Electrical Engineering ⇒ Topic : Electrodynamic Frequency Meter

Daniel
 
Electrodynamic Frequency Meter This is also known as a movingcoil 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 G_{1 }and G_{2} are the displacement constants of the two coils and I_{1} and I_{2} are the two currents, then their respective torques are T_{1}= G_{1}I_{1} cos θ and T_{2} = G_{2}I_{2} 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 (= G_{1}I_{1}_{ }cos θ) decreases while the torque T_{2} (= G_{2}I_{2} 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 I_{1}/I_{2} is made proportional to θ instead of tan θ.
i.e., deflection is proportional to the ratio of the two currents I_{1} and I_{2} 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 I_{1} flowing through coil X is the rectified current I_{c} drawn by the capacitor C. Similarly, direct current I_{2} 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 ∝ I_{1} / I_{2} _{} _{ 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. _{}  
 
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