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GLOSSARY
· Air Gap
A nonmagnetic discontinuity in a ferromagnetic circuit. For example, the space between the poles of a magnet, although filled with brass or wood or other nonmagnetic material, is nevertheless called an air gap.
· Alnico
Aluminum-nickel-cobalt-iron alloy with hard magnetic properties, formed by casting or sintering. Discovered in 1935, it owes its magnetic properties to elongated-shape magnetic particles of FeCo aligned in a lesser magnetic phase of Fe-Ni-Al. Most common alnico is Alnico 5, that has high remanence (1.2 - 1.3 T) and low coercivity (52 - 56 kA/m).
· Ampere-turn
· Anisotropic magnets
Anisotropic magnets have different magnetic characteristics in different directions. Generally, anisotropic magnets are pressed in a magnetic field and have for this reason a preferred direction. They have only in this preferred direction good magnetic properties and can be magnetised only in this direction.
· Anisotropy
The meaning is that specific physical properties may be different in various directions of the material. The magnetic powder of anisotropically manufactured permanent magnets are exposed to a strong magnetic field during the pressing procedure. The results in an higher magnetic value in the direction of the field than at right angles to it (see also: anisotropic magnets).
· Antiferromagnetism
Type of magnetic structure in which magnetic moments are aligned in anti-parallel way.
· B·Hmax
The greatest possible product from B and H within the demagnetisation curve. The greater the BHmax value of a material is, the smaller may be the magnetic volume required for a given application task, under identical conditions.
· Bonded magnet
Materials composed by ferrite or rare-earth powders embedded in rubber, resins or plastics. They may be formed by rolling, extrusion, compression and injection molding, and they can be flexible or rigid. They have lower levels of B compared with the original magnetic material, but they can be formed in complex shapes.
· Calibration
Magnetic calibration is a way to magnetize and stabilize the magnetic properties of permanent magnet materials and devices to fixed values. This operation eliminates differences between samples due to tolerances on magnetic properties or mechanical and constructive characteristics.
· Circuit, closed magnetic
Circuit where the magnetic flux is conducted continually around a closed path through ferromagnetic materials; for example, a steel ring.
· Circuit, open magnetic
Magnetic circuit in which the ferromagnetic material has not a closed external circuit and does not form a complete conducting circuit itself. For example, a permanent magnet ring interrupted by an air gap is an open magnetic circuit.
· Coercimeter
Instrument to measure the normal and intrinsic coercivity of a magnetic material. Generally, the measuring condition are open-circuit condition, with samples in elongated shapes.
· Coercivity
It is the magnetic field intensity Hc at which the magnetisation the ferromagnetic material, previously magnetised to complete saturation, returns to zero. One distinguishes between the coercivity Hcl (at which J = 0) and HcB (at which B = 0).
· Compass and pole indicator
· Curie temperature
It is the transition temperature above which a material loses its ferromagnetic properties and becomes paramagnetic.
· Curie, Pierre
French physical chemist and cowinner of the Nobel Prize for Physics in 1903. He and his wife, Marie Curie, discovered radium and polonium in their investigation of radioactivity.
·Demagnetization curve
The portion of the hysteresis loop in the second quadrant. This diagram gives the most important information about a permanent magnet: Br, HcB, HcJ, BHmax, working point, etc.
· Demagnetisation factor
In a magnetized magnet exists an internal demagnetization field Hd produced by the poles of the magnet itself. This field has opposite direction respect the magnetization M and it is related to it by the relationship Hd = N×M, where N is the demagnetization factor.
· Demagnetisation
Reduction of magnetisation by different means: an opposed field, an alternating and decreasing field or an increasing of the temperature (above the Curie temperature).
· Demagnetization field
A magnetizing force applied in such a direction as to reduce the remaining induction in a magnetized body.
· Diamagnetic material
· Eddy current
Current induced in a conductive material by a variation of magnetic flux in the material. Generally they are undesirable, both because of the power losses and of the shielding effects on variable magnetic fields.
· Eddy current loss
The portion of core loss due to currents circulating in the magnetic material, as a resultant electromotive forces induced by varying induction.
