In the core of any electromagnet, after turning off the current, part of the magnetic properties, called residual magnetism, always remains. The amount of residual magnetism depends on the properties of the core material and reaches a higher value for hardened steel and a lower value for soft iron.
However, no matter how soft the iron is, residual magnetism will still have a certain effect if, according to the operating conditions of the device, it is necessary to remagnetize its core, that is, demagnetize to zero and magnetize in the opposite direction.
Indeed, with any change in the direction of the current in the winding of an electromagnet, it is necessary (due to the presence of residual magnetism in the core) to first demagnetize the core, and only after that can it be magnetized in a new direction. This will require some kind of magnetic flux in the opposite direction.
In other words, the change in the magnetization of the core (magnetic induction) always lags behind the corresponding changes in the magnetic flux () created by the winding.
This lag of magnetic induction from the magnetic field strength is called hysteresis. With each new magnetization of the core, in order to destroy its residual magnetism, it is necessary to act on the core with a magnetic flux of the opposite direction.
In practice, this will mean spending some part of the electrical energy to overcome the coercive force, which makes it difficult to rotate the molecular magnets to a new position. The energy spent on this is released in the iron in the form of heat and represents losses due to magnetization reversal, or, as they say, hysteresis losses.
Based on the foregoing, iron that is subject to continuous magnetization reversal in a particular device (armature cores of generators and electric motors, transformer cores), should always be chosen soft, with a very small coercive force. This makes it possible to reduce hysteresis losses and thereby increase electrical efficiency machine or device.
Hysteresis loop
Hysteresis loop- a curve depicting the dependence of magnetization on the external field strength. The larger the loop area, the more work must be spent on magnetization reversal.
Let's imagine a simple electromagnet with an iron core. We will carry it through a full magnetization cycle, for which we will change the magnetizing current from zero to the OM value in both directions.
Initial moment: current strength is zero, iron is not magnetized, magnetic induction B = 0.
1st part: magnetization by changing the current from 0 to - + OM. The induction in the core iron will increase quickly at first, then more slowly. By the end of the operation, at point A, the iron is so saturated with magnetic lines of force that further increasing the current (above + OM) can give the most insignificant results, which is why the magnetization operation can be considered completed.
Magnetization to saturation means that the molecular magnets present in the core, which were in complete and then only partial disorder at the beginning of the magnetization process, are now almost all arranged in orderly rows, with north poles in one direction, south poles in the other, why are we at one end of the core? Now we have northern polarity, on the other - southern polarity.
2nd part: weakening of magnetism due to a decrease in current from + OM to 0 and complete demagnetization at current - OD. Magnetic induction, changing along the AC curve, will reach the OS value, while the current will already be zero. This magnetic induction is called residual magnetism, or residual magnetic induction. To destroy it, for complete demagnetization, it is necessary to give a current in the opposite direction to the electromagnet and bring it to the value corresponding to the ordinate OD in the drawing.
3rd part: magnetization in the opposite direction by changing the current from - OD to - OM1. Magnetic induction, increasing along the curve DE, will reach point E, corresponding to the moment of saturation.
4th part: weakening of magnetism by gradually reducing the current from - OM1 to zero (residual magnetism OF) and subsequent demagnetization by changing the direction of the current and bringing it to a value of + OH.
5th part: magnetization corresponding to the process of the 1st part, bringing magnetic induction from zero to + MA by changing the current from + OH to + OM.
P When the demagnetizing current decreases to zero, not all elementary or molecular magnets return to the previous disordered state, but some of them retain their position corresponding to the last direction of magnetization. This phenomenon of retardation or delay of magnetism is called hysteresis.
Hysteresis comes from a Greek word meaning retardation or lag. This concept is associated with such a physical quantity as the hysteresis loop, which determines one of the characteristics of the body. It is also connected in a certain way with physical quantities that characterize external conditions, such as the magnetic field.
General concepts of hysteresis
Hysteresis can be observed at those moments when a body in a specific period of time is dependent on external conditions. This state of the body is also considered at the previous time, after which a comparison is made and a certain relationship is derived.
