1. INTERNSHIP REPORT
FATIMA FERTILIZER
COMPANY LIMITED
Name: Husnain Bilal
Department of Electrical Engineering and
Instruments
LAHORE University of Management Sciences
Email: 16100037@lums.edu.pk

2. 1
Contents
Acknowledgement………………………………………………………….......4
Introduction ……………………………………………………………………..5
Hazards Area Classification…………………………………………………...6
Categories…………………………………………………………………………6
Zones………………………………………………………………………………7
Explosive Gases and Explosive Dusts………………………………………….8
Temperature Classification………………………………………………………9
Ingress Protection………………………………………………………………...9
Transformers …………………………………………………………………..12
Working of a Transformer…………………………………………..................12
Transformer Construction………………………………………………………13
Transformer Nameplate…………………………………………………..........14
Types of Transformer……………………………………………………..........15
Transformer Protection…………………………………………………………16
Circuit Breakers………………………………………………………………..19
Working of a Circuit Breaker………………………………………..................19
Arc Phenomenon………………………………………………………………..19
Types of Circuit Breaker………………………………………………………..19
Working of Different Circuit Breakers……………………………...................20
 MCB…………………………………………………………………….....20
 Air Circuit Breaker…………………………………………….................21

3. 2
 Vacuum Circuit Breaker………………………………………………....23
 SF6 Circuit Breaker……………………………………………………….24
Cables……………………………………………………………………………26
Voltages induced in cables…………………………………………................26
Construction of cables…………………………………………………………..28
Motors…………………………………………………………………..............33
Introduction to induction motor…………………………………………………33
Working Principle of induction motor…………………………………………..34
Why is three phase induction motor self-starting…………………………….34
Why is single phase induction motor not self-staring………………………...34
Synchronous motor……………………………………………………………..36
Construction of synchronous motor……………………………………………36
Main features of synchronous motor…………………………………………..37
Principle operation of synchronous motor…………………………………….37
Methods of starting synchronous motor……………………………………….38
Motor Starters………………………………………………………………….39
Advantages of using a motor starter…………………………………………..39
Types of Motor Starter…………………………………………………………..40
Full Voltage Motor Starter………………………………………………………40
 Direct Online………………………………………………………………40
 Forward Reverse Direct Online…………………………………………42
Reduced Voltage Motor Starter………………………………………………..43
 Star-Delta Starter…………………………………………………….......43

4. 3
 Soft Starter………………………………………………………………..45
Protection……………………………………………………………………….46
Objective of Power System Protection………………………………………..46
Functional Requirements of Protection Relay………………………………..46
Types of Relay…………………………………………………………………..47
Current Transformer…………………………………………………………….48
Voltage Transformer………………………………………………………........50
Variable Frequency Drives…………………………………………………...52
Introduction to VFDs…………………………………………………………….52
Working of VFDs………………………………………………………………...53
Merits of using Variable Frequency Drives……………………………………55
Applications of VFDs……………………………………………………………57

5. 4
ACKNOWLEDGEMENT
I would like to thank and present my gratitude to FFCL
management, engineers and staff for their cooperation and
guidance in the completion of 4 weeks internship program at FFCL.
The whole experience was excellent, there was so much to learn
and experience.

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A special thanks to the following people who had been guiding and
cooperating with throughout my internship program. In spite of their
busy schedule they took out time to explain to me the official
procedures and mechanics of work in the organization
Mr. Ahmad Abdullah
Mr. Arsalan Rauf

7. 6
Introduction
The Fatima Fertilizer Company Limited was incorporated on December 24, 2003. FFCL
is the first and only green field project which has materialized under the 2001 Fertilizer
Policy of the Government of Pakistan. Its foundation was laid by Prime Minister of
Pakistan on April 26, 2006 and is housed on 950 acres of land.
The fertilizer complex is a fully integrated production facility, capable of producing four
final products which are Urea, Calcium Ammonium Nitrate (CAN), Nitro Phosphate (NP)
and Nitrogen Phosphorous Potassium (NPK).
It has a 56MW captive power plant and has been allocated 110MMCFD of gas from the
MARI Gas fields.
During its construction phase the company engaged over 4000 engineers and
technicians from Pakistan, USA, China and Europe. The company provides its
employees with all necessary facilities.

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HAZARDS AREA CLASSIFICATIONS
There are different standards used for hazardous areas and electrical equipment
designed for use in those environments, depending upon where in the world they are to
be used. In Europe EN standards are used to gain compliance with the ATEX directive.
In the USA the standard is NEC (National Electric Code), with a variant called CEC
(Canadian Electric Code) used in Canada. In addition some countries have their own
approval standards (e.g. GOST for Russia and the former Soviet States, TISI for
Thailand, etc), however these are often based on IEC standards.
To simplify matters an attempt is being made to harmonize all major standards for use
in the IECEx scheme. The aim of the IECEx Scheme is to facilitate international trade in
electrical equipment intended for use in explosive atmospheres (Ex equipment) by
eliminating the need for multiple national certification while preserving an appropriate
level of safety.
Categories
Under the ATEX scheme, equipment is defined under categories. These ATEX
categories are levels of safety. The following information highlights the different
categories under the ATEX scheme and their expected zone of use. Zones will be
defined later in this report.
Category 1
Degree of Safety: Very High Level of Safety
Design Requirements: Two independent means of protection or safe with two
independent faults.
Application: Where explosive atmospheres are present continuously or for lengthy
periods.
Expected Zone of Use: Zone 0 (gas) and Zone 20 (dust)

9. 8
Category 2
Degree of Safety: High Level of Safety
Design Requirements: Safe with frequently occurring disturbances or with a normal
operating fault
Application: Where explosive atmospheres are likely to occur
Expected Zone of Use: Zone 1 (gas) and Zone 21 (dust)
Category 3
Degree of Safety: Normal Level of Safety
Design Requirements: Safe in normal operation
Application: Where explosive atmospheres are likely to occur infrequently and be of
short duration
Expected Zone of Use: Zone 2 (gas) and Zone 22 (dust)
Zones
Hazardous area are also classified into zones. We have already seen what category
equipment is supposed to be used in which zone. Next we will define these zones.
Zone 0 (gas) and Zone 20 (dust)
An area in which an explosive atmosphere is continuously present or for long periods or
frequently
Zone 1 (gas) and Zone 21 (dust)
An area in which an explosive atmosphere is likely to occur in normal operation
occasionally.
Zone 2 (gas) and Zone 22 (dust)
An area in which an explosive atmosphere is not likely to occur in normal operation and
if it occurs it will exist only for a short time

