Working of Electrodynamic Power Factor Meter

Working of Electrodynamic Power Factor Meter Points : Working of Electrodynamic Power Factor Meter Working of Electrodynamic Power Factor Meter Consider the case when load power factor is unity i.e. I is in phase with V. Then, I₁ is in phase with I₁, whereas I₂ lags behind by 90°. consequently, a torque will act on C₁, which will set its plane perpendicular to the common magnetic axis of coil F₁ and F₂i.e. corresponding to the pointer position of unit p.f. However, there will be not torque acting on coil C₂.

Now, consider the case when load power factor is zero i.e. I lags behind V by 90° (like current I₂). In that case, I₂ will be in phase with I whereas I₁ will be 90° out of phase. As a result, there will be no torque on C₁, but that acting on C₂ will bring its plane perpendicular to the common magnetic axis of F₁ and F₂. For intermediate values of power factor, the deflection of the pointer corresponds to the load power factor angle Φ or cos Φ, if the instrument has been calibrated to read the power factor directly.

For reliable readings, the instrument has to be calibrated at the frequency of the supply on which its is to be used. At any other frequency (or when harmonics are present), the reactance of L will change so that the magnitude and phase of current through C₂ will be incorrect and that will lead to serious errors in the instrument readings. For use on balanced 3-phase load, the instrument is modified, so as to have C₁ and C₂ at 1200 to each other, instead of 900, as in 1-phase supply.

C₁ and C₂ are connected across two different phases of the supply circuit, the stationary coils F₁ and F₂ being connected in series with the third phase (so that it carries the line current). Since there is no need of phase splitting between the currents of C₁ and C₂, I₁ and I₂ are not determined by the phase splitting.

Circuit and consequently, the instrument is not affected by variations in frequency or waveform.