Induction coil

How it works

Induction coil showing construction

An induction coil consists of two coils of insulated copper wire wound around a common iron core. One coil, called the primary winding, is made from relatively few (tens or hundreds) of turns of coarse wire. The other coil, the secondary winding, typically consists of many (thousands) turns of fine wire. An electric current is passed through the primary, creating a magnetic field. Because of the common core, most of the primary’s magnetic field couples with the secondary winding. The primary behaves as an inductor, storing energy in the associated magnetic field. When the primary current is suddenly interrupted, the magnetic field rapidly collapses. This causes a high voltage pulse to be developed across the secondary terminals through electromagnetic induction. Because of the large number of turns in the secondary coil, the secondary voltage pulse is typically many thousands of volts. This voltage is often sufficient to cause an electrical discharge, or spark, to jump across an air gap separating the secondary’s output terminals. For this reason, induction coils were called spark coils.

The size of induction coils was usually specified by the length of spark it could produce; an ’8 inch’ induction coil was one that could produce an 8 inch arc.

The interrupter

To operate the coil continuously, the DC supply current must be broken repeatedly to create the magnetic field changes needed for induction. Induction coils use a magnetically activated vibrating arm called an interrupter or break to rapidly connect and break the current flowing into the primary coil. The interrupters on small coils were mounted on the end of the coil next to the iron core. The magnetic field created by the current flowing in the primary attracts the interrupter’s iron armature attached to a spring, breaking a pair of contacts in the primary circuit. When the magnetic field then collapses, the spring closes the contacts again, and the cycle repeats.

Opposite potentials are induced in the secondary when the interrupter ‘breaks’ the circuit and ‘closes’ the circuit. However, the current change in the primary is much more abrupt when the interrupter ‘breaks’. When the contacts close, the current builds up slowly in the primary because the supply voltage has a limited ability to force current through the coil’s inductance. In contrast, when the interrupter contacts open, the current falls to zero suddenly. So the pulse of voltage induced in the secondary at ‘break’ is much larger than the pulse induced at ‘close’, it is the ‘break’ that generates the coil’s high voltage output. A “snubber” capacitor is used across the contacts to quench the arc on the ‘break’, which causes much faster switching and higher voltages. So the output waveform of an induction coil is a series of alternating positive and negative pulses, but with one polarity much larger than the other.

Mercury and electrolytic interrupters

The small ‘hammer’ interrupters described above were used on coils creating up to 8 inch (~120 kV) sparks. Larger coils used motor-driven interrupters. The largest coils, used in radio transmitters, used either electrolytic or mercury turbine ‘breaks’.

Construction details

To prevent the high voltages generated in the coil from breaking down the thin insulation and arcing between the secondary wires, the secondary coil uses special construction so as to avoid having wires carrying large voltage differences lying next to each other. The secondary coil is wound in many thin flat pancake-shaped sections (called “pies”), connected in series. The primary coil is first wound on the iron core, and insulated from the secondary with a thick paper or rubber coating. Then each secondary subcoil is coated with an insulating layer like paraffin, connected to the coil next to it, and slid onto the iron core, insulated from adjoining coils with paper disks. The voltage developed in each subcoil isn’t large enough to jump between the wires in the subcoil. Large voltages are only developed across many subcoils in series, which are too widely separated to arc over.

To prevent eddy currents, which flow perpendicular to the magnetic axis, and cause energy losses, the iron core is made of a bundle of parallel iron wires, individually coated with shellac to insulate them electrically.

History

Callan’s largest induction coil (Model of 1863), showing ‘pancake’ secondary construction. It was 42 inches (106 cm) long and could produce 15 inch (38 cm) sparks, corresponding to a potential of approximately 200,000 volts.

