Electrical Engineering ⇒ Topic : Single-Phase Electrodynamic Power Factor Meter
Single-Phase Electrodynamic Power Factor Meter
This instrument is based on the dynamometer principle. The mechanism is somewhat similar to that used in dynamometer wattmeter ; the principal differences being (i) there are two coils on the moving element oriented at right angles to each other as shown in Fig. 16.96, (ii) there are no control springs and currents are led to these, colis through fine ligaments which exert no controlling torque
Construction. Fig. (b) shows the essential parts of a single phase electrodynamic power factor meter. Like a dynamometer wattmeter, the fixed coil is split into two equal parts and carries the load current I. Pivoted between the fixed coils are two coils A and B rigidly fixed at angle of 900 apart. These coils move together and carry the pointer which indicates the power factor of the circuit directly on the scale. The coil A is connected through a resistor R across the line so that its current IA is proportional to and in phase with the supply voltage V. The coil B is connected through an inductance L across the line so that its current /B (proportional to supply voltage V) lags the supply voltage by 900. The values of R and L are so adjusted that the two coils carry the same current at normal frequency. Note that there is no controlling torque acting on the moving system ; the current being led into coils A and B by fine ligaments which exert no control
Theory. The currents in each of the moving coils (i.e., coil A and coil B) react with the current in the fixed coils to produce two torques. Connections to the coils are such that the two torques oppose each other. The moving system will come to rest at a position where the two torques are equal. Suppose the instrument is connected in a circuit whose p.f. is cos Φ lagging as shown in Fig. (b). As the currents are flowing through coils A and B as well as through the fixed coils, there will be two torques acting on the moving system
Thus the angular deflection of the moving system in degrees is numerically equal to the power factor angle Φ (i.e., phase angle between supply voltage V and line current I). The scale of the instrument is calibrated in terms of cos Φ to directly indicate the p.f. of the circuit.
Note. The meter will indicate correctly only at one frequency i.e., at the frequency at which the meter is calibrated. At the frequency of calibration, the values of R and L are so adjusted that IA= IB. At any other frequency, the two currents will be ***different. However, this is not a serious disadvantage because in most power distribution systems, the line frequency is held within narrow limits
!! OOPS Login [Click here] is required for more results / answer