A similar dependence is clearly visible in the example of the human body. To change his condition, you will need some period of time for relaxation. Therefore, the body’s reaction will always lag behind the reasons that caused the altered state. This lag is significantly reduced if changes in external conditions are also observed. However, in some cases, backlog reduction may not occur. As a result, an ambiguous dependence of quantities arises, known as hysteresis, and the phenomenon itself is called hysteresis.
This physical quantity can occur in a variety of substances and processes, but the concepts of dielectric, magnetic and elastic hysteresis are most often considered. Magnetic hysteresis usually appears in magnetic substances, such as ferromagnets. A characteristic feature of these materials is spontaneous or spontaneous inhomogeneous magnetization, which clearly demonstrates this physical phenomenon.
The mechanism of occurrence of the hysteresis loop
Hysteresis itself is a curve representing the changed magnetic moment of a substance that is affected by a periodically varying field strength. When a magnetic field acts on ferromagnets, the change in their magnetic moment does not occur immediately, but with a certain delay.
Every ferromagnet initially has spontaneous magnetization. The material itself includes individual fragments, each of which has its own magnetic moment. When these moments are directed in different directions, the value of the total moment turns out to be zero as a result of mutual compensation.
If a ferromagnet is exposed to a magnetic field, then all the moments present in individual fragments (domains) will be deployed along the external field. As a result, a certain general moment is formed in the material, directed in one direction. If the external action of the field ceases, then not all domains will be in their original position. This will require exposure to a sufficiently strong magnetic field designed to rotate the domains. Such a reversal is hampered by the presence of impurities and heterogeneity of the material. Therefore, the material has some residual magnetization, even when the external field is turned off.
To remove the residual magnetic moment, it is necessary to apply a field in the opposite direction. The field strength must be sufficient to completely demagnetize the material. This quantity is known as coercive force. A further increase in the magnetic field will lead to magnetization reversal of the ferromagnet in the opposite direction.
When the field strength reaches a certain value, the material becomes saturated, that is, the magnetic moment no longer increases. When the field is removed, a residual moment is again observed, which can again be removed. A further increase in the field leads to a saturation point with the opposite value.
Thus, a hysteresis loop appears on the graph, the beginning of which falls at zero values of the field and torque. Subsequently, the very first magnetization takes the beginning of the hysteresis loop from zero and the whole process begins to occur according to the closed loop schedule.
Any electromagnetic core, after exposure to electric current, retains a magnetic field for some time (residual magnetism). This value depends on the properties of the material, but residual magnetism is always present. To remagnetize the core, a magnetic flux in the opposite direction is necessary. The change in magnetic induction does not keep pace with the change in magnetic flux. This time delay in magnetization of the core due to a change in the direction of magnetic fluxes is called hysteresis.
To understand the essence of this phenomenon, it is necessary to consider the ability of substances to magnetize.
Magnetic properties of substances
All substances in the nature around us have magnetic properties to one degree or another. Even in ancient times, the amazing ability of some minerals to attract iron objects was known. Among the numerous navigational instruments necessary to plot the course of a ship or aircraft, a magnetic compass is always present.
In the most precise measuring instruments, permanent magnets are among the main parts. It is known that not only iron has strong magnetic properties. This includes cobalt, nickel, their alloys and some rare earth elements. All these substances and alloys are called ferromagnets. What they have in common is their ability to undergo spontaneous magnetization.
This property of ferromagnets is used to create permanent magnets. The presence of uncompensated magnetic moments in the atoms of a substance is a necessary condition for the occurrence of ferromagnetism.
In Einstein's experiment, based on the magnitude of twisting during magnetization of a sample, it was proven that ferromagnetism is associated with the spin magnetic moments of electrons. The exchange interaction of electrons at certain ratios of the diameter of the atom and the inner unfilled shell leads to parallel orientation of the spins.
It is possible only with a positive value of the exchange energy integral.
