Low Power Factor Disadvantages Tutorial

Poor power factor disadvantages:

In AC circuits, power consumed depends on the power factor. Thus the power factor plays an important role in AC circuits.

For single phase supply P = VIcosΦ

I= P / VcosΦ

For three-phase supply

P = √3.V.I.cosΦ

I = P/ √3.VcosΦ

From the above equations it is concluded that load current I is inversely proportional to the power factor for fixed power and voltage. So the lower the power, higher the load current and vice versa.

The larger current due to poor power factor results in the following disadvantages:

(1) Poor power factor means more line loss and low transmission efficiency

  • For a given cross-sectional area of the line conductors, line losses are proportional to 1/cos2Φ.
  • Poor power factor means more line losses and low transmission efficiency.

(2) More Capital Investment

  • Alternatively, if efficiency of transmission is to be kept same, poor power factor will require cross-section of the line conductors which will be proportional to 1/cosΦ.
  • This will increase capital investment in the transmission line.

(3) Large KVA rating of equipment – larger size and expensive

  • All electrical equipments in connection with generation, transmission, distribution of ac power such as alternators, transformers, switch gear, cables etc are rated on KVA basis

KVA = KW / cos Φ

  • The larger the power factor the larger is the KVA rating.
  • So at low power factor, the KVA rating of the equipment has to be made more, making the equipment larger and expensive.

(4) Greater Conductor size required

(5) Large Copper losses

  • At low power factor, the large current makes more I2R losses in all the elements of the supply system.
  • It leads to poor efficiency.

(6) Poor voltage regulation:

  • The large current at low power factor causes greater voltage drops in electrical equipments.
  • The result is reduced voltage at the supply end.
  • In order to keep the receiving end voltage within permissible limits, additional equipments like voltage regulators are required.

(7) Loss of generating capacity:

  • The lagging power factor reduces the handling capacity of all the elements of the system.
  • It is because the reactive component of current prevents the full utilization of installed capacity.

What are the causes of low power factor?

Causes of Poor Power factor:

  1. Most of the AC motors are induction motor type. They have low lagging power factor. During light loads the induction motors will work at a power factor or 0.2 to 0.3 and during full load condition they will work at 0.8 to 0.9.
  2. Arc lamps, electric discharge lamps and industrial heating furnaces operate at low lagging power factor.
  3. The load on the power system is varying. It will be high during morning and evening and will be low at other times. The supply voltage is increased during low load period, which increases the magnetization current. It results in low power factor.

How to avoid low power factor? (or) What are the various methods to avoid low power factor?

  1. As far as possible, over excited synchronous motors have to be used in place of induction motors.
  2. High speed induction motors have better power factor than low-speed induction motors. They are smaller in frame size and economical too. 
  3. The induction motors have maximum power factor when fully loaded. So we should try use induction motors at full load condition.

Read more:

Power Factor Improvement methods
Kelvin’s Law in Power System
Grid System Advantages
How to connect a wattmeter in a circuit?

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