Mechanical resonance type frequency meter is principally an electro-mechanical device. Also, we call this instrument the vibrating reed type frequency meter.
Conceptually, the principal of this frequency meter is quite simple. The instrument consists of a number of metal reeds with slightly different natural frequencies fitted side by side. Also, the instrument consists of an electromagnet placed nearer to these reeds. After that, we connect the electromagnet across the supply main. Consequently, there will be an alternating current flowing through the coil of the electromagnet. Therefore, the electromagnet produces an alternating magnetic flux with the same frequency of supply current. The natural frequency of any of the reeds matches to the double of the frequency of the alternating flux. Therefore the resonance occurs at that particular reed. As a result, this particular reed will vibrate with more amplitude than the others. We can see this vibration of that particular reed with our naked eyes.
Then we can asses that the frequency of the electrical signal coming from the supply main is the same as the natural frequency that vibrating reed.
Construction of Mechanical Resonance Frequency Meter
As we have already told, that mechanical resonance frequency meter (vibrating reed type) consists of a number of reeds. These reeds are of thin steel strips. The front portion of each reed bends with a 90° angle to make better visibility of the mechanical vibration. We fit them in a row with screws at the back end.
The dimension and weight of all the reeds are not the same. Instead, they differ slightly in each read from others. This is because for differing the natural frequency of one reed from others.
Practically this difference of natural frequency from one reed to its adjacent reed is either 1 or 0.5Hz.
Suppose we have designed a mechanical resonance frequency meter to measuring a frequency from 48 Hz to 51 Hz. If the difference of natural frequency of any reed from its adjacent reed is 1Hz, then there will be a total of 9 reeds in the instrument fitted in a row side by side. This is because for the above-mentioned frequency range there will be reeds of natural frequencies from 96 Hz (48X2) to 102 Hz (51X2)
The instrument also consists of a strong electromagnet with laminated core placed very nearby to the row of the reeds. The coil of the electromagnet has a resistance in series to limit the current in the coil.
Working of Mechanical Resonance Frequency Meter
When we connect the instrument with the supply main, the electromagnet sets up an alternating flux surround it. Due to this alternating flux, the reeds experience an alternating force for every half cycle of the flux wave. This is because when the flux will have a positive half cycle, the steel reeds will get attracted because of the opposite magnetic polarity of the electromagnet induced on the reeds. Again during the negative half cycle of the flux wave, the magnetic polarity of the electromagnet is reversed. Then also there will be opposite magnetic polarity induced in the reed in respect of that of the electromagnet. Hence again the reds get attracted by the electromagnet. So, for both negative and positive half cycles, the electromagnet attracts the steel reeds.
But there will be a particular reed whose natural frequency is double of the supply frequency. Therefore mechanical resonance occurs on that reed. Therefore the reed will vibrate most with the highest amplitude. As a result, the vibration of this reed will be easily visible. Hence, the supply frequency will be half of the natural frequency of that reed.
Although due to the alternating flux of the electromagnet all other reed will vibrate. But their vibration is so slight it cannot be visible easily.
In some occasions there may be two adjacent reeds vibrate strongly with equal amplitude. In that case, the supply frequency will be half of the average of the natural frequencies of these two adjacent reeds.
Advantages of Vibrating Reed Type Frequency Meter
- The indication of the frequency of an electrical signal does not depend on the pattern of the waveform of the signal.
- The indication also does not depend on the magnitude of the signal waveform. Only it requires a sufficiently large amplitude for visible reed vibration.
Disadvantage of Mechanical Resonance Frequency Meter
- The instrument cannot indicate properly a frequency which is in between 0.5 precision.