10. 9
Explosive Gases and Explosive Dust
It is very important to establish whether the hazardous area is due to the presence of an
explosive gas or explosive dust.
Explosive gases are classified into two groups.
Group 1
Group 1 gases are firedamp methane gas. These are usually associated with mining
applications
Group 2
Group II gases are all other explosive gases as listed opposite with relevant
subdivisions A, B or C according to the nature of the chemical content. These are
usually associated with surface applications.
Gas Group Representative
Test Gas
1 Methane
2A Propane
2B Ethylene
2C Hydrogen
If an area is classed as hazardous due to the presence of combustible dust, it is
important to establish if it is a metallic or non-metallic dust. The latest series of
standards for electrical apparatus in the presence of combustible dust that will provide

11. 10
protection concepts, installation and selection requirements will be the EN/IEC 61241
series.
Temperature Classification
The temperature classification codes define the maximum surface temperature of the
equipment being used. Care should be taken to ensure that this maximum surface
temperature is lower than the igniting temperature of the explosive gas or dust.
Temperature classifications are given in the table below:
Ingress Protection
Ingress Protection (IP) ratings are developed by the European Committee for Electro
Technical Standardization (CENELEC) (NEMA IEC 60529 Degrees of Protection

12. 11
Provided by Enclosures - IP Code), specifying the environmental protection the
enclosure provides.
The IP rating normally has two (or three) numbers:
1. Protection from solid objects or materials
2. Protection from liquids (water)
3. Protection against mechanical impacts (commonly omitted, the third number is
not a part of IEC 60529)

13. 12
This completes the summarization of all the information present on an equipment tag for
hazardous areas. Below are naming schemes according to IECEx and ATEX schemes.

14. 13

15. 14
Transformers
Working of a Transformer
The basic principle behind working of a transformer is the phenomenon of mutual
induction between two windings linked by common magnetic flux. A transformer consists
of two inductive coils; primary and secondary coils. They are electrically separated but
magnetically linked. In short, a transformer carries the operations shown below:
1. Transfer of electric power from one circuit to another.
2. Transfer of electric power without any change in frequency.
3. Transfer with the principle of electromagnetic induction.
4. The two electrical circuits are linked by mutual induction.
The governing equation of a transformer is:
𝑒 = 𝑀 ∗ 𝑑𝑖/𝑑𝑡

16. 15
Transformer Construction:
Transformer consists of the following parts
1) BUCHHOLZ RELAY: it is a very sensitive gas and oil
operated instrument which safely detect the formation of
gas or sudden pressure inside the oil transformer.
2) CONSERVATOR: it is used to provide adequate space for the
expansion of oil when transformer is loaded or when ambient
temperature changes.
3 SILICA GEL BREATHER: it sucks the moisture from the air
which is taken by transformer so that dry air is taken by
transformer.
4) DOUBLE DIAPHRAGM EXPLOSION VENT: it is used to discharge
excess pressure in the atmosphere when excess pressure is
developed inside the transformer during loading.
5) OIL LEVEL INDICATOR: it is used to show the oil level in

17. 16
the transformer.
6) Winding temperature indicator: used to show the
temperature of transformer winding.
7) RADIATORS: these are used for cooling of the transformer
oil.
Transformer Nameplate
Transformer nameplate gives the user the entire information about the device.

18. 17
(1) Phase indicate whether it is single phase or 3 phase transformer
(2) Vector number indicates the type of connection e.g. in this case this
transformer is connected in delta-High voltage side, Star-low voltage side and
is grounded. 11 tells how much high voltage leads/lag from low voltage side.
(3) Frequency at which the transformer can be operated, in this case 50Hz

19. 18
Types of Transformer
(a) Core type transformer
In core type transformer, windings are cylindrical former wound, mounted on the core
limbs as shown in the figure above. The cylindrical coils have different layers and each
layer is insulated from each other. Materials like paper, cloth or mica can be used for
insulation. Low voltage windings are placed nearer to the core, as they are easier to
insulate.
(b) Shell type transformer
The coils are former wound and mounted in layers stacked with insulation between
them. A shell type transformer may have simple rectangular form (as shown in above
fig), or it may have a distributed form.
(c) On the basis of their use
1. Power transformer: Used in transmission network, high rating
2. Distribution transformer: Used in distribution network, comparatively lower
rating than that of power transformers.
3. Instrument transformer: Used in relay and protection purpose in different
instruments in industries
o Current transformer (CT)
o Potential transformer (PT)

20. 19
(d) On the basis of cooling employed
1. Oil-filled self- cooled type
2. Oil-filled water cooled type
3. Air blast type (air cooled)
Transformer Protection
A transformer generally suffers from following types of transformer fault-
1. Over current due to overloads and external short circuits,
2. Terminal faults,
3. Winding faults,
4. Incipient faults.
All the above mentioned transformer faults cause mechanical and thermal stresses
inside the transformer winding and its connecting terminals. Thermal stresses lead to
overheating which ultimately affect the insulation system of transformer. Deterioration of
insulation leads to winding faults. Sometime failure of transformer cooling system, leads
to overheating of transformer. So the transformer protection schemes are very much
required.
The short circuit current of an electrical transformer is normally limited by its reactance
and for low reactance, the value of short circuit current may be excessively high. The
duration of external short circuits which a transformer can sustain without damage as
given in BSS 171:1936.

21. 20
The general winding faults in transformer are either earth faults or inter-turns faults.
Phase to phase winding faults in a transformer is rare. The phase faults in an electrical
transformer may be occurred due to bushing flash over and faults in tap changer
equipment. Whatever may be the faults, the transformer must be isolated instantly
during fault otherwise major breakdown may occur in the electrical power system.
Incipient faults are internal faults which constitute no immediate hazard. But it these
faults are over looked and not taken care of, these may lead to major faults. The faults
in this group are mainly inter-lamination short circuit due to insulation failure between
core lamination, lowering the oil level due to oil leakage, blockage of oil flow paths. All
these faults lead to overheating. So transformer protection scheme is required for
incipient transformer faults also. The earth fault, very nearer to neutral point of
transformer star winding may also be considered as an incipient fault.
Influence of winding connections and earthing on earth fault current magnitude.
There are mainly two conditions for earth fault current to flow during winding to earth
faults,
1. A current exists for the current to flow into and out of the winding.
2. Ampere-turns balance is maintained between the windings.

