The formula by which the self-induction emf is determined. Pulse generator of self-induction emf
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This phenomenon is called self-induction. (The concept is related to the concept of mutual induction, being, as it were, a special case of it).
The direction of the self-induction EMF always turns out to be such that when the current in the circuit increases, the self-induction EMF prevents this increase (directed against the current), and when the current decreases, it decreases (co-directed with the current). This property of self-induction emf is similar to inertial force.
The magnitude of the self-induction EMF is proportional to the rate of change of 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-inductive emf in the coil lags behind the current in phase by (that is, 90°), and the amplitude of this emf is proportional to the amplitude of the current, frequency and inductance (). After all, the rate of change of a function is its first derivative, a.
To calculate more or less complex circuits containing inductive elements, that is, turns, coils, etc. devices in which self-induction is observed, (especially completely linear, that is, not containing nonlinear elements) in the case of sinusoidal currents and voltages, the method of complex impedances is used or, in simpler cases, a less powerful one , but a more visual option is the vector diagram method.
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 Fourier series or integral and thus reduced to sinusoidal.
In more or less direct connection with this, we can mention the use of the phenomenon of self-induction (and, accordingly, inductors) in a variety of oscillating circuits, filters, delay lines and other various electronics and electrical circuits.
Self-inductance and current surge
Due to the phenomenon of self-induction in electrical circuit with an EMF source, when the circuit is closed, the current is not established instantly, but after some time. Similar processes occur when the circuit opens, and (with a sharp opening) the value of the self-induction EMF at this moment can significantly exceed the source EMF.
Most often in ordinary life it is used in automobile ignition coils. Typical ignition voltage with a 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 caused not only by a sharp interruption of the current, but also by the transformation ratio, since most often it is not a simple inductor coil that is used, but a transformer coil, the secondary winding of which is usually many times large quantity turns (that is, in most cases the circuit is somewhat more complex than the one whose operation would be fully explained through self-induction; however, the physics of its operation in this version partly coincides with the physics of the operation of a circuit with a simple coil).
This phenomenon is also used for ignition. fluorescent lamps in standard traditional scheme(Here we're talking about specifically 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 noticeable inductance: the resulting jump in EMF can lead to 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 Electricians”
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See what “Self-induction” is in other dictionaries:
Self-induction... Spelling dictionary-reference book
The appearance of induced emf in a conductive circuit when the current strength changes in it; special cases electromagnetic induction. When the current in the circuit changes, the magnetic flux changes. induction through the surface limited by this contour, resulting in ... Physical encyclopedia
Excitation of electromotive force of induction (emf) in an electrical circuit when changing electric current in this chain; special case 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, female. (physical). 1. units only The phenomenon that when the current changes in a conductor, an electromotive force appears in it, preventing this change. Self-induction coil. 2. A device with... ... Dictionary Ushakova
- (Self induction) 1. A device with inductive reactance. 2. The phenomenon that when an electric current changes in magnitude and direction in a conductor, an electromotive force arises in it, preventing this... ... Marine Dictionary
Induction of electromotive force in wires, as well as in electrical windings. machines, transformers, apparatus and instruments when the magnitude or direction of electricity flowing through them changes. 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 adhesion to the circuit magnetic flux, caused by electric current in this circuit... Source: ELECTRICAL ENGINEERING. TERMS AND DEFINITIONS OF BASIC CONCEPTS. GOST R 52002 2003 (approved... ... Official terminology
Noun, number of synonyms: 1 excitation of electromotive force (1) Dictionary of synonyms ASIS. V.N. Trishin. 2013… Synonym dictionary
self-induction- Electromagnetic induction caused by a change in the magnetic flux interlocking with the circuit, caused by 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 Guide
SELF-INDUCTION- a special case of electromagnetic induction (see (2)), consisting in the occurrence of an induced (induced) emf in the circuit and caused by changes in time magnetic field, created by a changing current flowing in the same circuit.... ... Big Polytechnic Encyclopedia
Books
- Set of tables. Physics. Electrodynamics (10 tables), . Educational album of 10 sheets. Electric current, current strength. Resistance. Ohm's law for a section of a circuit. Dependence of conductor resistance on temperature. Connection of wires. EMF. Ohm's law…
Self-induction is the induction of EMF in a conductor when the electric current in this conductor changes.
When voltage is applied to the electromagnet coil, the current does not increase immediately. It increases gradually. The increase in current is inhibited by the resulting voltage, which is opposite to the applied one. This voltage is the electromotive force (EMF) of self-induction. The EMF value gradually decreases, and the current in the electromagnet increases to the nominal value.
The interaction of electric and magnetic fields is the cause of self-induction
Electric and magnetic fields are interconnected: electric current or changing electric field creates a magnetic field.
In turn, the changing magnetic field creates an electric field.