Michael Faraday discovered the principle of induction, Faraday’s induction law, in 1831 and did the first experiments with induction between coils of wire. The induction coil was invented by the Irish scientist Nicholas Callan in 1836 at the St. Patrick’s College, Maynooth and improved by William Sturgeon and Charles Grafton Page. The early coils had hand cranked interrupters, invented by Callan and Antoine Masson. The automatic ‘hammer’ interrupter was invented by C. E. Neeff, P. Wagner, and J. W. M’Gauley. Hippolyte Fizeau introduced the use of the quenching capacitor. Heinrich Ruhmkorff generated higher voltages by greatly increasing the length of the secondary, in some coils using 5 or 6 miles of wire. In the early 1850s, after examining an example of a Ruhmkorff coil, which produced a small spark of around 2 inches (50 mm) when energized, American inventor Edward Samuel Ritchie perceived that it could be made more efficient and produce a stronger spark by redesigning and improving its secondary insulation. His own design divided the coil into sections, each properly insulated from each other. Ritchie’s induction coil proved superior to other designs of the day, initially producing a spark of 10 inches (25 cm) in length; later versions could produce an electrical bolt 24 inches (61 cm) or longer in length. The full story of Page’s invention of the induction coil in its modern guise is told in Robert Post, “Physics, Patents, and Politics: A Biography of Charles Grafton Page” (Science History Publications, 1976. In 1857, one of Ritchie’s induction coils was exhibited in Dublin, Ireland at a conference of the British Association, and later at the University of Edinburgh in Scotland. Ruhmkorff himself purchased a Ritchie induction coil, utilizing its improvements in his own work.

Induction coils were used to provide high voltage for early gas discharge and Crookes tubes and for X-ray research. They were also used to provide entertainment (lighting Geissler tubes, for example) and to drive small “shocking coils”, Tesla coils and violet ray devices used in quack medicine. They were used by Hertz to demonstrate the existence of electromagnetic waves, as predicted by James Maxwell and by Tesla and Marconi in the first research into radio waves. Their largest industrial use was probably in early wireless telegraphy radio transmitters and to power cold cathode x-ray tubes. By about 1920 they were supplanted in both these applications by vacuum tubes. However their largest use was as the ignition coil or spark coil in the ignition system of internal combustion engines, where they are still used, although the interrupter contacts are now replaced by solid state switches. A smaller version is used to trigger the flash tubes used in cameras and strobe lights.

Wireless charging

Toyota’s heavy duty division, Hino Motors, is testing a new kind of hybrid electric vehicle without a plug (hybrid outboard chargeable vehicle). The energy in the batteries doesn’t come from a plug and a charging point, but it comes from a wireless charging system built into the road. A series of induction coils built into the road resonate energy at certain frequency, like radio waves. The bus is able to capture those waves and store the energy in its batteries.

Early patents

U.S. Patent 52,054 The induction-coil, instead of being made movable upon the magnet

U.S. Patent 72,616 This compound coil is made like any ordinary induction-coil

U.S. Patent 74,905 The inner end of the induction-coil are surrounded by the prime coil

U.S. Patent 76,654 The induction-coil consists of a metallic conductor, copper is generally preferred

U.S. Patent 78,495 Energizing the primary wire of the induction-coil, the iron core becomes magnetized

U.S. Patent 90,626 Making use of an induction-coil

U.S. Patent 734,197 a split-coil improvement (1903).

U.S. Patent 1,092,417 Induction coil comprising a soft iron core (Mar 5, 1913)

See also

Charging station

Ignition coil

Spark gap transmitter

Transformer

Tesla coil

Electromagnetism

Faraday’s law of induction

Ignition system

Inductor

Magnetic field

Footnotes

^ Collins, Archie F. (1908). The Design and Construction of Induction Coils. New York: Munn & Co.. http://books.google.com/books?id=dJNPAAAAMAAJ&pg=PA98.  p.98

^ Faraday, Michael (1834). “Experimental researches on electricity, 7th series”. Phil. Trans. R. Soc. (London) 124: 77122. doi:10.1098/rstl.1834.0008. 

^ Fleming, John Ambrose (1896). The Alternate Current Transformer in Theory and Practice, Vol.2. The Electrician Publishing Co.. http://books.google.com/books?id=17sKAAAAIAAJ&pg=PA16.  p.16-18

^ http://www.nuim.ie/museum/ncallan.html

^ Severns, Rudy. “History of soft switching, Part 2″. Design Resource Center. Switching Power Magazine. http://www.switchingpowermagazine.com/downloads/Oct 01 soft.pdf. Retrieved 2008-05-16. 