· Electric angle
In the electric motor terminology, concerning the cross section of a circular machine with p couple of pole, the mechanical angle α corresponds to an electric angle ß
ß = α • p
(In a 4 poles motor, 45 geometric degrees correspond to 2 x 45 = 90 electric degrees
ß = α • p
(In a 4 poles motor, 45 geometric degrees correspond to 2 x 45 = 90 electric degrees
· Electromagnet
Magnet consisting of a solenoid with iron core, which has a magnetic field existing only during the time of current flow through the coil.
· ELF meter
Instrument to measure ELF magnetic field. It displays directly the rms value of magnetic field. Commonly used to measure field levels at 50-60 Hz.
· ELF field
· Energy density (B·H)
Product of the magnetic induction B and the field strength H. The SI unit is J/m3. It is represented as a drawn rectangle under the demagnetisation curve. See also (B·H)max value.
· Ferrimagnetism
Type of magnetic structure in which magnetic moments of different elements are aligned in anti-parallel way. If the elements have different moments, the net moment is not zero, and the material exhibits magnetic properties similar to a ferromagnet. Hard and soft ferrites are ferrimagnetic materials.
· Ferrite, hard
Compound with general chemical composition MO· 6Fe2O3, whereas M stands for metal element (generally, M = Ba or Sr). All hard ferrites have an hexagonal structures. The magnetic form could be isotropic or anisotropic. Hard ferrite is actually the most used magnetic material in the world, mainly for its low cost.
· Ferrite, soft
Compound with general chemical composition MO· Fe2O3, whereas M stands for metal element (generally, M = Cu-Zn, or Mg-Zn or Mn-Zn). Soft ferrites are used in high frequency applications (transformers, antennas, etc)
· Ferromagnetism
Type of magnetic structure in which magnetic moments are aligned in parallel way. Under the Curie temperature, moments align spontaneously, without need of external magnetic field. Practical magnetic materials, soft and hard, are all ferromagnetic materials.
· Field lines
It is a common way to visualise the magnetic field. The field in one point of one field line has the same direction of the line and intensity proportional to the density of the lines in that point.
· Flux density (B)
· Flux leakage
The portion of the magnetic flux which does not pass through the air gap, or useful part of the magnetic circuit.
· Fluxmeter
Instrument to measure the magnetic flux linked with a measuring coil. The working principles of modern fluxmeters is the electronic integration of voltage induced in the coil by a change of flux.
· Gauss (G)
The CGS unit of magnetic induction B. It is correlated to the SI units (tesla) by the relationship: 1 G = 10-4 T.
· Gauss, Carl Friedrich
· Gauss's law
Gauss's law for electricity states that the electric flux across any closed surface is proportional to the net electric charge enclosed by the surface. The law implies that isolated electric charges exist and that like charges repel one another while unlike charges attract.
· Gaussmeter
Instrument to measure the magnetic induction B. Generally, it uses a Hall sensor (see), that gives a voltage proportional to the field.
· Hall probe
Probe to measure magnetic fields, containing an Hall sensor (see). Hall probes are made in two configurations: transversal and axial. Transversal probes measure magnetic field perpendicularly to the probe (like field produced by yokes or in air gaps). Axial probes measure field in the direction of the axis of the probe (like field from solenoids and long coils).
· Hall sensor
It is a four-terminal solid-state device capable of producing an output voltage proportional to the product of the input current and the magnetic field B perpendicular to the plane of the Hall sensor.
· Hard magnet
Ferromagnetic material with high coercivity (more than 1000 A/m). A hard magnet can store its magnetic energy in a permanent way, because its demagnetizing field is not enough to fully demagnetise it. The reverse is that it is more difficult to magnetize it.
· Helmholtz coils
The Helmholtz coils consist of a set of two identical coils fixed in the same axis at a distance equal to the radius of the coils. This configuration gives a wide homogeneous volume inside. It permits to measure the magnetic polarization JW of the working point (see) of a permanent magnet. It is also possible to create a uniform magnetic field supplying a current to the coil.
