Introduction
One of the revolutionary inventions in electrical engineering field was Tesla coil. Nikola Tesla invented Tesla coil in 1891. Tesla coil is primarily used to produce high voltage and high frequency alternating current electricity. Tesla coils are capable of producing high voltages better than artificial means of producing the same. “It takes the output from a 120vAC to a several kilovolt transformer and driver circuit and steps it up to an extremely high voltage. Voltages can get to be well above 1,000,000 volts and are discharged in the form of electrical arcs” (UCSC). Very popular in early part of the century Tesla coils were used in radio circuit, transmitters, electrotherapy and many such applications. However, with the advent of newer technologies and electronic circuits it lost its relevance in many fields. This paper will throw light on the history of Tesla Coil, its theory, components and functions.
Brief History
Nikola Tesla was experimenting on the possibility of transmitting electricity through air. He successfully accomplished that using his looping coil structure famously known as the Tesla coil. Tesla coil was later used by Marconi to design radio. Currently, Tesla coils are mostly used in school science projects and exhibitions. Most of the science enthusiasts create their small Tesla Coil at home to see the beautiful electrical sparks. In school and exhibitions, students are encouraged to build Tesla coil as it demonstrates many basic properties of electricity. Even in some of the movies Tesla coil is used to create nice effects. Hollywood movie ‘Terminator’ has used Tesla coil to create electrical sparks in some of the scenes. Tesla coil was once very popular in the industry but since the advent of semiconductor circuits it has lost its due relevance. Nowadays it is only used for the purpose of laboratory experiments and exhibitions.
Theory
A conventional transformer has windings which are tightly coupled and the voltage gain between primary and secondary line is determined by the ratio of number of turns between the two lines. This works very well at normal voltage but at high voltage the proximity of these two lines may disrupt the electrical insulation, leading to mishaps and hence the conventional transformers require a sophisticated design at higher voltage. On the other hand Tesla coils are loosely wound. Both primary and secondary lines are loosely coupled with a large air gap between them (Tesla’s original design). The magnetic coupling between primary and secondary line is only 10-20% (compared to almost 95-99% of conventional transformers). As the energy transfers from primary to secondary line, the voltage builds up in the secondary side. A well designed Tesla coil even with its loose coupling and significant spark gap loss can transfer over 80% of the energy. Tesla coil because of its large gap between windings can acquire significantly large voltages than conventional transformers. In Tesla coil the voltage gain is proportional to the square root of the ration between secondary and primary inductances. The main design aspect of the Tesla coil is that the resonant frequency of the secondary is matched to that of the primary.
Components
Figure 1: A typical Tesla Coil design (Gallatinriver)
Tesla coil is an air core transformer operating in a resonant mode. As regards the components of Tesla coil, Nicola Tesla wrote in an article in 1919 "The essential parts of an oscillator are: a condenser, a self-induction coil for charging the same to a high potential, a circuit controller, and a transformer which is energized by the oscillatory discharges of the condenser" (Jeff Behary).The main components of the coil are touched upon in the discussion below.
Power transformer is a very basic transformer that is used to equalize the normal voltage from 120V/220V to 1KV/5KV. The higher the power transformer voltage and current rating, the better it is suited for the Tesla Coil use.
Safety Gap is placed across high voltage lines of the supply side transformer or the power transformer. Safety Gap is used to safeguard the whole Tesla Coil. It disconnects the circuit and protects the power transformer when voltage goes above the safety limit. This phenomenon can happen sometimes due to resonance frequency kickbacks from Tesla Coil.
RF Chokes create very high resistance to the voltage generated by Tesla Coil. These chokes prevent the RF energy from flowing back to the secondary windings of the power transformer.
Capacitor is one of the most important components of the whole design of Tesla Coil. The capacitor voltage rating should be well above the high voltage rating of the power transformer. This type of high voltage transformer is often very expensive. Many people make their own capacitors using plastic and aluminum foil.
Primary coil of Tesla Coil is different from that of a normal transformer primary coil. Diameter of the primary coil is often made twice the diameter of secondary to keep them loosely coupled so that reasonable transfer of energy takes place between the two.
