Lecturer:Engr: Sajid Hussain Qazi
of low power
Defination: Power factor (P.F) is the ratio between active power and apparent power
Active power/Apparent power or P.F=Kw/Kva
Power factor involves the relationship between two types of power: Working
Power and Reactive Power
Most loads in electrical distribution systems are inductive, which means that
they require an electromagnetic field to operate
Inductive loads require two kinds of current:
Working Power – performs actual work of creating heat, light, motion, etc
Reactive Power – sustains the electromagnetic field
PF measures how effectively/efficiently electrical power is being used
Examples of Electric Equipment and Their Power Factor
Different types of electric equipment have different Power Factors and consequently
different efficiencies and current requirements:
Name of Equipment
Lightly loaded induction
Loaded induction motor
Power Factor Percent
.30 - .70
All types of resistance heating
devices (e.g. toaster, space
Working Power (kW)
Apparent Power (kVA)
1. Real Power or Working Power (kW) - Measured
2. Reactive Power (kVAR) - Measured
3. Apparent Power (kVA) - Calculated
c = a+ b
kVA = kW + kVAR
Active and reactive powers are designated by P & Q respectively.
The average power in a circuit is called active power and the power that supplies
stored energy to reactive elements is called reactive power.
P.F For Inductive load: Inductive loads cause the current to lag behind the voltage.The wave form of
voltage and curret then are “out of phase” with each other. The more out phase
they become then the lower the power factor will be.Power factor is usually
expressed as cos phi(φ).
Consider a canal boat is being pushd by a horse.
If the horse could walk on water then angle is φ(phi) would be zero and
cos(0)=1.Meaning all the horse power is being used to pull the load.
However the relative position of the horse influnces the power.
As the horse gets closer to barge, angle φ1 increases and power is wasted, but as the
horse positioned further away, then angle φ2 gets closer to zero and less power is
ACTIVE POWER (P)
Also known as “real power” or simply “power”. Active power is the rate of
producing, transfaring, or using elctrical energy. It is measured in watts and
often expressed in kilowatts(KW) or Megawatts(MW). The term “active” or
“real” power are used in place of the term “power” alone to differentiate it from
Or other defination of active power can be that Active power the actual power (
Kw ) or Real power Consumed by a load .If we are not maintaining the power
factor at the appreciable value that is if cos0 is decreasing then sino is increased
that is VI SIN o that is reactive power.
cos o = Active power / Apparent Power = Kw / KVA.
Reactive power = VI SINo = Kvar.
The power which is actually utilised in the circuit is known as active power.
REACTIVE POWER (Q)
Reactive power is nothing but which is delivered by the inductive load and does
not do any usefulwork . It is measured in Kvar or simply Var.
Reactive power is that component of power for which we don't pay any money,
but we put our own system to a pressure. It is like somebody climbing upstairs
and coming down without doing anything
APPARENT POWER (S)
The product of the voltage (in volts) and the current (in amperes). It comprises
both active and reactive power..
It is measured in “volt-amperes”(va) and often expressed in “kilovoltamperes”(kva) or “Megavolt-amperes”(Mva).
1. When current and voltage wave forms are in phase as shown
below then corresponding power wave form of an electrical
network would be:-
2. When current and voltage wave forms are out of phase as
shown below then corresponding power wave form of an
electrical network would be:-
Effect Of Low P.F On Inductive Load
Causes Of Low Power Factor
A poor power factor can be the result of either a significant phase difference
between the voltage and current at the load terminals or it can be due to a high
harmonic content or distorted/discontineous current wave form. Poor load
current phase angle is generally the result of an inductive load such as an
induction motor , power transformere, lightening ballasts, welder or induction
furnanc, Induction generators, wind mill generators and high intensity
Dis-Advangates Of a Low Power Factor
Increases heating losses in transformer and distribution equipments.
Reduce plant life.
Unstablise voltage levels.
Increase power losses.
Upgrade needs costly equipments.
Decreases energy efficiency.
Increases electricity cost by paying power factor surcharges.
Correction Of Power Factor
Most loads on an electrical distribution system fall into one of three catogries;
resistive, inductive or capactive. World wide the most common is likely to be
inductive (almost 85%). Typical examples of this includes transformers,
fluorescent lightening and AC induction motors.
Most inductive loads use a conductive coil winding to produce an
electromagnetic field, allowing the motor to function.
All inductive loads require two kind o f power to operate:
Active Power:- (KW)- to produce the motive force
Reactive Power:- (KVAR)- to energize the magnetic field
The operating power from the distribution system is composed of both active
(working) and reactive (non-working) elements. The active power does useful work
in driving the motor whereas the reactive power only provides the megnetic field
The amount of power capacitor KVAR required to correct a system to a desired
power factor level, is the difference between the amount of KVAR in the
uncorrected system and the amount of desired KVAR in the corrected system
The most efficient location for power factor capacitor is at the load. Capacitors
work from the point of installation back to the generating source. Individual motor
correction is not always practical, sometimes it is more practical to connect larger
capacitors on the distribution bus or install an automatic syatem at the incoming
service along with fixed capacitors at the load
After Correction Of Power Factor
After Correction Of Power Factor
Utilization Of Power With & Without Capacitor
Capactive Powet Factor Correction (PFC) is applied to electric circuits as a means
of minimising the inductive componenet of the current and there by reducing the
losses in the supply. The introduction of power factor correction capactiors is a
widely recognised method of reducing an electrical load, thus minimising wasted
energy and hence improving the efficiency of a plant and reducing the electricity
It is not usually necessary to reach unity, i.e. Power factor 1, since most supply
companies are happy with a PF of 095 to 0.98.
By installing suitably sized switched capacitors into the circuit, the the power factor
is improved and the value becomes nearer to 1 thus minimising wasted energy and
improving the efficiency of a plant.
Power factor can be increased by synchronous motor as well.
Advantages Of Power Factor Correction
Eliminate power factor penalities
Increase system capacity
Reduce line losses in distribution systems
Improve voltage stability
Increase equipment life
Save on utility cost
Enhance equipment operation by improving voltage
Improve energy efficiency
Reduction in size of transformers, cables and switchgear in new installations
Delay costly upgrades
Less total plant KVA for the same KW working power
Improves voltage regulation due to reduced line voltage drop
1- Internet (www.authorstream.com)
Bird) Page no: 223, 239, 240, 241
• 2- Book (Electrical & Electronic Principles &
Technology By John