What is Power Factor?
Power factor is the ratio of active power P in volt-ampere to Apparent power Q in volt-ampere. or Power factor is the cosine of the angle between the applied voltage and current flowing through the a.c circuit.
Power factor = Active Power P / Apparent power Q = P/Q
Power factor = VI Cos Ï• / VI = Cos Ï•
The power factor is equal to the cosine of the phase difference between the applied voltage V and current I flowing through the circuit.
Ideally, the power factor should be 1 and practically it should be as high as possible.
Depending upon the polarity of the power factor, the power factor can be classified as lagging power factor and leading power factor.
The power factor can be leading or lagging or unity. This depends on whether the current I leads, lags or is in phase with applied voltage V.
In inductive circuits, current lags behind the applied voltage hence Ï• is negative and the power factor is lagging.
In the capacitive circuit, the Current lead the applied voltage hence Ï• is positive and the power factor is leading.
In the resistive circuit, Ï• = 0 hence power is unity.
The nature of the power factor is dependent on the type of circuit as shown in the below table
| Type of circuit | Value of Ï• | Nature of Power Factor |
|---|---|---|
| Purely resistive | Ï• = 0 | Cos Ï• = 1 Power factor unity |
| Purely inductive | Ï• = -90 | Cos Ï• = 0 Power factor should be zero lagging due to negative sign of Ï• |
| Purely capacitive | Ï• = +90 | Cos Ï• = 0 Power factor should be zero leading due to positive sign of Ï• |
| L-R series circuit | 0≤Ï•≤-90 | 0≤PF≤1 PF is lagging due to negative sign of Ï• |
Causes of Low Power Factor
The low power factor indicates that a very small percentage of total power is being actually utilized. The remaining power simply travels to and fro.
So if the power factor is low, then a large power is required to be generated to delivered the required to load. The low power factor is the result of large inductive loads such as motors. The current drawing by large machines lags behind the supply voltage by a large angle. So the power factor is low.
The reason for the low power factor is connected inductive load. In an inductive circuit current lags behind the supplied voltage. So power factor in the inductive circuit is lagging.
The inductive load in the power system is a three-phase induction motor which operates at 0.8 lagging power factor, welding machine, lamps and transformer are operates at lagging power factor. The induction motor has a power factor that is very small on a low load condition from 0.2 to 0.3 and increases to 0.8 to 0.9 at full load conditions.
The induction motor and arc lamps, discharge lamps and industrial heating furnaces have a low lagging power factor.
The load in the power system is varying. The load in the morning is high and low at other times. During low load conditions, supply voltage increased due to this magnetisation current increased and power factor decreased.
Disadvantages of low power factor
1. Large KVA rating of the equipment
The electrical equipment like transformer, alternator and switchgear are rated in kilo volt-ampere.
KVA = Kw / Cos Ï•
Now it is clear from the above equation that the Kva rating of electrical equipment is inversely proportional to the power factor. If the power factor is low then the Kva rating of electrical equipment increases. So the low power factor increased the Kva rating of electrical equipment. making the equipment larger and more expensive.
2. Greater conductor size
For transmitting a fixed amount of power at a fixed voltage, the conductor carries more current at a low power factor. SO at a low power factor, current ratting increased so conductor size increased and cost making the conductor also increased.
3. Larger copper losses
At a low power factor, the conductor carries more current so I2R losses increased in all the equipment of the power system. This results in poor efficiency in the power system
4. Poor voltage regulation
The current increased at low power factor which causes a voltage drop in transformer, alternator, transmission and in distribution.
Due to voltage drop, receiving end voltage is decreased which give poor voltage regulation and extra equipment like voltage regulators are required for increasing the receiving voltage.
5. Reduced handling capacity of the system
The low power factor reduced the handling capacity of all the equipment of the power system. The reactive component of the current in the power system prevents the full utilization of the installed capacity of the equipment.
From the above discussion, we note below points
1. At low power factor, the high current required by the power system equipment which is increased the economic cost of equipment.
2. At the low power factor, the KVA rating of the power system equipment is increased.
3. At low power factor, the current is high and copper losses increase which gives poor efficiency.
4. At the low power factor, the larger current produced more voltage drop in the power system which gives the poor voltage regulation.
5. At the low power factor , capital and running costs are increased.