What is alternating current?
Many decades in electrotechnical practice exclusively direct current was applied. Refused it only when the need for transmission of energy on the long distance from places where there are cheap natural stocks (the falling water, low-grade coal, peat), in those numerous points where there are energy consumers ripened.
But such distant transmission of energy can be carried out only by means of electric current of high tension. The tension is higher, the smaller current intensity is necessary at the same power.
It is very approximately possible to consider economically expedient voltage of 1000 V = 1 kV on transmission line length kilometer. For example, for electric power transmission for 200 km voltage of 220 kV is desirable. But it is very difficult to develop electrical energy at power plant thus very high tension what is required for its transfer for hundreds of kilometers. Also the device of receivers for electric power consumption would be very difficult (for example, electric motors) at such high tension.
Therefore, the indispensable condition of long distance power transmission and its distribution is the possibility of transformation, i.e. transformation with small losses, electric current of big force and low tension in electric current of small force and high tension or inverse transformation. But such transformation is feasible only by means of the electromagnetic device of alternating current — the transformer. Because of enormous advantages of transformation the modern power industry is constructed on alternating-current application.
In principle alternating current it would be possible to call any electric current changing in time, but in the equipment alternating current it is accepted to call that which periodically changes in size and the direction, and mean value of this current for the period is equal to zero. Alternating-current frequency is expressed that through the certain period called by the period of T, alternating-current changes repeat (fig. 1).
The cycle of alternating-current changes is called the cycle. Therefore, the period is duration of one alternating-current cycle. The number of the periods (or cycles) in the second is called f frequency. Therefore, frequency — size, the return to f period = 1/T. Frequency is measured in hertz (Hz), hertz = 1 sec. Alternating-current frequency is equal to one hertz if its period is equal to one second.
In USA for electric generating plants (in particular, for all power plants) the standard frequency of 50 Hz is accepted. It is easy to specify the reasons of such choice: with reduction in the frequency even up to 40 Hz electric lamps of incandescence considerably for the eye blink; with increase in frequency the self-induction EMF significantly preventing electric power transmission on wires in proportion increases.
These reasons do not belong to electric power transmission in rather small quantities without wires by means of electromagnetic waves therefore in radio engineering high frequencies from 100 kHz = 100 x 1000 Hz (long waves) up to 30000 megahertz (centimeter HF waves) are applied.
Besides, for the special technical purposes alternating currents of the most different frequencies are used: for example, for electric furnaces alternating currents with a frequency from 500 Hz to 50 MHz.Top