^ American Academy of Arts and Sciences, Proceedings of the American Academy of Arts and Sciences, Vol. XXIII, May 1895 – May 1896, Boston: University Press, John Wilson and Son (1896), pp. 359-360

^ Page, Charles G., History of Induction: The American Claim to the Induction Coil and Its Electrostatic Developments, Boston: Harvard University, Intelligencer Printing house (1867), pp. 104-106

^ Rogers, W. B. (Prof.), Brief Account of the Construction and Effects of a very Powerful Induction Apparatus, devised by Mr. E.S. Ritchie, of Boston, United States, British Association for the Advancement of Science, Report of the Annual Meeting (1858), p. 15

^ American Academy, pp. 359-360

^ American Academy, pp. 359-360

^ Page, pp. 104-106

^ http://www.ecogeek.org/content/view/1431/

Further reading

Norrie, H. S., “Induction Coils: How to Make, Use, and Repair Them”. Norman H. Schneider, 1907, New York. 4th edition.

Collins, Archie F. (1908). The Design and Construction of Induction Coils. New York: Munn & Co.. http://books.google.com/books?id=dJNPAAAAMAAJ&pg=PA98. 

Fleming, John Ambrose (1896). The Alternate Current Transformer in Theory and Practice, Vol.2. The Electrician Publishing Co.. http://books.google.com/books?id=17sKAAAAIAAJ&pg=PA16.  Has detailed history of invention of induction coil

External links

Battery powered Driver circuit for Induction Coils

The Cathode Ray Tube site

Categories: Transformers (electrical) | Electrical breakdown

15 FOOT TALL TESLA COIL WITH MAX. SPARKS OF 26 FEET. RUNS UP TO 55000 WATTS IN NEWCASTLE OKLAHOMA.
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How Does a Tesla Coil Work

One time when I was a young boy I saw a documentary on tv, that happened to show a large tesla coil in operation at some college, from that moment on the Tesla Coil bug had bitten me. I knew someday I would have to own one of these wonderful devices. The loud noise, the powerful arcs of flowing electric was completely amazing. Now that I am getting older, I am the proud owner of three tesla coils, a wimhurst machine, van de graff generator, odin coil, and other high voltage devices.

Before I start telling you how a tesla coil works, I should take a little time to tell you about it’s inventor, Nikola Tesla. Nikola Tesla was born on July 10, 1856, he was an inventor, a mechanical and an electrical engineer. Born in Smiljan, Croatian Krajina, Austrian Empire, he was an ethnic Serb subject of the Austrian Empire and later became an American citizen. Tesla invented numerous things and had obtained around 300 patents worldwide for his ideas. Some of Tesla’s patents include such amazing devices as the AC motor, bifilar coil, various devices that use rotating magnetic fields, AC polyphase power distribution system, wireless communication devices, radio frequency oscillators, voltage magnification by standing waves, robotics, logic gates for secure radio frequency communications, devices for x-rays, apparatus for ozone generation, devices for ionized gases, devices for high field emission, devices for charged particle beams, means for increasing the intensity of electrical oscillations, voltage multiplication circuitry, devices for high voltage discharges, devices for lightning protection, the bladeless turbine, and many more.

If you do some searching on the internet or other sources for tesla coils, most places try to explain these devices in terms that only a physics expert can understand. I have broken down the tesla coil into it’s 6 main parts or components, with easy to understand terminology on the operation. The 6 main components are the transformer, capacitor, spark gap, primary coil, secondary coil and discharge sphere.

The TRANSFORMER takes 110 volt wall socket current or current from another source, and steps it up to several thousand volts (depending on the transformer). CAPACITOR – Acts like a battery, and takes the stepped-up voltage and stores it until it’s fully charged. Once fully charged, the SPARK-GAP fires, releasing all of the capacitors stored energy in a sngle powerful burst or pulse. From the SPARK-GAP, the current flows to the large PRIMARY COIL. The PRIMARY COIL, is usually made from thick copper wire or even pipe, consisting of 5-25 turns or coils. This large copper coil produces a strong magntic field as the current flows through it, the current is then picked up by the SECONDARY COIL. The SECONDARY COIL, acts like another transformer, and consists of numerous turns or coils of smaller gauge copper wire. Here the current continues building by the effects of the magnetic field on the copper, until it reaches massive voltage levels. This current moves up into the DISCHARGE SPHERE. The DISCHARGE SPHERE also acts like a simple capacitor (storage battery), before discharging the current as sparks and corona.

There is a website that describes the tesla coil and numerous other high voltage devices in detail, this website is called: How a Tesla Coil Works and it may be found at this url: http://www.rb59.com/tesla-coil/tesla-coil.html

You may publish this article in your ezine, newsletter or on your web site as long as it is reprinted in its entirety and without modification except for formatting needs or grammar corrections.

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