Induction coil

An induction coil or "spark coil" (archaically known as a Ruhmkorff coil) is a type of disruptive discharge coil. It is a type of electrical transformer used to produce high-voltage pulses from a low-voltage DC supply. To create the flux changes necessary to induce voltage in the secondary, the DC current in the primary is repeatedly interrupted by a vibrating mechanical contact called an "interrupter". The term 'induction coil' is also used for a coil carrying high-frequency AC producing eddy currents to heat objects placed in the interior of the coil, in induction heating or zone melting equipment.

How it works

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 tens or hundreds of turns of coarse wire. The other coil, the "secondary winding," typically consists of many thousands of 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 sometimes called spark coils.

The size of induction coils was often 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 attracted an 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.

Although opposite potentials are induced in the secondary when the interrupter 'breaks' the circuit and 'closes' the circuit, the current change is much more abrupt when the interrupter 'breaks', so the pulse of voltage induced in the secondary at 'break' is much larger. 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. [cite book|last=Collins|first=Archie F.|title=The Design and Construction of Induction Coils|date=1908|publisher=Munn & Co.|location=New York|url=http://books.google.com/books?id=dJNPAAAAMAAJ&pg=PA98 p.98] 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 sparking between the secondary wires, the secondary coil is constructed so as to avoid having wires carrying large voltage differences lying next to each other. The secondary coil is wound in many thin 'pancake' shaped sections, 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. 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, the iron core is made of a bundle of parallel iron wires, individually coated with shellac to insulate them electrically. This prevents eddy currents, which flow perpendicular to the magnetic axis.

History

Michael Faraday discovered the principle of induction, Faraday's law, in 1831 and did the first experiments with induction between coils of wire. [cite journal|last=Faraday|first=Michael|date=1834|title=Experimental researches on electricity, 7th series|journal=Phil. Trans. R. Soc. (London)|volume=124|pages=77–122|doi=10.1098/rstl.1834.0008] The induction coil was invented by the Irish scientist Nicholas Callan in 1836 at the St. Patrick's College, Maynooth [cite book|last=Fleming|first=John Ambrose|date=1896|title=The Alternate Current Transformer in Theory and Practice, Vol.2|publisher=The Electrician Publishing Co.|url=http://books.google.com/books?id=17sKAAAAIAAJ&pg=PA16 p.16-18] [http://www.nuim.ie/museum/ncallan.html] and improved by William Sturgeon, C. G. 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 suggested the use of the quenching capacitor. [cite web|last=Severns|first=Rudy|title=History of soft switching, Part 2|work=Design Resource Center|publisher=Switching Power Magazine|url=http://www.switchingpowermagazine.com/downloads/Oct%2001%20soft.pdf|accessdate=2008-05-16] Heinrich Ruhmkorff generated higher voltages by greatly increasing the length of the secondary, in some coils using 5 or 6 miles of wire.

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.

Today, a type of induction coil remains in common use as the ignition coil or spark coil in the ignition system of internal combustion engines. 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 [http://www.ecogeek.org/content/view/1431/] .

Early patents

* The induction-coil, instead of being made movable upon the magnet
* This compound coil is made like any ordinary induction-coil
* The inner end of the induction-coil are surrounded by the prime coil
* The induction-coil consists of a metallic conductor, copper is generally preferred
* Energizing the primary wire of the induction-coil, the iron core becomes magnetized
* Making use of an induction-coil
* a split-coil improvement (1903).
* Induction coil comprising a soft iron core (Mar 5, 1913)

See also

* Charging station
* Ignition coil
* Spark gap transmitter
* Transformer
* Tesla coil

Footnotes

Further reading

* Norrie, H. S., "Induction Coils: How to Make, Use, and Repair Them". Norman H. Schneider, 1907, New York. 4th edition.
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* Has detailed history of invention of induction coil
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External links

* [http://www.rmcybernetics.com/projects/DIY_Devices/homemade_ignition_coil_driver.htm Battery powered Driver circuit for Induction Coils]
* [http://members.chello.nl/~h.dijkstra19/page8.html The Cathode Ray Tube site]