Electrical Engineering ⇒ Topic : 3Phase Electrodynamic Power Factor Meter

Daniel
 
3Phase Electrodynamic Power Factor Meter For the measurement of power factor of a balanced 3phase load, a 3phase power factor meter may be used. The general principle of operation is the same as for the singlephase instrument. Construction. The construction of a 3phase electrodynamic power factor meter See Fig.(a) is almost the same as for a single phase instrument. The two fixed coils (i.e., current coils) are connected in series with one phase (phase R in the present case) and carry the line current I. The two moving coils A and B are fixed at 120° to each other and are connected across two different phases through high noninductive resistances. In the figure shown, the coil A is connected across R and Y and coil B across R and B. Since the moving coils A and B are connected to the two phases of the 3phase supply, *splitting device is not required.
figure (a) Theory. Fig. (b) shows the **phasor diagram of the circuit. Here V_{RN}, V_{YN} and V_{BN} are the phase voltages of the supply that are equal in magnitude but 120° apart. The voltage across coil A is the line voltage V_{RY}(= V_{RN}  V_{YN } ......phasor difference) and that across coil B is the line voltage V_{RB }(= V_{RN}  V_{BN }. . . phasor difference). Let Φ = load p.f. angle ; θ = angular deflection from the plane of reference
FIGURE (B) Thus the angular deflection of the pointer is equal to the phase angle Φ of the circuit to which the meter is connected. The scale is graduated in terms of power factor (i.e., cos Φ) instead of Φ.  
 
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