Electrical Engineering ⇒ Topic : 3-Phase Electrodynamic Power Factor Meter

3-Phase Electrodynamic Power Factor Meter

For the measurement of power factor of a balanced 3-phase load, a 3-phase power factor meter may be used. The general principle of operation is the same as for the single-phase instrument.

Construction. The construction of a 3-phase 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 non-inductive 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 3-phase 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.