Power Factor - Definition, Formula, Disadvantages of Low Power Factor

What is Power Factor?

Definition 1: Power Factor is defined as the cosine angle between the voltage and the current. 

Definition 2: Power Factor is defined as the ratio of the active power to the apparent power.

Definition 3: Power Factor is defined as the ratio of the resistance to the impedance.

 

Disadvantages of Low Power Factor

The power factor plays an important role in A.C. circuits as power consumed in A.C. circuits is depending on the power factor.

    Now, we know that the current in a single-phase circuit is given by  

and the current in a three-phase circuit is given by  

It is clear from the above equation that for a fixed amount of power and voltage, the load current is inversely proportional to the power factor. Hence, for the smaller power factor, the higher is the load current and for the higher load factor, the smaller is the load current. The large current due to low power factor results in the following disadvantages. 

(i)               Greater Conductor Size: 

To transmit and distribute a given amount of power at a constant voltage, the conductor will have to carry more current at a low power factor. Hence, the greater size of the conductor required to carry more current. 

(ii)             Large kVA rating of the equipment: 

In an electrical circuit, the electrical equipment i.e., alternators, transformers, motors, switchgear, etc., are always rated in kVA.

Now, 

Hence, it is clear from the above equation that the kVA rating of the equipment is inversely proportional to the power factor. Thus, for the smaller power factor, the larger the kVA rating and for the larger power factor, the smaller the kVA rating. Therefore, at a low power factor, the kVA rating of the equipment has to be made larger, making the equipment larger and more expensive.  

(i)               Large copper losses:

The large current at low power factor causes increased I²R losses in all the equipment. This results in poor efficiency.

 (iii)         Poor voltage regulation: 

The large current at low power factor causes greater voltage drops in transformers, alternators, motors, transmission lines, and distribution lines. Hence, reduced voltage available to the utilization devices which impacts their performance. For example, due to reduced voltage, the starting torque of the motors is reduced, lightning becomes dimmer, etc. To maintain the required voltage within permissible limits, the voltage regulator is required.