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

                                                              figure (a)

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 I_c drawn by the capacitor C. Similarly, direct current I₂ flowing through coil Y is the rectified current I_R 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

                                                                                  _{FIGURE (B)}

Assuming sinusoidal waveform, the direct currents /1 and 12 will be proportional to the r.m.s. values of I_C an I_R 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.