Alternating current electrical equipment is designed to operate within a given frequency range. In some instances the equipment is designed to operate at one particular frequency, as are electric clocks and time switches. For example, electric clocks are commonly designed to operate at 60 cps. If the supply frequency is reduced to 59 CPS, the clock will lose one minute every hour.
Transformers and ac machinery are designed to operate at a specified frequency. If the supply frequency falls more than 10 percent from the rated value, the equipment may draw excessive current, and dangerous overheating will result. It is, therefore, necessary to control the frequency of electric power systems. Frequency meters are employed to indicate the frequency so that corrective measures can be taken if the frequency varies beyond the prescribed limits.
Frequency meters are designed so that they will not be affected by changes in voltage. Because ac systems are designed to operate normally at one particular frequency, the range of the frequency meter may be restricted to a few cycles on either side of the normal frequency. There are several types of frequency meters, including the vibrating reed type, the fixed coil and moving coil type, the fixed coil and moving disk type, and the resonant circuit type. Of these types, the vibrating reed frequency meter is used most often in aircraft systems, and is discussed in some detail.
Vibrating Reed Frequency Meter
The vibrating reed type of frequency meter is one of the simplest devices for indicating the frequency of an ac source. A simplified diagram of one type of vibrating reed frequency meter is shown in figure 8-261.
The current whose frequency is to be measured flows through the coil and exerts maximum attraction on the soft iron armature twice during each cycle (A of figure 8-261). 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 cps are mounted on the bar (B of figure 8-261). The reed having a frequency of 110 cps is marked "55" cycles; the one having a frequency of 130 cps is marked "65" cps; the one having a frequency of 120 cps is marked "60" cps, and so forth. In some instruments the reeds are the same lengths, but are weighted by different amounts at the top so that they will have different natural rates of vibration.
When the coil is energized with a current having a frequency between 55 and 65 cps, all the reeds are vibrated slightly; but the reed having a natural frequency closest to that of the energizing current (whose frequency is to be measured) vibrates through a larger amplitude. The frequency is read from the scale value opposite the reed having the greatest amplitude of vibration.
An end view of the reeds is shown in the indicator dial (C of figure
8-261). If the energizing current has a frequency of 60 cps, the reed
marked "60" cps will vibrate the greatest amount, as shown.