Alternating electric current is a current with direction and force changing in time. Those currents that change only in magnitude are called ripple currents. In industry and everyday life, an alternating sinusoidal current is most often used .
The conversion of direct current to alternating electric can be performed as follows. We put in a uniform constant magnetic field a coil of wire. With a uniform rotation of this coil around the axis, the magnetic flux will continuously change both in magnitude and direction. As a result of this, according to the law of electromagnetic induction, a variable in direction and magnitude electromotive force (EMF) is formed in the coil . If such a coil is connected to an external circuit, then in it we will get an alternating electric current.
When the plane of the rotating coil becomes perpendicular with respect to the lines of force of a given magnetic field, the magnetic flux passing through it is the largest (Φ = Φmax), but its rate of change is zero (ΔΦ / Δt = 0), since, passing through this position, the conductors of the loop slip along the field lines of force without crossing them. This means that the induction EMF formed in the coil becomes zero (E = 0).
When the plane of the turn is parallel to the field lines of force, the flow penetrating it is equal to zero (Φ = 0), but the rate of change in this position is the highest ((ΔΦ / Δt) max), since the conductors of the turn move perpendicular to the field lines.
EMF arising in this case in a turn has the greatest value (E = Emax). With further rotation of the coil, the rate of change of the flow penetrating the coil will increase; hence, EMF in absolute value will increase from 0 to Emax. So, the level of induction EMF in a rotating coil during one revolution varies from –Emax to + Emax.
Open the coil of wire and attach it to the oscilloscope. When the coil rotates in a magnetic field, the oscilloscope will record all changes in current, by which it will be possible to judge the change in the electromotive force in the coil during one revolution.
The current arising in the coil when it is reversed uniformly in a uniform magnetic field, as the oscillogram shows, changes sinusoidally. Such a current is called alternating sinusoidal.
The time interval during which the electromotive force performs one oscillation is called the period of the alternating current.
The letter designation of the oscillation period is T. The number of oscillations per 1 second is the frequency of the current, which is denoted by the letter f. Her unit of measure is hertz (Hz):
f = 1 / T, or T = 1 / f.
If the EMF value at some arbitrary time moment is denoted by e (its instantaneous value), and the largest value (amplitude) by Emax, then the law expressing the dependence of e on time in the case of a sinusoidal current can be expressed as the following expression:
e = Emax˖sin (2 / T) t.
In most countries, industrial and domestic use an alternating electric current with a frequency of 50 Hz, a duration of 0.02 seconds.
Obtaining alternating electric current from mechanical energy is carried out using special machines called generators. The principle of their work is based on the law of electromagnetic induction. The simplest generator circuit can be represented as a frame rotating around an axis in the magnetic field of an electromagnet or a permanent magnet. When the frame rotates, a variable electromotive force is formed in it. By connecting the frame to the external circuit, we get an alternating electric current. An alternator with a fixed magnetic system and rotating turns is rarely built.
In almost all such generators, the winding (armature) is fixed, and the magnetic system (inductor) rotates. The real part of the generator is called the stator, and the movable part is called the rotor.