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electromagnet,device in which magnetismmagnetism,
force of attraction or repulsion between various substances, especially those made of iron and certain other metals; ultimately it is due to the motion of electric charges.
..... Click the link for more information. is produced by an electric current. Any electric current produces a magnetic field, but the field near an ordinary straight conductor is rarely strong enough to be of practical use. A strong field can be produced if an insulated wire is wrapped around a soft iron core and a current passed through the wire. The strength of the magnetic field produced by such an electromagnet depends on the number of coils of wire, the magnitude of the current, and the magnetic permeability of the core material; a strong field can be produced from a small current if a large number of turns of wire are used. Unlike the materials from which permanent magnets are made, the soft iron in the core of an electromagnet retains little of the magnetism induced in it by the current after the current has been turned off. This property makes it more useful than a permanent magnet in many applications. Electromagnets are used to lift large masses of magnetic materials, such as scrap iron. They are essential to the design of the electric generator and electric motor and are also employed in doorbells, circuit breakers, television receivers, loudspeakers, atomic particle accelerators, and electromagnetic brakes and clutches. Electromagnetic propulsion systems can provide motive power for spacecraft. Electromagnets are also essential to magnetic levitationmagnetic levitation
, support and often propulsion of objects or vehicles by the use of magnets. The magnets used in magnetic levitation suspend an object free of contact with any surface, making it particularly appropriate for high-speed (275–300
..... Click the link for more information. systems. Such systems often use a special kind of electromagnet whose coil is made of a superconducting metal. Because the coils of a superconducting electromagnet offers no resistance to the flow of electricity, no energy is wasted by the development of heat, and the magnetic field produced by the magnet can be very strong. Superconducting magnets are used in magnetic-resonance imaging, and can also be used for energy storage. The first practical electromagnet was invented early in the 19th cent. by William Sturgeon.
an electrical engineering device that usually consists of a current-carrying coil and a ferromagnetic core. The core is magnetized—that is, acquires the properties of a magnet—when an electric current is passed through the coil. Electromagnets are used mainly to produce either a magnetic flux, as in electric machinery, or a force, as in driving mechanisms.
Regardless of differences in design, all electromagnets usually comprise the following parts, which serve the same purposes in any electromagnet: a current-carrying coil, a magnetizable core, and an armature. The coil windings are made of insulated aluminum or copper wire; in some electromagnets, the windings are fabricated from a superconducting material (seeSUPERCONDUCTOR MAGNET). The core is the fixed part of an electromagnet’s magnetic circuit. The armature, which imparts a force to components that actuate a mechanism, is the movable part of the magnetic circuit. The magnetic circuit of an electromagnet is made of a soft magnetic material, usually electrical sheet, high-grade structural steel, cast steel, cast iron, an iron-nickel alloy, or an iron-cobalt alloy. To reduce losses due to eddy currents, a magnetic circuit is fabricated from a stack of sheets.
Depending on the method of producing the magnetic flux and on the nature of the magnetizing force, electromagnets are divided into three groups. DC electromagnets constitute two groups, namely, ordinary electromagnets and polarized electromagnets. The third group comprises AC electromagnets.
In ordinary electromagnets, the pull depends only on the magnitude of the magnetic flux and is independent of the direction of the current in the coil. When the current in the coil is switched off, the magnetic flux and, consequently, the pull are virtually equal to zero.
In polarized electromagnets, two separate magnetic fluxes, called the polarizing flux and the operating flux, are produced. The polarizing flux is usually produced by the field of a permanent magnet or, in some cases, of another electromagnet. The operating flux is produced by the magnetizing force of the operating coil. If the current is switched off, a pull created by the polarizing flux acts on the armature. The operation of a polarized electromagnet depends both on the magnitude of the overall magnetic flux and on the direction of the current in the operating coil.
In AC electromagnets, the coil is energized by an AC source and the magnetic flux varies periodically in magnitude and direction. As a result of the variation, the pull fluctuates from zero to some maximum value at twice the frequency of the magnetizing current.
Electromagnets may also be classified according to a number of other criteria, such as the method of energizing the coil, the nature of their operation, and the operating and release characteristics. According to the method of energizing the coil, a distinction is made between series electromagnets and shunt electromagnets. Electromagnets may be designed for continuous, intermittent, or pulsed operation. According to operating and release characteristics, a distinction is made between quick-acting and slow-acting electromagnets.
The broadest and most important area of application for electromagnets is electric machinery and devices that are components of automation systems or of equipment that controls or protects electrical engineering installations. In various mechanisms, an electromagnet is used as an actuator for developing the necessary rotation of a working member or for producing a holding force. Examples of such electromagnets include the electromagnets used in load-lifting machines, clutches, brakes, starters, contactors, switches, and moving-iron instruments. The electromagnets in the traction drives employed in high-speed transportation might be used for magnetic suspension of a vehicle. A developing field of application for electromagnets is medical equipment. For scientific purposes, electromagnets are used in experimental chemistry, biology, and physics.
In connection with the broad range of applications, the designs, sizes, and power consumption of electromagnets vary over a wide range. Depending on the purpose for which electromagnets are used, the weight of an electromagnet may range from fractions of a gram to hundreds of tons; the power consumption may range from fractions of a watt to tens of megawatts.
REFERENCESGordon, A. V., and A. G. Slivinskaia. Elektromagnity postoiannogo toka. Moscow-Leningrad, 1960.
Karasik, V. R. Fizika i tekhnika sil’nykh magnitnykh polei. Moscow, 1964.
Ter-Akopov, A. K. Dinamika bystrodeislvuiushchikh elektromagnitov. Moscow-Leningrad, 1965.
Slivinskaia, A. G. Elektromagnity i postoiannye magnity. Moscow, 1972.
M. I. OZEROV
A soft-iron core that is magnetized by passing a current through a coil of wire wound on the core. Electromagnets are used to lift heavy masses of magnetic material and to attract movable magnetic parts of electric devices, such as solenoids, relays, and clutches.
The difference between cores of an electromagnet and a permanent magnet is in the retentivity of the material used. Permanent magnets, initially magnetized by placing them in a coil through which current is passed, are made of retentive (magnetically “hard”) materials which maintain the magnetic properties for a long period of time after being removed from the coil. Electromagnets are meant to be devices in which the magnetism in the cores can be turned on or off. Therefore, the core material is nonretentive (magnetically “soft”) material which maintains the magnetic properties only while current flows in the coil. All magnetic materials have some retentivity, called residual magnetism; the difference is one of degree.
In an engineering sense the word electromagnet does not refer to the electromagnetic forces incidentally set up in all devices in which an electric current exists, but only to those devices in which the current is primarily designed to produce this force, as in solenoids, relay coils, electromagnetic brakes and clutches, and in tractive and lifting or holding magnets and magnetic chucks.
Electromagnets may be divided into two classes: traction magnets, in which the pull is to be exerted over a distance and work is done by reducing the air gap; and lifting or holding magnets, in which the material is initially placed in contact with the magnet. Examples of the latter type are magnetic chucks and circular lifting magnets. For examples of the first type. See Brake, Clutch, Relay