Power factor is the ratio of the real power dissipated in the load to the apparent power of the load. Thus,
P.f = cosθ = P/S (1.4)
Angle θ is the angle between voltage and current. For a pure resistance, therefore, θ =00.
0; for a pure capacitance, θ = -900. For a circuit For a pure inductance, θ = 90
containing resistance and inductance, θ will be somewhere between 000; for a and 90
circuit containing resistance and capacitance, θ will be somewhere between 00 and – 900.
1.2.2 Factors Affecting the Value of Power Factor
- Increase in supply voltage Due to increase in supply voltage, which usually occurs during low loads period, the magnetizing current of inductive reactance increase and power factor of the plant as a whole comes down.
- Improper Maintenance and Repairs The power factor at which motors operates falls due to improper maintenance and repairs of motors.
- Industrial Heating of Furnance Induction furnances operates at a very low lagging power factor due to heating of the furcance.
1.3 METHODS OF POWER FACTOR CORRECTION
- Synchronous Motors These machines draw leading KVAr when they are over excited and, especially, when they are running idle. They are employed for correcting of the power factor in bulk and have the special advantage that the amount of correction can be varied by changing their excitation.
- Static Capacitors They are installed to improve the power factor of a group of a.c motors and are practically loss-free. Since their capacitances are not variable, they tend to over-compensate on light loads, unless arrangements for automatic switching off the capacitor bank are made.
- Phase Advancers The power factor of induction motors is being improved by equipping the machines with a phase advancer, which supply exciting current to the motor circuit. With the arrangement, the phase angle between the supply voltage and current would be decrease, thereby increasing the power factor.