Figs. 1,230 to 1,233.—Curves showing some phase relations between current and pressure. Fig. 1,230, synchronism of current and pressure, expressed by the term "in phase," meaning simultaneous zero values, and simultaneous maximum values of the same sign; fig. 1,231, in quadrature, current leading 90°; fig. 1,232 in quadrature, current lagging 90°; fig. 1,233, in opposition, meaning that the phase different between current and pressure is 180°.
Maximum Volts and Amperes.—In the operation of an alternator, the pressure and strength of the current are continually rising, falling and reversing. During each cycle there are two points at which the pressure or current reaches its greatest value, being known as the maximum value. This maximum value is not used to any great extent, but it shows the maximum to which the pressure rises, and hence, the greatest strain to which the insulation of the alternator is subjected.
Fig. 1,234.—Elementary alternator developing one average volt. If the loop make one revolution per second, and the maximum number of lines of force embraced by the loop in the position shown (the zero position) be denoted by N, then each limb will cut 2N lines per second, because it cuts every line during the right sweep and again during the left sweep. Hence each limb develops an average pressure of 2N units (C.G.S. units), and as both limbs are connected in series, the total pressure is 4N units per revolution. Now, if the loop make f revolutions per second instead of only one, then f times as many lines will be cut per second, and the average pressure will be 4N f units. Since the C.G.S. unit of pressure is so extremely small, a much greater practical unit called the volt is used, which is equal to 100,000,000, or 108 C.G.S. units is employed. Hence average voltage = 4Nf ÷ 108. The value of N in actual machines is very high, being several million lines of force. The illustration shows one set of conditions necessary to generate one average volt. The maximum pressure developed is 1 ÷ .637 = 1.57 volts; virtual pressure = 1.57 × .707 = 1.11 volts.
Average Volts and Amperes.—Since the sine curve is used to represent the alternating current, the average value may be defined as: the average of all the ordinates of the curve for one-half of a cycle.
Ques. Of what use is the average value?
Ans. It is used in some calculations but, like the maximum value, not very often. The relation between the average and virtual value is of importance as it gives the form factor.
Virtual Volts and Amperes.—The virtual[1] value of an alternating pressure or current is equivalent to that of a direct pressure or current which would produce the same effect; those effects of the pressure and current are taken which are not affected by rapid changes in direction and strength,—in the case of pressure, the reading of an electrostatic voltmeter, and in the case of current, the heating effect.