Ans. A change of phase of an alternating pressure or current.
Figs. 1,226 and 1,227.—Diagram and sine curves illustrating synchronism. If two alternators, with coils in parallel planes, be made to rotate at the same speed by connecting them with chain drive or equivalent means, they will then be "in synchronism" that is, the alternating pressure or current in one will vary in step with that in the other. In other words, the cycles of one take place with the same frequency and at the same time as the cycles of the other as indicated by the curves, fig. 1,226. It should be noted that the maximum values are not necessarily the same but the maximum and zero values must occur at the same time in both machines, and the maximum value must be of the same sign. If the waves be distorted the maximum values may not occur simultaneously. See fig. 1,348.
Synchronism.—This term may be defined as: the simultaneous occurrence of any two events. Thus two alternating currents or pressures are said to be "in synchronism" when they have the same frequency and are in phase.
Ques. What does the expression "in phase" mean?
Ans. Two alternating quantities are said to be in phase, when there is no phase difference between; that is when the angle of phase difference equals zero.
Thus the current is said to be in phase with the pressure when it neither lags nor leads, as in fig. 1,228.
A rotating cylinder, or the movement of an index or trailing arm is brought into synchronism with another rotating cylinder or another index or trailing arm, not only when the two are moving with exactly the same speed, but when in addition they are simultaneously moving over similar portions of their respective paths.
