Tesla transformer
A Tesla transformer , also known as a Tesla coil , is a resonance transformer , named after its inventor Nikola Tesla , for generating high-frequency alternating voltage . It is used to generate high voltage . Its functional principle is based on the resonance of magnetically loosely coupled electrical oscillating circuits .
In contrast to power transformers , which are used in the high voltage area and whose application is in the field of electrical power engineering , the average power of Tesla transformers ranges from a few watts to a few kilowatts , despite the high instantaneous power . Due to their low power, they serve as a relatively safe high voltage source for various show experiments; they are of no importance for electrical power engineering.
history
One of Nikola Tesla's goals was to transmit electrical energy wirelessly. The Tesla transformer is only suitable to a limited extent for this purpose - it does generate electromagnetic waves; however, these can only be recovered in a small distance and only partially in a receiving circuit. The Wardenclyffe Tower on Long Island in the USA, built for this purpose in 1901, was demolished again in 1917 due to lack of money.
Tesla transformers in the form described here are primarily used to demonstrate the effect of high, high-frequency electrical alternating voltages.
construction
Two very different resonant circuits with the same resonance frequency are loosely magnetically coupled and form a transformer. If the primary and secondary circuits are in resonance , a high voltage of more than 100 kV occurs due to the resonance increase on the secondary coil. The ratio of turns of the primary and secondary coil in the Tesla transformer alone is not responsible for the transformation of the input voltage. Rather, due to the loose coupling, an increase in resonance can take place. The resonant circuit is formed from the long secondary coil and its own capacitance as well as the capacitance of the head electrode to earth. The lower end of the secondary coil is grounded. In particular, the area of the coil close to the earth is located in the magnetic field of the exciting primary coil.
Tesla transformers work with high frequencies between 30 kHz and 500 kHz, which is why you do not need a common iron core for coupling between the coils as with other transformers. The secondary coil is a single-layer long cylinder coil with a few 100 to 2000 turns. It lies in the magnetic field of a short, larger-diameter primary coil with a few turns. This achieves a sufficient insulation distance, especially to the upper, so-called “hot” end of the secondary coil, which carries a high alternating voltage.
The single-layer, uniform winding of the secondary coil causes a field control (uniform electrical field curve), so that partial discharges along this coil are avoided. The electrical potential rising upwards also determines the shape of the primary coil, which may be close to the secondary coil at the bottom, but expediently often widens conically upwards. As a result, the electric field strength between the two coils remains below the breakdown field strength .
In large systems, the output voltage can reach several megavolts. The high-frequency alternating voltage (a few 10 to a few 100 kHz) at the “hot” end of the secondary coil (a toroidal electrode is often attached there) generates gas discharges , known as streamers , in the surrounding air . The thermal load on the electrode remains so low that no electric arc occurs. The phenomenon is a corona discharge (tuft discharge) and is similar to Elmsfeuer .
A distinction is made between two different types: pulse and carrier Tesla transformers. Both are based on the excitation of the natural resonance of the secondary coil. They differ in that, in one case, the excitation takes place in a pulsed manner through capacitor discharge and, in the other, continuously through a powerful high-frequency generator. The impulse transformer is the better known type. A mixed form works with a high-frequency generator operated in pulses.
Impulse Tesla transformer
The primary circuit consists of a switch (in the so-called SGTC (Spark Gap Tesla Coil) a spark gap , represented by arrows in the circuit diagram), a capacitor from about 5 nF to several 100 nF and a short coil with about 5 ... 15 turns and a large diameter. This coil often has taps so that the inductance and thus the resonance frequency can be adjusted. The capacitor is charged to at least 5 kV by a short-circuit-proof voltage source (AC voltage symbol on the left in the picture) until the switch closes or the spark gap ignites. At this moment, damped electrical high-frequency oscillations of very high instantaneous power up to the megawatt range arise in the now closed primary oscillating circuit .
These vibrations are inductively transmitted to the Tesla tower, which is a long cylinder coil with a few hundred turns. Due to its own capacitance between the upper and lower end or ground, this coil forms an oscillating circuit with the same resonance frequency as the primary circuit.
Ideally, the spark gap of the primary excitation disappears after a few microseconds, as soon as all of the energy from the capacitor has been transferred to the secondary coil. If the spark gap is still ionized when the capacitor is being recharged by a powerful supply voltage source, an arc can remain, which overloads the supply. The quick extinction can be ensured by an extinguishing spark gap (see also extinguishing spark transmitter ), in which the spark is divided into sections of about 0.2 mm. The plasma of the spark can be cooled sufficiently quickly by means of massive, flat metal parts, so that it does not reignite when the voltage rises again. Furthermore, the wear is distributed over a large area. Constructions with rotating sector disks are also known, whereby the ignition takes place periodically with the speed (rotating spark gap).