Secondary coil is a tightly wound coil around any non-conducting core. In general plastic PVC pipes are used. The bottom of the secondary coil needs to have a solid ground connection.
Discharge Electrode is a toroid on one side of the secondary coil. This toroid adds capacitance to the secondary side of the circuit. The larger the toroid the lesser the natural frequency is.
The RF ground connection is one of the most important connections of the Tesla Coil. One side of the secondary coil needs to be grounded properly. Proper grounding in electrical term means low inductance and low impedance. Low inductance and impedance ensures that the high current at the base of the secondary coil is carried away to the ground. Some of the old designs may show the primary and secondary coils connected at the base but that design is no longer used in any circuits as that may damage the power transformer.
Functions of Tesla Coil
In earlier days Tesla coil was highly used in the radio industry. It was widely used for many laboratory experiments to show different properties of electricity. Before new technologies came to replace Tesla coils, the resonant transformers of high frequency and high voltage were used in many commercial applications. Though all the commercial applications are not in use today, some earlier and current applications include induction and dielectric heating, induction coils, ozone generators, medical x-ray devices, particle accelerators, quack medical devices, generation of extremely high voltages and spark gap radio transmitters (Mitch Tilbury, 2008, p 1). Currently Tesla coil is mostly used for the purpose of entertainment and research. Ball lightning experiments are still done using Tesla Coil. Aircraft industry uses Tesla coil to simulate a lightning strike on an airplane. Tesla coils are used to make some of the most interesting toys available in the market like Plasma Ball and Luminglas. Tesla Coil design has undergone some changes and is used in applications in which large voltage is required in a small setup. For example, solid state Tesla Coils are one of the most popular and growing devices used in all types of cathode ray tubes and computer power supplies.
Conclusion
Tesla Coil was once a very popular electrical device used in many appliances. However, in the last century slowly but surely usage of Tesla Coil has reduced drastically. The primary usage remains in the laboratory and exhibitions. Few usage of Tesla coil remains in aerospace industry and ball lighting experiment. Modified Tesla coils continue to gain its popularity in cathode ray tubes and computer power supplies as solid state tesla Coils. There is a possibility for Tesla coil to gain popularity again in future, if not in its old form, the concept of Tesla coil to transmit energy over air will surely be used. Already the major area in which Tesla coil is being experimented is the electrical charging of the battery for an automobile without plugging in to any socket. With more and more vehicles becoming battery operated, this will become a critical device for the automobile industry in the near future.
References
Uth, Robert (2000). Tesla coil: Master of Lightning. PBS.org. Retrieved on 10th June 2013 from <http://www.pbs.org/tesla/ins/lab_tescoil.html>
Tilbury, Mitch (2007). The Ultimate Tesla Coil Design and Construction Guide. McGraw-Hill Professional. New York: Retrieved on 10th June 2013 from <http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CC0QFjAA&url=http%3A%2F%2Fnashaucheba.ru%2Fv12333%2F%3Fdownload%3D1&ei=-Qu3UZH5HcrJyAHct4GoAQ&usg=AFQjCNEqKpqfpC8ufp0_Hk1KhAt-GD0GAw&sig2=mGntforSUgZMcBpeQ5_KqQ&bvm=bv.47534661,d.aWM>
Aderhold, Dan (2010). The Future of Electric Vehicles, Where’s the Plug? , Retrieved on 10th June 2013 from <http://www.teslamotors.com/forum/forums/future-electric-vehicles-where%E2%80%99s-plug>
Behary, Jeff. Redefining Tesla Technologies, Retrieved on 10th June 2013 from <http://www.electrotherapymuseum.com/Articles/TriFold.pdf >
What is a Tesla Coil?, UCSC.edu, Retrieved on 10th June 2013 from <http://scipp.ucsc.edu/edu/tesla/teslacoil/whatisateslacoil.html>
Figure 1: Tesla Coil Design, Gallatinriver, Retrieved on 10th June 2013 from
<http://home.gallatinriver.net/mhammer/schem.jpg>
