Application Notes

EAP-030030

How can the electric power of an electric circuit with resistance and inductance be calculated?

The electric power is calculated by:

P =VI cos Ø

Where,

P = electric power in the unit of Watt

V = effective value of voltage (V)

I = effective value of electric current (A)

Ø = phase difference (degree or radian)

When voltage V and current I are both sinusoidal functions of time, a plot of both will show a displacement between them. This displacement is phase difference or phase angle Ø. By agreement, the phase difference is described as what current does with respect to the voltage, lag or lead. Cos Ø is called a power factor that indicates what fraction of [(Voltage V) x (Current I)] becomes the useful power.

Most electric circuits have resistance and inductance. In such electric circuit, the current lags the voltage by the phase difference. Then, the graph for the instantaneous value of electric power is plotted as shown below. The effective electric power consumption in the circuit is the deference of the positive and the negative instantaneous electric power.