· Hysteresis loop
It is a closed curve obtained for a material by plotting the values of magnetic induction B, for the ordinates and magnetising force (or field) H, for abscissa, when the material passes through a complete cycle.
· Hysteresis loss
The energy per unit of mass expended to perform a complete hysteresis cycle. Graphically, it is the area of the cycle (in J/m3).
· Hysteresisgraph
Instrument to measure hysteresis loop and magnetic characteristics of hard magnetic materials. It consists of an electromagnetic yoke, power supply, fluxmeters and measuring coils.
· Irreversible loss
Not recoverable reductions of magnetic properties, due to many factors (temperature, self-demagnetisation, ageing, metallurgical changes, etc.). It is possible to distinguish between permanent losses and losses that could be recoverable by new magnetization.
· Isotropic magnets
Isotropic magnets have no preferred direction. Generally, the properties are lower than anisotropic magnets, but it is possible to magnetize and then use the magnet in every direction.
· Initial permeability
· Intrinsic coercivity
Hci, the magnetising force required to bring to zero the intrinsic induction (J) of a magnetic material.
· Joule
· Joule, James Prescott
English physicist who established that the various forms of energy--mechanical, electrical, and heat--are basically the same and can be changed, one into another. Thus he formed the basis of the law of conservation of energy, the first law of thermodynamics.
· Load line
Theoretic line on the second quadrant B-H , function of the geometry of a sample or of the magnetic circuit, whose inclination depends on its demagnetization factor (see). Elongated shape magnets have a little inclination, while short shape magnets have a bigger inclination. The intersection between loading line and second quadrant part of the hysteresis cycle defines the working point of the magnet (see).
· Magnetic axis
In the electric motor terminology, concerning the cross section of a circular machine, is the axis that divides a pole in two parts. (See neutral axis)
· Magnetic material
Generally, all ferromagnetic materials. They are subdivided in two categories: soft magnetic material (those materials with coercivity less than 1 kA/m) and hard magnetic materials (coercivity higher then 1 kA/m).
· Magnetic flux Φ
The magnetic flux of B over an area S is defined as:
If the magnetic induction B is uniform and constant, perpendicular to coil of N turns and area S, the flux linked to the coil is:
The SI unit of flux is weber (Wb).
1 Wb = 1 T ·m²
If the magnetic induction B is uniform and constant, perpendicular to coil of N turns and area S, the flux linked to the coil is:
The SI unit of flux is weber (Wb).
1 Wb = 1 T ·m²
· Magnetic induction (B)
The magnetic induction B could be seen as the total magnetic field, resulting from the field originated by electrical currents (H) and field produced by magnetic material (M): B = µ0·(H + M).
The SI unit is the tesla (T): 1 T = 104 G.
The SI unit is the tesla (T): 1 T = 104 G.
· Magnetic pole
Place at which the magnetic flux emerges from the magnet. Magnetic lines comes out from North pole and enter in South pole.
· Magnetic polarisation (J)
It is defined as the product of magnetization M and vacuum permeability: J = µ0·M.
Its unit is tesla.
Its unit is tesla.
· Magnetic shield
Protection shield that magnetically “short circuits” the magnetic field to protect the external area from the magnetic field.
· Magnetic susceptibility
It describes the interdependance between magnetisation M and magnetic field strength H:
The susceptibility is correlated to the permeability by the:
The susceptibility is correlated to the permeability by the:
· Magnetisation M
Magnetic moment per volume unit. SI unit is A/m. It is the magnetic field produced by magnetic matter.
· Magnetiser
Device that stores and discharges energy on a coil to produce a magnetic field, with the aim to magnetize a permanent magnet or a device containing permanent magnet. Modern magnetisers contains capacitors that are charged to a voltage V (energy = ½CV2) and could be switched to discharge stored energy in form of a pulse of current in a magnetizing coil.
· Magnetising coil (or fixture)
This generates an intense magnetic field to magnetise a permanent magnets or a device containing permanent magnets. This may be in the form of a standard magnetising fixture or more often a fixture designed specifically for a magnetising application. It is highly dependent upon the magnet to be magnetised.
