Electromagnet

An electromagnet consisting of a coil, in which forms a magnetic field when current flows through. In the coil is usually an open iron core which leads the magnetic field and amplified. The invention of the electromagnet succeeded the Englishman William Sturgeon in 1826. Was first detected the electromagnetic effect in 1820 by the Danish physicist Hans Christian Ørsted.

Action principle

A current-carrying conductor creates a magnetic field in its vicinity ( discovery by Hans Christian Ørsted in 1820).

The direction of the magnetic field lines of a single turn of the coil can be with the corkscrew rule (including right-hand rule) determine: If the conductor is so embraced by the hand thought that the splayed thumb in the direction from the positive to the negative pole ( direction of current ) shows, then the fingers are pointing the direction of the field lines of the magnetic field. The fields of the individual turns add up to a winding cross-sectional total circumferential field. The field lines as well as in a single turn (all current directions of the turns are in the same direction! ) And leave the iron core - there is the north magnetic pole is formed. All field lines occur at the magnetic south pole back in the iron core.

The magnetic field lines are concentrated in the interior of 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 with distance from fast, so electromagnets have a large effect only at close distances.

If work can be done, the magnetic circuit must be ferromagnetic and non-homogeneous, ie an interruption contained in the iron core, which is to be reduced by the work.

The Lenz's Law states in substance, that a force or motion is directed so that (in this case the current flow ) it counteracts its cause. Consequently, a magnetic circuit is a current-carrying coil is striving to reduce its magnetic resistance and also to close air gaps: This increases the inductance and the coil, a voltage is induced which has the same polarity as the supply voltage - the flow is reduced during the to each other, moving the iron parts of the magnetic circuit.

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

Theory

For a long electromagnetic coil of length l { unit: m (meters ) } and the number of turns n { dimensionless }, through which a current I { unit: A ( amperes) } flows, the magnetic field strength H { unit calculates: A / m } to

And the magnetic flux density B { unit: T ( Tesla) } for

It is the magnetic field constant and the permeability of the area enclosed by the coil room.

In vacuo ( or approximated in air) the relative permeability in the ferromagnetic materials is their value 4-15000 until the material-dependent magnetic saturation.

Types and characteristics

Train, folding anchor and holding magnets

They are the actuation ( train, printing and folding armature magnets ) as a coupling or transport. They differ by the anchor form:

• Train and printing magnets have rod-shaped anchor
• Contactor actuating coil having the I or T-shaped anchor, and in accordance with an E-shaped yoke
• The hinged armature (see also hinged armature relay ) pivots an angled anchor plate to one of the edges of the yoke
• At coupling magnet ( magnetic coupling ) of the anchor is a disc
• Holding and transporting the cargo to use magnets as an "anchor ." Examples are magnetic separators and magnetic crane.

With DC -powered magnets have a highly non-linear force-displacement curve at approach of the armature to the yoke. Touch both, the force is greatest. With the distance it falls almost hyperbolic. The reason is the increasing with the decrease of the air gap magnetic flux density. The low at the beginning of the tightening force makes them unsuitable for applications that require a large force immediately. Ways out are:

• Excessive voltage as torque support
• Structural design of the magnetic poles ( the armature and yoke ): Andre Hungen increase the force of large strokes
• Proportional solenoids (for example for proportional valves ) have an effective in decreasing distance expectant magnetic shunt.

The situation is different for AC voltage: Here the reduced case of a large air gap inductance causes an increased current flow when tightening. AC pull magnets (or relay and contactor coils) have therefore already at the beginning of attracting a large force.

In order to maintain the force at AC traction magnet while the current zero-crossings, is given to short-circuit windings as in a shaded pole one - they produce in a part of the magnetic circuit a phase shifted magnetic field. Another possibility is three- pull magnets, however, this requires three separate leg of the yoke and the armature.

When switching off the current can be caused by self-induction surges that cause in turn sparks or arcs. This can lead to the destruction of the switch. As a remedy, varistors used in direct current protection diodes in AC.

Relay, contactor

Electromechanical relays are usually constructed with a hinged armature mechanism operated by a lever or the contacts. Relays are built with DC or AC coils. A contactor used mostly plunger solenoids for DC or AC. The tightening forces to the contact closure are substantially larger than at the relay, and therefore the electromagnets for greater than relays.

