Техническое чтение для энергетиков. Бухарова Г.П. - 29 стр.

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31
A direct current (D.C.) flows continuously through a conducting circuit in one
direction only, although it may not be steady so far as magnitude is concerned. It is
unidirectional in character. An alternating current (A.C.), on the other hand,
continually reverses in direction, as its name implies. Starting from zero, it grows in
one direction, reaches a maximum, dies down to zero again, after which it rises in the
opposite direction, reaches a maximum, again dying down to zero. It is thus
continually changing in magnitude as well as direction, and this continual change
causes certain effects of far-reaching importance.
It can be shown that high voltages are desirable for the economic transmission of a
given amount of electric power. Take, for example, the transmission of 1000 kW. If
the transmission voltage is 100 volts the current must be 10,000 amperes, but if the
transmission voltage is 10,000 volts the current is only 100 amperes. The cross-
section of the cables transmitting the power is determined by the current to be carried,
and so in the former case the cables would need to be very much larger than in the
latter case. It is true that the high-voltage cable would need to have more insulation,
but even so, it would be very much cheaper than the larger low-voltage cable. A high
voltage is therefore essential for the economic transmission of electric power. Again,
a.c. generators can be designed and built for much higher voltages than can d.c.
generators, the voltage of the latter being limited by the problem of sparking at the
commutator, a component which is absent in the a.c. generator. Then there is the
most important factor that it is easy to transform a.c. power from one voltage to
another by means of the transformer, an operation that is denied to the d.c. system.
The transformer also enables the voltage to be stepped down at the receiving end of
the transmission line to values which can readily be used by the various consumers. If
necessary, it can be converted to the d.c. form for actual use, although this is not
often necessary. There are certain processes for which D.C. is either essential or at
any rate desirable but the utilization of electric power in the a.c. form is growing
steadily. At the present day, by far the greater part* of the generation, transmission,
and utilization of electric power is carried out by means of A.C.
* by far the greater part значительно большая часть; by far употребляется перед сравнительной
степенью прилагательного для усиления его значения
ELECTRIC CIRCUIT
The electric circuit is the subject to be dealt with in the present article. But what
does the above term really mean? We know the circuit to be a complete path which
carries the current from the source of supply to the load and then carries it again from
the load back to the source.
The purpose of the electrical source is to produce the necessary electromotive
force required for the flow of current through the circuit*
The path along which the electrons travel must be complete otherwise no
electric power can be supplied from the source to the load. Thus we close the circuit

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