Being, as it were, a special case of it).
The direction of the EMF of self-induction always turns out to be such that when the current in the circuit increases, the EMF of self-induction prevents this increase (directed against the current), and when the current decreases, it decreases (co-directed with the current). With this property, the EMF of self-induction is similar to the force of inertia.
The value of the EMF of self-induction is proportional to the rate of change of the current:
.The proportionality factor is called self-induction coefficient or inductance circuit (coil).
Self-induction and sinusoidal current
In the case of a sinusoidal dependence of the current flowing through the coil on time, the self-induction EMF in the coil lags the current in phase by (that is, 90 °), and the amplitude of this EMF is proportional to the current amplitude, frequency and inductance (). After all, the rate of change of a function is its first derivative, and .
To calculate more or less complex circuits containing inductive elements, i.e. turns, coils, etc. devices in which self-induction is observed, (especially, completely linear, that is, not containing non-linear elements) in the case of sinusoidal currents and voltages, the method of complex impedances is used or, in simpler cases, a less powerful but more visual version of it is the method of vector diagrams.
Note that everything described is applicable not only directly to sinusoidal currents and voltages, but also practically to arbitrary ones, since the latter can almost always be expanded into a series or Fourier integral and thus reduced to sinusoidal ones.
In more or less direct connection with this, one can mention the use of the phenomenon of self-induction (and, accordingly, inductors) in a variety of oscillatory circuits, filters, delay lines, and various other circuits in electronics and electrical engineering.
Self-induction and current surge
Due to the phenomenon of self-induction in an electric circuit with an EMF source, when the circuit is closed, the current is not established instantly, but after some time. Similar processes also occur when the circuit is opened, while (with a sharp opening) the value of the self-induction emf can at this moment significantly exceed the source emf.
Most often in ordinary life it is used in car ignition coils. Typical ignition voltage at 12V battery voltage is 7-25 kV. However, the excess of the EMF in the output circuit over the EMF of the battery here is due not only to a sharp interruption of the current, but also to the transformation ratio, since most often not a simple inductor coil is used, but a transformer coil, the secondary winding of which, as a rule, has many times more turns ( that is, in most cases, the circuit is somewhat more complex than that which would be fully explained by self-induction; however, the physics of its operation in this version partly coincides with the physics of the circuit with a simple coil).
This phenomenon is also used to ignite fluorescent lamps in a standard traditional circuit (here we are talking about a circuit with a simple inductor - a choke).
In addition, it must always be taken into account when opening contacts, if the current flows through the load with a noticeable inductance: the resulting jump in the EMF can lead to a breakdown of the intercontact gap and / or other undesirable effects, to suppress which in this case, as a rule, it is necessary to take a variety of special measures.
Notes
Links
- About self-induction and mutual induction from the "School for an Electrician"
Wikimedia Foundation. 2010 .
