Power Factor

InstructorEngr. Muhammad Yaseen

Power FactorIntroduction The cosine of angle made between the voltage and current is called the

power factor. In AC circuits, there is always the phase deference between the voltage and

current, which is calculated in terms of power factor. If the load is inductive the current lags behind the voltage and the power

factor is lagging. If the load is capacitive the current leads the voltage and the power factor is

leading. The value of power factor can never be more than unity. It is usual practice to attach the word “lagging” or “leading” with the

numerical value of the power factor to signify whether the current lags or leads the voltage.

Thus if the circuit has a p.f of 0.5 and current lags the voltage, we generally write p.f. as 0.5 lagging or sometimes expressed as a percentage (50% lagging)

Power Factor

Power FactorCauses of Low Power Factor Ideally the power factor must be close to unity, where as low power

factor is undesirable. Following are the causes of low power factor:1. Most of AC motors (i.e. induction motor) are used to work at low

lagging power factor, which causes the low power factor.2. The load on power station is not constant but is varying; it is

more at peak time (Day time) and reduces at night time. During low load period the supply voltage increases which increases the magnetizing current , this decreases the power factor.

3. The arc lamps and industrial heating furnace operates on low power factor.

Power FactorEffect of Low Power Factor The low power factor causes the following effects:1. Line losses (I2R) will be more.2. Lower the power factor higher will be the rating of electrical

equipment, which makes the equipment more complex and larger in size.

KW = kVA × CosØkVA = KW / CosØ

3. The useful load that can be transmitted is reduced.4. There will be a poor voltage regulation5. It reduces the handling capacity of the plant.

Power FactorMethods of Improving Power Factor: Normally the power factor of the system ranges between 0.8 to 0.9, however

in case of low P.F. there is a need of improving power factor. There are three main methods used to improve the power factor of the system:

i. Static Capacitorii. Phase Advanceriii. Synchronous Condenser

i) Static Capacitor The power factor can be improved by connecting capacitors in parallel with

the equipment operating at lagging power factor. The capacitor (Generally known as static capacitor) draws a leading current

and partly or completely neutralizes the lagging reactive component of load current. This rises the power factor of load.

Power Factor For three-phase loads, the capacitors can be connected in delta or

star as shown in below fig. Static capacitors are invariably used for power factor improvement

in factories.

Advantages:i. They have low losses.ii. They require little maintenance as there are no rotating parts.iii. They can be easily installed as they are light and require no foundation.iv. They can work under ordinary atmospheric conditions.

Disadvantages:v. They have short service life ranging from 8 to 10 years.vi. They are easily damaged if the voltage exceeds the rated value.vii. Once the capacitors are damaged, their repair is uneconomical.

Power Factor

Power Factor

Power Factorii) Phase advancer Phase advancer are used to improve the power factor of induction motor. The low power factor of an induction motor is due to the fact that its

stator winding draws exciting current which lags behind the supply voltage by 90o .

If the exciting ampere turns can be provided from some other a.c. source, then the stator winding will be relieved of exciting current and the power factor of motor can be improved.

This job is accomplished by the phase advancer which is simply an a.c. exciter.

The phase advancer is mounted on the same shaft as the main motor and is connected in the rotor circuit of motor.

The major disadvantage of phase advancers is that they are not economical for motors below 200 H.P.

Power Factoriii) Synchronous Condenser A synchronous motor takes a leading current when over-excited

and, therefore, behaves as a capacitor. An over-excited synchronous motor running on no load is known as

“Synchronous condenser”. When such a machine is connected in parallel with the supply, it

takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved.

Synchronous condensers are generally used at major bulk supply sub-stations for power factor improvement.

Power Factor

Power FactorAdvantages:i. By using the field excitation, the magnitude of current drawn by the

motor can be changed by any amount. This helps in achieving stepless control of power factor.

ii. The motor windings have high thermal stability to short circuit currents.iii. The fault can be removed easily. Disadvantages:iv. There are considerable losses in the motor.v. The maintenance cost is high.vi. It produces noise.vii. Except in sizes above 500 kVA, the cost is greater than that of static

capacitors of the same rating.viii. As synchronous motor has no self-starting torque, therefore, an

auxiliary equipment has to be provided for this purpose.

Power FactorPower Triangle:The analysis of power factor can also made

in terms of power drawn by a.c. circuit.OA = VI cos Ø and represents the active power

in the watts or kW.

AB = VI sin Ø and represents the reactive power in VAR or kVAROB = VI and represents the apparent power in VA or kVA.The following points may be noted from the power triangle:(i) The apparent power in an a.c. circuit has two components viz.,

active and reactive power at right angles to each other.OB2 = OA2 + AB2

(Apparent power)2 = (Active power)2 + (Reactive power)2

(kVA)2 = (kW)2 + (kVAR)2

Power Factor(ii)Power factor, cos Ø = OA / OB

= active power/ apparent power

= kW / kVAThus the power factor of a circuit may also be defined as the ratio of

active power to the apparent power.This is perfectly general definition and can be applied to all cases,

whatever be the waveform

(iii)The lagging reactive power is responsible for the low power factor. It is clear from the power triangle that smaller the reactive power component, the higher is the power factor of the circuit

sin sincos

tan

kWkVAR kVA

kVAR kW

Power Factor

The power factor correction can also be illustrated from power triangle.

Thus referring to figure, the power triangle OAB is for the power factor cos Ø1 , where the power triangle OAC is for the improved power factor cos Ø2

It may be seen that active power (OA) does not change with power factor improvement.

However the lagging kVAR of the load is reduced by the power factor correction equipment , thus improving power factor to cos Ø2 .

Leading kVAR supplied by p.f. correction equipment:

1 2

BC AB ACkVAR kVAR

1 2

1 2

tan tan

tan tan

OA

kW