What are electromagnets



A Electromagnet consists of a coil in which a magnetic field is created when a current flows through it. In the coil there is usually an open iron core that guides and strengthens the magnetic field. The Englishman William Sturgeon invented the electromagnet in 1826.

The amplification of the magnetic field by ferromagnetic materials only works up to their saturation flux density, which is around 1 to 2 Tesla. Very strong electromagnets must therefore be manufactured without a core material.

Working principle

A current-carrying conductor creates a magnetic field around itself (Ørsted 1820).

The direction of the magnetic field lines of a single turn of the coil can be determined using the corkscrew rule (also known as the right-hand rule): If the conductor is held in the hand, the splayed thumb points in the direction from the plus to the minus pole (technical current direction ) then the fingers indicate the direction of the field lines of the magnetic field. The fields of the individual turns add up to a total field surrounding the winding cross-section. The field lines run just like with a single turn (all current directions of the turns are in the same direction!) And leave the iron core - this is where the magnetic north pole is formed. All field lines re-enter the iron core at the magnetic south pole.
The magnetic field lines are concentrated inside the coil. The magnetic flux density is highest in the center of the coil.
Outside the coil, the magnetic flux density is lower; it decreases rapidly with distance, so that electromagnets only have a great effect at short distances.

If work is to be done, the magnetic field circuit must be ferromagnetic and inhomogeneous, i.e. contain an interruption.
Lenz's rule states that a force or movement is directed in such a way that it counteracts its cause (in this case the flow of current). Consequently, a magnetic circuit around a current-carrying coil tries to reduce its magnetic resistance and, for example, to close air gaps: This increases the inductance and a voltage is induced in the coil that has the same polarity as the supply voltage - the current is reduced during the Moving the iron parts of the magnetic circuit towards each other.

Iron parts of the magnetic circuit consist of a yoke (fixed part) and moving parts such as tie rods, hinged armatures or iron parts to be transported (magnetic crane).

theory

For an electromagnetic coil of length l {Unit of measurement: m (meter)} and the number of turns n {without a unit of measure} through which a stream I. {Unit of measurement: A (Ampère)} flows, the magnetic field strength is calculatedH {Unit: A / m} to

or the magnetic flux densityB. {Unit of measurement: T (Tesla)} to

.

Where μ is0 the magnetic field constant and μr the permeability of the space enclosed by the coil.

.

In a vacuum or in air, the relative permeability is μr = 1, in ferromagnetic materials its value is between 4 and 15,000 until the material-dependent magnetic saturation is reached.

Pulling and holding magnets operated with DC voltage have a strongly non-linear force-displacement characteristic. The cause is the increasing magnetic flux density as the air gap decreases. The low force at the beginning of the tightening makes them unsuitable for applications that require a great deal of force immediately. One way out is to use excessive tension to aid in getting dressed.
It is different with AC voltage: Here, the reduced inductance with a large air gap causes an increased current flow when tightening. AC pull magnets (or relay and contactor coils) therefore have a great deal of force right from the start of their attraction.
In order to maintain the force of alternating current pull magnets during the current zero crossings, short-circuit windings are used as in a shaded-pole motor - these generate a phase-shifted magnetic field in part of the magnetic circuit. Another possibility are three-phase pull magnets, but these require three separate legs of the yoke and armature.

Properties of electromagnets

DC solenoidAC solenoid
constant high power consumptionPower consumption strongly dependent on armature position
longer switching timefast switching
When switching off, the switching element often needs protection, e.g. by means of a free-wheeling diodeSuppression element (Boucherot element) recommended
large dropout delay with free-wheeling diodelow dropout delay
Residual air gap required as adhesive protectionShaded pole / short-circuit winding required to avoid noise
Switching time can be reduced by overvoltageSwitching time cannot be influenced

Applications

1st coil With ferromagnetic core (mostly made of iron)

  • Actuating magnets for relays and contactors
  • Door opener magnet, magnets in buzzers and door gongs
  • Magnetic clutches (e.g. in vacuum pumps or air conditioning compressors in vehicles) and brakes (e.g. with return springs in lawnmowers and on cranes)
  • Pull magnets, push magnets
  • Lifting magnets (magnetic crane in steelworks)
  • Magnetic rail brake on rail vehicles
  • Magnets to set points in rail vehicles
  • AC magnets in diaphragm pumps (e.g. air pumps for aquariums) and vibratory feeders
  • Excitation field generation in electric motors (e.g. vacuum cleaners) and generators (car alternator, power station)
  • Separators for "ferromagnetic" / "non-ferromagnetic" material separation (magnetic separators, e.g. for sorting waste)
  • Deflection magnets in particle accelerators and for electron beams (picture tube)
  • Deflection and focusing magnets (electron microscope, electron beam welding)
  • Nitrogen-cooled pulse magnets for high-field examinations

2nd coil without ferromagnetic core material

  • Field generation for traveling wave tubes
  • Actuating coil for reed contacts
  • superconducting magnets in nuclear magnetic resonance tomographs and for research
  • uncooled magnetic coils for high-field examinations (only pulse operation - the coil often has to be replaced after each experiment)
  • Bitter magnet (named after its developer at the National Magnet Laboratory of MIT Francis Bitter), consisting of a stack of around 250 conductor and insulator plates, fields of up to 20 Tesla in continuous operation, up to 100 Tesla in pulse operation achievable by water cooling

See also: List of electronic components

Category: Magnetism