# Electromagnet

Electromagnet on a crane for picking up scrap iron

An electromagnet consists of a coil in which a magnetic field is created as a result of an electric current .

In the coil there is usually an open iron core that guides and strengthens the magnetic field. The invention of the electromagnet was achieved by the Englishman William Sturgeon in 1826. The electromagnetic effect was first demonstrated in 1820 by the Danish physicist Hans Christian Ørsted .

## Working principle

A current-carrying conductor causes a magnetic field in its surroundings (discovery by Hans Christian 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 that encircles 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, that is, contain an interruption in the iron core, which is to be reduced by the work.

The Lenz's Law states in substance, that a force or movement is directed so as to its cause - in this case, the current flow - counteracts. As a result, a magnetic circuit around a coil with current through it strives to reduce its magnetic resistance and also 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 decreases during 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 a long electromagnetic coil of length l {unit of measure: m (meter)} and the number of turns n {without unit of measure} through which a current I {unit of measure: A (ampère)} flows, the magnetic field strength H {unit: A is calculated / m} to

${\ displaystyle H = I \ cdot {\ frac {n} {l}} \,}$

or the magnetic flux density B {unit of measurement: T ( Tesla )} to

${\ displaystyle B = \ mu _ {0} \ cdot \ mu _ {r} \ cdot I \ cdot {\ frac {n} {l}} \,}$.

Here μ 0 is the magnetic field constant and μ r is the relative permeability of the space enclosed by the coil. By definition, μ r = 1 in a vacuum , and the value of μ r is also very close to one in air ; in ferromagnetic materials, however, μ r can assume values ​​between 4 and 15,000 until the material-dependent magnetic saturation is reached.

## Designs and special features

Electromagnet with variable air gap for experimental applications, with which magnetic flux densities of up to 2 Tesla can be generated.

### Pull, hinged anchor and retaining magnets

They are used for actuation (pull, pressure and hinged armature magnets), as a coupling or for transport. They differ in the anchor shape:

• Pull and push magnets have rod-shaped armatures
• Contactor actuating coils have I- or T-shaped armatures and an E-shaped yoke accordingly
• With the hinged armature (see also hinged armature relay ) an angled armature plate swivels around one of the edges of the yoke
• with coupling magnets ( magnetic coupling ) the armature is a disk
• Holding and transport magnets use the transported goods as "anchors". Examples are also magnetic separators and magnetic cranes.

Magnets operated with DC voltage have a strongly non-linear force-displacement characteristic when the armature approaches the yoke. When they both touch, the force is greatest. With the distance it sinks almost hyperbolically . 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. The ways out are:

• Excessive tension as a dressing aid
• structural design of the magnetic poles (armature and yoke):
• Turning increases the force even with large strokes
• Proportional solenoids (for example for proportional valves ) have a magnetic shunt that becomes effective as the distance decreases

It is different with alternating voltage : Here the reduced inductance with a large air gap causes an increased current flow when tightening. AC pull magnets (or also relay and contactor coils ) therefore have a great deal of force at the beginning of their attraction.

Basic timing diagram for pull magnet with shaded pole (F ~ H²)

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 create 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.

When the current is switched off, self-induction can cause overvoltages, which in turn cause sparks or arcs . These can destroy the switch. As a remedy are in direct current protection diodes , with alternating current varistors and discretely antiserially connected zener diodes (or integrated power suppressor ), and - - or in combination with Boucherotgliedern used.

### Relay, contactor

Electromechanical relays are usually constructed with a hinged armature mechanism that actuates the contact or contacts via a lever. Relays are built with DC or AC coils. A contactor mostly uses plunger electromagnets for direct or alternating current. The attraction forces to make the contact are much greater than with relays, which is why the electromagnets are larger than with relays.

### Moving coil magnets

Pull magnets with plunger for direct current

Moving coils can also be built into pull and push magnets. A common English term is also voice coil , because microphones or speakers are built with it. Either there is also a parallel guide or the user himself has to guarantee that the coil is guided in the permanent magnet through the design. With moving coil magnets , as with electrodynamic loudspeakers, a coil ( cylinder coil ) moves in the air gap of a permanent magnet due to the Lorentz force . Compared to the designs described above, they have an almost linear force / displacement characteristic (depending on the non-linear boundary conditions of the technical implementation). The moving mass is low, so the dynamics are high. However, the achievable force per mass is lower.

### Solenoid armature magnets

In contactors , greater forces are required to close the contacts than in relays, which is why electromagnets are used for this, which pull an iron core into the fixed coil. These are built for both direct and alternating current operation.

### Solenoids

Electromagnets with and without a yoke, but without a moving armature or the like, are usually not referred to as electromagnets. Relevant terms are solenoid ( cylinder coil ), Helmholtz coil , deflection magnet , dipole magnet .

