Electrical Engineering ⇒ Topic : Range of vibrating Reed frequency meter

Range. The usual range of such frequency meters is 6 Hz say from 47 Hz to 53 Hz. They maybe used in electric power system where the maximum variation of frequency is only a few cycles. The frequency range of this meter may be doubled by polarising the electromagnet i.e., by using a d.c. winding on the electromagnet in addition to the a.c. winding. The magnitude of direct current through the d.c. winding is so adjusted that steady flux produced by it is equal in magnitude to the alternating flux of the a.c. winding.

• If only a.c. winding is excited, the alternating flux acting alone will cause two vibrations ofeach reed for one cycle of supply as explained above. The reed whose natural frequency is twice that of the alternating current in the electromagnet will vibrate most vigorously due to mechanical resonance.
• If both a.c. and d.c. windings are excited, the fields (fluxes being equal in magnitude) will cancel during one half-cycle and reinforce during the opposite half-cycle. In other words, the resultant flux will be zero in one half-cycle and double in the other half-cycle. The result is that each reed will receive one impulse for each cycle of the supply. Consequently, the reed whose natural frequency is equal to that of the alternating current in the electromagnet will respond the most. It is clear that range of the frequency meter will be doubled with polarisation.

Let us illustrate polarisation of the electromagnet with a numerical example. Suppose the frequency to be measured has a value of 50 Hz. If the electromagnet is unpolarised (i. e . , only a.c. winding is excited), then the reed whose natural frequency is 100 Hz will respond the most. On the other hand, if the electromagnet is polarised (i. e . , both a.c. and d.c. windings are excited), then the reed whose natural frequency is 50 Hz will respond the mos