- Bourdon, Robert Gregory
- Juan Amar
See what "Self-induction" is in other dictionaries:
self-induction- self-induction ... Spelling Dictionary
SELF-INDUCTION- the occurrence of induction emf in a conducting circuit when the current strength changes in it; special cases of electromagnetic induction. When the current in the circuit changes, the magnetic flux changes. induction through the surface bounded by this contour, resulting in ... Physical Encyclopedia
SELF-INDUCTION- excitation of the electromotive force of induction (emf) in an electric circuit when the electric current in this circuit changes; special case of electromagnetic induction. The electromotive force of self-induction is directly proportional to the rate of change of current; ... ... Big Encyclopedic Dictionary
SELF-INDUCTION- SELF-INDUCTION, self-induction, for women. (physical). 1. only units The phenomenon that when a current changes in a conductor, an electromotive force appears in it, preventing this change. Self-induction coil. 2. A device that has ... ... Dictionary Ushakov
SELF-INDUCTION- (Self induction) 1. A device with inductive resistance. 2. The phenomenon consisting in the fact that when an electric current changes in magnitude and direction in a conductor, an electromotive force arises in it that prevents this ... ... Marine Dictionary
SELF-INDUCTION- guidance of the electromotive force in the wires, as well as in the windings of electr. machines, transformers, apparatus and instruments when changing the magnitude or direction of the electric current flowing through them. current. The current flowing through the wires and windings creates around them ... ... Technical railway dictionary
self induction- electromagnetic induction caused by a change in the interlocking with the circuit magnetic flux, caused by electric current in this circuit... Source: ELEKTROTEHNIKA. TERMS AND DEFINITIONS OF BASIC CONCEPTS. GOST R 52002 2003 (approved ... ... Official terminology
self-induction- noun, number of synonyms: 1 electromotive force excitation (1) ASIS synonym dictionary. V.N. Trishin. 2013 ... Synonym dictionary
self-induction- Electromagnetic induction, caused by a change in the magnetic flux interlocking with the circuit, due to the electric current in this circuit. [GOST R 52002 2003] EN self induction electromagnetic induction in a tube of current due to variations… … Technical Translator's Handbook
SELF-INDUCTION- a special case of electromagnetic induction (see (2)), consisting in the occurrence of an induced (induced) EMF in the circuit and due to changes in time magnetic field created by a varying current flowing in the same circuit. ... ... Great Polytechnic Encyclopedia
Books
- Induction, mutual induction, self-induction - it's simple. Theory of absoluteness , Gurevich Harold Stanislavovich , Kanevsky Samuil Naumovich , The process of interaction of electrons changing electromagnetic field with the electrons of the conductors in this electromagnet field is called electromagnetic induction. As a result… Category: Physics Series: Nature of the Far East Publisher: At the Nikitsky Gate, Manufacturer:
Self-induction is the appearance in the conductor of an electromotive force (EMF) directed in the opposite direction relative to the voltage of the power source when current flows. In this case, it occurs at the moment when the current strength in the circuit changes. A changing electric current generates a changing magnetic field, which in turn induces an EMF in the conductor.
This is similar to the formulation of Faraday's law of electromagnetic induction, where it says:
When a magnetic flux passes through a conductor, an emf arises in the latter. It is proportional to the rate of change of the magnetic flux (math. time derivative).
E=dФ/dt,
Where E is the EMF of self-induction, measured in volts, F is the magnetic flux, the unit of measurement is Wb (weber, it is also equal to V / s)
Inductance
We have already said that self-induction is inherent in inductive circuits, so we will consider the phenomenon of self-induction using the example of an inductor.
An inductor is an element that is a coil of insulated conductor. To increase the inductance, the number of turns is increased or a core made of a magnetically soft or other material is placed inside the coil.
The unit of inductance is Henry (H). Inductance measures how strongly a conductor opposes electric current. Since around each conductor through which current flows, a magnetic field is formed, and if you place the conductor in an alternating field, a current will appear in it. In turn, the magnetic fields of each turn of the coil add up. Then around the coil through which the current flows, a strong magnetic field will arise. When its strength changes in the coil, the magnetic flux around it will also change.
According to Faraday's law of electromagnetic induction, if a coil is pierced by an alternating magnetic flux, then a current and an EMF of self-induction will appear in it. They will impede the current that flowed in the inductor from the power supply to the load. They are also called extracurrent EMF self-induction.
The formula for the EMF of self-induction on inductance is:
That is, the greater the inductance, and the more and faster the current changes, the stronger the surge in EMF will be.
With an increase in current in the coil, an EMF of self-induction occurs, which is directed against the voltage of the power source, respectively, the increase in current will slow down. The same happens when decreasing - self-induction will lead to the appearance of an EMF, which will maintain the current in the coil in the same direction as before. It follows that the voltage at the coil terminals will be opposite to the polarity of the power source.
In the figure below, you can see that when the inductive circuit is turned on / off, the current does not appear abruptly, but changes gradually. The laws of commutation also speak of this.
Another definition of inductance sounds like this: the magnetic flux is proportional to the current, but in its formula, the inductance acts as a proportionality factor.
