2-12
The normal speed range is from 110 to 25,000 rpm. At speeds below 600 rpm, "flicker" becomes a
problem because the human eye cannot retain successive images long enough to create the illusion of
continuous motion. The life of the strobotron lamp is approximately 250 hours if used at flashing speeds
of less than 5,000 rpm, or 100 hours if used at higher speeds.
ELECTRICAL OUTPUT FREQUENCY
All alternating voltage sources are generated at a set frequency or range of frequencies. A
FREQUENCY METER provides a means of measuring this frequency. The electrical output frequency of
ac power generators can be measured by a vibrating reed, a tuned circuit, or by a crossed-coil, iron-vane
type meter. The vibrating-reed device is the simplest type of frequency meter. It has the advantage of
being rugged enough to be mounted on generator control panels.
A simplified diagram of a vibrating-reed frequency meter is shown in figure 2-8, views A through D.
In view A, you can see that the current to be measured flows through the coil and exerts maximum
attraction on the soft-iron armature twice during each cycle. The armature is attached to the bar, which is
mounted on a flexible support. Reeds of suitable dimensions to have natural vibration frequencies of 110,
112, 114, and so forth, up to 130 hertz are mounted on the bar (view B). The reed with a frequency of 110
hertz is marked 55 hertz; the one with a frequency of 112 hertz is marked 56 hertz; the one with a
frequency of 120 hertz is marked 60 hertz, and so forth.
Figure 2-8.—Vibrating-reed frequency meter.
