What is Power Factor

What is power factor - Power factor is defined as the ratio of real power to apparent power in the circuit. Power in the circuit are Apparent Power (VA), Real Power (Watt), and Reactive Power (VAR).

In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor.

Figure 1

[Figure 1]

Instantaneous and average power calculated from AC voltage and current with a zero power factor (Φ = 90, Cos Φ = 0). The blue line shows all the power is stored temporarily in the load during the first quarter cycle and returned to the grid during the second quarter cycle, so no real power is consumed.

Figure 2

[Figure 2]

Instantaneous and average power calculated from AC voltage and current with a lagging power factor (Φ = 45, Cos Φ = 0.71). The blue line shows some of the power is returned to the grid during the part of the cycle labelled φ

S = Apparent Power ; P = Real Power ; Q = Reactive Power

Also by the theorem of Pythagoras,

S² = P²+Q²

Another theorem:

S = P + Q

S = Cos Φ . P

S = Sin Φ . Q

Simple Analogy:

MOTOR POWER FACTOR

The power factor for a three-phase electric motor can be expressed as:

PF = P / (V I √3), where,

P =  power applied / real power (W, watt)

V = voltage (V, volts)

I = current (A, ampere)

PF = power factor

For instance, an industrial plant draws 300 A at 380 V and the supply transformer and backup UPS is rated 300 A x 380 V = 114 kVA. If the power factor of the loads is only 0.8. So, 91,2 KW of real power is consumed by system.

LEADING AND LAGGING CURRENT

Leading current is when voltage leads the current in an inductor. Lagging current is when current leads the voltage in a capacitor. Practical loads have resistance, inductance, and capacitance.

1. Resistance - with a purely resistive load current and voltage changes polarity in step and the power factor will be 1. Electrical energy flows in a single direction across the network in each cycle.
2. Inductance loads - transformers, motors and wound coils – consumes reactive power with current waveform lagging the voltage.
3. Capacitance loads - capacitor banks or buried cables – generates reactive power with current phase leading the voltage.

Note:

Just remember ELI the ICE man" or "ELI on ICE"

where,

E = voltage

I = current

L = inductor

C = capacitive

ELI = E is the first letter and I is the last. So, E leads I in the L (inductor).

ICE = I is the first letter and E is the last. So, I leads E in the C (capacitor).