Continuous-action or permanent magnets, in contrast to electromagnets have the advantage that their magnetic field does not continuously need to be regenerated by a flow of electrical current but instead is able to sustain flux on a lasting basis. To achieve this, magnetizable material such as iron, nickel or cobalt needs to be exposed to a magnetic field and this leads to permanent static magnetization. All alloys from which permanent magnets can be manufactured share in common a hysteresis curve that extends across a large surface area.
Having said that, permanent magnets can also be demagnetized at an early stage, i.e. prematurely, if they are exposed to a strong opposing field or to heating action, i.e. to an increase in temperature. Even a diminishing reciprocal magnetic field can trigger this process.
The field lines in a permanent magnet, from the point they enter the South pole, describe an arc-shaped track towards the North pole, where they re-emerge.
Electrical charge is a specific instance of the general notion of physical charge and is one of the fundamental concepts of physics. Forces acting between these charges have a multiplicity of impacts on the human environment. Virtually all visible physical processes can be traced back in some shape or form to the action of electrical charges.
A body can carry a positive (lack of electrons) or a negative charge (a surplus number of electrons). A charge can arise through the effect of friction if negatively charged electrons are removed or accumulated.
The movement of electrical charges is fundamental to the creation of magnetic field and the phenomenon of magnetic attraction.
Electrical charges are not directly perceptible for human beings. However, we can certainly see or feel the consequences of their presence around us. Every one of us will have seen the lightning that accompanies a thunderstorm, which is electricity resulting from friction, or will have experienced an electric shock when trying to open a door after becoming electrically charged from friction contact with a carpet, or when taking certain items of clothing on or off. These are all manifestations of the forces that result from electrical charges.
In electromagnets, the magnetic field is generated by a coil with an electrical flux. These are the second largest group of magnets, alongside the continuous-action or permanent magnets.
The magnetic field is generated by a conductor carrying an electrical current or, more precisely, by the charges being moved therein. A single wire coil is all that is required, but often several coils are used. The number of windings per coil is also significant for the magnetic field. The more windings a coil has, the greater the magnetic field it generates because the fields in the individual windings group together to form a single cumulative field.
Due to the fact that generation of the magnetic field depends on the introduction of electrical current, the benefit of using electromagnets lies in the fact that, in contrast to continuous-action magnets, the magnetic field can be controlled or regulated
To determine the direction of the lines in a magnetic field, you can employ what is known as the 'right-hand rule'. The trick here involves the user visualizing the conductor in such a way that the thumb points from positive to negative pole, while the extended index finger (90° to the thumb) points in the direction of the Lorentz force while the middle finger, also extended at 90°, points in the direction of the magnetic field.
Field lines - magnetic flux
Field lines illustrate the magnetic field in magnitude and direction. Their density is a measure of the magnetic flux density. The magnetic flux is constant between two adjacent field lines.
Ferromagnetism denotes the phenomenon whereby defined ferromagnetic solids (the word being derived from the Latin ferrum = iron) retain at least part of their magnetization even after becoming separated from an external magnetic field. This also means that these materials are also attracted by other magnets and endeavor to form a magnetic field.
This process is the opposite of induction in respect of diamagnetic or paramagnetic material. However, after separation from the magnetic field no magnetic flux remains
This process is characterized by a hysteresis loop. This is an indicator of magnetic behavior as a function of the magnetic field.
The most typical ferromagnetic elements are iron, cobalt and nickel. Certain alloys made from these ferromagnetic elements manifest identical properties.
In the case of sintered magnets, the magnet material is in its highest performance composition. We speak about this type of magnets, if the main material (e.g. NdFeB) is not being mixed with a further material. They are contrary to the today mainly spread plastic bonded magnets.
In sintered form, magnets achieve its strongest attraction (remanence) and its highest consistency (coercivity field strength), so they are highest performance. Mainly the homogeneity as well as resistance are a characteristic for good quality.
