Effects of Low Power Factor

Effects of low power factor

1. Load Current: Low power factor tends to increasing the load current.

• Power in an AC circuit can be given as: P = VI cosɸ
• Therefore, cosɸ = P / VI
  • I ∝ 1 / cosɸ

Similar relationship can be derived for 3 phase circuit too. We can see that current is inversely proportional to pf.

2. Losses: As stated above, for low pf, the current drawn is high. Hence copper losses (I2R losses) will also be high. This decreases the efficiency of the equipment.

3. Overheating of the equipment: I2R losses produce heat (Joule’s law). Hence, the temperature rise will be relatively more for low PF which will further increase the stress on the insulation.

4. Size of conductor: Low power factor causes higher load current. If the load current increases, the size of the conductor required will also increase. This will further increase the conductor cost.

5. KVA rating of the machine: Machines are not rated in kW while manufacturing because the power factor of supply is unknown. Instead, they are rated in kVA.

According to definition, Cosɸ = Active power (kW) / Apparent power (kVA)

Hence, kVA rating = 1 / cosɸ

Therefore, for low pf, equipment of larger kVA rating is needed. But larger kVA rating means larger size of the equipment’s. If size increases, the cost also increases.

6. Voltage Regulation: It is defined as the difference between sending and receiving end voltage per unit sending end voltage. When power is transferred from one end to another, the voltage drops due to several reasons. This voltage drop should be within permissible limits.

P = VI cosɸ, Therefore I ∝ 1 / V

For low power factor, current will be more and hence voltage drop will be increased. Hence, the voltage regulation at low power factor is poor.

7. Active and Reactive power (Power Transfer Capacity): Active and reactive power both are transferred over the line together. Active power is needed for supplying the load. Reactive power is needed to maintain the voltage of the line. But if reactive power is more, then active power transferred is decreased. For low pf, active power is low because, cosɸ = Active power (W) / Apparent power (VA).

This results in uneconomic operation.