Text 4. Terms and definitions

In the following paragraphs the terms commonly used to distin­guish between the various properties, or ways measuring the proper­ties of alternating current are described.

Cycle and Period. — The complete series of changes consisting of the growth and decay of the voltage or current in one direction, together with its growth and decay in the reverse direction, is called one cycle. A voltage or current that is reproduced at equal intervals is said to be periodic, and the minimum time interval elapsing be­fore the same instantaneous value recurs is called the periodic time or the period.

Frequency. — The number of complete cycles per second through which a voltage or current passes is called the frequency. It is always expressed in cycles per second, the reciprocal of this number being the periodic time.

Frequencies are now standardized for power purposes in the prin­cipal countries of the world, the standard for Great Britain being 50 cycles per second, while the U.S.A. has adopted 60 cycles per second.

Wave Form. — The shape of the graph of the voltage or current, when plotted against time as a base, is called the wave form or wave shape. The ideal aimed at is that of the sine wave, i.e. the graph has the same shape as the graph of a sine function in mathematics. The sine goes through 360° in a complete cycle, and so the time scale on the wave form of a voltage or current is usually represented in de­grees, 360° being considered the equivalent of the time correspond­ing to 1 cycle, or the periodic time. Since these degrees may not always coincide with the geometrical degrees through which the ro­tating member of a machine has been rotated, they are called electri­cal degrees, in order to distinguish them from geometrical degrees. One cycle is thus regarded as the equivalent of 360% and 180° is considered as a half-cycle, i.e. the duration of the voltage or current

in one direction only.

Notes

cycle - оберт, такт

decay - розпад

to eclapse - минати

to recur - повторюватись

frequency - частота

reciprocal - взаємний

to coincide - збігатися

Comprehension questions:

1. What do we call cycle and period?

2. What is the definition of frequency?

3. Give the characteristics of wave form.

Text 5. Phase and phase difference

During the interval of time necessary for a current to pass through one complete cycle, it passes through many phases. In fact it has a different phase for each different interval of time. Consider the analo­gous case of the phases of the moon. Starting from the time of new moon the moon passes through its various phases, measured by its age from the time of the new moon, up to full moon, and then on­wards through the various stages of the waning moon. Similarly an alternating current goes through various phases, starting from zero, rising up to maximum, and dying down again to zero. In the A.C. case, however, this comprises only half a cycle, for the whole series of values are then repeated in the opposite (negative) direction. Oth­er electrical quantities, such as voltage, power, etc., in addition to current go through various phases from 0° to 360^a, indeed all quanti­ties which vary in a periodic manner do so.

When two voltages or two currents are considered together, how­ever, or when a voltage and current are considered simultaneously,

the frequency being the same, they may not pass through the same phase at the same instant of time. For example, two currents may be such that, although their frequency may be the same, their phase at a particular instant of time may be different. One may pass through its maximum value at the instant when the other has a zero value, or some other value not its maximum value; the two currents, etc., are then said to have a phase difference, this being quoted in de­grees. If one current has its maximum value at the same time that the other is zero, the two currents are said to be in quadrature; they have a phase difference of 90°. Phase difference, because it is con­stant in a circuit where steady conditions obtain, is much more important in a.c. work than the actual phase which varies from instant to instant.

Phase difference is illustrated in Fig. 2, where two currents are shown not in phase. The phase difference is measured by the distance between the points where the two graphs cross the base line in the same direction. This distance is mea­sured in electrical degrees, the scale being obtained by considering the dis­tance corresponding to one complete cy­cle as 360°. The current that is ahead in phase is said to lead the other cur­rent, while this current is said to lag behind the first current. Similarly, when considering two voltages, one is said to lead and the other to lag. Again, a voltage may lead the current that it produces, the current lagging behind the voltage.

Fig. 2. Illustrating Leading Current Lagging Current Angle of Lag

Notes

analogous - аналогічний, подібний

onward - що рухається вперед

similarly - подібно

to wane - зменшувати(ся)

simultaneously - одночасно

to quote - цитувати

circuit - цикл; кругообіг

scale - розмір

to lag - відставати

to lag behind - запізнюватися

Comprehension questions:

1. How can you characterize various phases of alternating current?

2. When may two voltages or two currents not pass through the same phase at the same instant of time?

3. What can you say about phase difference measuring?