Ultimately, a spin orientation is established in a ferromagnet that ensures the minimum value of the sum of the magnetic and exchange interaction energies.
An area with uniform spontaneous magnetization is called a domain. The most energetically favorable arrangement of domains is one in which they create a closed magnetic circuit.
Between neighboring domains with different directions of magnetization there are transition layers called domain boundaries or walls. There is a gradual rotation of the magnetization vector in them.
Ferromagnetic properties of substances exist only in a certain temperature range. The temperature at which ferromagnetic materials completely lose their ferromagnetic properties is called the Curie point. The shape and size of domains on the surface of a ferromagnet can be seen under a microscope
In the elementary crystalline cell of iron, the edges of the cube correspond to the direction of the easiest magnetization of the iron crystal. The diagonals of the faces determine the direction of the average magnetization.
The direction of the most difficult magnetization coincides with the diagonals of the cube. The area on the graph characterizes the energy of magnetic anisotropy.
In the absence of an external field, the magnetic moments of the domains are oriented along the directions of easy magnetization. In general, the sample is demagnetized.
In weak fields, the growth of domains occurs, the direction of magnetization of which makes a smaller angle with the direction of the external field.
This process is reversible. If the external field is removed, the sample will be demagnetized. As the external field increases, further growth of domains occurs, which is stopped due to crystal defects. When the field reaches a certain value, the walls of the growing domains jump over the obstacle. Due to this obstacle, the magnetization curve has a stepped character.
Abrupt changes in magnetization create voltage pulses in the solenoid coil. With a further increase in the field, the magnetization vector rotates from the easy magnetization axis towards the external field until they coincide.
Hysteresis
This area is called the region of technical saturation of the ferromagnet, and the corresponding field value is called the saturation field. If the field is reduced from this value to zero, residual magnetization will remain in the sample.
Hysteresis is the phenomenon of magnetization lagging behind the external field strength. Closing domains, creating a closed magnetic circuit, reduce stray fields and reduce the free energy of the sample.
It is defined as the difference between the magnetic saturation of the ferromagnet and the magnetization of the closing domains. To demagnetize a sample, a negative field called coercive force must be applied to it. When the field reaches saturation value, complete magnetization reversal of the ferromagnet will occur.
On the graph you can determine another property that hysteresis has. With the next change in the field, the magnetization curve closes a loop, which is called a hysteresis loop.
The hysteresis loop for the saturation condition is called the limit loop. Its area is proportional to the energy loss due to magnetization reversal of the sample. Ferromagnets, when magnetized, change their linear dimensions. This phenomenon is called magnetostriction.
There are two main groups of ferromagnetic materials:
- Magnetically hard.
- Magnetic soft.
One of the main requirements for soft magnetic materials is their high coercivity. Soft magnetic materials are magnetized to saturation in low fields and have low magnetization reversal losses. The energy loss of the transformer depends on these parameters.
For example, in a 100 x 10 6 VA power line with transformers at the ends, the annual losses are about 5 million kilowatt-hours. One of the best representatives of soft magnetic materials is considered to be permalloy, an alloy of iron and nickel. The magnetization of permalloy in weak fields is tens of times greater than the magnetization of iron. The magnetic ordered structures in some substances differ from the magnetic structure of ferromagnets.
If in iron, cobalt and nickel the spin magnetic moments are directed parallel, then in chromium and manganese they are antiparallel. Such substances are called antiferromagnets.
In this case, magnetic sublattices with spontaneous magnetization are compensated. If the crystals of a substance do not have complete compensation of the magnetic sublattices, then it is called a ferrimagnet. Ferrite is one example of ferrimagnets that is widely used in technology. The structure of ferrites is similar to the structure of spinel minerals, in which non-ferromagnetic metal ions are replaced by ferromagnetic ones.
Hysteresis in electrical engineering and electronics
From the variety of examples of the use of ferromagnetic materials, we will talk about their use in memory devices. For rapid storage of information, memory on ferrite rings is used. One ferrite core is enough to store one bit of information. Special magnetic disks (Schmidt triggers) serve as long-term high-capacity storage devices.