22. 21
The value of winding earth fault current depends upon position of the fault on the
winding, method of winding connection and method of earthing. The star point of the
windings may be earthed either solidly or via a resistor. On delta side of the transformer
the system is earthed through an earthing transformer. Grounding or earthing
transformer provides low impedance path to the zero sequence current and high
impedance to the positive and negative sequence currents.
Star Winding with Neutral Resistance Earthed
In this case the neutral point of the transformer is earthed via a resistor and the value of
impedance of it, is much higher than that of winding impedance of the transformer. That
means the value of transformer winding impedance is negligible compared to
impedance of earthing resistor. The value of earth current is, therefore, proportional to
the position of the fault in the winding. As the fault current in the primary winding of the
transformer is proportional to the ratio of the short circuited secondary turns to the total
turns on the primary winding, the primary fault current will be proportional to the square
of the percentage of winding short circuited. The variation of fault current both in the
primary and secondary winding is shown below.
Star Winding with Neutral Solidly Earthed
In this case the earth fault current magnitude is limited solely by the winding impedance
and the fault is no longer proportional to the position of the fault. The reason for this non
linearity is unbalanced flux linkage

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Circuit Breakers
Working of a circuit breaker
A circuit breaker is an equipment that breaks a circuit either manually or automatically
under all conditions at no load, full load or short circuit. Two contacts called electrode
remains closed under normal operating conditions. When fault occurs on any part of the
system, the trip coil of the circuit breaker get energized and contacts are separated.
Arc Phenomenon

24. 23
An arc is struck when contacts are separated. The current is thus able to continue.
Thus the main duty of a circuit breaker is to distinguish the arc within the shortest
possible time.
The arc provides the low resistance path to the current and the current in the circuit
remains uninterrupted. The arc resistance depends upon the following factors.
 Degree of ionization
 Length of the arc
 Cross Section of the arc
Types of Circuit Breaker
 MCCB
 MCB
 Air Circuit Breaker
 Oil Circuit Breaker
 Vacuum Circuit Breaker
 SF6 Circuit Breaker
Working of Different Circuit Breakers
 MCB
Nowadays we use more commonly miniature circuit breaker or MCB in low voltage
electrical network instead of fuse. There are two arrangement of operation of
miniature circuit breaker. One due to thermal effect of over current and other due to
electromagnetic effect of over current. The thermal operation of miniature circuit

25. 24
breaker is achieved with a bimetallic strip whenever continuous over current flows
through MCB, the bimetallic strip is heated and deflects by bending. This deflection of
bimetallic strip releases mechanical latch. As this mechanical latch is attached with
operating mechanism, it causes to open the miniature circuit breaker contacts. But
during short circuit condition, sudden rising of current, causes electromechanical
displacement of plunger associated with tripping coil or solenoid of MCB. The plunger
strikes the trip lever causing immediate release of latch mechanism consequently open
the circuit breaker contacts. This was a simple explanation of miniature circuit breaker
working principle.
 Air Circuit Breaker
This type of circuit breakers, is those kind of circuit breaker which operates in air at
atmospheric pressure. After development of oil circuit breaker, the medium voltage air

26. 25
circuit breaker (ACB) is replaced completely by oil circuit breaker in different countries.
But in countries like France and Italy, ACBs are still preferable choice up to voltage 15
KV. The working principle of this breaker is rather different from those in any other
types of circuit breakers. The main aim of all kind of circuit breaker is to prevent the
reestablishment of arcing after current zero by creating a situation where in the contact
gap will withstand the system recovery voltage. The air circuit breaker does the same
but in different manner. For interrupting arc it creates an arc voltage in excess of the
supply voltage. Arc voltage is defined as the minimum voltage required maintaining the
arc. This circuit breaker increases the arc voltage by mainly three different ways,
1. It may increase the arc voltage by cooling the arc plasma. As the temperature of
arc plasma is decreased, the mobility of the particle in arc plasma is reduced,
hence more voltage gradient is required to maintain the arc.
2. It may increase the arc voltage by lengthening the arc path. As the length of arc
path is increased, the resistance of the path is increased, and hence to maintain
the same arc current more voltage is required to be applied across the arc path.
That means arc voltage is increased.
3. Splitting up the arc into a number of series arcs also increases the arc voltage.
The first objective is usually achieved by forcing the arc into contact with as large an
area as possible of insulating material. Every air circuit breaker is fitted with a chamber
surrounding the contact. This chamber is called 'arc chute'. The arc is driven into it. If
inside of the arc chute is suitably shaped, and if the arc can be made conform to the
shape, the arc chute wall will help to achieve cooling. This type of arc chute should be
made from some kind of refractory material. High temperature plastics reinforced with
glass fiber and ceramics are preferable materials for making arc chute.

27. 26
The second objective that is lengthening the arc path, is achieved concurrently with fist
objective. If the inner walls of the arc chute is shaped in such a way that the arc is not
only forced into close proximity with it but also driven into a serpentine channel
projected on the arc chute wall. The lengthening of the arc path increases the arc
resistance.
The third technique is achieved by using metal arc slitter inside the arc chute. The
main arc chute is divided into numbers of small compartments by using metallic
separation plates. These metallic separation plates are actually the arc splitters and
each of the small compartments behaves as individual mini arc chute. In this system
the initial arc is split into a number of series arcs, each of which will have its own
mini arc chute. So each of the split arcs has its own cooling and lengthening effect
due to its own mini arc chute and hence individual split arc voltage becomes high.
These collectively, make the overall arc voltage, much higher than the system
voltage.