· Magnetizing field
Magnetic field created or used with the aim of magnetize a magnetic material. It is most commonly produced by a current flowing through a coil of wire and its magnitude is proportional to the current, and to the number of turns.
· Matching coil
Coil fed by a matching transformer, with the aim to reduce the voltage monitoring the magnetising current.
· Maxwell (Mx)
· Maxwell, James Clerk
Scottish physicist best known for his formulation of electromagnetic theory. He is regarded by most modern physicists as the scientist of the 19th century who had the greatest influence on 20th-century physics, and he is ranked with Sir Isaac Newton and Albert Einstein for the fundamental nature of his contributions.
· Moment, magnetic m
For a current loop, it is the product of the current and the area of the loop. The direction is perpendicular to the surface of the loop. SI units is A×m2.
· Mu-metal
Fe-Ni alloy (77 % Ni, 17 % Fe, 5 Cu, 1 % Cr) with very low coercivity (Hc ≈ 1 ÷ 4 A/m) and high permeability ( µ, up to 100000). It is used for magnetic shields.
· Neodymium-Iron-Boron (NdFeB)
Type of permanent magnet material, discovered in 1983 independently by Sumitomo and General Motors. The complete formula is Nd2Fe14B, even if other addictive elements are often present, like Dy, to enhance coercivity. It is characterised by high values of Br, HcB, BHmax. Its properties are due to high crystalline anisotropy.
· Neutral axis
In the electric motor terminology, concerning the cross section of a circular machine, is the axis that divides two poles. (See magnetic axis)
· Normal magnetization curve
It is the geometrical place of vertices of symmetrical cycles performed at different excitation field Hv.
· Nuclear magnetic resonance (NMR)
A magnetic field is capable to split an energetic level of some nuclei in two different levels, separated by an energy amount ΔE proportional to the intensity of the magnetic field. An electromagnetic field with an energy of ΔE can undergo a transition between the two levels. This condition is called nuclear magnetic resonance (NMR). Measuring the energy (by measuring the frequency) of the electromagnetic wave that gives the resonance, it is possible to measure the magnetic field that split the two levels with a very high accuracy (some p.p.m.).
· Oersted (Oe)
· Ørsted, Hans Christian
Danish physicist and chemist who discovered that electric current in a wire can deflect a magnetized compass needle, a phenomenon the importance of which was rapidly recognized and which inspired the development of electromagnetic theory.
· Operating point
· Operating temperature
It is the highest temperature which a magnet can be subjected to without showing permanent magnetisation losses. Other factors as a mechanical or chemical strain can further reduce the maximum application temperature.
· Oxide magnet (hard ferrite)
Ceramic magnetic material, e.g. barium or strontium ferrite.
· Paramagnetic material
Material having a permeability which is slightly greater than that of a vacuum, and which is approximately independent of the magnetizing force.
· Paramagnetism
Type of magnetic structure in which magnetic moments are aligned in the same direction of an external magnetic field. In absence of external field, magnetic moments are randomly oriented, so that the net result is a zero magnetization. The paramagnetism is a weak effect: 10-4 > X > 10-8
· Peak current meter
Instrument for measure the peack current of a pulse. Uses for monitoring the behaviour of a magnetizer.
· Permalloy
· Permanent magnet
· Permeability, absolute (µ)
It is the ratio between B and H. It could be seen as an indication of the amplification of the field by the magnet. SI unit is T×m/A or H/m.
· Permeability, relative (µr)
It is the ratio between absolute permeability m and permeability of free space µ0:µr = µ/µ0. It is dimensionless. Magnetic material could be distinguished by their relative permeability: µr < 1 for diamagnetic materials, µr > 1 for paramagnetic materials, µr >> 1 for ferromagnetic materials. The relative permeability of some alloys arrives up to 105.
· Permeability of free space (µ0)
· Permeability recoil µ
· Permeability, incremental (µΔ)
It is the ratio of the cyclic change of B to the corresponding cyclinc change of H in a specified static field H:
1 ΔB
µΔ = ____ . ____
µ0 ΔH
The limiting value of µΔ for ΔH → 0 is the reversible permeability (see).
1 ΔB
µΔ = ____ . ____
µ0 ΔH
The limiting value of µΔ for ΔH → 0 is the reversible permeability (see).
