Moving-iron Frequency Meter
Working Principle of Moving-iron Frequency Meter:
The action of Moving-iron Frequency Meter depends on the variation in current drawn by two parallel circuits – one inductive and the other non-inductive—when the frequency changes.
Construction of Moving-iron Frequency Meter:
The construction and internal connections are shown in Figure A. The two coils A and B are so fixed that their magnetic axes are perpendicular to each other. At their centres is pivoted a long and thin soft-iron needle which aligns itself along the resultant magnetic field of the two coils. There is no control device used in the instrument.
It will be noted that the various circuit elements constitute a Wheatstone bridge which becomes balanced at the supply frequency. Coil A has a resistance RA in series with it and a reactance LA in parallel.
Similarly RB is in series with coil B and LB is in parallel. The series inductance L helps to suppress higher harmonics in the current waveform and hence, tends to minimize the waveform errors in the indication of the instrument.
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Working of Moving-iron Frequency Meter
On connecting the instrument across the supply, currents pass through coils A and B and produce opposing torques. When supply frequency is high, currents through coil A is more whereas that through coil B is less due to the increase in the reactance offered by LB.
Hence, magnetic field of coil A is stronger than that of coil B. Consequently, the iron needle lies more nearly to the magnetic axis of coil A than that of B. For low frequencies, coil B draws more current than coil A and, hence, the needle lies more nearly parallel to the magnetic axis of B than to that of coil A. The variations of frequency are followed by the needle as explained above.
The instrument can be designed to cover a broad or narrow range of frequencies determined by the parameters of the circuit
Read article – Units of Resistivity
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