28. 27
 Vacuum Circuit Breaker
A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes
place in vacuum. The technology is suitable for mainly medium voltage application. For
higher voltage vacuum technology has been developed but not commercially viable.
The operation of opening and closing of current carrying contacts and associated arc
interruption take place in a vacuum chamber in the breaker which is called vacuum
interrupter. The vacuum interrupter consists of a steel arc chamber in the center
symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum
interrupter is normally maintained at 10 - 6 bar. The main aim of any circuit breaker is to

29. 28
quench arc during current zero crossing, by establishing high dielectric strength in
between the contacts so that reestablishment of arc after current zero becomes
impossible. The dielectric strength of vacuum is eight times greater than that of air and
four times greater than that of SF6 gas. This high dielectric strength makes it possible to
quench a vacuum arc within very small contact gap. For short contact gap, low contact
mass and no compression of medium the drive energy required in vacuum circuit
breaker is minimum. When two face to face contact areas are just being separated to
each other, they do not be separated instantly, contact area on the contact face is being
reduced and ultimately comes to a point and then they are finally de-touched. Although
this happens in a fraction of micro second but it is the fact. At this instant of de-touching
of contacts in a vacuum, the current through the contacts concentrated on that last
contact point on the contact surface and makes a hot spot. As it is vacuum, the metal on
the contact surface is easily vaporized due to that hot spot and create a conducting
media for arc path. Then the arc will be initiated and continued until the next current
zero.
 𝑺𝑭 𝟔 Circuit Breaker
SF6 has excellent insulating property. SF6 has high electro-negativity. That
means it has high affinity of absorbing free electron. Whenever a free electron
collides with the SF6 gas molecule, it is absorbed by that gas molecule and forms

30. 29
a negative ion. The attachment of electron with SF6 gas molecules may occur in
two different ways,
These negative ions obviously much heavier than a free electron and therefore
over all mobility of the charged particle in the SF6 gas is much less as compared
other common gases. We know that mobility of charged particle is majorly
responsible for conducting current through a gas. Hence, for heavier and less
mobile charged particles in SF6 gas, it acquires very high dielectric strength. Not
only the gas has a good dielectric strength but also it has the unique property of
fast recombination after the source energizing the spark is removed. The gas has
also very good heat transfer property. Due to its low gaseous viscosity (because
of less molecular mobility) SF6 gas can efficiently transfer heat by convection. So
due to its high dielectric strength and high cooling effect SF6 gas is approximately
100 times more effective arc quenching media than air. Due to these unique
properties of this gas SF6 circuit breaker is used in complete range of medium
voltage and high voltage electrical power system. These circuit breakers are
available for the voltage ranges from 33KV to 800KV and even more.

31. 30

32. 31
Cables
Voltages induced in cables
SINGLE-CORE aluminium wire armored cables are often employed in high current
industrial applications. Such cables are available in sizes up to 1000 mm2, whereas it is
difficult to obtain multicore steel wire armored cables in sizes above 400 mm2. Single-
core cables have a smaller bending radius than the equivalent SWA multicore cable and
are, in general, easier to handle. Tables 4D3 and 4E3 in BS 7671 give information on
current-carrying capacities and voltage drop for single-core cables with non-magnetic
armor. Note that single-core cables armored with steel wire or tape must not be used for
a.c. circuits. (Regulation 521-02-01 refers).
In any armored cable system the armor is an exposed conductive-part and has to be
connected to earth as required by Regulation 413-02-06 for TN systems or Regulation
413-02-18 for TT systems. The connection with earth has to be made at a minimum of
one point, usually one end. For a single-core armored cable, carrying an a.c. load
current, a voltage will be induced in the armor. Similarly a voltage will be induced in a
metallic screen or sheath of a single-core cable. The magnitude of the induced voltage
depends on factors which include the load current, the length of the cable, the armor
diameter and the cable spacing. The armor is effectively the secondary of a transformer
and the conductor is the primary.
Consider a single-phase circuit formed using two single-core armored cables, the
armors are earthed, generally at the supply end. There are two possible configurations
for the connection of the armors of the line and neutral conductors at the load end of the
circuit:

33. 32
Solid bonded system: The armors are interconnected forming a loop
Single point bonded system: The armors are left unconnected. Note that the same
two configurations exist in a 3-phase circuit formed by the use of three (or four) single-
core armored cables.
Solid Bonded System

34. 33
Single Point Bonded System
Construction of Cables

35. 34
Usually stranded copper (Cu) or aluminium (Al). Copper is dense and heavier, but more
conductive than aluminium. Electrically equivalent aluminium conductors have a cross-
sectional area approximately 1.6 times larger than copper, but are half the weight
(which may save on material cost).
Conductor Screen
A semi-conducting tape to maintain a uniform electric field and minimize electrostatic
stresses (for MV/HV power cables).
Insulation
Commonly thermoplastic (eg. PVC) or thermosetting (eg. EPR, XLPE) type materials.
Mineral insulation is sometimes used, but the construction of MI cables are entirely
different to normal plastic / rubber insulated cables. Typically a thermosetting (eg. EPR,
XLPE) or paper/lead insulation for cables under 22kV. Paper-based insulation in
combination with oil or gas-filled cables are generally used for higher voltages.
Plastics are one of the more commonly used types of insulating materials for electrical
conductors. It has good insulating, flexibility, and moisture-resistant qualities. Although
there are many types of plastic insulating materials, thermoplastic is one of the most
common. Plastic insulation is normally used for low- or medium-range voltage.
Insulation Screen
A semi-conducting material that has a similar function as the conductor screen (ie.
control of the electric field for MV/HV power cables).
Conductor Sheath
A conductive sheath / shield, typically of copper tape or sometimes lead alloy, is used
as a shield to keep electromagnetic radiation in, and also provide a path for fault and

36. 35
leakage currents (sheaths are earthed at one cable end). Lead sheaths are heavier and
potentially more difficult to terminate than copper tape, but generally provide better
earth fault capacity.
Filler
The interstices of the insulated conductor bundle is sometimes filled, usually with a soft
polymer material.
Bedding / Inner Sheath
Typically a thermoplastic (eg. PVC) or thermosetting (eg. CSP) compound, the inner
sheath is there to keep the bundle together and to provide a bedding for the cable
armour.
Individual Screen (Instrument Cables)
An individual screen is occasionally applied over each insulated conductor bundle for
shielding against noise / radiation and interference from other conductor bundles.
Screens are usually a metallic (copper, aluminium) or semi-metallic (PETP/Al) tape or
braid. Typically used in instrument cables, but not in power cables.
Overall Screen (Instrument Cables)
An overall screen is applied over all the insulated conductor bundles for shielding
against noise / radiation, interference from other cables and surge / lightning protection.
Screens are usually a metallic (copper, aluminium) or semi-metallic (PETP/Al) tape or
braid. Typically used in instrument cables, but not in power cables.

