AC FUNDAMENTALS - PART - 09 - COMPLEX POWER AND POWER FACTOR

COMPLEX POWER 

P = real power in kW

Q = reactive power in kVAR

Complex power S = P + jQ or P – jQ

The apparent power S = √P² + Q² in volt-amperes (VA)

S = P + jQ = VI* means the load is inductive

S = P - jQ = V*I means the load is capacitive

Real power (P) and Reactive power (Q) increases as the square of voltage magnitude.

If frequency increases the real power decreases whereas reactive power increases.

POWER FACTOR

In a.c. circuit, there is generally a phase difference ɸ between voltage and current.

If the circuit is inductive, the current lags behind the voltage and the power factor is referred to as LAGGING.

If the circuit is capacitive, the current lead the voltage and power factor is said to be LEADING.

The ration of active power to the volt-amperes in an a.c. circuit is defined as power factor (p.f).

Power factor = Active power / Apparent power

Power factor = P/S = [VIcosθ] / [VI] = cosθ

Power factor = P/S = R/Z = Vr/V

The term cosθ is called as power factor of the circuit.

The cosine of angle between voltage and current in an a.c. circuit is known as power factor.

The angle ‘θ’ is called as power factor angle.

The maximum value of power factor is one.

Power factor of a purely resistive circuit is one.

Power factor of a purely inductive circuit is zero.

Power factor of a purely capacitive circuit is zero.

Lagging or leading with the numerical value of power factor to signify whether the current lags or leads the voltage.

If the circuit has a p.f. of 0.6 and the current lags the voltage, we write p.f as 0.6 lagging.

Sometimes p.f is expressed as a percentage. Thus 0.6 lagging p.f. may be expressed as 60%.

If power factor value is one, then the real power is equal to the apparent power i.e. P=S. That means the whole apparent power drawn by the circuit is being utilized by it.

If p.f is 0.5 or 50% means that it will utilize the 50% of the apparent power.

Thus the power factor of a circuit is a measure of its effectiveness in utilizing the apparent power drawn by it. The greater the power factor of a circuit, the greater is its ability to utilize the apparent power.