FERRODYNAMIC TYPE FREQUENCY METER construction Ferrodynamic type frequency meter consist of a fixed coil which is connected across the supply whose frequency is to be measured. This coil is called magnetising coil. The magnetising coil is mounted on a laminated iron core. The iron core has a cross-section which varies gradually over the length, being maximum near the end where the magnetising coil is mounted and minimum at the other end. A moving coil is pivoted over the iron core. A pointer is attached to the moving coil. The terminals of the moving coil are connected to a suitable capacitor C.There is no provision for the controlling force.

WORKING:
The operation of the instrument can be understood from the three phasor diagram. The magnetising coil carries a current I and this current produces a flux Φ. If we neglect the resistance of the coil and the iron losses in the core, flux Φ is in phase with current I. Flux Φ, being alternating in nature, induces an emf E in the moving coil. This emf lags behind the flux by 90°. The emf induced circulates a current Im in the moving coil. The phase of this current Im depends upon the inductance L of the moving coil and the capacitance C.

In fig. (a), the circuit of the moving coil is assumed to be inductive and therefore current Im lags behind the emf E by an angle α. The torque acting on the moving coil is thus: Td α Im cos(90°+α)
In fig (b), the moving coil circuit is assumed to be largely capacitive and therefore current Im leads the emf E by an angle β and therefore the deflecting torque is: Td α Im cos(90°-β)
This torque is obviously opposite to what was in the case represented in (a).
In fig (c) the inductive reactance is supposed to be equal to the capacitive reactance and, therefore, the circuit is under resonance conditions. Thus the moving coil circuit is purely resistive and so current Im is in phase with emf E. This is because inductive reactance XL = 2∏FL equals the capacitive reactance XC = 1/2∏fc, and, therefore, the net reactance of the circuit is zero.
The deflecting torque is, therefore, Td α imcos90° =0.
Hence the deflecting torque on the moving coil is zero when the inductive reactance equals the capacitive reactance.
Coming to actual operation of the instrument, for affixed frequency the capacitive reactance is constant but the inductive reactance of moving coil is not constant. The value of capacitance C is so chosen that the moving coil occupies a convenient mean position on the iron core when the frequency is at its normal value. Fig shows the position of the moving coil for normal frequency. This means at this position, the value of inductance is such that for normal frequency the inductive reactance is equal to the capacitive reactance. Now suppose that the frequency increases above the normal value. Under this condition the inductive reactance becomes larger then the capacitive reactance. This is because XL α f and capacitive reactance Xc α 1/f. thus the circuit becomes largely inductive and, therefore, there is a torque produced. This torque tries to pull the moving coil to an equilibrium position, i.e., a position where the inductive reactance should equal the capacitive reactance. A decreased inductive reactance is obtained if the moving coil moves away from the magnetising coil. The coil will come under the rest under the conditions of resonance. If the frequency decreases below the normal, value of capacitive reactance becomes more than the inductive reactance and hence the torque is produced. And therefore moving coil deflects towards the magnetising coil.

ADVANTAGE:
An advantage of this instrument is that great sensitivity is achieved with its use as the inductance of moving coil changes gradually with variation of its position on the iron core.
Reference :-
A.K SAWHNEY

SUBMITTED BY:
CHITRA TIBREWAL
(101113006)
KUMAR AVANISH
(101113007)