The power factor of a circuit is defined in one of the following ways:
(i) Power factor = cos ϕ = cosine of the angle of lead or lag
(ii) Power factor = R/Z = Impedance / Resistance
(iii) Power factor = VI cos φ / VI
= True power / Apparent power
Some examples for power factors:
(i) Power factor = cos 0° = 1 for a pure resistive circuit because the phase angle ϕ between voltage and current is zero.
(ii) Power factor = cos(±π /2 )= 0 for a purely inductive or capacitive circuit because the phase angle ϕ between voltage and current is ±π /2 .
(iii) Power factor lies between 0 and 1 for a circuit having R, L and C in varying proportions.