As can be seen in the 2nd circuit diagram, the capacitor and spark gap can also be interchanged so that the capacitor is parallel to the voltage source instead of the spark gap.
The voltage supply of the primary oscillating circuit must withstand a brief short circuit when the capacitor is charged. It is often implemented with a 50 Hz transformer (mains transformer) operated on the mains, which initially generates a voltage between 5 and 30 kV. Suitable are e.g. B. the short-circuit-proof ignition transformers of oil burners. High-frequency chokes between the mains transformer and the spark gap can somewhat reduce high-frequency mains interference.
Instead of the spark gap, thyratrons , IGBTs (Insulated Gate Bipolar Transistors) or thyristors are also used. These components have to switch the high currents, often several kA, and are therefore expensive. However, such a solution works reproducibly, quietly and free of wear. The electronic control enables the switching processes to be determined exactly.
Girder Tesla transformer
The coil of carrier Tesla transformers has the same structure as that of pulse Tesla transformers. However, no capacitor discharge is used for the supply, but a continuously working high-frequency generator that works with transistors (abbreviation SSTC from English solid state tesla coil ) or electron tubes (abbreviated VTTC from English vacuum tube Tesla coil ). It must be matched to the natural resonance of the high-voltage coil or its feedback signal must be obtained from this. The transformer structure sometimes has an additional (auxiliary) winding for this purpose.
In the so-called DRSSTC (abbreviation DRSSTC from English dual resonant solid state Tesla coil ) the primary circuit is a series resonant circuit that is effectively fed with a square wave. As a result, an increase in resonance is already effective on the primary side.
With continuously operating devices, it is usually less possible to generate long brush discharges than with pulse Tesla transformers - the power requirement for ionization and generation of the discharges increases significantly with the voltage and is easier to provide from a capacitor in pulse mode.
If the resonance conditions change, there is a risk of a maladjustment of the generator and thus the risk of its overload. Electron tubes can withstand overloading better than transistors.
Both of the aforementioned findings led to carrier Tesla transformers in which the generator generates higher power in pulse mode. Often every second half-wave of the mains voltage is used so that the devices pulse with 50 Hz.
Applications
Technical importance
The structure of the Tesla transformer is very similar to the concept of early radio systems according to Marconi and others, especially the pop-spark transmitter and the extinguishing spark transmitter , which were banned in the 1920s due to their large bandwidth. Tesla transformers lead to interference in radio reception due to the spark discharges and the resonant fundamental wave in the long wave range, the short spark duration leads to cracking noises in a wide range up to decimeter waves.
There are currently hardly any useful applications for Tesla transformers of the form described above. In essence, it is an impressive, instructive device from the pioneering days of electrical engineering.
Leaks can be found on non-conductive vacuum containers (e.g. glass) because the air begins to glow there when the largely evacuated interior is excited with high-frequency high voltage.
The principle of wireless transmission of energy propagated by Tesla is used for the transmission of very small powers in the range from microwatts to a few milliwatts, but does not require high voltage. There are RFID chips and sensors that draw from a high-frequency electromagnetic field. The field is generated by toroidal coils that are brought closer to the sensors and at the same time serve to receive the signals from the sensors. There are also attempts to generate a correspondingly high field in an entire room in order to feed low-power sensors located therein.
A similar functional principle to that of the Tesla transformer is given in resonance converters which, in addition to other circuit parts, also consist of a resonance transformer. Resonance converters are used, among other things, to supply power to fluorescent tubes and are used to generate electrical voltages in the range of a few 100 V for the operation of cold cathode tubes . Some electronic ballasts for fluorescent lamps are also based on the principle of resonance converters, since high electrical voltages can be generated with relatively little effort.
Other such applications are electronic ignition transformers for arc lamps, oil and gas burners and arc splicers, and arc and plasma welding devices .
In some types of plasma tweeters , Tesla transformers are used to generate the high voltage.
Experiments
With Tesla transformers, a series of physical relationships can be impressively demonstrated in show experiments. They are used in teaching and in shows.
Since Tesla transformers are not encapsulated like conventional test transformers and are designed without transformer oil and are only insulated by the surrounding air, the high electrical edge field strengths at exposed points lead to corona discharges (brush discharges or streamers ). There air is ionized and becomes plasma. It produced free radicals , ozone and nitrogen oxides in the sequence. Characteristic noises arise from thermal expansion. The high temperature of the streamers is sufficient to ignite flammable objects.
If you approach the high-voltage parts with a fluorescent lamp or other gas discharge lamp, the gas discharge lamps light up without being electrically connected. This is a consequence of the displacement current . A similar effect also occurs under overhead lines , which are operated with maximum voltage and can be observed especially in the dark. Nikola Tesla used this effect, which evokes astonishing reactions, especially among laypeople, in his demonstrations such as the Columbia Lecture in New York in May 1891. At that time he used Geissler tubes .