Let us consider the processes in a conducting circuit when the electric current in it changes (for example, it is turned on or off).
- An emf is induced in a conductor placed in a changing magnetic field.
- If the magnitude of the electric current changes in a conductor, a changing magnetic field appears.
- A changing magnetic field created by a current in a conductor induces a self-inductive emf in the same conductor.
Not all electrical circuits experience self-induction. An incandescent light bulb flashes instantly when current is applied, and goes out instantly when it is turned off, and in an electromagnet, to which a constant voltage is applied and turned off, the processes are extended over time. A light bulb and an electromagnet have different inertia.
In mechanics, the measure of inertia is mass: to set a massive object in motion, you need to apply force for some time.
In electrical engineering, the measure of inertia is a quantity called inductance. It is indicated by the symbol L. The unit of inductance is Henry (H), as well as derived units: milliHenry (mH), microHenry (μH), and so on. The greater the inductance of the circuit, the longer and more powerful the transient processes occur. An incandescent light bulb has a very small inductance, while an electromagnet has a large inductance.
In radio engineering and electrical engineering, chokes are used - parts that have standardized inductance values.
The figure shows a diagram of an experiment demonstrating the phenomenon of self-induction.
A coil wound on a ferrite core has significant inductance. The power source is a battery with a nominal value of one and a half volts. While the toggle switch is on, the light bulb lights dimly because the battery voltage is not enough for it. After opening the toggle switch, the light flashes brightly and then goes out.
Why does the light flash after turning off the power supply? Through it, the self-induction EMF induced in the coil at the moment the voltage is turned off is discharged.
But why does the light not just continue to burn, but flashes brighter than when the toggle switch was on? The self-induced emf exceeds the rated voltage of the battery. Let's consider what this effect depends on.
What does self-induced emf depend on?
The self-inductive emf that occurs in an electrical circuit depends on its inductance and the rate of change of current in the circuit.
The rate of change of current has important. If it turns off instantly, that is, the rate of change is very large, then the self-induction EMF is also large. The induced voltage is discharged through parallel branches of the circuit (in the experiment with a light bulb - through a light bulb).
Self-induction and transient processes in electrical circuits
Inductance electric stove or incandescent light bulbs is very small, and the current in these electrical appliances, when turned on and off, appears or disappears almost instantly. The inductance of the electric motor is high, and it “goes into operation” within a few minutes.
If you turn off the current in a large electromagnet with great value induction, allowing high speed If the current decreases, a spark flashes between the contacts of the switch, and in the case of a large current, a voltaic arc may ignite. This dangerous phenomenon, therefore, in circuits with high inductance, the current is reduced gradually using a rheostat (an element with variable electrical resistance).
Safe Power Shutdown – serious problem. All switches are subject to “shock loads” that arise due to the self-induction EMF when the current is turned off, and the switches “spark.” For each type of switch, the maximum current value that can be switched is indicated. If the current exceeds permissible value, the switch may flash electric arc.
In hazardous industries, coal mines, and petroleum product storage facilities, simple sparking of switches is unacceptable. Explosion-proof switches are used here, reliably protected by a sealed plastic housing. The price of such switches is tens of times higher than ordinary ones - this is a necessary payment for safety.
Physics 10-11 grade. SELF-INDUCTION
Each conductor through which electric current flows is in its own magnetic field.
When the current strength changes in the conductor, the m.field changes, i.e. the magnetic flux created by this current changes. A change in magnetic flux leads to the emergence of a vortex electric field and an induced emf appears in the circuit.
This phenomenon is called self-induction.
Self-induction is the phenomenon of the occurrence of induced emf in an electrical circuit as a result of a change in current strength.
The resulting emf is called Self-induced emf
Manifestation of the phenomenon of self-induction
Circuit closure
When there is a short circuit in the electrical circuit, the current increases, which causes an increase in the magnetic flux in the coil, and a vortex electric field appears, directed against the current, i.e. A self-induction emf arises in the coil, preventing the increase in current in the circuit (the vortex field inhibits the electrons).
As a result L1 lights up later, than L2.
Open circuit
When the electrical circuit is opened, the current decreases, a decrease in the flux in the coil occurs, and a vortex electrical field appears, directed like a current (trying to maintain the same current strength), i.e. A self-induced emf arises in the coil, maintaining the current in the circuit.
A current that changes in magnitude always creates a changing magnetic field, which, in turn, always induces an emf. With any change in current in a coil (or in general in a conductor), a self-inductive emf is induced in it; it depends on the rate of change of current. The greater the rate of change of current, the greater the self-induction emf.
The magnitude of the self-induction EMF also depends on the number of turns of the coil and its size. The larger the diameter of the coil and the number of its turns, the greater the self-induction emf. This dependence has great importance in electrical engineering. The direction of the self-induction EMF determines Lenz's Law:
The self-induced emf has Alwayssuch a direction in which it prevents the change in the current that caused it.
