The main feature of a coil is inductance, and this is caused by the connecting many individual conductors in series. The conductor wire is wound on a support core. If the core is of ferromagnetic material, then the coils are made of iron core. An air-core coil has no magnetisable core. Two main type of coils may be distinguished: the cylindrical coil open on both sides, and the closed circular ring coil.
The coil possesses an ohmic resistance, which depends on the leader material, the leader length, and its cross section. When the electric current flows through a coil, it develops a magnetic field. A cylinder coil attached to DC voltage works in the long run as a bar magnet. The magnetic field depends on the nuclear material of the coil, on the number of turns, and on coil geometry. All sizes together form the inductance of a coil.
Alternating current reduces the magnetic field in a coil. The coil takes up energy, stores it in the magnetic field, and delivers it again, pushing the energy back and forth, pushed without effect. This energy is called reactive energy and the resistance reactance. If the core is made of iron, then eddy currents from the continuing remagnetisation arise. This warms up the core, leading to further loss of energy. A coil with iron losses takes up more current than is calculated from the reactance. The iron loss resistance corresponds to an efective resistance in the equivalent electric circuit, which is switched parallel to the reactance.
The inductive reactance is affected by the frequency of the AC voltage, and its inductance. The inductive reactance is the greater, the greater the inductance of the coil, and the higher the frequency of the applied AC voltage is. The symbol of inductance is “L”, and the unit is designated as “H”, in honor of the physicist Joseph Henry. A coil has inductance 1 Henry, if the amperage changes evenly in 1 second by 1 ampere, and a tension is induced by 1 V.
It is possible to measure the inductance easily with a precise LCR meter.« Back