37. 36
Armour
For mechanical protection of the conductor bundle. Steel wire armour or braid is
typically used. Tinning or galvanising is used for rust prevention. Phosphor bronze or
tinned copper braid is also used when steel armour is not allowed.
 SWA - Steel wire armour, used in multi-core cables (magnetic),
 AWA - Aluminium wire armour, used in single-core cables (non-magnetic).
Outer Sheath
Applied over the armour for overall mechanical, weather, chemical and electrical
protection. Typically a thermoplastic (eg. PVC) or thermosetting (eg. CSP) compound,
and often the same material as the bedding. Outer sheath is normally colour coded to
differentiate between LV, HV and instrumentation cables. Manufacturer’s markings and
length markings are also printed on the outer sheath.
Non-Metallic
The category of non-metallic protective coverings is divided into three areas. These
areas are:
(1) according to the material used as the covering,
(2) according to the saturant in which the covering was impregnated, and
(3) according to the external finish on the wire or cable.
These three areas reflect three different methods of protecting the wire or cable. These
methods allow some wire or cable to be classified under more than one category. Most
of the time, however, the wire or cable will be classified based upon the material used
as the covering regardless of whether or not a saturant or finish is applied.

38. 37
Fibrous Braid
Fibrous braid is used extensively as a protective covering for cables. This braid is
woven over the insulation to form a continuous covering without joints. The braid is
generally saturated with asphalt, paint, or varnish to give added protection against
moisture, flame, weathering, oil, or acid. Additionally, the outside braid is often given a
finish of stearin pitch and mica flakes, paint, wax, lacquer, or varnish depending on the
environment where the cable is to be used.
Metallic Sheath
Cables or wires that are continually subjected to water must be protected by a
watertight cover. This watertight cover is either a continuous metal jacket or a rubber
sheath molded around the cable.
Lead-sheathed cable is one of three types currently being used: alloy lead, pure lead,
and reinforced lead. An alloy-lead sheath is much like a pure lead sheath but is
manufactured with 2-percent tin. Reinforced lead sheath consists of a double lead
sheath. A thin tape of hard-drawn copper, bronze, or other elastic metal (preferably
nonmagnetic) is wrapped around the inner sheath. This tape gives considerable
additional strength and elasticity to the sheath, but must be protected from corrosion.
For this reason, a second lead sheath is applied over the tape.
Metallic Armour
Metallic armour provides a tough protective covering for wires and cables. The type,
thickness, and kind of metal used to make the armour depend on three factors:
(1) The use of the conductors,
(2) The environment where the conductors are to be used, and

39. 38
(3) The amount of rough treatment that is expected.

40. 39
Motors
Introduction to Induction motor
One of the most common electrical motor used in most applications which is known as
induction motor. This motor is also called as asynchronous motor because it runs at a
speed less than synchronous speed. In this, we need to define what synchronous speed
is. Synchronous speed is the speed of rotation of the magnetic field in a rotary machine
and it depends upon the frequency and number poles of the machine. An induction
motor always runs at a speed less than synchronous speed because the rotating
magnetic field which is produced in the stator will generate flux in the rotor which will
make the rotor to rotate, but due to the lagging of flux current in the rotor with flux
current in the stator, the rotor will never reach to its rotating magnetic field speed i.e. the
synchronous speed. There are basically two types of induction motor that depend
upon the input supply - single phase induction motor and three phase induction motor.
Single phase induction motor is not a self-starting motor which we will discuss later and
three phase induction motor is a self-starting motor. Now in general we need to give two
supply i.e. double excitation to make a machine to rotate. For example if we consider a
DC motor, we will give one supply to the stator and another to the rotor through brush
arrangement.
Working principle of Induction motor
In induction motor we give only one supply, so it is really interesting to know that how it
works. It is very simple, from the name itself we can understand that there is induction
process occurred. Actually when we are giving the supply to the stator winding, flux will
generate in the coil due to flow of current in the coil. Now the rotor winding is arranged

41. 40
in such a way that it becomes short circuited in the rotor itself. The flux from the stator
will cut the coil in the rotor and since the rotor coils are short circuited, according to
Faraday's law of electromagnetic induction, current will start flowing in the coil of the
rotor. When the current will flow, another flux will get generated in the rotor. Now there
will be two flux, one is stator flux and another is rotor flux and the rotor flux will be
lagging to the stator flux. Due to this, the rotor will feel a torque which will make the rotor
to rotate in the direction of rotating magnetic flux. So the speed of the rotor will be
depending upon the ac supply and the speed can be controlled by varying the input
supply. This is the working principle of an induction motor of either type.
Why is Three Phase Induction Motor Self Starting?
In three phase system, there are three single phase line with 120° phase difference. So
the rotating magnetic field is having the same phase difference which will make the rotor
to move. If we consider three phases a, b and c, when phase a is magnetized, the rotor
will move towards the phase a winding, in the next moment phase b will get magnetized
and it will attract the rotor and then phase c. So the rotor will continue to rotate.
Why Single Phase Induction Motor is not Self Starting?
But what about single phase. It will be having only one phase still it makes the rotor to
rotate, so it is quite interesting. Before that we need to know why single phase induction
motor is not a self-starting motor and how the problem is overcome. We know that the
ac supply is a sinusoidal wave and it produces pulsating magnetic field in uniformly
distributed stator winding. Since pulsating magnetic field can be assumed as two
oppositely rotating magnetic fields, there will be no resultant torque produced at the
starting and due to this the motor does not run. After giving the supply, if the rotor is
made to rotate in either direction by external force, then the motor will start to run. This

42. 41
problem has been solved by making the stator winding into two winding, one is main
winding and another is auxiliary winding and a capacitor is fixed in series with the
auxiliary winding. This will make a phase difference when current will flow through the
both coils. When there will be phase difference, the rotor will generate a starting torque
and it will start to rotate. Practically we can see that the fan does not rotate when the
capacitor is disconnected from the motor but if we rotate with hand it will start to rotate.
So this is the reason of using capacitor in the single phase induction motor. There are
several advantages of induction motor which makes this motor to have wider
application. It is having good efficiency up to 97%. But the speed of the motor varies
with the load given to the motor which is a disadvantage of this motor. The direction of
rotation of induction motor can easily be changed by changing the sequence of three
phase supply, i.e. if RYB is in forward direction, the RBY will make the motor to rotate in
reverse direction. This is in the case of three phase motor but in single phase motor, the
direction can be reversed by reversing the capacitor terminals in the winding.

