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Phase of Alternating Current
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Phase.
Physical quantity which represents both the instantaneous value and direction of alternating quantity at any instant is called it's phase. It's a dimensionless quantity and it's unit is radian.
If an alternating quantity is expressed as X = X0 sin(wt ± Æ0 ) then the argument of sin(wt + Æ ) is called it's phase. Where w t = instantaneous phase (changes with time) and Æ0= initial phase (constant w.r.t. time)
(1) Phase difference (Phase constant)
The difference between the phases of currents and voltage is called phase difference. If alternating voltage and current are given by V=V0 sin(wt + Æ1) and i=i0 sin(wt + Æ2 ) then phase difference Æ = Æ1 – Æ2 (relative to current) or Æ = Æ2 - Æ1 (relative to voltage)
Note : Phase difference, generally is given relative to current.
The quantity with higher phase is supposed to be leading and the other quantity is taken to be lagging.
(2) Graphical representation
(3) Time difference
If phase difference between alternating current and voltage is Æ then time difference between them is given as
T.D. = (T/2p) x Æ
(4) Phasor and phasor diagram
The study of ac circuits is much simplified if we treat alternating current and alternating voltage as vectors with the angle between the vectors equals to the phase difference between the current and voltage. The current and voltage are more appropriately called phasors. A diagram representing alternating current and alternating voltage (of same frequency) as vectors (phasors) with the phase angle between them is called a phasor diagram.
While drawing phasor diagram for a pure element (e.g. R, L or C) either of the current or voltage can be plotted along X-axis.
But when phasor diagram for a combination of elements is drawn then quantity which remains constant for the combination must be plotted along X-axis so we observe that
(a) In series circuits current has to be plotted along X-axis.
(b) In parallel circuits voltage has to be plotted along X-axis.
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