Of particular practical importance is one particular case of the phenomenon of electromagnetic induction, called self-induction. So, when the induction coil generates current, then at the same time a magnetic flux arises, which increases with increasing current. With a change in magnetic flux, the coil induces an electromotive force (EMF), the magnitude of which is proportional to the change in the speed of the magnetic flux.
Since in this case the conductor induces an electromotive force in itself, this phenomenon is called self-induction. The phenomenon of self-induction in electrical circuits is sometimes compared with the manifestation of inertia in mechanics.
The electromotive force induced in the induction coil under the influence of changes in its own magnetic flux is called the electromotive force of self-induction.
According to Lenz’s law, during the entire growth of the magnetic flux, which unifies the turns of the coil, the self-induction EMF in the coil is directed against the electromotive force of the source included in this circuit and counteracts the increase in current in the coil circuit.
When the current in the coil reaches a constant value, the magnetic flux stops the change, and the self-induction EMF in the coil becomes equal to zero.
In self-induction, as in any process of electromagnetic induction, the induced electromotive force is proportional to the speed at which the magnetic flux coupled to the circuit along which current flows changes. The magnitude of the magnetic flux in the absence of iron in the coil is proportional to the speed with which the current (∆I / ∆t) that creates this flux changes.
Thus, the magnitude of the electromotive force of self-induction arising in the conductor is proportional to the speed with which the current in it changes.
If we take conductors of different shapes, it turns out that having the same rate of change of current, the electromotive forces of self-induction arising in them will be different.
So, if you take a coil and then stretch it in one turn, then at the same speed with which the current changes, the EMF of the coil self-induction will be greater. This is due to the fact that each power line, linking the turns of the coil, engages with it more times than with one turn.
The value characterizing the relationship between the speed with which the current changes in the circuit, and the resulting self-induction EMF is the inductance of the circuit.
Denote the inductance of the coil by the letter L; then the dependence of the magnitude of the electromotive force of self-induction on the speed with which the current changes can be expressed by the following formula:
E = - L (∆I / ∆t)
From here
units L = (unit E ˖ unit t) / (unit I)
Assuming that in this formula ∆t = 1 sec, ∆I = 1 ampere and E = 1 volt, we get:
units L = 1 (in ˖ s / a)
Such a unit is called henry (Hg).
Consequently,
1 H = 1 (in ˖ s / a)
So, Henry is the inductance of a coil in which a change in current of 1 ampere per second excites an electromotive force of self-induction, equal to 1 volt.
To measure small inductances, thousandths of henry - milligenry (mH) and millionths of henry - microgenry (μH) are used.
In addition, another unit is often used - a centimeter of inductance, with 1 μH = 1000 cm of inductance.
Thus,
1 GN = 1000 mH = 1,000,000 μH = 1,000,000,000 cm
The inductance of the coil depends on its number of turns, shape and size. The larger the number of turns in the self-induction coil, the greater its inductance.
Also, self-induction, the inductance of the coil increases significantly when introduced into its core from iron or some other magnetic material.
The windings of electromagnets in generators and motors have great inductance, at the moment of opening the circuit, when the rate of change of electric current (∆I / ∆t) is very high, a large self-induction emf can occur in these windings, which, if not taken, will lead to breakdown winding insulation.