c. The rotating coil or armature is represented in cross-section in VI, figure 2-12.

The curve shown with the sectional view is a graphic representation of the AC

produced. One complete revolution of the coil produces one cycle AC, mathematically

termed a sine curve or sine wave. This curve demonstrates the instantaneous values of

voltage or current as they vary with time, and is the normal waveform for AC. The curve

between two successive corresponding points on the wave (A to B or C to D, VI, figure

2-12) represents one cycle of AC.

d. The AC most commonly used in the United States is 60-cycle (60 complete

cycles occur per second). The number of cycles an AC completes per second is called

its frequency. Each cycle consists of two impulses (alternations or pulsations) or

changes in current direction; in 60-cycle AC, there are 120 impulses per second.

e. Since the current and voltage change from zero to maximum in each

alternation, the measurements of an AC are not the same as those of a DC. However,

AC is as effective in producing heat as a proportionately large DC. Thus, a simple

relationship can be used to relate their effectiveness, providing a useful means of

converting one value to the other. The values of AC are assumed to be effective

values, unless otherwise specified. With pure sine wave AC, relationships between

maximum and effective values of the current and maximum and effective voltage are:

maximum current = 1.414 x effective current

(maximum voltage)

(effective voltage)

or

effective current = 0.707 x maximum current

(effective voltage)

(maximum voltage)

The fundamental difference between an AC and a DC generator is the method in

which the current is collected. The distinguishing structural characteristic of the DC

generator is the commutator. The terminals of the armature are not connected to slip

rings, but to the segments of a commutator. The commutator changes alternating

current into pulsating direct current before the current enters the external circuit.

Although the voltage and current are at zero value once during each pulsation, the

current flow is still only in one direction. The type of current produced by a simple DC

complicated DC generator does not produce a truly uniform direct current

(figure 2-13B).

MD0950

2-20