The term “power factor” (PF) is an expression of the relationship between the peak voltage and the peak current. Stated differently, the power factor is an expression of how far out of phase the voltage and current are to each other.
What is power factor?
Figure 1 shows both voltage and current peaks occurring at the same time in a circuit that has a PF of one.
When the voltage and current values peak at the same time, the power factor is one. When they do not, the power factor is less than one. Many facilities have a lagging power factor of around 0.87.
When the voltage changes, the current flowing in the system also changes, and the electrical system reacts to this change by creating a kind of resistance called reactance (X).
Figure 1–2 shows the voltage and current peaks out of phase with each other. The voltage lags the current peak. The two types of reactance are inductive reactance (XL) and capacitive reactance (Xc). Their effect on the power factor of a circuit is opposite to one another.
Inductive reactance results in a lagging power factor. Most commercial and industrial facilities have a lagging power factor. Inductive reactance moves current flow after the flow of voltage, and capacitance moves the flow of current ahead of the flow of the voltage.
When a circuit has only resistor-type loads, such as an electrical space heater without a fan motor, the power factor of the circuit is one.
When the circuit has only inductor-type loads, such as transformers and motors, the current flow peaks after the voltage peaks. That is, the system has a lagging power factor.
When the circuit has only capacitors, the current flow moves ahead of the flow of the voltage, resulting in the system having a leading power factor.
Figure 3 shows the current peak lagging behind the voltage peak in a capacitive circuit.
The amount of inductive and capacitive reactance varies from system to system. Typically a facility will have a mixture of resistance-, capacitance-, or inductive-type loads referred to as impedance.
While all electrical systems (like all the roads in a town) and the smaller parts, called feeder and branch circuits (side streets), have some amount of capacitance, it is primarily the amount of inductive reactance in the circuit that determines the system’s power factor.
It is possible to mix the correct amount of capacitors with the correct amount of inductors so that the circuits’ power factor is one.
Active | Reactive | Apparent Power
When the facility’s load has a power factor of less than one, the electrical utility must generate, transmit, and distribute more power than is used by the facility. Facilities pull or demand two types of power from the supply, but only one does useful work.
Total power is composed of true power (that does useful work) and reactive, or apparent, power (that does not do useful work—produces only heat).
What is active power?
The power which is consumed in the circuit is called active power or true power. It is denoted by P. The watt-meters indicate the active power of the circuit. The current in phase with the voltage produces true or active power.
Hence, True power, P = voltage x current in phase with the voltage
P = VI cos φ
The active power or true power of the circuit is expressed in watts or kilowatts. The true power produces heat in heaters, torque in motors, light in lamps, etc. Once this power is used in the circuit, it cannot be recovered.
What is reactive power?
The reactive component of current (i.e. I sin φ) when multiplied with circuit voltage results in reactive power. It is denoted by Q.
Hence, Reactive power, Q = VI sin φ
It is expressed in VAR (reactive volt-amperes) or KVAR (kilovolt-ampere reactive). The reactive power does not do any useful work in the circuit. It is the power that is supplied by the source during the first quarter cycle and returns to the source during the next quarter cycle. However, it determines the power factor of the circuit.
The result of nonproductive reactive power is that current-carrying components (conductors and switchgear) must be sized larger to carry both true and reactive power.
What is apparent power?
The product of RMS values of current and voltage is called the apparent power and is measured in VA (volt-amperes) or KVA (kilo-volt ampere).
Hence, Apparent power, S = VI
It is so called because it appears that the product of current and voltage is power. But in AC circuits, except in purely resistive circuits, there is usually a phase difference between current and voltage so VI does not give true power or active power.
What are disadvantages of low power factor?
In AC circuits power factor plays an important role in the power system. Since the power of an AC circuit is given by the relation:
P = VIcos φ
or I = P ÷ (V cos φ)
It is clear from the above relation, that for a fixed power at a constant voltage, the current drawn by the circuit increases with the decrease in power factor. Following are the main disadvantages of low power factor in ac circuit:
- At low power factors, conductors have to carry more current for the same power, therefore, they require a larger area of cross-section.
- At low power factors, conductors have to carry more current for the same power which increases copper losses (I2R) and results in the poor efficiency of the system.
- At low power factors, voltage drop (IR) increases, which results in poor regulation of the system.
- The kVA rating of electrical equipment and machines connected to power system such as transformers switch gears, alternators, etc. will be more at low power factors since it is inversely proportional to power factor (kVA = kW/cos φ.)
To improve the power factor (p.f) of an AC circuit a capacitor is connected parallel to the circuit.