Electrical Engineering ⇒ Topic : Electrodynamic Frequency Meter
Electrodynamic Frequency Meter
This is also known as a moving-coil frequency meter and is a ratiometer type of instrument.
Principle. The working principle of a ratiometer type frequency meter is shown in Fig. (a) It consists of two coils X and Y rigidly fixed together with their planes at right angles to each other and mounted on the same spindle. The coils are placed in the field of a permanent magnet. There is no mechanical control torque and the current leads to the two coils are fine ligaments designed to have a negligible effect on the position of the coils.
If G1 and G2 are the displacement constants of the two coils and I1 and I2 are the two currents, then their respective torques are T1= G1I1 cos θ and T2 = G2I2 sin θ. The currents in the two coils are in such directions See Fig. (a) that the two torques oppose each other. It is clear that if angular deflection e increases, torque T (= G1I1 cos θ) decreases while the torque T2 (= G2I2 sin θ) increases.However, an equilibrium position is possible for some angle
By modifying the shape of the pole faces and the angle between the coils, the ratio I1/I2 is made proportional to θ instead of tan θ.
i.e., deflection is proportional to the ratio of the two currents I1 and I2 and hence the name ratiometer type instrument.
Construction. Fig. (b) shows the various parts of an electrodynamic frequency meter. It consists of two coils X and Y rigidly fixed at right angles to each other and placed in the magnetic field of a permanent magnet. The two coils are connected through respective bridge rectifiers across the supply whose frequency is to be measured. The direct current I1 flowing through coil X is the rectified current Ic drawn by the capacitor C. Similarly, direct current I2 flowing through coil Y is the rectified current IR drawn by the resistor R. A pointer is attached to the coils which moves over a calibrated scale.
Working. The meter is connected across the supply whose frequency f is to be measured. The direct currents I1 and I2 flowing through coils X and Y respectively produce two torques which oppose each other. The pointer will come to rest at angle e for which the two torques are equal.
In the equilibrium position, θ ∝ I1 / I2
Assuming sinusoidal waveform, the direct currents /1 and 12 will be proportional to the r.m.s. values of IC an IR respectively.
i. e., deflection of the pointer is directly proportional to the supply frequency f Clearly, such metershave linear scale.
An inspection of exp. (i) reveals that deflection is independent of supply voltage. Therefore, such instruments can be used over a fairly wide range of voltages. However, if the voltage becomestoo low, the distortion introduced by the rectifiers prevents an accurate indication of frequency.
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