Page 318 - Basic Electrical Engineering

In Fig. 3.21, the power triangle diagram has been developed from the
simple voltage–current relationship in an R–L series circuit. First we have
shown I laggingV by the power factor angle ϕ. The in-phase component of I

is I cos ϕ and quadratuse component is I sinϕ as have been shown in Fig. 3.21
(a).

Multiplying all the sides of the triangle ABC by KV (kilo-volt), we can
draw the power triangle as in Fig. 3.21 (b)

kVA cos ϕ = kW
kVA sin ϕ = kVAR

In the power triangle diagram, if θ is taken as zero, i.e., if the circuit is

resistive, reactive power, Q becomes zero. If the circuit is having pure
inductance or capacitance, ϕ = 90, active power, P becomes zero. Reactive

power will be present whenever there is inductance or capacitance in the
circuit. Inductors and capacitors are energy-storing and energy-releasing

devices in the form of magnetic and electric fields, respectively, and are of
importance in the field of electrical engineering.

3.2.5 R—C Series Circuit

Consider a circuit consisting of a pure resistance R and connected in series

with a pure capacitor C across an ac supply of frequency f as shown in Fig.
3.22.

When the circuit draws a current I, then there are two voltage drops.

i. Drop across pure resistance V = I × R
R
ii. Drop across pure capacitance V = I × X C
C

Where and I, V , V are the RMS values
C
R

The phasor diagram for such a circuit can be drawn by taking the current
as a reference phasor represented by OA as shown in Fig. 3.23. The voltage

drop V across the resistance is in phase with current and is represented by
R