#### Disc windings

Disc winding made of a 16 T magnet for 20 kA, approx. 40 cm diameter, with a point of breakdown from a crash

High flux densities, even without superconductivity, can be achieved using magnets in which each coil turn consists of a slotted disc made of copper . Iron cores cannot be used because they were saturated at 2 Tesla. The central hole is used to hold the sample. The next plate is electrically separated by an intermediate insulating layer and thus forms the next turn. The bores made radially on the outside (picture on the right) are used to accommodate assembly bolts; in addition, many small bores are distributed over the surface, through which coolant flows. Because of the shorter current path on the smaller circumference inside, higher electrical current densities occur there, so there are more holes per area than outside. The plates are put together to form a stack of plates that is roughly the same height as it is wide. Such magnets are also called Bitter Magnet , the Bitter disk . They were invented in 1933 by the American physicist Francis Bitter .

With disk diameters of approx. 40 cm, bore diameters of approx. 5 cm, disk thicknesses of approx. 2 mm, currents up to 20 kA, number of disks of 250 and a great deal of water cooling, z. B. achieve flux densities of up to 16 Tesla; with a bore diameter of 3 cm up to 19 Tesla. The electrical power requirement here reaches 5 MW (approx. 1 V per turn).

Such magnets are used to hold the flux density records for artificial continuous magnetic fields. This is 37.5 T in the High Field Magnet Laboratory (HFML) in Nijmegen (32 mm bore). Such flux densities cannot be achieved with superconducting magnets - the transition temperature drops with the field and the superconductivity breaks down at the critical field strength . However, combined systems are in operation (hybrid magnets) in which a bitter magnet is placed inside a superconducting magnet. In the National High Magnetic Field Laboratory in Florida / USA, currently 45 T, the strongest artificial continuous field in the world is achieved. For this purpose, there is a 33.5 Tesla disc magnet coil (32 mm bore) in a 11.5 Tesla superconducting magnet. The power requirement is 30 MW.

#### Pulse operation

In pulse operation, thanks to the heat capacity of the coil material, high flux densities can be achieved for a short time without the heat output having to be cooled down immediately ( integral of the current heat over time). Such coils must be mechanically stabilized for mechanical stability. To serve u. a. Fiber composite materials, coil wires made of high-strength materials such as copper-clad steel or beryllium bronze as well as outer bandages made of steel strip. The current pulses are provided by capacitors . The pulse durations result from the heat capacity and the strength and amount to a few milliseconds. See also Gauss rifle .

Such reusable pulse magnet coils, cooled down with liquid nitrogen, can be implemented for high field examinations up to about 100 Tesla and are developed and tested at the Institute High Field Magnetic Laboratory in Dresden .

Pulse magnetic coils are also used for magnet forming , among other things . Here, however, the fields are damped oscillations with frequencies in the two-digit kHz range, the pulse durations are less than 100 µs.

In experiments with magnetic flux densities of a few thousand Tesla for scientific purposes, it is often accepted that the coils are mechanically or thermally destroyed with each attempt. An additional increase in the flux density can be achieved with simultaneous compression of the coil or the field by means of explosive charges; See also the flow compression generator or in the chapter on impulse technology from Sakharov , the inventor of the flow compression generator.

## Properties of actuating magnets

DC solenoid AC solenoid
constant high power consumption Current consumption strongly dependent on armature position
longer switching time fast switching
When switching off, protection of the switching element is often necessary (for example by means of a freewheeling diode ) Suppression element (Boucherot element) recommended
large dropout delay with free-wheeling diode low dropout delay
Residual air gap required as adhesive protection Shaded pole / short-circuit winding required to avoid humming noises
Switching time can be reduced by overvoltage Switching time cannot be influenced

## Applications

Stator windings of an electric motor

1. Coil with ferromagnetic core (mostly made of iron)

2. Coil without ferromagnetic core material

## literature

• Klaus D. Linsmeier, Achim Greis: Electromagnetic actuators. Physical principles, designs, applications. In: Die Bibliothek der Technik, Volume 197. Verlag Moderne Industrie, ISBN 3-478-93224-6 .
• Günter Springer: Expertise in electrical engineering. 18th edition, Verlag - Europa - Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .
• Horst Stöcker: Pocket book of physics. 4th edition, Verlag Harry Deutsch, Frankfurt am Main 2000, ISBN 3-8171-1628-4 .
• The big book of technology. Publishing house for knowledge and education, Bertelsmann GmbH publishing group, Gütersloh 1972.
• Kallenbach, et al. (2008): Electromagnets . 3rd edition, Vieweg + Teubner Verlag, Wiesbaden, ISBN 978-3-8351-0138-8 .
• Greg Boebinger, Al Passner, Joze Bevk: High- performance electromagnets . Spectrum of Sciences, March 1996, pp. 58-63