Plasma discharges, similar to those in a plasma lamp , also arise in the filling gas of large incandescent lamps , the power connection of which is brought so close to the tip of the Tesla transformer that sparks jump over. It is usually safe to touch the glass bulb if you keep a sufficient distance from the connections and the system used does not have too much electrical power . Components of the glass bulb often fluoresce when excited by the ultraviolet radiation of the plasma.
High-frequency currents (including those from a Tesla transformer) can flow painlessly through the human body to a certain extent, as the pain reaction is based on ionic conduction and this cannot follow the alternating field sufficiently quickly. Current flow through the body takes place even without electrical contact, because a person standing next to the system on earth has an electrical capacity of a few 10 pF, which is constantly recharged by the alternating voltage of the Tesla transformer. The current strength that can be tolerated without thermal damage can light up a 100 mA incandescent lamp connected between the body and the Tesla transformer. The contact to the skin must be extensive in such experiments, otherwise painful punctual burns can occur. However, humans must never establish a direct connection between the earth and the top load, or between the terminals in a bipolar Tesla transformer, because the Tesla transformer is so detuned that the resonance frequency can become too low. In addition, a connection to the power supply unit can be established and thus 50 Hz currents would flow through the body. Therefore, such experiments are usually only carried out in the field of the Tesla transformer with galvanic isolation (see capacitive coupling ).
The corona discharge at the tips creates an ion wind .
Well-known Tesla systems
Electrum , the largest plant still in operation, is in Auckland, New Zealand. It has an output of 130 kW and a height of approx. 12 m. At full power, lightning strikes with a length of 15 m. Electrum is on private property and can therefore no longer be visited.
The largest conical Tesla transformer in the world can be seen at the Mid America Science Museum in Hot Springs , Arkansas . This transformer arrangement can generate voltages of up to 1.5 MV.
From August to November 2007, an approximately 4 m high Tesla transformer from EnBW (EnergyTower) was shown in the Science Center phæno in Wolfsburg. This Tesla transformer, the largest in Europe, generates lightning cascades over 5 m long (notarized on August 17, 2007).
Tesla systems are also located in the Technorama in Winterthur (Switzerland), in Vienna (technical museum, high-voltage laboratory), at Graz University of Technology in the Nikola Tesla Hall and in many other technical museums or experimental exhibitions called science centers .
There are several Tesla transformer projects by hobby enthusiasts (English tesla coiler ) and also commercial public exhibitions that use Tesla transformers .
hazards
Tesla transformers generate high electrical voltages and electromagnetic alternating fields. This creates the following dangers during the operation of a Tesla system:
- Danger to life from electric shocks ( electrical accidents ) if the distance to high-voltage parts is too close.
- Punctual burns on approach and sparks on the skin.
- Ultraviolet radiation damage from the discharges.
- Irritation and difficulty breathing due to the formation of ozone and nitrogen oxides.
- Interference with pacemakers or implantable cardioverter defibrillators .
Tests with high voltage should therefore be carried out in appropriately shielded rooms such as a high-voltage test field or high-voltage laboratory.
Depending on their design, Tesla transformers generate alternating electric and magnetic fields in the frequency range below long waves up to decimeter waves , which are emitted or emitted as conducted interference due to the connection to the power grid . Operation can interfere with electronic devices, radio communications and radio reception.
Tesla systems in culture
Since the 1990s, the Tesla transformer-based Violet Wand has been popular in the BDSM scene for erotic electrical stimulation .
The Tesla transformer is mentioned in the films Coffee and Cigarettes , Duell der Magier , The Prestige , xXx - Triple X and in the classic Metropolis as well as in the computer games Command & Conquer: Red Alert I / II / Yuri's Rache / III under the name Tesla coil , the video game Tomb Raider: Legend , Blazing Angels 2, an add-on from Fallout 3 (Broken Steel), Grand Theft Auto II (Electro Gun), Secret Missions of WW II , Tremulous , Return to Castle Wolfenstein , the radio play series Revelation 23 , episode 11 : »Die Hindenburg« and the smartphone game Clash of Clans .
See also
literature
- Günter Wahl: Tesla Energy learning package . Franzis, 2005, ISBN 3-7723-5210-3 .
- Günter Wahl: Tesla energy . Franzis, 2000, ISBN 3-7723-5496-3 .
- E. Nicolas: How do I build myself - vol. 26 - devices for Tesla currents . Survival Press, 2011, pp. 32 (reprint of the original edition from around 1900).
Web links
- DRSSTC projects
- Project documentation of many carrier Tesla transformers
- JavaScript to calculate your own Tesla coils
- "Spielerien" from Peter Terren