Page 315 - Basic Electrical Engineering

2
2
Q = I X = I Z sin ϕ = I(ZI) sin ϕ
L
or, Q = VI sin ϕ VAR (3.14)

These three powers are shown in the power triangle of Fig. 3.21 (b) from
where it can be seen that

3.2.4 Power in an AC Circuit

Let us now develop a general expression for power in an ac circuit by
considering the instantaneous values of voltage and current. A sinusoidal

voltage v is expressed as

Figure 3.20 Wave forms of voltage, current, and power in an R–L series circuit

v = V sin ωt
m

In a circuit when current is lagging the voltage by an angle ϕ, current i is
expressed as