Tauchspulmagnete

Moving coil can be installed in train and print magnet. A common English term is also voice coil because microphones or speakers are built with it. Either is also a parallel guide available or the user needs to carry out the construction itself a tour of the coil in a permanent magnet. In Tauchspulmagneten a coil ( solenoid ) moves like the electrodynamic loudspeaker in the air gap of a permanent magnet by the Lorentz force. You have, compared to the designs described above to a linear force / displacement characteristic. The moving mass is low, therefore the momentum is high. However, the achievable power per mass is lower.

Solenoid plunger magnets

In contactors greater forces than in relay to close the contacts are required, which is why it uses electromagnets that pull an iron core in the fixed coil. These are built for both DC and for AC operation.

Solenoids

Solenoids with and without the yoke, but without moving the anchor or the like are generally not referred to as an electromagnet. Relevant keywords are solenoid ( solenoid ), Helmholtz coil, bending magnet, dipole magnet.

Magnetic disks

High flux densities without superconductivity can be achieved by magnets in which each coil consists of a slotted disc of copper. The central hole serves to receive the sample.

The next plate is placed with some overlap with the previous. Here is a direct electrical contact. The remaining area is electrically separated by an insulating intermediate layer, and so is the next turn.

The outward radial bore holes are used for receiving mounting bolts, moreover, are distributed over the surface many small holes introduced. These allow the passage of cooling fluid. Because of the higher inside the electric current density (due to the shorter current path on the smaller circumference ) more holes are provided inwardly relative.

The plates are assembled to form a plate stack which is approximately the same as width. With disc diameters of about 40 cm, hole diameters of about 5 cm, slice thickness of approximately 2 mm, currents up to 20 kA, slice numbers of 250 and a large amount of water cooling can thus reach up to 16 Tesla flux densities; for a bore diameter of 3 cm to 19 Tesla. This is no iron core application because this would already at 2 Tesla in saturation. The test specimen is positioned directly in the center of the magnet.

The electric power demand reaches up to 5 MW (about 1 V per turn ). For continuous operation, the fully converted into heat electrical power must be dissipated by an adequately dimensioned cooling.

In the pulsed operation a high flux density can be achieved with non-cooled coil due to the heat capacity of the coil material for a short time. In experiments with even higher magnetic flux densities, however, the coils are destroyed mechanically or thermally at each attempt.

Applications

1 coil with a ferromagnetic core (usually made of iron)

• Operating solenoids relays and contactors
• Door opener magnet, refrigerator magnet and door buzzers, gongs
• Magnetic couplings ( in vacuum pumps or air compressors for automotive applications) and brakes ( with return spring in lawn mowers and cranes )
• Pull magnets, drawer magnets
• Lifting magnets (magnetic crane in steel mills )
• Magnetic track brake for rail vehicles
• To provide magnets course of rail vehicles
• AC magnets in membrane pumps or metering pumps ( air pump for aquariums, additives or fuels) and vibratory feeders
• Exciter field generation in electric motors (as in the vacuum cleaner) and generators ( automotive alternator, power plant)
• Separators for separating materials "ferromagnetic " / " non-ferromagnetic " ( magnetic separator for waste sorting)
• Bending magnets in particle accelerators for charged particle beams
• Deflection and focusing magnets ( electron, electron beam welding, cathode ray tubes )
• Nitrogen -cooled pulse magnets for high field studies. Best example is the European record holder for high magnetic fields, the Institute High Magnetic Field Laboratory Dresden.
• Electromagnetically actuated fuel injectors in diesel engines with common-rail injection method
• With magnetic filters (mainly electric magnetic filter ) are ferromagnetic solids ( finely divided iron oxides) filtered from the circulation condensates of power plants and the circulating water of district heating networks.

Second coil without ferromagnetic core material

• Field generation for traveling wave tubes
• Field generation for induction furnaces
• Operating coil for reed contacts
• Superconducting magnets in magnetic resonance tomography and research, as in nuclear fusion reactors based on the fusion by magnetic confinement
• Uncooled magnetic coils for high-field studies (only pulse operation - often the coil must be replaced after each experiment )
• Forming, welding, joining and cutting metals
• Bitter Magnet ( named after its developer at the National Magnet Laboratory at MIT Francis Bitter ), consisting accessible from a stack of about 250 conductors and insulator plates by water cooling fields up to 20 Tesla in continuous operation up to 100 Tesla in pulsed operation