When speaking about sintered magnets we divide it in sturdy hard ferrite magnets and in high performance rare-earth magnets.
MAGNETIC CLAMP SYSTEMS
Magnetic clamp systems are of great significance to plant construction and to mechanical engineering, and their purpose is to secure, cover and assist in the assembly of applications involving systems that require unusually high levels of clamping force. This is achieved by direction of the magnetic flux by means of 'pole shoes', rather than simply distributing flux evenly around the magnet. This directs the flux so that it emerges at a desired location and is able to exert its impact there: this is particularly effective due to the high level of concentration of force at a local point or points. On contact with the mating part, the clamping force is many times higher, with an air gap, than it is on systems with open magnets. Thanks to this enhanced mounting capability, applications are easier to transport and secure.
Hysteresis characteristics are typical of ferromagnetic materials. This means that the output parameter depends not only on its point of origin but also on the preceding output parameter. This is depicted in the form of a hysteresis curve or loop. With ferromagnetic materials, the magnetic flux in the material is indicated in accordance with the strength of the surrounding magnetic field. Here it becomes apparent that flux does not drop to 0 when the field disappears, but instead manifests pure residual magnetization (remanence). It also clearly manifests that a reversal of the magnetic field leads to the lower end of the hysteresis curve being tracked.
COERCITIVE FIELD STRENGTH
The magnetic coercivity is the strength necessary to demagnetize a sustainable magnet so that it no longer has any magnetic flux. A material previously magnetized to saturation has a flux density of 0 after contact with this magnetic strength. In addition to this, one also speaks of the coercive field strength of magnetic polarization, at which the magnet additionally loses its magnetic polarization completely.
The coercivity measures the strength that a permanent magnet has to avoid demagnetization in the presence of a counter field. The higher the value, the more difficult it is to demagnetize the magnet.
The value is one of the most important magnetic properties of a permanent magnet. The temperature also has an effect on the coercivity. This is described for the different materials by the temperature coefficients.
COUPLINGS AND BRAKES
Permanent magnet couplings can transfer forces without contact and wear. Also the driving and driven coupling components can be seperated by a closed container wall. There are different types of permanent magnet couplings, which will be described in the following. It is always important to keep the air gap between the components of the coupling as small as possible, to maximise efficiency..
Magnetic forces occur in a magnetic field. It transfers the forces applied by magnets and affects objects in its surroundings. A magnetic field is created through the movement of electric charges. This can can be triggered by an electrical current, the temporal change of an electric field or materials that have already been magnetised, such as permanent magnets.
Humans can only sense magnetic fields indirectly. It can be recognised by the behaviour of magnetised objects in its sphere of influence. Iron filings spread out around the magnet also make it visible. The filings align themselves parallel to the field lines and thereby reveal the magnetic field.
The strength, direction and/or sense of direction of the fields can also be determined using the field lines. The greater the proximity of the field lines, the stronger the magnetic field. The force that is applied to a body runs tangential to the field line. Magnetic field lines always run from the north pole of a magnet to its south pole.
Magnetic fields always apply forces to moving loads as well as magnets and objects that can be magnetised. They are also the key to the alignment and the poles of such objects. Opposite poles attract, like poles repel each other. This basic rule ensures movement in the magnetic field because magnets always want to align themselves in a way that their opposite poles attract each other.
A very well know magnetic field is the magnetic field of our earth. On the one hand, it protects us from the charged particles of solar winds and, one the other hand, it serves as a guide for many animals, e.g. migratory birds.
MAGNETIC FIELD OF THE EARTH
Not only bodies but also entire planets may have a magnetic field. For this, defined conditions must be in place. The Earth also has a magnetic field that surrounds it. This principle is similar to that of the 'small' magnetic fields of electromagnets or those surrounding ferromagnetic materials. Here too, movement must take place between charge carriers. Energy stored in the Earth's core acts as an energy source that is also responsible for the movement of the conducting material. In the case of our Earth, this is performed by the liquid outer part of the Earth's core that contains a high proportion of iron, and is therefore conductive.