It is also used in special hysteresis electric motors, noise reduction devices (contact bounce, oscillations, etc.) when switching logic circuits.
Many electronic devices have thermal hysteresis. During operation, the devices heat up, and after cooling, some properties no longer take their initial values. When a microcircuit, printed circuit board, or semiconductor crystals heat up, they expand and mechanical stress appears. During cooling, this tension remains to some extent.
In electrical engineering there are various devices whose operating principle is based on electromagnetic phenomena. Where there is a core on which a coil of conductive material, such as copper, is wound, interactions due to magnetic fields are observed. These are relays, starters, contactors, electric motors and magnets. Among the characteristics of cores there is such a characteristic as hysteresis. In this article we will look at what it is, as well as the benefits and harms of this phenomenon.
Definition of the concept
The word “Hysteresis” has Greek roots and is translated as lagging or lagging. This term is used in various fields of science and technology. In a general sense, the concept of hysteresis distinguishes the different behavior of a system under opposite influences.
This can be said in simpler words. Let's say there is some kind of system that can be influenced in several directions. If, when acting on it in the direct direction, after stopping the system does not return to its original state, but is established in an intermediate state, then in order to return it to its original state, it is necessary to act in a different direction with some force. In this case, the system has hysteresis.
Sometimes this phenomenon is used for useful purposes, for example, to create elements that operate at certain threshold values of acting forces and for regulators. In other cases, hysteresis has a detrimental effect; let’s consider this in practice.
Hysteresis in electrical engineering
In electrical engineering, hysteresis is an important characteristic for the materials from which the cores of electrical machines and devices are made. Before proceeding with the explanations, let's look at the magnetization curve of the core.
An image on a graph of this type is also called a hysteresis loop.
Important! In this case, we are talking about hysteresis of ferromagnets; here it is a nonlinear dependence of the internal magnetic induction of the material on the magnitude of the external magnetic induction, which depends on the previous state of the element.
When current flows through a conductor, a magnetic field appears around the latter. If you wind a wire into a coil and pass current through it, you get an electromagnet. If you place a core inside the coil, its inductance will increase, as will the forces arising around it.
What does hysteresis depend on? Accordingly, the core is made of metal; its characteristics and magnetization curve depend on its type.
If you use, for example, hardened steel, the hysteresis will be wider. When choosing so-called soft magnetic materials, the schedule will narrow. What does this mean and what is it for?
The fact is that when such a coil operates in an alternating current circuit, current flows in one direction or the other. As a result of magnetic forces, the poles are constantly flipped. In a coil without a core this happens in principle simultaneously, but with a core things are different. It gradually becomes magnetized, its magnetic induction increases and gradually reaches an almost horizontal section of the graph, which is called the saturation section.
After this, if you begin to change the direction of the current and magnetic field, the core will have to re-magnetize. But if you simply turn off the current and thereby remove the source of the magnetic field, the core will still remain magnetized, although not so much. In the following graph this is point "A". In order to demagnetize it to its original state, it is necessary to create a negative magnetic field strength. This is point "B". Accordingly, the current in the coil must flow in the opposite direction.
The value of the magnetic field strength for complete demagnetization of the core is called coercive force and the lower it is, the better in this case.
Magnetization reversal in the opposite direction will take place similarly, but along the lower branch of the loop. That is, when operating in an alternating current circuit, part of the energy will be spent on reversing the magnetization of the core. This leads to the fact that the efficiency of the electric motor and transformer decreases. Accordingly, this leads to its heating.
Important! The lower the hysteresis and coercive force, the lower the losses due to magnetization reversal of the core.
In addition to what was described above, hysteresis is also characteristic of the operation of relays and other electromagnetic switching devices. For example, opening and closing current. When the relay is turned off, a certain current must be applied in order for it to work. In this case, the current for holding it in the on state can be much lower than the turn-on current. It will turn off only when the current drops below the holding current.
