20th September 2020

Power Factor (PF)

What is power factor

Power factor is defined as ” the ratio of the active power absorbed by the *apparent power absorbed by the load.
Apparent power defined as the product of the voltage and current absorbed by the load.

The value of power factor varies from -1 to +1 and it is a unitless quantity. The “- ” symbol denotes that the load is capacitive and “+” symbol denotes that the load is inductive in nature.

“Power factor is a measure of the degree to which a given load matches that of a pure resistance”

PF = PLoad/ Pr. PLoad is the average power of load and Pr is the average power of a resistive load;

The load can be of three types on the basis of impedance –  resistive load, capacitor load, and an inductive load.

Disadvantages of Low Power Factor

  1. The KVA ratings of Generator and Transformer required is inversely proportional to the power factor. i.e. For same KW rating if the power factor decreases than the required KVA will increase.
  2. Power loss will increase. Power loss proportional to the square of current if the power decreases the current will increase for the same amount of active power(KW).
  3. Large conductor size required. The high current requirement for the same KW rating if the power factor decreases.
  4. Poor Voltage Regulation for Lagging power factor. The Low Lagging power produces large voltage drops which result in Poor voltage regulation.

Causes of Low Power Factor

  1. Induction motors have a lagging power factor, but the power factor decreases as the load driven by the motor decreases. For Example, some induction motors have 0.2-0.3 no-load power factor.
  2. Industrial arc and induction furnaces have a very low power factor.
  3. Electrical welding also has a very low power factor. These loads are severe loads on the basis of the power factor.

A consumer with low power factor draws more reactive power from the grid, this creates the problem for the distribution company since they have to use large conductor size. but generators cannot deliver power at such a low power factor. So the distribution company sometimes uses power factor correction devices to improve power factors.

An International norm was made according to which every customer have to maintain their power factor minimum of 0.85 pf lagging. If consumer utilizes on pf lower 0.85 then a penalty is imposed. So the power factor correction is required.

Relevant articles: Introduction to Power Factor Improvement, Power factor improvement using a power capacitor.