The strong temperature differences in the Earth's core assure rotating movement in the liquid part of the core, causing induction to take place that causes the magnetic field around the Earth. The sequence of movements designated Geodynamo does not have to be very powerful in order to generate this field.
Another decisive aspect is that the core of the Earth rotates more rapidly than the Earth's mantle. There is speculation to the effect that, for this process, the impact of tidal forces exerted by Moon and Sun may have a decisive role to play, although clear-cut scientific confirmation of this has yet to be produced.
The fact that the geographic North Pole is, in physical terms, a magnetic South Pole, is now widely recognized. Furthermore, a distinction is made between the geographical, magnetic and geomagnetic Pole. The last of these is a theoretical pole point in the assumed magnetic field of the Earth, inclined at 11° to the axis of the Earth. The other magnetic poles are based on very specific calculations. In both cases however, these 'poles' are not at fixed locations, in contrast to the geographic Poles.
Some animals - unlike human beings - can perceive the magnetic field of the Earth and orient themselves with its help. The best known examples of this are migratory birds that change their habitat every year for defined seasonal periods. Typically, in the case of European migratory birds, they fly down to warmer climes to spend the winter.
Magnetism can be defined as an interaction of the forces existing between magnetizable objects. It is expressed in the ability of these objects to attract or repel. Underlying this action are the movements in electrical charges. These generate a magnetic field that acts in turn on those charges, thus creating the phenomenon of magnetism.
Magnetism is almost imperceptible to human beings and it therefore took a long time before the correct explanation was discovered, situating it in the context of electrical charges. Until that time, it was very much a thing of concealed mystery. In most cases, magnetic phenomena are depicted by a magnetic field with field lines. This also makes it possible to depict the force exerted, known as the Lorentz Force, which acts in a plane perpendicular to the lines of the magnetic field.
The occurrence of magnetism is associated with the movement of electrical charges. This encompasses the movement of electrons around atomic nuclei as well as the 'spin' of electrons (i.e. their innate rotational movement), both of which generate magnetic 'moments' that add together vectorially to yield 'atomic moment'. If the total amounts to zero, the material is referred to as 'diamagnetic'. With paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic materials, the total of moments does not add up to zero.
Paramagnetism occurs in materials whose atoms have at least one electron shell that is not completely filled. Examples are O, Al, Pt, Ti, various transition metals, rare earth metals, and actinides. These atoms possess a permanent magnetic moment. Neighbouring atoms are not coupled to each other. In an external magnetic field, the atoms align their magnetic moments in the direction of the external field. Here 1+4·10-4<µr<1+10-8.
Ferromagnetism occurs in materials whose atoms have a particular electron shell occupation ans also a particular relationship between zheir interatomic spacing distance and their atomic radii. Examples are Fe, Co, Ni and compounds such as Alnico. Neighbouring atomic magnetic moments couple paralell to each other and from domains with a total magnetic moment of a certain size and orientation. Here5·105>µr>100.
Antiferromagnetic materials also from domains. However, they have two different sublattices, whose magnetic moments are antiparalell. That is, they are of equal magnitude and opposite direction. These materials behave like paramagnetic substances. Examples are a-Mn, (a-Mn, FeO, Fe2O3, FeS, CoO).
Domains with magnetic moments from different sublattices, pointing in opposite directions, characterise ferrimagnetism. The magnetic moments are of different magnitude, so the material behaves like a ferromagnet. (Cubic ferrites, such as MnO⋅FeO, are soft magnetic materials, whereas hexagonal ferrites such as BaO⋅6Fe2O3, are hard magnetic materials).
Usually the magnetic flux is determined to a tolerance of about ± 10%. For more exact technical applications, the magnetic flux must be set to tighter tolerances.