Power Factor

Power Factor; Power Factor Definition; Define Power Factor; Power factor correction/ improvement methods;

Definition – Power factor is defined as the cosine of angle between voltage & current in an a.c. circuit. or

It is the ratio of Active Power (output) to Apparent Power (Input). In other words, it the ratio of power consumed by the load to the total power given to it.

It is denoted by Cos Ø. The value of power factor depends on the types of load.

Power Factor Cos Ø = Output (Active Power) in W/ Input (Apparent Power) in VA

Power Factor (pf) plays an important role in power consumption. Value of power factor depends on the type of load. Broadly there are three types of load resistive, inductive & capacitive load. Value of pf is unity on resistive load, pf is lagging on inductive load & pf is leading on capacitive load

(Note -Power factor happens in AC circuits only not in DC circuits because in DC system voltage & current don’t vary with time.)

EFFECT OF TYPES OF LOAD ON AC CIRCUIT–

A -Resistive load–(Ex – Incandescent Lamps & Heaters) –

When AC voltage is applied to a resistive load, current follows the same cycle of voltage with respect to time(but amplitude may not be same), hence there is no phase difference between voltage & current (Ø = 0 deg).

B -Inductive load –(Ex – Induction motors, Transformers, choke, Induction Heaters, electromechanical relays etc.) –

When AC voltage is applied to an inductive load, it resists the flow of current & it builds up slowly and becomes steady. That is why current lags behind voltage which creates phase difference between voltage & current. The value of phase difference between voltage & current varies between 0 to 90deg depending on the value of inductance.

This can be understood by the diagram given below –

C – Capacitive load –(Ex – Flash of camera, synchronous motors and capacitors) –

When AC voltage is applied to a capacitive load, the current flows to build voltage across capacitor, so current leads voltage with respect to time which creates phase difference between voltage & current. The value of phase difference between voltage & current is between 0 to 90 deg.

EFFECT OF POWER FACTOR ON POWER CONSUMPTION–

Power factor plays as important role in power consumption. There are three categories of power factor – 1) Unity power factor, 2) Lagging power factor & 3) Leading power factor. Let us understand one by one –

Let us say Power, P = VI CosØ. If the value of V&I is constant, power is directly proportional to the pf (Cos Ø).

Power on Unity Power Factor– Where Ø = 0

So, P = VI Cos0 = VI X 1 = VI ………. (1)

Power on Lagging Power Factor– Where Ø ˃ 0 & max upto 90 deg (lagging)

So, P = VI CosØ

Where, value of CosØ varies from 0.01 to 0.99 which depends on phase angle(˃ 0deg to 90 deg) between voltage & current. Hence the value of power will be less than the value of power in equation –(1).

It’s clear that if phase difference between voltage & current (on inductive load) is more, the power factor will be less&power will be less. Now let us understand effect of power factor in electrical system –

P = VI CosØ or I = P / V CosØ or I ά 1/CosØ. Current is inversely proportional to the power factor. It means low power factor will draw high current & vice versa to get same power.
Due to above reason, for a particular load of a building, more current will be drawn by the load.
When more current will be drawn, higher capacity of equipment (like transformer, switchgears etc) will be used.
High current will cause high I’2R losses. So higher size of conductors will be used.
High current means higher capacity generating plants will be installed.
High current means higher voltage drop in the line which causes low voltage regulation.
It causes less efficiency of generating plants due to higher loses which happens due to high current,
Government keeps a watch on power factor of premises due to above reasons & has guidelines on low power factor if it goes below a certain value.

Let us understand the effect of power factor on power demand through the table given below – (let us say power demand is 2300 W in a building) –

Eample-1 –

	Power Utilized	˭	Voltage	X	Current	X	Power Factor	Wastage of power due to low PF(in watts)
	P (in Watts)		V		I		PF	P – P1,P2,P3,..
P	2300		230		10		1	0
P1	2070		230		10		0.9	230
P2	1840		230		10		0.8	460
P3	1610		230		10		0.7	690
P4	1380		230		10		0.6	920
P5	1150		230		10		0.5	1150

Note – In other words we can say that due to low power factor, full power is not utilized & power goes waste. This wastage of power is called reactive power. The power which is actually utilized is called active power. The power which is available (includes both active & reactive power) to the building is called apparent power.

Eample-2 –Let us understand by this example that how more current is drawnt o get same power in the system when power factor is low.

Formula of power for single phase supply – P = V I CosØ,

I = P / V CosØ or In other words we can say that if P & V are constant then current I is inversely proportional to power factor (pf). The value of pf depends on the angle Ø which varies from 0 to 90 depending on the inductive load. Cos0 has maximum value, 1 while Cos90 has zero value (0).

Therefore, current, I varies with the value of pf’s value inversely i.e. high pf value causes low current & low pf value causes high current.