Transformer and Mutual Inductance
If you place two coils in close proximity, for example, on the same core, then the phenomenon of mutual induction will be observed. Let's pass an alternating current through the first one, then its alternating current will pierce the turns of the second one and an EMF will appear on its outputs.
This EMF will depend on the length of the wire, respectively, the number of turns, and also on the magnitude of the magnetic permeability of the medium. If they are placed just next to each other, the EMF will be low, and if you take a core made of magnetically soft steel, the EMF will be much higher. Actually, this is how the transformer works.
Interesting: this mutual influence of the coils on each other is called inductive coupling.
Benefit and harm
If you understand the theoretical part, it is worth considering where the phenomenon of self-induction is applied in practice. Consider the examples of what we see in everyday life and technology. One of the most useful applications is a transformer, we have already considered the principle of its operation. Now they are less and less common, but previously fluorescent tubular lamps were used daily in lamps. The principle of their work is based on the phenomenon of self-induction. You can see her diagrams below.
After applying voltage, the current flows through the circuit: phase - inductor - spiral - starter - spiral - zero.
Or vice versa (phase and zero). After the starter is activated, its contacts open, then (a coil with a large inductance) seeks to maintain the current in the same direction, induces a large self-induction EMF and the lamps are ignited.
Similarly, this phenomenon applies to the ignition circuit of a car or motorcycle that runs on gasoline. In them, a mechanical (breaker) or semiconductor key (transistor in the computer) is installed in the gap between the inductor and the minus (ground). This key, at the moment when a spark should form in the cylinder to ignite the fuel, breaks the power circuit of the coil. Then the energy stored in the core of the coil causes an increase in the EMF of self-induction and the voltage at the electrode of the spark plug increases until the breakdown of the spark gap occurs, or until the coil burns out.
In power supplies and audio equipment, it is often necessary to remove unnecessary ripples, noises or frequencies from the signal. For this, filters of different configurations are used. One option is LC, LR filters. Due to the current growth inhibition and resistance alternating current, respectively, it is possible to achieve the set goals.
Self-induction EMF harms the contacts of switches, circuit breakers, sockets, automata and other things. You may have noticed that when you pull out the plug of a working vacuum cleaner from the outlet, a flash inside it is very often noticeable. This is the resistance to a change in current in the coil (motor winding in this case).
In semiconductor switches, the situation is more critical - even a small inductance in the circuit can lead to their breakdown when peak values Uke or Usi are reached. To protect them, snubber circuits are installed, on which the energy of inductive surges is dissipated.
Conclusion
Let's summarize. The conditions for the occurrence of self-induction EMF are: the presence of inductance in the circuit and a change in current in the load. This can occur both during operation, when changing modes or disturbing influences, and when switching devices. This phenomenon can harm the contacts of relays and starters, as it leads to the opening of inductive circuits, for example, electric motors. To reduce the negative impact, most of the switching equipment is equipped with arc chutes.
For useful purposes, the phenomenon of EMF is used quite often, from the filter for smoothing current ripples and frequency filters in audio equipment, to transformers and high-voltage ignition coils in cars.
We hope that now it has become clear to you what self-induction is, how it manifests itself and where it can be used. If you have any questions, ask them in the comments below the article!
materials
As it was established empirically, around any conductor with current is created a magnetic field. We know, magnetic flux F, passing through the contour is directly proportional induction B magnetic field and horses S, covered by the conductor The induction B of the magnetic field created by the current-carrying conductor at each point in space around this conductor is proportional to current strength I in the conductor. Therefore, the magnetic flux Ф through a given conducting circuit (S = const) is directly proportional to the current strength in the circuit:
Proportionality factor L between current strength I and the magnetic flux through the circuit is called loop inductance or coefficient of self-induction. It depends on the area covered by the contour, on its shape, the properties of the environment in which the contour is located.
The SI unit for inductance is Henry (H).
L \u003d F / I,
1 Gn \u003d 1Wb / 1A.
A circuit in which an electric current with a force of 1 Ampere creates a magnetic flux of 1 Weber has an inductance of 1 Henry.
