The term induction in electrical engineering means the occurrence of current in an electric closed circuit, if it is in a changing magnetic flux. Electromagnetic induction was discovered just two hundred years ago by Michael Faraday. Much earlier, Andre Ampere, who conducted similar experiments, could have done this. He inserted a metal rod into the coil, and then, bad luck, he went into another room to look at the galvanometer needle - and suddenly it moves. And the arrow was doing its job properly - deviating, but while Ampere wandered around the rooms - was returning to zero. This is how the phenomenon of self-induction waited a good ten years, until the coil, instrument and researcher were simultaneously in the right place.
The main point of this experiment was that the induction emf only occurs when the magnetic field passing through the closed loop changes. But you can change it as you like - either change the magnitude of the magnetic field itself, or simply move the field source relative to the same closed loop. EMF, which occurs in this case, was called "mutual induction EMF". But this was only the beginning of discoveries in the field of induction. Even more surprising was the phenomenon of self-induction, which was discovered by Joseph Henry at about the same time. In his experiments, it was found that the magnetic field of the coil not only induced a current in another coil, but when the current in this coil changed, it induced an additional EMF in it. This is what they called EMF of self-induction. In electrical phenomena, the direction of the current is of great interest. It turned out that in the case of 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, there is nothing easier. We assemble two electric circuits: the first is a series-connected inductor and a bulb, and the second is just a bulb. We connect them to the battery through a common switch. When you turn on, you can see that the light in the circuit with the coil lights up “reluctantly”, and the second light bulb, faster “to rise”, turns on instantly. What's happening? After switching on, a current begins to flow in both circuits, and it changes from zero to its maximum, and just the current changes and an inductor is expected, which generates the self-induction EMF. If there is an EMF and a closed circuit, it means 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 circuit parameters and stop growing, and if there is no change in current, there is no self-induction EMF either. Everything is simple. A similar picture, but “exactly the opposite”, is observed when the current is turned off. True to its "bad habit" to counteract any change in current, the self-induction EMF supports its flow in the circuit after a power outage.
The question immediately became - what is the phenomenon of self-induction? It was found that the rate of change of current in the conductor affects the EMF of self-induction, and you can write:
E = L • dI / dt
This shows that the self-induction EMF E is directly proportional to the rate of change of current dI / dt and the proportionality coefficient L, called the 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 he was named after the unit of inductance - Henry (GN). It is the inductance of the current circuit that determines the phenomenon of self-induction. One can imagine that inductance is a kind of “storage” of magnetic energy. In the case of an increase in the current in the circuit, electrical energy is converted into magnetic energy, delays the growth of current, and when the current decreases, the magnetic energy of the coil is converted into electric and maintains the current in the circuit.
Probably, everyone had to see a spark when turning off the plug from the outlet - this is the most common version of the manifestation of EMF self-induction 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 500 V. It would seem that the question of what the phenomenon of self-induction consists of is not so complicated. But in fact, EMF self-induction is a big technical problem. The bottom line is that when the circuit breaks, when the contacts have already opened, self-induction supports the flow of current, and this leads to the burnout of the contacts, because In technology, circuits with currents of hundreds or even thousands of amperes are switched. This often refers to the EMF of self-induction of tens of thousands of volts, and this requires an additional solution to 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 the circuit 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.