Electrical Studys

POWER FACTOR DEFINITION

     The cosine of the angle between voltage and current in an ac circuit is known as power factor. If we consider an AC electrical system, power factor is simply defined as the ratio of the real power flowing to the load, to apparent power in the circuit.

       Power factor = P/S = VI Cosθ/VI = Cosθ

Here θ is the phase angle between the current and the voltage.

EXPLANATION

   In an ac circuit there is generally a phase difference between voltage and current. If we consider a pure resistive circuit, current is in phase with circuit voltage i.e, θ = 0, hence p.f (power factor), Cosθ = 1. But in case of purely

inductive or capacitive circuit, current is 90° out of phase with circuit voltage i.e. θ = 90°, hence p.f Cosθ = 0. In inductor circuits current always lags behind the voltage and their power factors are mentioned as lagging p.f . Whereas, in capacitive circuits the p.f is mentioned as leading p.f as in this case current leads the voltage. Circuits containing purely resistive heating elements (filament lamps, cooking stoves, etc.) have a power factor of 1.0. Circuits containing inductive

or capacitive elements (electric motors, solenoid valves, lamp ballasts, etc.)often have a power factor below 1.0. It may be noted that the value or p.f can never be more than one.

IMPORTANCE OF POWER FACTOR

   The powr factor plays an important role in the power system. As we know that power in ac circuit is given by the relation:

   P = VICosθ or, for our convenience we can write it as I = P/VCosθ

  In the power system the voltage is almost constant. Hence for a fixed power at constant voltage, the current drawn by the circuit increases with decrease in p.f . Thus at low p.f ac circuits draw more current from their main supply.

  In practical if p.f is considerably less, then it affects the whole system badly. The cost of generation and transmission increases. Alternatively all components of the system such as generators, conductors, transformers, and switchgear would be increased in size (and cost) to carry the extra current. The power supplying utilities generally charge the customers who doesn't maintain the p.f in the considerable limit.

METHODS OF POWER FACTOR IMPROVEMENT

   A high p.f is generally desirable in a transmission system to reduce transmission losses and improve voltage regulation at the load. It is often desirable to adjust the power factor of a system to near 1.0. To improve the p.f of an a.c ciruit a capacitor is connected across the circuit i.e parallel to the circuit. But in practical following are used to improve p.f of the system-

1 Use of Static Capacitor
2 Use of Synchronous Motors
3 Use of Synchronous Condensers
4 Use of Phase Advancers etc.