A direct current I flowing through a circuit with an inductance L creates a magnetic flux around the circuit Ф, equal to
What happens to this circuit when the current is turned off?
The current will stop, the magnetic field will disappear. Recall that the disappearance of the magnetic flux is its change from the initial value Ф = LI to zero. According to the law of electromagnetic induction, changes in the magnetic flux through the circuit should cause the appearance of an EMF of induction equal to:
Ε is = -∆Ф/∆t = -L (∆I/∆t)
The phenomenon of the occurrence of an emf of induction caused by a change in the current strength in the circuit itself called self-induction.
With a change in the current in the circuit, the magnetic flux through the surface, which is limited by this circuit, also changes proportionally. According to the law of electromagnetic induction, a change in the magnetic flux leads to the excitation of an inductive EMF in this circuit.
The unit of inductance 1 Henry has such a circuit, in which, when the current strength changes by 1 Ampere in 1 second, an EMF of self-induction of 1 Volt arises.
According to Lenz's rule, The self-induction EMF, with a decrease in the current strength in the circuit, acts in the direction of maintaining the current strength unchanged; with an increase in the current strength in the self-induction EMF circuit, it prevents the current from increasing.
To detect the phenomenon of self-induction, one can use electrical circuit shown in the diagram:
In this circuit, a resistor and an iron core coil are connected in parallel, electric lamps are connected in series with the resistor and coil. The electrical resistance of the resistor is equal to the electrical resistance of the DC coil, therefore, when they are connected in parallel to a current source, the lamps should burn equally brightly.
Let's do some experiments. When the circuit is closed, the lamp in the coil circuit lights up noticeably later than the lamp in the resistor circuit. This can be explained by the fact that the coil with an iron core has a large inductance, the self-induction EMF prevents the current from increasing when turned on.
Both lamps flash when the power source is turned off. The current in the circuit of the coil and the resistor creates an EMF of self-induction that occurs when the current in the coil decreases. This shows that the magnetic field is not only capable of acting on driving charges, but also has a certain amount of energy. It is due to the energy of the magnetic field that a current appears in the circuit when it is disconnected from the current source.
When the current strength changes in the conductor, a vortex electric field arises in the latter. This field slows down the electrons when the current strength increases and accelerates them when the electric current in this conductor decreases.
The phenomenon of self-induction plays a very important role in electrical and radio engineering. The inductance of the circuit has a significant effect on the passage of an alternating electric current through the circuit.
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The term induction in electrical engineering means the occurrence of current in an electrical closed circuit if it is in a changing state. It was discovered only two hundred years ago by Michael Faraday. Much earlier, this could have been done by André Ampère, who conducted similar experiments. He inserted a metal rod into the coil, and then, bad luck, went into another room to look at the galvanometer needle - and suddenly it would move. And the arrow regularly did its job - it deviated, but while Ampere wandered around the rooms - it returned to zero. This is how the phenomenon of self-induction waited for another ten years, until the coil, the device and the researcher were at the same time in the right place.
The main point of this experiment was that the induction emf occurs only when the magnetic field passing through the closed circuit changes. But you can change it as you like - either change the value of the magnetic field itself, or simply move the source of the field relative to the same closed loop. The emf, which arises in this case, was called the “emf of mutual induction”. But this was only the beginning of discoveries in the field of induction. Even more surprising was the phenomenon of self-induction, which he discovered at about the same time. In his experiments, it was found that the coil not only induced a current in another coil, but also when the current in this coil changed, it induced an additional EMF in it. So it was called the EMF of self-induction. Of great interest is the direction of the current. It turned out that in the case of the EMF of self-induction, its current is directed against its “parent” - the current due to the main EMF.
