This article is about a specific type of
transformer. For the more general electrical component, see
Inductor.
An
induction coil or "spark coil" (
archaically known as a
Ruhmkorff coil after
Heinrich Ruhmkorff) is a type of
disruptive discharge coil. It is a type of electrical
transformer used to produce high-voltage pulses from a low-voltage
direct current (DC) supply. To create the flux changes necessary to induce voltage in the secondary, the direct current in the primary is repeatedly interrupted by a vibrating mechanical
contact called an
interrupter. Developed beginning in 1836 by
Nicholas Callan and others, the induction coil was the first type of transformer.
The term 'induction coil' is also used for a coil carrying high-frequency
alternating current (AC), producing eddy currents to heat objects placed in the interior of the coil, in
induction heating or
zone melting equipment.
Antique induction coil used in schools, Bremerhaven, Germany
[edit] How it works
Induction coil showing construction, from 1920.
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) 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' (20 cm) induction coil was one that could produce an 8 inch arc.
[edit] 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.
[edit] 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.
[1] The largest coils, used in radio transmitters, used either electrolytic or mercury turbine 'breaks'.
[edit] 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.
[edit] 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.
[2] The induction coil was invented by the Irish scientist and Catholic priest
Nicholas Callan in 1836 at the
St. Patrick's College, Maynooth[3][4] 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. Neef, P. Wagner, and J. W. M'Gauley.
Hippolyte Fizeau introduced the use of the quenching capacitor.
[5] Heinrich Ruhmkorff generated higher voltages by greatly increasing the length of the secondary, in some coils using 5 or 6 miles (10 km) 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 (60 cm) or longer in length.
[6][7] 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,
[8] and later at the
University of Edinburgh in Scotland.
[9] Ruhmkorff himself purchased a Ritchie induction coil, utilizing its improvements in his own work.
[10][11]
Induction coils were used to provide high voltage for early
gas discharge and
Crookes tubes and other high voltage 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
Lodge and
Marconi in the first research into radio waves. Their largest industrial use was probably in early
wireless telegraphy spark-gap radio transmitters and to power early
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.
Automobile
ignition coil, the largest remaining use for induction coils
[edit] 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.
[12]
[edit] 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)
http://en.wikipedia.org/wiki/Induction_coil