In other words, a decrease in the current in the coil entails the appearance of a self-induction emf directed in the direction of the current, i.e., preventing its decrease. And, conversely, when the current increases in the coil, a self-induction emf appears, directed against the current, i.e., preventing its increase. If the current in the coil does not change, then no self-induction emf occurs. The phenomenon of self-induction is especially pronounced in a circuit containing a coil with a steel core, since steel significantly increases the magnetic flux of the coil, and therefore the magnitude of the self-induction emf.
The phenomenon of self-induction can be demonstrated by conducting the following experiment. Let's assemble an electrical circuit consisting of a battery, a disconnector and two parallel circuits: in the first - a light bulb and a resistor, and in the second - a light bulb and a coil, and the resistance of both light bulbs is the same, and the resistance of the resistor and coil is also the same.
1. When the disconnector is turned on, lamp L1 will light up with a delay, since the self-inductive emf of the coil prevents a rapid increase in current in the lamp L1 circuit (Fig. 1a and 1b).
2. When the disconnector is turned off, both lamps will flash briefly, since the self-induction emf of the coil is higher than the battery emf. When the self-induction emf dries up, both lamps go out simultaneously (Fig. 2a and 2b).
The phenomenon of self-induction has both positive and negative properties, and both of them manifest themselves during the operation of devices and electrical circuits of the metro rolling stock:
- Inductive shunt, connected parallel to the excitation windings of traction motors, smoothes out oscillations high voltage on the contact rail (or during short-term separation of pantographs). The inductance of this shunt is comparable to the inductance of the excitation windings, and its EMF is always directed opposite the EMF of the OF TED. Thus, when the high voltage is reduced or removed from the contact rail, the EMF of the inductive shunt prevents the current from decreasing, and when the voltage increases, it prevents the current from increasing, which prevents the occurrence of an emergency mode in the power circuit and the formation of a circular fire along the electric motor commutator.
- If you open a circuit containing a coil with high inductance, then when the contacts open, an electric arc will form, which can lead to destruction of the switching device, so in such cases it is necessary use an arc extinguishing device or (for low-voltage circuits) connect a capacitor in parallel with the contacts.
Electric current passing through a conductor creates a magnetic field around it. The magnetic flux F through the circuit of this conductor is proportional to the induction module B of the magnetic field inside the circuit, and the magnetic field induction in turn is proportional to the current strength in the conductor. Therefore, the magnetic flux through the loop is directly proportional to the current in the loop:
The proportionality coefficient between the current strength I in the circuit and the magnetic flux F created by this current is called inductance. Inductance depends on the size and shape of the conductor, on magnetic properties environment in which the conductor is located.
Unit of inductance.
The unit of inductance in the International System is the henry. This unit is determined based on formula (55.1):
The inductance of the circuit is equal if at force direct current 1 A magnetic flux through the circuit is equal to
Self-induction.
When the current in the coil changes, the magnetic flux created by this current changes. A change in the magnetic flux passing through the coil should cause the appearance of an induced emf in the coil. The phenomenon of the occurrence of induced emf in
of an electrical circuit as a result of a change in current strength in this circuit is called self-induction.
In accordance with Lenz's rule, the self-inductive emf prevents the current from increasing when the circuit is turned on and the current from decreasing when the circuit is turned off.
The phenomenon of self-induction can be observed by assembling an electrical circuit from a coil with high inductance, a resistor, two identical incandescent lamps and a current source (Fig. 197). The resistor must have the same electrical resistance, like the coil wire. Experience shows that when the circuit is closed, an electric lamp connected in series with the coil lights up somewhat later than a lamp connected in series with a resistor. The increase in current in the coil circuit during closure is prevented by the self-induction emf, which occurs when the magnetic flux in the coil increases. When the power source is turned off, both lamps flash. In this case, the current in the circuit is maintained by the self-induction emf that occurs when the magnetic flux in the coil decreases.
The self-induction emf arising in a coil with inductance according to the law of electromagnetic induction is equal to
The self-inductive emf is directly proportional to the inductance of the coil and the rate of change of current in the coil.
Using expression (55.3), we can give a second definition of the unit of inductance: an element of an electrical circuit has inductance if, with a uniform change in the current strength in the circuit by 1 A in 1 s, a self-inductive emf of 1 V arises in it.
Magnetic field energy.
When the inductor coil is disconnected from the current source, an incandescent lamp connected parallel to the coil gives a short-term flash. The current in the circuit arises under the influence of self-induction emf. The source of energy released in the electrical circuit is the magnetic field of the coil.
The energy of the magnetic field of the inductor can be calculated in the following way. To simplify the calculation, consider the case when, after disconnecting the coil from the source, the current in the circuit decreases with time according to a linear law. In this case, the self-induction emf has a constant value equal to