Alternating current. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electricity for beginners
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As you know, the electric current is constant and variable. But only alternating current is widely used. This is due to the fact that the voltage and strength of the alternating current can be converted with little or no loss of energy.
Alternating current is obtained using alternating current generators using the phenomena of electromagnetic induction. On fig. 8 shows a primitive installation for generating alternating current.
Rice. 8. The simplest installation for generating alternating electric current
The principle of operation of the installation is simple. The wire frame rotates in a uniform magnetic field at a constant speed. With its ends, the frame is fixed on rings that rotate with it. Springs are tightly attached to the rings, acting as contacts.
A changing magnetic flux will continuously flow through the surface of the frame, but the flux created by the electromagnet will remain constant. In this regard, an EMF of induction will appear in the frame.
In order to determine whether the magnetic flux passing through the surface of the frame changes, you just need to compare the position of the frame in certain periods of time. To do this, take a close look at Fig. 9.
Rice. 9. Changes in the position of the frame in different periods of time
The starting point will be the position of the frame shown in Fig. 9, a. At this moment, the plane of the frame is perpendicular to the magnetic lines, and the magnetic flux will have a maximum value. Parallel to the magnetic lines, the frame will rise in a quarter of the period.
In this case, the magnetic flux will become equal to zero, because not a single magnetic line passes through the surface of the frame.
To determine the EMF of induction, you need to know not the magnitude of the flux, but the rate of its change. At the reference point, the induction EMF is zero, and at the third (Fig. 9, c) - the maximum value.
Based on the provisions of the frame, it can be seen that the induction emf changes both the value and the sign. Thus, it is a variable (see graph in Fig. 9).
If the frame has only active resistance, then the current that occurs in the circuit under the action of the induction EMF will change over time, like the EMF itself. Such a current is called an alternating sinusoidal current.
The period of an alternating current is the length of time during which the current performs one complete oscillation (this unit is denoted by the letter T).
The number of complete oscillations in 1 s is called the frequency of the current and is denoted by the letter f. Frequency is measured in hertz (Hz).
In industry and everyday life in most countries, alternating current with a frequency of 50 Hz is used.
Author: Smirnova L.N.
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