What are the causes and disadvantages of Low Power Factor?

Power Factor

The cosine of angle between voltage and current in an AC circuit is called the power factor of the circuit. In other words, the power factor is defined as the ratio of active power to the apparent power in the circuit, i.e.,

$$\mathrm{Power\: factor,cos\:\phi\:=\:\frac{Active\: power\: (in \:Watts)}{Apparent\: power\: (in\: VA)}}$$

Where, $\phi$ is the power factor angle.

Causes of Low Power Factor

From the economic point of view, the low power factor of the load connected to a power system is undesirable. The main causes of low power factor are given as follows −

  • The AC electric motors consist of inductive winding. Therefore, they have low lagging power factor. Generally, these motors work at a power factor which is very small (0.2 to 0.3) at light load and increases to 0.8 to 0.9 at full load.

  • The load on the power system varies time to time. It being high during morning and evening and low at other times. During the period of low load, the supply voltage of the system is increased which increases the magnetisation current. Consequently, the power factor is decreased.

  • Industrial appliances such as electric discharge lamps, arc lamps and industrial heating furnaces operate at very low lagging power factor.

Disadvantages of Low Power Factor

In case of AC circuits, the power factor plays a vital role as the current drawn by the load depends upon the power factor as −

For single-phase supply system −


$$\mathrm{\therefore Load\: Current,\mathit{I_{L}}\:=\:\frac{\mathit{P}}{\mathit{V_{L}}cos\:\phi }\:\:\:\:...\left ( 1 \right )}$$

For three-phase system −


$$\mathrm{\therefore Load\: Current,\mathit{I_{L}}\:=\:\frac{\mathit{P}}{\sqrt{3}\mathit{V_{L}}cos\:\phi }\:\:\:\:...\left ( 2 \right )}$$

Hence, from equations (1) and (2), it can be seen that if the power and voltage are constant, then the load current is inversely proportional to the power factor of the load, i.e. the lower is the power factor, the higher is the load current and vice-versa.

The low power factor of the load results in the following disadvantages −

Large kVA Rating of Equipment

The electrical source equipment such as alternators, transformers, switchgears, etc. are always rated in kVA because the power factor of the load is not known when these equipment are manufactured in factory. Hence, from the definition of power factor, we get,

$$\mathrm{kVA\: rating\:=\:\frac{kW}{cos\phi }}$$

That is, the kVA rating of the equipment is inversely proportional to power factor. Therefore, the lower is the power factor, the higher is the kVA rating of the equipment. Hence, at low power factor, the kVA rating the equipment need to be made more which increases the size and cost of the equipment.

Larger Conductor Size

For transmitting and distributing a constant amount of power at a constant voltage, the conductor will have to carry more current at load power factor. Consequently, it requires the conductor of larger size.

Greater Copper Losses

As at the low power factor, the load current is greater. Therefore, the large load current causes more $\mathit{I}^{\mathrm{2}}\mathit{R}$ losses in all the components of the supply system. This in turn results in poor efficiency of the system.

Reduces the Handling Capacity of the System

The low power factor decreases the handling capacity of all the components of the system. It is because the increased reactive component of the load current prevents the full utilisation of installed capacity.

Poor Voltage Regulation

The increased current at low power factor causes greater voltage drop in alternators, transformers, transmission lines and distribution systems. As a result, a reduced voltage is available at the receiving end, i.e., the voltage regulation of the system is poor which impairing the performance of the utilisation device.