Hysteresis in electronics
In electronic devices, hysteresis has mainly useful functions. Let's say this is used in threshold elements, for example, comparators and Schmidt triggers. Below you see a graph of its states:
This is necessary in cases where the device is triggered when the signal X is reached, after which the signal can begin to decrease and the device does not turn off until the signal drops to level Y. This solution is used to suppress contact bounce and random spikes, as well as in various regulators.
For example, a thermostat or temperature controller. Typically, its principle of operation is to turn off the heating (or cooling) device at the moment when the temperature in the room or other place has reached a predetermined level.
Let's look at two options briefly and simply:
- No hysteresis. Switches on and off at a given temperature. However, there are nuances here. If you set the temperature controller to 22 degrees and heat the room to this level, then as soon as the room reaches 22 it will turn off, and when it drops to 21 again it will turn on. This is not always the right solution because your controlled appliance will turn on and off too often. In addition, in most household and many industrial tasks there is no need for such precise temperature support.
- With hysteresis. To create a certain gap in the permissible range of adjustable parameters, hysteresis is used. That is, if you set the temperature to 22 degrees, then as soon as it is reached, the heater will turn off. Let's assume that the hysteresis in the regulator is set to a gap of 3 degrees, then the heater will start working again only when the air temperature drops to 19 degrees.
Sometimes this gap is adjusted to your liking. In simple versions, bimetallic plates are used.
We looked at the phenomenon and application of hysteresis in electrical engineering. The result is as follows: in electric drives and transformers it has a detrimental effect, but in electronics and various regulators it also finds useful applications. We hope the information provided was useful and interesting for you!
Materials
To better understand what magnetic hysteresis is, you need to understand where and under what conditions it occurs.
Basic Concepts
A magnetic field– this is one of the components of the electromagnetic field, characterized by its force effect on moving charged particles.
Magnetic induction vector B– this is the main force value of the magnetic field.
Magnetization M is a quantity that characterizes the magnetic state of a substance.
Magnetic field strength is a characteristic of the magnetic field, which is equal to the difference between magnetic induction and magnetization.
Ferromagnetic material is a material whose magnetization depends on the strength of the external magnetic field.
Let's say we have a coil, inside of which there is a core made of ferromagnetic material. Typically, such a core consists of iron, nickel, cobalt and various compounds based on them. If you connect it to an alternating current source, then a magnetic field is formed around the coil, which will change according to the law
B (H) graph
Section 0-1 is called the initial magnetization curve. Thanks to it, we can see how the magnetic induction changes in a demagnetized coil.
After saturation (that is, point 1) with a decrease in the magnetic field strength to zero (section 1-2), we see that the core remained magnetized by the value of the residual magnetization Br. This is called the phenomenon of magnetic hysteresis.
From a physics point of view, residual magnetization is explained by the fact that in ferromagnets there are strong magnetic bonds between molecules, due to which randomly directed magnetic moments are created. Under the influence of an external field, they take the direction of the field, and after it is removed, some of the magnetic moments remain directed. Therefore, the substance remains magnetized.
After changing the direction of the current in the coil, demagnetization continues (section 2-3) until the x-axis is crossed. Section 3-0 is called the coercive force Hc. This is the value that is necessary to destroy the field in the core. Then, similarly, the core is magnetized to saturation (section 3-4) and demagnetized back in sections 4-5 and 5-6, followed by magnetization to point 1. This entire graph is called a magnetic hysteresis loop.
If you repeatedly magnetize the core with a magnetic field strength and induction lower than at saturation, you can obtain a family of curves from which you can subsequently construct the main magnetization curve (0-1-2). This curve is often required in electrical calculations of magnetic systems.
Depending on the width of the hysteresis loop, ferromagnetic materials are divided into hard magnetic and soft magnetic. Hard magnetic substances have high values of residual magnetization and coercive force. Soft magnetic substances, such as electrical steel, are used in transformers, electrical machines, and electromagnets due to their low coercive force and high magnetic permeability.