43. 42
Synchronous Motor
Electrical motor in general is an electro-mechanical device that converts energy from
electrical domain to mechanical domain. Based on the type of input we have classified it
into single phase and 3 phase motors. Among 3 phase induction motors and
synchronous motors are more widely used. When a 3 phase electric conductors are
placed in a certain geometrical positions (In certain angle from one another) there is an
electrical field generate. Now the rotating magnetic field rotates at a certain speed, that
speed is called synchronous speed. Now if an electromagnet is present in this rotating
magnetic field, the electromagnet is magnetically locked with this rotating magnetic field
and rotates with same speed of rotating field. Synchronous motors is called so because
the speed of the rotor of this motor is same as the rotating magnetic field. It is basically
a fixed speed motor because it has only one speed, which is synchronous speed and
therefore no intermediate speed is there or in other words it’s in synchronism with the
supply frequency. Synchronous speed is given by
Ns =
120𝑓
𝑝
Construction of Synchronous Motor

44. 43
Normally its construction is almost similar to that of a 3 phase induction motor, except
the fact that the rotor is given dc supply, the reason of which is explained later. Now, let
us first go through the basic construction of this type of motor
From the above picture, it is clear that how this type of motors are designed. The stator
is given is given three phase supply and the rotor is given dc supply.
Main Features of Synchronous Motors
1. Synchronous motors are inherently not self-starting. They require some
external means to bring their speed close to synchronous speed to before they
are synchronized.
2. The speed of operation of is in synchronism with the supply frequency and hence
for constant supply frequency they behave as constant speed motor irrespective
of load condition
3. This motor has the unique characteristics of operating under any electrical power
factor. This makes it being used in electrical power factor improvement.

45. 44
Principle Operation of Synchronous Motor
Synchronous motor is a doubly excited machine i.e. two electrical inputs are provided to
it. Its stator winding which consists of a 3 phase winding is provided with 3 phase supply
and rotor is provided with DC supply. The 3 phase stator winding carrying 3 phase
currents produces 3 phase rotating magnetic flux. The rotor carrying DC supply also
produces a constant flux. Considering the frequency to be 50 Hz, from the above
relation we can see that the 3 phase rotating flux rotates about 3000 revolution in 1 min
or 50 revolutions in 1 sec. At a particular instant rotor and stator poles might be of same
polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be
N-S causing attractive force. But due to inertia of the rotor, it is unable to rotate in any
direction due to attractive or repulsive force and remain in standstill condition. Hence it
is not self-starting.
To overcome this inertia, rotor is initially fed some mechanical input which rotates it in
same direction as magnetic field to a speed very close to synchronous speed. After
some time magnetic locking occurs and the synchronous motor rotates in synchronism
with the frequency.
Methods of Starting of Synchronous Motor
1. Synchronous motors are mechanically coupled with another motor. It could be
either 3 phase induction motor or DC shunt motor. DC excitation is not fed
initially. It is rotated at speed very close to its synchronous speed and after that
DC excitation is given. After some time when magnetic locking takes place
supply to the external motor is cut off.
2. Damper winding: In case, synchronous motor is of salient pole type, additional
winding is placed in rotor pole face. Initially when rotor is standstill, relative speed
between damper winding and rotating air gap flux in large and an emf is induced
in it which produces the required starting torque. As speed approaches

46. 45
synchronous speed, emf and torque is reduced and finally when magnetic
locking takes place, torque also reduces to zero. Hence in this case synchronous
is first run as three phase induction motor using additional winding and finally it is
synchronized with the frequency.
Motor Starters
Advantages of using a motor starter
When an induction motor is connected across the full line voltage, the starting surge of
current momentarily reaches as high a value as 400% to 600% or more of the rated full-
load current.
At the moment the motor starts, the rotor is at a standstill. At this instant, the stator field
cuts the rotor bars at a faster rate than when the rotor is turning. This means that there
will be relatively high induced voltages in the rotor which will cause heavy rotor current.
The resulting input current to the stator windings will be high at the instant of starting.
Motor Starters provides the following protection:
Overload, Short Circuit, Phase Unbalance etc.
Circuit Breakers (Thermal + Magnetic)
Control (Magnetic Contactors, Timers)

47. 46
Types of Motor Starters
There are different types of motor starters divided into many categories. But we will
divide them into two main category and then further into sub-categories. The two types
of starts are:
Full Voltage motor starters
Reduced Voltage motor starters
Full Voltage motor starters
Direct Online (DOL)
The simplest form of motor starts is the Direct Online (DOL) motor starter which is used
in many fertilizer inudstries. Main features of DOL are:
For low- and medium-power three-phase motors
High starting torque
Very high mechanical load
High current peaks
Voltage dips

48. 47
A direct on line starter can be used if the high inrush current of the motor does not
cause excessive voltage drop in the supply circuit. The maximum size of a motor
allowed on a direct on line starter may be limited by the supply utility for this reason.
DOL starting is sometimes used to start small water pumps, compressors, fans and
conveyor belts.
Advantages Disadvantages
Economical and cheap Does not reduce starting current
Simple to establish and operate Mechanically harsh
Simple control circuitry There is a big voltage dip due to high
voltage current
Provides 100% starting torque High starting torque which might be
unnecessary at times
advantages and disadvantages of DOL starter

49. 48
Forward Reverse Direct Online (RDOL)

50. 49
Certain applications require forward and reverse direction of motor rotation e.g. cranes.
It has the same working principle as of DOL except it also works in reverse.
Reduced Voltage Starter
Start-Delta Motor Starter
Voltage reduction during star-delta starting is achieved by physically reconfiguring the
motor windings. Principle behind star-delta starter is simple. During starting the motor
windings are connected in star configuration which reduces the voltage across each
winding. After a period of time the windings are reconfigured as Delta and the motor
runs normally

51. 50
Advantages Disadvantages
The operation of the star-delta method
is simple and rugged
High transmission and current
peaks: To reach the rated speed,
a switch over to delta position is
necessary, and this will very often
result in high transmission and
current peaks. In some cases the
current peak can reach a value
that is even bigger than for a D.O.L
start.
It is relatively cheap compared to other
reduced voltage methods.
Low Starting
Torque: Applications with a load
torque higher than 50 % of the

52. 51
motor rated torque will not be able
to start using the start-delta starter.
Good Torque/Current Performance
Soft Starter
 A soft starter is any device which reduces the torque applied to the electric motor.
 Generally consists of solid state devices like thyristors to control the application of
supply voltage to the motor.
 The starter works on the fact that the torque is proportional to the square of the
starting current, which in turn is proportional to the applied voltage.
 Thus the torque and the current can be adjusted by reducing the voltage at the time
of starting the motor.
 The basic principle of soft starter is by controlling the conduction angle of the SCRs
the application of supply voltage can be controlled.
Block Diagram

53. 52
Protection
Objective of Power System Protection
The objective of power system protection is to isolate a faulty section of electrical
power system from rest of the live system so that the rest portion can function
satisfactorily without any severer damage due to fault current. Actually circuit breaker
isolates the faulty system from rest of the healthy system and this circuit breakers
automatically open during fault condition due to its trip signal comes from protection
relay. The main philosophy about protection is that no protection of power system can
prevent the flow of fault current through the system, it only can prevent the continuation
of flowing of fault current by quickly disconnect the short circuit path from the system.

