Electrical Engineering ⇒ Topic : Vibrating-Reed Frequency meter

Vibrating-Reed Frequency meter

It is the simplest type of frequency meter and uses the principle of mechanical resonance for the measurement of frequency. In this type, a number of thin, flat steel strips (called reeds) of different **natural frequency are excited electromagnetically from the source whose frequency is to be determined. The amplitude of vibration will be greatest for the reed whose natural frequency (or resonant frequency) is equal to the frequency of the source. Thus a row of such reeds with a calibrated scale is a means of measuring frequency.

Construction. Fig. a (i) shows the principal parts of a vibrating-reed frequency meter. It consists of a number of thin steel strips (called reeds) attached to a bar which has flexible supports.An electromagnet with a laminated core is mounted close to the reeds as shown. The coil of this electromagnet, in series with a resistance R, is connected across the supply [like a voltmeter as shown in Fig. a (ii)] whose frequency is to be determined The lengths of the reeds are such that every reed has a natural frequency 0.5 Hz greater than the preceding one. The free end of each reed faces the observer. White painted flags are attached to the free ends of the reeds so that their vibrations may be seen more easily.

                                                    figure (a)

Working. When the electromagnet is connected across the supply whose frequency is to be determined, the bar is attracted twice per cycle as the current reaches its positive and negative peaks. Thus the bar makes two vibrations for each cycle of the alternating current through the electromagnet. Since the reeds are attached to the bar, they also tend to vibrate accordingly. But the reed whose natural frequency is exactly *double the supply frequency will vibrate with maximum amplitude due to mechanical resonance. Thus, by noting which reed vibrates most vigorously, we can read the frequency of the supply from the scale.

                                                                      figure (b)

Fig. b shows the dial of a vibrating-reed frequency meter. Note that the free ends of the reeds (white painted flags) face the reader. Fig.b (i) shows the case when the frequency meter is not connected in the circuit so that no reed vibrates. If the supply frequency to be measured is 50 Hz, then the 50 Hz reed will vibrate most vigorously due to mechanical resonance as shown in Fig. b(ii).

Some vibrations of 49.5 Hz and 50.5 Hz reeds [See Fig.b (ii)] may be noticed but very little vibrations will be seen on 49 Hz and 51 Hz reeds. Fig. b (iii) shows the condition when the supply frequency is mid-way between 49.5 Hz and 50 Hz i.e., the supply frequency is 49.75 Hz.

Note that both 49.5 Hz and 50 Hz reeds vibrate at amplitudes which are equal but considerably less than the maximum amplitude at resonance.