Is it possible to observe the phenomenon of self-induction? As they say, nothing is easier. We will assemble the first two - a series-connected inductor and a light bulb, and the second - only a light bulb. Connect them to the battery through a common switch. When turned on, you can see that the light in the circuit with the coil lights up “reluctantly”, and the second light, faster “to rise”, turns on instantly. What's happening? In both circuits, after switching on, the current begins to flow, and it changes from zero to its maximum, and it is just the change in current that the inductor coil waits for, which generates the self-induction EMF. There is an EMF and a closed circuit, which means that there is also its current, but it is directed opposite to the main current of the circuit, which, in the end, will reach the maximum value determined by the parameters of the circuit and stop growing, and since there is no change in current, there is no self-induction EMF. Everything is simple. A similar picture, but with “exactly the opposite”, is observed when the current is turned off. Faithful to her bad habit” to counteract any change in current, the self-induction EMF maintains its flow in the circuit after the power is turned off.
Immediately the question arose - what is the phenomenon of self-induction? It was found that the EMF of self-induction is affected by the rate of change of current in the conductor, and we can write:
From this it can be seen that the EMF of self-induction E is directly proportional to the rate of change of the current dI / dt and the coefficient of proportionality L, called inductance. For his contribution to the study of the question of what the phenomenon of self-induction consists of, George Henry was rewarded by the fact that the unit of inductance, the henry (H), bears his name. It is the inductance of the current flow circuit that determines the phenomenon of self-induction. It can be imagined that inductance is a kind of “storage” of magnetic energy. In the case of an increase in current in the circuit, the electrical energy is converted into magnetic energy, which delays the increase in current, and when the current decreases, the magnetic energy of the coil is converted into electrical energy and maintains the current in the circuit.
Probably, everyone had to see a spark when the plug was turned off from the socket - this is the most common variant of the manifestation of self-induction EMF in real life. But in everyday life, currents of a maximum of 10-20 A are opened, and the opening time is about 20 ms. With an inductance of the order of 1 H, the EMF of self-induction in this case will be equal to 500 V. It would seem that the question of what the phenomenon of self-induction consists of is not so complicated. But in fact, self-induction EMF is a big technical problem. The bottom line is that when the circuit breaks, when the contacts have already dispersed, self-induction maintains the flow of current, and this leads to burnout of the contacts, because. in technology, circuits with currents of hundreds and even thousands of amperes are switched. Here we are often talking about self-induction EMF of tens of thousands of volts, and this requires additional solution technical issues related to overvoltages in electrical circuits.
But not everything is so gloomy. It happens that this harmful EMF is very useful, for example, in ICE ignition systems. Such a system consists of an inductor in the form of an autotransformer and a chopper. A current is passed through the primary winding, which is turned off by a breaker. As a result of an open circuit, an EMF of self-induction of hundreds of volts occurs (while the battery gives only 12V). Further, this voltage is additionally transformed, and a pulse of more than 10 kV is supplied to the spark plugs.
We have already studied that a magnetic field arises near a current-carrying conductor. And also studied that a variable magnetic field generates a current (the phenomenon of electromagnetic induction). Consider an electrical circuit. When the current strength changes in this circuit, a change in the magnetic field will occur, as a result of which an additional voltage will appear in the same circuit. induction current. Such a phenomenon is called self-induction, and the resulting current is called self-induction current.
The phenomenon of self-induction- this is the occurrence in a conducting circuit of an EMF created due to a change in the current strength in the circuit itself.
Loop inductance depends on its shape and size, on magnetic properties environment and does not depend on the current strength in the circuit.
EMF of self-induction is determined by the formula:
The phenomenon of self-induction is similar to the phenomenon of inertia. Just as in mechanics it is impossible to instantly stop a moving body, so the current cannot instantly acquire a certain value due to the phenomenon of self-induction. If a coil is connected in series with the second lamp in a circuit consisting of two identical lamps connected in parallel to a current source, then when the circuit is closed, the first lamp lights up almost immediately, and the second with a noticeable delay.
When the circuit is opened, the current strength decreases rapidly, and the resulting self-induction EMF prevents the magnetic flux from decreasing. In this case, the induced current is directed in the same way as the original one. The self-induced emf can be many times greater than the external emf. Therefore, light bulbs very often burn out when the light is turned off.