54. 53
For satisfying this quick disconnection the protection relays should have following
functional requirements.
Functional Requirements of Protection Relay
Reliability
The most important requisite of protective relay is reliability. They remain inoperative for
a long time before a fault occurs; but if a fault occurs, the relays must respond instantly
and correctly.
Selectivity
The relay must be operated in only those conditions for which relays are commissioned
in the electrical power system. There may be some typical condition during fault for
which some relays should not be operated or operated after some definite time delay
hence protection relay must be sufficiently capable to select appropriate condition for
which it would be operated.
Sensitivity
The relaying equipment must be sufficiently sensitive so that it can be operated reliably
when level of fault condition just crosses the predefined limit.
Speed
The protective relays must operate at the required speed. There must be a correct
coordination provided in various power system protection relays in such a way that for
fault at one portion of the system should not disturb other healthy portion. Fault current
may flow through a part of healthy portion since they are electrically connected but
relays associated with that healthy portion should not be operated faster than the relays
of faulty portion otherwise undesired interruption of healthy system may occur. Again if

55. 54
relay associated with faulty portion is not operated in proper time due to any defect in it
or other reason, then only the next relay associated with the healthy portion of the
system must be operated to isolate the fault. Hence it should neither be too slow which
may result in damage to the equipment nor should it be too fast which may result in
undesired operation.
Types of Relays
Types of protection relays are mainly based on their characteristic, logic, on actuating
parameter and operation mechanism. Based on operation mechanism protection relay
can be categorized as electromagnetic relay, static relay and mechanical relay. Actually
relay is nothing but a combination of one or more open or closed contacts. These all or
some specific contacts the relay change their state when actuating parameters are
applied to the relay. That means open contacts become closed and closed contacts
become open. In electromagnetic relay these closing and opening of relay contacts are
done by electromagnetic action of a solenoid. In mechanical relay these closing and
opening of relay contacts are done by mechanical displacement of different gear level
system. In static relay it is mainly done by semiconductor switches like thyristors. In
digital relay on and off state can be referred as 1 and 0 state. There are different types
of protection relay and some of them are listed below:
 Differential Protection
 Over-Current
 Under Current
 Over Voltage
 Under Voltage
 Over Frequency
 Under Frequency

56. 55
 Reverse Power
 Loss of excitation
Current Transformer
A CT is an instrument transformer in which the secondary current is substantially
proportional to primary current and differs in phase from it by ideally zero degree. A CT
is similar to an electrical power transformer to some extent, but there are some
difference in construction and operation principle. For metering and indication purpose,
accuracy of ratio, between primary and secondary currents are essential within normal
working range. Normally accuracy of current transformer required up to 125% of rated
current; as because allowable system current must be below 125% of rated current.
Rather it is desirable the CT core to be saturated after this limit since the unnecessary
electrical stresses due to system over current can be prevented from the metering
instrument connected to the secondary of the CT as secondary current does not go
above a desired limit even primary current of the CT rises to a very high value than its
ratings. So accuracy within working range is main criteria of a CT used for metering
purpose. The degree of accuracy of a metering CT is expressed by CT accuracy class
or simply current transformer class or CT class. But in the case of protection, the CT
may not have the accuracy level as good as metering CT although it is desired not to be
saturated during high fault current passes through primary. So core of protection CT is so
designed that it would not be saturated for long range of currents. If saturation of the core
comes at lower level of primary current the proper reflection of primary current will not come to
secondary, hence relays connected to the secondary may not function properly and protection
system losses its reliability. Suppose you have one CT with current ratio 400/1 A and its
protection core is situated at 500 A. If the primary current of the CT becomes 1000 A the
secondary current will still be 1.25 A as because the secondary current will not increase after
1.25 A because of saturation. If actuating current of the relay connected the secondary circuit of
the CT is 1.5 A, it will not be operated at all even fault level of the power circuit is 1000 A.

57. 56
The degree of accuracy of a protection CT may not be as fine as metering CT but it is
also expressed by CT accuracy class or simply current transformer class or CT
class as in the case of metering current transformer but in little bit different manner. In a
power transformer, if load is disconnected, there will be only magnetizing current flows
in the primary. The primary of the power transformer takes current from the source
proportional to the load connected with secondary. But in case of CT, the primary is
connected in series with power line. So current through its primary is nothing but the
current flows through that power line. The primary current of the CT, hence does not
depend upon whether the load or burden is connected to the secondary or not or what
is the impedance value of burden. Generally CT has very few turns in primary whereas
secondary turns is large in number. Say Np is number of turns in CT primary and Ip is
the current through primary. Hence the primary AT is equal to NpIp AT. If number of
turns in secondary and secondary current in that current transformer are Ns and Is
respectively then Secondary AT is equal to NsIs AT.
Voltage Transformer

58. 57
Potential transformer or voltage transformer gets used in electrical power system for
stepping down the system voltage to a safe value which can be fed to low ratings
meters and relays. Commercially available relays and meters used for protection and metering,
are designed for low voltage. This is a simplest form of potential transformer definition.
A voltage transformer theory or potential transformer theory is just like a theory of
general purpose step down transformer. Primary of this transformer is connected across
the phase and ground. Just like the transformer used for stepping down purpose,
potential transformer i.e. PT has lower turns winding at its secondary. The system
voltage is applied across the terminals of primary winding of that transformer, and then
proportionate secondary voltage appears across the secondary terminals of the PT. The
secondary voltage of the PT is generally 110 V. In an ideal potential transformer or
voltage transformer, when rated burden gets connected across the secondary; the
ratio of primary and secondary voltages of transformer is equal to the turns ratio and
furthermore, the two terminal voltages are in precise phase opposite to each other. But
in actual transformer, there must be an error in the voltage ratio as well as in the phase
angle between primary and secondary voltages.
The errors in potential transformer or voltage transformer can be best explained by
phasor diagram, and this is the main part of potential transformer theory.
The difference between the ideal value Vp/KT and actual value Vs is the voltage error or
ratio error in a potential transformer, it can be expressed as,
The voltage applied to the primary of the potential transformer first drops due to the
internal impedance of the primary. Then it appears across the primary winding and then

59. 58
transformed proportionally to its turns ratio, to the secondary winding. This transformed
voltage across the secondary winding will again drop due to the internal impedance of
the secondary, before appearing across burden terminals. This is the reason of errors in
potential transformer.

