Along with electrified friction pieces of amber permanent magnets were for the ancient people it is the first material evidence of electromagnetic phenomena (lightning at the dawn of history is definitely related to the sphere of manifestations of the intangible forces). An explanation of the nature of ferromagnetism has always occupied the inquiring minds of scientists, however, currently the physical nature of the permanent magnetization of some substances, both natural and artificially created, are still not fully disclosed, leaving a large field of activity for current and future researchers.
Traditional materials for permanent magnets
They began to be actively used in industry, since 1940 with the advent of the alnico alloy (AlNiCo). Prior to this, permanent magnets from different grades of steel were used only in compasses and magnetos. Alnico made it possible to replace them with electromagnets and use them in devices such as motors, generators and loudspeakers.
This penetration into our everyday life has received a new impulse with the creation of ferrite magnets, and since then, permanent magnets have become commonplace.
The revolution in magnetic materials began around 1970, with the creation of a samarium-cobalt family of hard magnetic materials with an unprecedented density of magnetic energy. Then a new generation of rare-earth magnets based on neodymium, iron and boron with a much higher density of magnetic energy was discovered than for Samarium-cobalt (SmCo) and with an expected low cost. These two families of rare-earth magnets have such high energy densities that they not only can replace electromagnets, but they can be used in areas that are inaccessible to them. Examples include a tiny stepper motor on permanent magnets in wristwatches and sound transducers in Walkman type headphones.
Gradual improvement of the magnetic properties of materials is shown in the diagram below.
Neodymium permanent magnets
They represent the latest and most significant achievement in this field over the past decades. For the first time, they were announced almost simultaneously at the end of 1983 by metal experts from Sumitomo and General Motors. They are based on the intermetallic compound NdFeB: a neodymium alloy, iron and boron. Of these, neodymium is a rare-earth element extracted from a mineral monazite.
The huge interest that these permanent magnets caused was due to the fact that for the first time a new magnetic material was obtained, which is not only stronger than the previous generation, but is more economical. It consists mainly of iron, which is much cheaper than cobalt, and of neodymium, which is one of the most abundant rare-earth materials, whose reserves on Earth are greater than lead. In the main rare-earth minerals, monazite and banezite are five to ten times more neodymium than samarium.
The physical mechanism of constant magnetization
To explain the operation of permanent magnet, we have to look inside it down to the atomic scale. Each atom has a set of spins of their electrons, which together form its magnetic moment. For our purposes, we can consider each atom as a small bar magnet. When a permanent magnet is demagnetised (either by heating it to high temperatures or external magnetic field), each atomic moment is oriented randomly (see Fig. below) and no regularity is observed.
When it is magnetized in a strong magnetic field, all the atomic moments are oriented in the direction of the field and as it engages "in lock" with each other (see Fig. below). This clutch allows you to save the field of a permanent magnet when removing the external field, as well as to resist demagnetization when changing its direction. A measure of the strength of the coupling of the atomic moments is the magnitude of the coercive force of the magnet. More on that later.
At a deeper presentation of the magnetization mechanism, they do not operate with the concepts of atomic moments, but use the notions of miniaturized (in the order of 0.001 cm) regions inside the magnet, initially having a constant magnetization, but oriented randomly in the absence of an external field, so that a strict reader may, if desired, The mechanism is not to the magnet as a whole. but to a separate domain.
Induction and magnetization
The atomic moments are summed up and form the magnetic moment of the entire permanent magnet, and its magnetization M indicates the magnitude of this moment per unit volume. The magnetic induction B indicates that the permanent magnet is the result of the external magnetic force (field strength) H applied to the primary magnetization, as well as the internal magnetization M, due to the orientation of the atomic (or domain) moments. Its value in the general case is given by the formula:
where μ0 is a constant.
In a constant annular and homogeneous magnet, the field strength H inside it (in the absence of an external field) is zero, since, according to the law of total current, the integral of it along any circle inside such a ring core is equal to:
H ∙ 2πR = iw = 0. whence H = 0.