· Permeability, initial (µi)
· Permeameter
Instrument to measure hysteresis loop, normal magnetization and permeability of soft magnetic materials. It consists of an electromagnetic yoke, power supply, fluxmeter, gaussmeter and measuring coils.
· Permeance
· Potential coil
Coil to measure the potential difference of H between two points. It is useful to determine the H of the working point of a permanent magnet.
· Pulse, periodic, aperiodic
Different behaviours of a magnetiser. The mode of operation is imposed by either the magnetiser internal power circuit and the parameters of the magnetiser discharging circuit with the magnetising coil.
· Rare earth
Set of elements with similar chemical and physical characteristics. Their electronic configurations gives them magnetic properties. Some of them (Sm, Nd) with transition metals (Fe, Co) give the most powerful permanent magnets (see also NdFeB, Sm-Co).
· Reluctance R
In a magnetic circuit, it is the analogous to the resistance in an electrical circuit. It is proportional to the length of magnetic path and inversely proportional to the cross section and permeability.
· Residual induction (Br)
It is the remaining magnetisation in a magnetic material, which has been magnetised to saturation in a closed circuit, when the magnetization field H is zero.
· Reversible losses
Reversible losses are changes of magnetic properties that are recovered returning in the initial condition. For example, reversible losses of Br at high temperature can be recovered returning to room temperature.
· Rogowski coil
Closely and evenly wound coil of N turns/m on a non-magnetic, usually plastic, former of constant cross sectional area. The most commonly application is the Rogowski current transducers.
· Samarium-Cobalt magnets
First rare-earth magnet, discovered in 1970. There are two main compositions: SmCo5 and Sm2Co17. They are characterised by a very high Br, HcJ, BHmax, and very good temperature behaviour. The cost of this compound limits its use; SmCo is now used only in special case, while NdFeB is preferred in most of cases.
· Saturation
The condition in which every magnetic moment of a ferromagnetic material is aligned in the same direction. The magnetization (M) and the intrinsic induction J is the maximum possible in the material. The saturation is easy for soft materials (up to few hundreds of gauss) and more difficult for hard magnets (up to 40 - 50 kG).
· Shear angle
· Sintering
Heat treatment procedure at high temperature during which powder pressed parts are compacted and homogenised. Hard ferrite and NdFeB are produced by sintering. The sintering temperature for ferrite is approx. 1220 °C ÷ 1250 °C, for NdFeB is approx. 1050 °C ÷ 1200 °C.
· Soft magnetic material
Ferromagnetic material with low coercivity (Hc < 1000 A/m). Soft magnetic material generally have also high permeability. The use of soft magnetic material are: amplification of magnetic field (electromagnets), low reluctance guide for magnetic flux (magnetic circuits), low magnetic loss devices (transformers).
· Stabilisation
A treatment of a magnetic material designed to increase the permanency of its magnetic properties or condition. Generally, it prevent modifications of the magnetic properties in time or under external influences.
· Temperature coefficients
· Tesla (T)
· Tesla, Nikola
Serbian-American inventor and researcher who discovered the rotating magnetic field, the basis of most alternating-current machinery.
· Thyristor
Solid state switching devices. Used in magnetiser as a capacitor discharge switch.
· Weber (Wb)
SI unit of the magnetic flux. 1 Wb = 1 V×s = 10-8 maxwell. Weber, Wilhelm Eduard German physicist who, with his friend Carl Friedrich Gauss, investigated terrestrial magnetism and in 1833 devised an electromagnetic telegraph. The magnetic unit, termed a weber, formerly the coulomb, is named after him.
· Working point
Intersection between loading line (see) and demagnetization curve. It is the couple of points (HW, BW), or (HW, JW), in the second quadrant of the hysteresis cycle of a permanent magnet that corresponds to its actual demagnetization field and induction. For long magnet, HW is little and BW is similar to Br. JW can be measured with Helmholtz coils (see), HW can be measured with potential coil (see).
· Yoke
Electromagnet with moving part that permits to regulate the air gap. It is used to produce uniform magnetic field.