'free energy, or 'zero
point' energy information, which some people seem to associate with
tesla coils,
this is not the site for you!
5) Resonant Circuit
If a capacitor is placed across an inductor you will have a resonant
circuit. As the capacitor discharges, it sends current into the inductor
that will then store this as energy in its magnetic field. But as the
capacitor discharges the current feed into the inductor diminishes. This
then causes its magnetic field to collapse and generate an opposing
voltage that goes back into charging up the capacitor, and the cycle
starts all over again. The number of times that this 'back and forwards'
cycle happens per second, is its resonant frequency, expressed in Hertz
(Hz). Because of resistive losses the current is reduced every cycle
down to zero. There is no such thing as free energy!
A Tesla coil has two separate parts, the primary and secondary
circuits, each of these consists of a capacitor and an inductor, so they
each become resonant circuits when voltage is applied. The secret
behind a good Tesla coil is making both resonant at the same frequency,
allowing them to interact with one another.
Using the analogy of the swing again, the actual swing part becomes the
secondary coil, while the role of the person pushing is taken by the
primary coil. This gives the secondary (the swing) a push at just the
correct time. The push in this case consisting of additional power,
though for this to occur it is imperative that they both have the same
resonant frequency.
This resonant frequency is a product of the capacitance value in Farads (C), and the inductor's value in Henries (L).
Using different values of capacitance and inductance will give a different frequency.
In the circuit of Fig 1 above, the capacitor (
'C') is charged up by a high voltage source, like my example of the sponge soaking up water.
Once the capacitor attains a high enough voltage the spark gap
fires and conducts (Fig 2 below). The spark gap is now a short-circuit
that completes the resonant circuit (shown in red) of the primary
inductor and capacitor.
The spark gap firing is virtually an instantaneous discharge of
the capacitor energy into the inductor and is the same as the example of
the sponge being instantaneously squeezed out.
The inductor stores this energy in its magnetic field with the lines of
force cutting into the secondary coil and inducing a voltage into it.
Once the capacitor is empty the current flow into the inductor stops,
and its magnetic field collapses causing a reverse current to flow back
into the capacitor again.
This back and forth diminishing cycle (called the 'Primary
Ringdown') of capacitor to inductor and back, continues untill there is
insufficient current flowing to keep the spark gap conducting. The point
to remember is that every time the cycle occurs, more energy is
transferred to the secondary, so the inductors magnetic field stores
less energy on each cycle.
Unfortunately every time the spark gap conducts, losses occur in
the form of heat and light, so you want the minimum number of cycles
that are consistent with getting all of the available energy transferred
to the secondary.
Usually after two, three, or possibly four cycles the majority of the
energy has been transferred and the primary current has dropped enough
to allow the spark gap to stop conducting (called quenching).
Once the spark gap has quenched it allows the capacitor to get a fresh charge and the whole affair can start again.
The amount of energy available to be sent to the primary (measured in Joules) is equal to the 0.5 x C x V^2
C = Farads
V = voltage that the gap fires at.
You can see here that doubling the value of C (provided your
power source is robust enough) will give you twice the power. But
doubling the voltage that the capacitor is charged up to will give 4
times the power, because the voltage value is squared, that's why if you
want spark length its best to go for a higher voltage power source.
While the primary circuit is resonating and transferring its energy,
the following is also occurring in the secondary circuit at the same
time .........
The toroid on top of the coil acts like a capacitor with respect
to the surrounding ground. This is easier to see in the diagrams below.
Fig
1 is the same as Fig
2, because in reality the toroid
discharges through the air to earth. If you now replace the toroid with
the symbol for a capacitor (Fig
3) and re-arrange things, you end up with Fig
4.
This means that the secondary coil is
also a resonant
circuit and it behaves much like the primary circuit. The secondary's
energy is therefore also resonating back and forth between the coil and
the toroid. However it does not dampen down the same as the primary
does, in fact it is steadily increasing.
This is because at just the right time-point in its cycle (like you
pushing that swing in the example) another magnetic field from the
primary circuit, which remember is also resonating at the same
frequency, transfers a bit more of
its stored energy into the secondary circuit.
Therefore as the primary ringdown is occurring causing the
primary to loose its energy, the secondary is gaining power in what is
called the Secondary Ring-up.
Remember the primary and the secondary need to have the same resonant
frequencies for them to interact successfully. Typically this is in the
hundreds of Kilo-Hertz.
Eventually the voltage on the surface of the toroid at the top, rises so
high that the curved surface cannot retain the charge anymore, and
breakout occurs. This will either be a misty purple corona discharge or,
if all components are suitably balanced to one another, a whitish solid
streamer down to earth or into the air.
In a perfect Tesla coil once breakout has occurred this would be
the end of the matter, allowing a fresh charging cycle to start all over
again. What usually occurs though is that as the secondary's field
starts collapsing it starts to transfer its energy back into the primary
again. This is because the hot ionized spark gap in the primary
charging circuit is still able to conduct the somewhat reduced energy
now being returned by the secondary.
This means that any remaining energy in the collapsing secondary,
that could have gone into prolonging the discharge, is wasted by being
sent back into the primary instead. This can result in the whole of the
primary to secondary transfer cycle occurring again, and in the worst
cases even three of four times.
What is the problem with that you say? Well firstly it is better
to have all the energy forming one high charge, rather than several
cycles of successive diminishing charges.
And secondly no new energy from the power source can be added to the
circuit until the spark gap has quenched, and that can't happen untill
the present cycle stops.
There are various ways to overcome the problem of these unwanted cycles. In a so called static spark gap you can use either
suction or a fan to both remove the hot ionized air from between the electrodes and also cool them, as both actions help quenching.
Another method is the rotary spark gap. In these the spark gap
consists of a fixed electrode while the other revolves, much the same as
a distributor in a car engine. These spark gaps come in two different
types. Asynchronous [ARSG] and Synchronous [
SRSG],
the latter being where the position of the rotating electrodes in each
revolution, is directly linked to the mains frequency cycle, whilst the
Asynchronous system is not.
With the synchronising system, you arrange for the revolving electrodes
to come into alignment with the fixed ones when the AC cycle is
near its peak (normally you aim for approx' 1mS or so afterwards).
This allows the capacitor to discharge into the primary, at the optimum
time in its charging cycle. The revolving electrodes also disturb the
surrounding air to assist with their own cooling.
It's not the actual separating action of the revolving electrodes
that quenches the arc though, this is because a spark can be stretched
quite significantly once struck. Quenching occurs naturally at one of
the primary notches, and hopefully this quenching notch will occur once
the electrodes have moved sufficiently out of alignment, before the
capacitor has recharged sufficiently to start the cycle all over again.
Asynchronous gaps, because they use revolving electrodes, also
fire at an even rate, but in their case it is independent of where the
AC cycle is. This means that the capacitor may not be fully charged at
the time of firing. It can also mean that the opposite situation can
arise where a higher than normal voltage can occur across the capacitors
and the HV supply. For this reason Asynchronous systems should not be
used with an
NST, because they can sometimes be rather fragile when subjected to high voltage spikes.
Even rotary spark gaps have their short-falls though. As the
voltage that the spark gap handles is increased the timing tends to
become advanced. This is because the higher voltages from the bigger
transformers (usually 15,000>) means the spark is able to jump the
gap between the static electrode and the rapidly approaching rotating
one,
before they actually line up.
With a Synchronous spark gap [
SRSG] this can be overcome by adjusting the
Phasing
of the AC input to the motor using inductors and capacitors. This way
the position of the revolving electrodes can be finely adjusted in
relation to the fixed ones.
Who Builds Tesla Coils?
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