Consequently, the magnetization in the ring magnet:
In a non-closed magnet, for example, in the same annular, but with an air gap of width lzaz in the core of length lsir. in the absence of an external field and the same induction B inside the core and in the gap in accordance with the law of total current, we obtain:
Since B = μ0 (Hsir + Msir ), then, substituting its expression in the previous one, we get:
In the air gap:
where B is determined from the given Msir and found Hsir .
Since namagnichennosti state when N increases from zero, due to the orientation of all atomic moments in the direction of the external field are rapidly increasing M and B, varying along the section "a" basic curve of magnetization (see figure below).
When all the atomic moments are aligned, M comes to its saturation value, and a further increase in B occurs solely because of the applied field (section b of the main curve in the figure below). When the external field is reduced to zero, induction B decreases not along the original path, but along section "c" due to the cohesion of atomic moments, which tends to keep them in the same direction. The magnetization curve begins to describe the so-called hysteresis loop. When H (external field) approaches zero, the induction approaches the residual value, determined only by atomic moments:
Once the direction is changed to H, H and M act in opposite directions, and B is reduced (the portion of the curve "d" in figure). The value of a field which decreases to zero is called the coercivity of the magnet B HC. When the value of the applied field is large enough to break the grip of nuclear moments, they focus in the new field orientation and the direction of M is reversed. The value of the field at which this occurs is called the intrinsic coercive force of the permanent magnet M HC. So, there are two different, but related, coercive forces associated with a permanent magnet.
The figure below shows the main demagnetization curves for various materials for permanent magnets.It is seen from it that the largest residual induction Br and the coercive force (both complete and internal, that is, determined without taking into account the strength of H, only by the magnetization M) is possessed by NdFeB magnets.
Surface (ampere) currents
The magnetic field of the permanent magnets can be considered as field some related currents flowing on their surfaces. These currents are called amirovsky. In the ordinary sense currents inside permanent magnets available. However, when comparing the magnetic fields of the permanent magnets and the field currents in the coils, the French physicist Ampere suggested that magnetism of matter can be explained by the occurrence of the microscopic currents constituting the same microscopic closed loops. And indeed, the analogy between a solenoid and a long cylindrical magnet is almost complete: there is a North and South pole of the permanent magnet and the poles of the solenoid, and the picture of the force lines of their fields is also very similar (see figure below).
Are there currents inside the magnet?
Imagine that the entire volume of some permanent magnet rod (arbitrary cross-sectional shape) is filled with microscopic amirovsky currents. Cross-section of the magnet to such currents shown in the figure below. Each of them has a magnetic moment. With the same orientation along the direction of the external field, they form the resultant magnetic moment, which is different from zero. It also determines the existence of a magnetic field in the apparent absence of ordered motion of charges, in the absence of current through any section of the magnet. It is also easy to understand that within it the currents of contiguous contiguous contours are compensated. Only currents on the surface of the body that form the surface current of a permanent magnet are uncompensated. Its density turns out to be equal to the magnetization M.
How to get rid of moving contacts
The problem of creating a contactless synchronous machine is well known. Its traditional design with electromagnetic excitation from the poles of the rotor with coils involves the supply of current to them through movable contacts - contact rings with brushes. The disadvantages of such a technical solution are well known: these are the difficulties in servicing, and low reliability, and large losses in mobile contacts, especially when it comes to powerful turbo and hydrogenerators, whose excitation circuits consume considerable electrical power.
If you make this a permanent magnet generator, the contact problem immediately goes away. However, there is the problem of reliable fastening of the magnets on the rotating rotor. Here can be useful experience in the tractor. There has long been switched reluctance generator with permanent magnets placed in the grooves of the rotor, filled with fusible alloy.
The motor on permanent magnets
In recent decades widespread valve DC motors. This unit is actually a motor and electronic switch its armature acting as the collector. The motor is a synchronous motor with permanent magnets located on the rotor, as in Fig. above, with a fixed armature winding on the stator. Electronic switch circuit design is an inverter DC voltage (or current) supply.
The main advantage of this engine is its non-contact. Its specific element is the photo-, induction or Hall sensor of the rotor position, which controls the operation of the inverter.