Техническое чтение для энергетиков. Бухарова Г.П. - 27 стр.

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29
thermionic valve. Whatever the source of origin, the electric current is fundamentally
the same in all cases, but the manner in which it varies with time may be very
different. This is shown by the graph of the current plotted against time as a base, and
a number of examples are illustrated in Fig. 1.
(a) represents a steady direct current (D.C.) of unvarying magnitude, such as is
obtained from an accumulator.
(b) represents a D.C. obtained from a d.c. generator, and consists of a steady D.C.
superimposed on which is a uniform ripple of relatively high frequency, due to the
commutator of the d.c. generator. As the armature rotates the commutator segments
come under the brush in rapid succession and produce a ripple in the voltage which is
reproduced in the current.
(c) represents a pulsating current varying periodically between maximum and
minimum limits. It may be produced by adding a D.C. to an A.C. or vice versa. The
d.c. component must be the larger if the current is to remain unidirectional. All the
first three types, of current are unidirectional, i.e. they flow in one direction only.
(d) represents a pure alternating current (A.C.). The current flows first in one
direction and then in the other in a periodic manner, the time of each alternation
being constant. In the ideal case the current varies with time according to a sine law,
when it is said to be sinusoidal. Considering the time of a complete cycle of current
(a positive half-wave plus a negative half-wave) as equal to 360°, the instantaneous
values of the current are proportional to the sine of the angle measured from the zero
point where the current is about to rise in the positive direction*.
(e) represents a type of A.C. with a different wave form. Such an A.C. is said to have
a peaked wave form, the term being self explanatory.
(f) represents an A.C. with yet another different wave form. Such an A.C. is said to
have a flat-topped wave form, the term again being self-explanatory. Both this and
the previous example represent cases of A.C. having non-sinusoidal wave forms.
(g) represents an example of an oscillating current, and is similar in shape to (d)
except that it has a much higher frequency. An oscillating current is usually regarded
as one having a frequency determined by the constants of the circuit, whereas an
alternating current has a frequency determined by the apparatus supplying the circuit.
(h) represents another type of oscillating current which is known as damped. The
current again has a constant frequency, but its amplitude is damped, i.e. it dies down,
after which it is brought back to its original value.
(i) represents yet another type of oscillating current, this time known as a modulated
current. The amplitude varies rhythmically between maximum and minimum values.
It may even die down to zero.
(j) The next three examples represent various types of transient currents. These
transient currents usually die away extremely rapidly, and times** are generally
measured in microseconds. The first example shows a current dying away to zero,
and is an example of a unidirectional transient. Theoretically it takes an infinite time
to reach absolute zero.
(k) represents a simple a.c. transient. The current gradually dies down to zero as in
the previous case, but this time it is an A.C. that is dying away.

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