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.Permanent magnets, their description and principle of operation

Traditional materials for permanent magnets

They have been actively used in industry since 1940 with the appearance of the alloy Alnico (AlNiCo). Prior to this, permanent magnets of various steel grades were used only in compasses and magneto. Alniko made it possible to replace electromagnets and use them in devices such as motors, generators and loudspeakers.

It was their penetration into our daily life that received a new impetus 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 rigid magnetic materials with a hitherto unseen magnetic energy density. Then a new generation of rare-earth magnets based on neodymium, iron and boron with a much higher magnetic energy density than that of samarium-cobalt (SmCo) and with the expected low cost was discovered. These two families of rare-earth magnets have such high energy densities that they not only can replace electromagnets, but be used in areas inaccessible to them. Examples include a tiny permanent magnet stepper motor in a wristwatch and sound transducers in Walkman headphones.

A gradual improvement in the magnetic properties of materials is presented 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, their discovery was announced almost simultaneously at the end of 1983 by specialists in metals from Sumitomo and General Motors. They are based on the NdFeB intermetallic compound: an alloy of neodymium, iron and boron. Of these, neodymium is a rare-earth element extracted from the mineral monazite.

The enormous interest that caused these permanent magnets has arisen because for the first time a new magnetic material was obtained, which is not only stronger than the previous generation, but more economical. It consists mainly of iron, which is much cheaper than cobalt, and of neodymium, which is one of the most common rare-earth materials, which has more reserves on Earth than lead. The main rare-earth minerals of monazite and bastanesite contain five to ten times more neodymium than samarium.

Physical mechanism of permanent 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.

With a deeper presentation of the magnetization mechanism, they do not operate with concepts of atomic moments, but use concepts of miniature (about 0.001 cm) areas inside the magnet, which initially have constant magnetization, but are oriented in the absence of an external field in a random way, so that the strict physical reader the mechanism is not to the magnet as a whole. and to its separate domain.

Induction and Magnetization

The atomic moments are summed and form the magnetic moment of the entire permanent magnet, and its magnetization M indicates the magnitude of this moment per unit volume. Magnetic induction B shows that a permanent magnet is the result of an external magnetic force (field strength) H applied during primary magnetization, as well as internal magnetization M, due to the orientation of atomic (or domain) moments. Its value is generally given by the formula:

where µ0   is a constant.

In a constant ring and uniform 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.

Therefore, the magnetization in the ring magnet:

In an open magnet, for example, in the same ring, but with an air gap of width lzAZ   in the core length lsir. in the absence of an external field and the same induction B inside the core and in the gap according to 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:

moreover, B is determined by the given Msir   and found Hsir .

Magnetization curve

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 atomic moments are aligned, M comes to its saturation value, and further increase in B is solely due to the applied field (section b of the main curve in the figure below). When the external field decreases to zero, induction B decreases not along the original path, but along section “c” due to the coupling of atomic moments, which tend 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 a 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 of various materials for permanent magnets.It shows that the greatest residual induction is Br   and the coercive force (both full and internal, that is, determined without taking into account the intensity H, only by magnetization M) is precisely the 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 in the direction of the external field, they form a nonzero resulting magnetic moment. It determines the existence of a magnetic field in the apparent absence of an orderly movement of charges, in the absence of current through any section of the magnet. It is also easy to understand that inside it the currents of adjacent (adjoining) circuits are compensated. Uncompensated are only the currents on the surface of the body, forming the surface current of the permanent magnet. Its density is equal to the magnetization M.

How to get rid of moving contacts

Known problem of creating a contactless synchronous machine. Its traditional design with electromagnetic excitation from the poles of the rotor with coils involves the supply of current to them through the moving contacts - contact rings with brushes. The disadvantages of this technical solution are well known: they are difficulties in servicing, low reliability, and large losses in moving contacts, especially when it comes to powerful turbo and hydrogenerators, in the excitation circuits of which considerable electrical power is consumed.

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.

Permanent magnet motor

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 contactlessness. Its specific element is a photo, induction or Hall rotor position sensor that controls the operation of the inverter.

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