The table given below will help to understand the relation between current & pf –

	Power Utilized	˭	Voltage	X	Current	X	Power Factor
	P (in Watts)		V		I		PF (cosØ)
P	2300		230		10		1
P	2300		230		11.11		0.9
P	2300		230		12.50		0.8
P	2300		230		14.29		0.7
P	2300		230		16.67		0.6
P	2300		230		20		0.5

CAUSES OF LOW POWER FACTOR –

Most of the ac motors are of induction type(1-ph and 3-ph) which have low PF. These motors works at a PF which is extremely small on light load 0.2 to 0.3) and rises to 0.8 or o.9 at full load.
Arc lamps, electric discharge lamps and industrial heating furnaces operate at low PF,
Power on Leading Power Factor– In our industries, mostly induction equipment are used which causes low power factor in the system which is highly undesirable as it causes an increase in current resulting in losses of active power in all elements of power system from power generation to utilization. Hence it is important to have power factor as close to unity as possible.

Power Factor Correction Methods or Power Factor Improvement Methods-

In order to improve the power factor, equipment which cause leading power factor are used in parallel with load. These are –

Static Capacitors,
Synchronous motor (or condenser),
Phase advancement. Details are given below –

Static Capacitors – Banks of static capacitors are installed in the main LT panel & connected parallel with main supply.
An over excited synchronous motor can be used for pf correction as when it runs, current leads the voltage by some angle. An over excited synchronous moto running on no load is known as synchronous condenser. When such motor is connected in parallel with main supply, it takes leading current which partly neutralizes the lagging effect of current (or load). Thus the power factor is improved.
Phase advancer is simply an a.c exciter. It is mounted on the same shaft of the induction motor & is connected with the rotor. It provides exciting ampere turns to the rotor circuit at slip frequency. By providing more ampere turns then required, the induction motor can be made to operate on leading power factor like an over excited synchronous motor.

However the main disadvantage of phase advancers is that they are not economical.

Frequently asked questions (FAQs) –

Q1) A power factor of zero indicates?

Ans) A zero power factor (Cos 90) indicates that power factor is very low which will impact the power demand of the equipment in two ways. The impact of power factor on power requirement is given in above two tables is given in detail in the section given above “Effect of power factor on power consumption”.

Q2) What is unit of power factor?

Ans) Power factor has no unit. It is represented by CosØ whose value varies between 0-1 depending on the type of load load. Various types of load details are given in above section ‘ Effect of types of load on AC Circuit’.

Q3) What is power factor meter?

Ans) Power factor meter is a devise/instrument that monitors power factor in electrical system. Power factor meter helps to control the power factor in electrical system because low power factor (or lagging power factor) is not good for electrical system. The impact of low power factor is given in detail in the section which is given above “Effect of power factor on power consumption”.

Power factor meter may be digital or analogue type. These days digital power factor meters are very popular & used to monitor power factor in electrical panels. Normally power factor meter is installed in incoming feeder of HT & LT panels.

Q4) What is maximum value of power factor?

Ans) The maximum value of power factor is 1 which comes from Cos0. Cos0 means there is no phase difference between voltage & current. Detail of phase difference between voltage & current is given in section given above ‘EFFECT OF TYPES OF LOAD ON AC CIRCUIT’. All electricity consumer should try to maintain power factor close to 1 to avoid penalties from electricity authority.

Q5) Formula for single phase & three phase power factor?

Ans) Power factor can be calculated from the formula of single phase & three phase power. Formulas are given below –

Power factor for single phase – P= V I CosØ or CosØ = P / V I

Power factor for three phase supply – P= Г3 V I CosØ or CosØ = P / Г3 V I

Where P = Power, V= Voltage, I = Current & CosØ = Power Factor

Q6) Why power factor happens only in AC supply & not in DC supply?

Ans) It is because in DC system, voltage & current don’t vary with time while both voltage & current varies with in AC supply & power factor comes into picture. Detail of different power factors are given is given in above sections.

Q7) What is the full form of APFCR & what is the function of APFCR in electrical system?

Ans) The full form of APFCR is ‘Automatic Power Factor Correction Relay’. It is an electronic relay which is installed in main LT panel. The function of this relay is to correct the power factor through capacitor banks which are installed in the main LT panel. As we have under- stood that capacitors can improve the power (because of its leading nature) when they are installed parallel in electrical supply in main LT panel. The capacitor banks are installed in groups of 25KVA, 50 KVA etc & switched on/off automatically through APFCR. Number of switching on/off of capacitor banks depends on the value of power factor, if power factor value is less then more number of banks will be switched on & they will be switched off automatically one by one when power factor improves.

Note: According to section 3.4 of NBC 2016, the value of power factor should be 0.9 or better.

National Building Code of India 2016