60. 59
Variable Frequency Drives
Introduction to VFDs
It is interesting to know that the first A.C. drive (400 HP) based on thyratron
cycloconverter-fed WRIM was installed in 1932 by F.E. Alexanderson of General
Electric in the Logan Power Station of Pacific Gas and Electric Company. From then
industrial drives have evolved rapidly by dedicated effort of many scientists and
engineers all over the world resulting in development of advanced drive technology
such as Variable Frequency Drive (VFD). VFD is a power electronics based device
which converts a basic fixed frequency, fixed voltage sine wave power (line power) to a
variable frequency, variable output voltage used to control speed of induction motor(s).
It regulates the speed of a three phase induction motor by controlling the frequency and
voltage of the power supplied to the motor.

61. 60
Working of VFDs
Any Variable Frequency Drive or VFD incorporates following three stages for
controlling a three phase induction motor.
Rectifier Stage
A full-wave power diode based solid-state rectifier converts three-phase 50 Hz power
from a standard 220, 440 or higher utility supply to either fixed or adjustable DC voltage.
The system may include transformers for high voltage system.
Inverter Stage
Power electronic switches such as IGBT, GTO or SCR switch the DC power from
rectifier on and off to produce a current or voltage waveform at the required new
frequency. Presently most of the voltage source inverters (VSI) use pulse width
modulation (PWM) because the current and voltage waveform at output in this scheme
is approximately a sine wave. Power Electronic switches such as IGBT; GTO etc. switch

62. 61
DC voltage at high speed, producing a series of short-width pulses of constant
amplitude. Output voltage is varied by varying the gain of the inverter. Output frequency
is adjusted by changing the number of pulses per half cycle or by varying the period for
each time cycle. The resulting current in an induction motor simulates a sine wave of
the desired output frequency. The high speed switching action of a PWM inverter results
in less waveform distortion and hence decreases harmonic losses.
Control System
Its function is to control output voltage i.e. voltage vector of inverter being fed to motor
and maintain a constant ratio of voltage to frequency (V/Hz). It consists of an electronic
circuit which receives feedback information from the driven motor and adjusts the output
voltage or frequency to the desired values. Control system may be based on SPWM
(Sine Wave PWM), SVPWM (Space Vector modulated PWM) or some soft computing
based algorithm.

63. 62
Induction Motor Characteristic under Variable Frequency Drive
In an induction motor voltage induced in stator, E is proportional to the product of the
slip frequency and the air gap flux. The terminal voltage can be considered proportional
to the product of the slip frequency and flux, if stator drop is neglected. Any reduction in
the supply frequency without a change in the terminal voltage causes an increase in the
air gap flux which will cause magnetic saturation of motor. Also the torque capability of
motor is decreased. Hence while controlling a motor with the help of VFD or Variable
Frequency Drive we always keep the V/f ratio constant.
Merits of using Variable Frequency Drives
Energy Saving
Primary function of VFD in industry is to provide smooth control along with energy
savings. The variable speed motor drive system is more efficient than all other flow
control methods including valves, turbines, hydraulic transmissions, dampers, etc.
Energy cost savings becomes more pronounced in variable-torque ID fan and pump

64. 63
applications, where the load’s torque and power is directly proportional to the square
and cube of the speed respectively.
Increased Reliability
Adjustable speed motor-drive systems are more reliable than traditional mechanical
approaches such as using valves, gears, louvers or turbines to control speed and flow.
Unlike mechanical control system they don’t have any moving parts hence they are
highly reliable.
Speed Variations
Beyond energy saving, applications such as crushers, conveyors and grinding mills can
use the motor and VFD’s packages to provide optimal speed variations. In some crucial
applications, the operating speed range can be wide, which a motor supplied with a
constant frequency power source cannot provide. In the case of conveyors and mills, a
VFD and motor system can even provide a “crawl” speed foe maintenance purposes
eliminating the need for additional drives.
Soft Starting
When Variable Frequency Drives start large motors, the drawbacks associated with
large inrush current i.e. starting current (winding stress, winding overheating and
voltage dip on connected bus) is eliminated. This reduces chances of insulation or
winding damage and provides extended motor life.
Extended Machine Life and Less Maintenance
The VFD’s greatly reduce wear to the motor, increase life of the equipment and
decrease maintenance costs. Due to optimal voltage and frequency control it offers
better protection to the motor from issues such as electro thermal overloads, phase

65. 64
faults, over voltage, under voltage etc. When we start a motor (on load) with help of a
VFD, the motor is not subjected to “instant shock” hence there is less wear and tear of
belt, gear and pulley system.
High Power Factor
Power converted to rotation, heat, sound, etc. is called active power and is measured in
kilowatts (kW). Power that charges builds magnetic fields or charges capacitor is called
reactive power and is measured in kVAR. The vector sum of the kW and the kVAR is
the Apparent Power and is measured in KVA. Power factor is the ratio of kW/KVA.
Typical AC motors may have a full load power factor ranging from 0.7 to 0.8. As the
motor load is reduced, the power factor becomes low. The advantage of using VFD’s is
that it includes capacitors in the DC Bus itself which maintains high power factor on the
line side of the Variable Frequency Drive. This eliminates the need of additional
expensive capacitor banks.
Slip Power Recovery
The fundamental power given to rotor by stator is called air gap power Pg. The
mechanical power developed is given by
The term 'sP' is called slip power.

66. 65
Applications of VFDs
1. They are mostly used in industries for large induction motor (dealing with variable
load) whose power rating ranges from few kW to few MW.
2. Variable Frequency Drive is used in traction system. In India it is being used by
Delhi Metro Rail Corporation.
3. They are also used in modern lifts, escalators and pumping systems.
4. Nowadays they are being also used in energy efficient refrigerators, AC’s and
Outside-air Economizers.