91Ó°ÊÓ

Figure 31-26 shows a phasor diagram.

We can predict whether the current lead or lag the emf by observing the given phasor diagram. Phasor Diagrams are a graphical way of representing the magnitude and directional relationshipbetween two or more alternating quantities. Then inserting the formula for inductive reactance into the formula for inductive current, we can find whether the length of the current phasor increases or decreases if the angular speed of the phasor is increased when the scale of the diagram is maintained.

Formulae:

The inductive reactance of the inductor,${\mathit{X}}_{\mathbf{L}}\mathbf{=}{\mathit{Ó\neg }}_{\mathbf{d}}\mathit{L}$ . . . (i)

The current equation for an inductor from Ohm’s law, ${\mathit{I}}_{\mathbf{L}}\mathbf{=}\frac{{\mathbf{V}}_{\mathbf{L}}}{{\mathbf{X}}_{\mathbf{L}}}$. . .(ii)

From the given figure we can conclude that the current is lagging behind the emf.

Hence, the phasor diagram of Fig.31-26 corresponds to an alternating emf source connected to an inductor.

The current through the inductor can be given using equation (i) in equation (ii) as follows:

$I_L=\frac{V_L}{{\mathrm{Ó\neg }}_{d}L}$

From this, we can conclude that the length of the current phasor is decreased if the angular speed of the phasors is increased when the scale of the diagram is maintained.