- - - ---- - - ----~-----

IUNIT 11

PRINCIPLES AND DESIGN OF ELECTRICAL MACHINE

---------------------------------------------------
~.1 INTRODUCTION
-....· 1 1.2.2 Factors for Consideration
• Electrical machine design involves application of The design should be carried out based on the given
science and technology to produce cost-effective, specification using available materials economically and to
durable, quality and efficient machines. Also the . achieve the following three key factors.
machines should be designed as per standard
1. Cost
specifications. The requirements like low cost and high
quality will be conflicting in nature and so a 2. Durability
compromise should be made between them. 3. Compliance with performance criteria as laid down
• Aim of design is to determine the dimensions of each in specifications.
part of the machine, !he material specification, prepare 11.3 DESIGN FACTORS
the drawings and furnish to manufacturing units. The mechanical force required for movement in rotating
Design has to be carried out keeping in view the
electrical machines can be produced both by
optimizing of the cost, volume and weight and at the
1. Electrostatic fields
same time achieving the desired performance as per
specification. Knowledge of latest technological trends 2. Electromagnetic fields
to supply a competitive product is a must. Design Both_the fields stores some energy
should conform to stipulations specified by 1. Electrostatic Fields
International / National standards. Design is the most In electrostatic machines, the energy density is limited
important activity.
by the dielectric strength of the medium used. For Air
• The designer should be familiar with the following 3
dielectric medium the energy density about 40J/m •
aspects:
Voltages that can be developed and used by normal
► Thorough knowledge of international/national means, the forces produced by electrostatic effects are
standards. very weak.
► Properties of good electrical materials (like 2. Electromagnetic Fields
copper), magnetic materials (like silicon steels),
In electromagnetic machines, magnetic effect is used
insulating materials (like Epoxy mica), mechanical
for production of force and there is no comparable
and metallurgical properties of all types of steel.
restriction in magnetic fields. Maximum value of flux
► Governing laws of electrical circuits. density that can be used is about l.6wb/m
2
.
► Laws of hea~ transfer. A small current can produce large mechanical force by
► Prices of materials used, foreign exchange rate s, electromagnetic means and therefore all the modern
types of duties levied on products. electrical machines are electromagnetic type
► Labour rates of both ski lied and unskilled l~bour 1.4 CONSTRUCTIONAL ELEMENTS OF
► Knowledge of competitor's p rod ucts. TRANSFORMER
11.2 DESIGN OF-MACHINE • The transformer is a static electromagnetic device used
Design is defined as a creative physical rec;1lization of to transfer electrical energy from a high potential

theoretical concepts. (voltage) circuit to low potential (voltage) circuit or

vice-versa.
1.2.1 Engineering Design
· • t' of science technology • It consists of two or more windings which link with a
Engineering Design is a app 11ca ion ' 'f d
common magnetic field. An iron core serves as a path
and invention to produce machines to perform speci ,e
for magnetic flux.
tasks with optimum economy and efficiency.
(1.1)
 .
..J
D DESIGN OF ELECTRICAL MACliJNe
PRIN. OF ELEC. MACHINE DESIGN (BATU ELECT.) (1.2) pRJNOPLES AN ~
• The constructional elements of the transfo.nner are called stator an d
the rotating electromagnet is called

windings, core, tank and cooling tubes or radiators. rotor. f

• A simple transformer has two windings and they are •
.
The basic cons
tructional elements o a rotating
d
. h' are stator and rotor. In c machines
called high voltage winding and low voltage winding. electrical mac me • d'
. f field core and wm mg. The rotor
One of the winding is connected to supply and it is the stator consists o . .
. f ture core and winding. In ac
called primary. The other winding is connected to load comprises o arma .
. the stator has armature core and winding.
and it is called secondary. h
mac mes · d'
· t s of field core and wm mg.
• The two different types of transformer constructions
The rotor cons1s
• • I eIements of various electrical
The construct1ona
are core type and shell type. In core type transformer
the windings surround the core and in shell type machines are listed here.
transformer the core surround the windings. The core 1. Constructional Elements of DC Machine,
and winding assembly is housed in the tank. Cooling Stator Rotor
tubes or radiators are provided around the tank • Armature core
surface in order to increase the effective cooling
• Yoke or Frame
• Armature winding
surface. • Field winding
• Poles • Commutator
1.5 CONSTRUCTIONAL ELEMENTS OF
• Pole shoe • Others Brush
ROTATING MACHINE
• Field winding interpole • Brush holder
• The rotating electrical machines converts electrical
2. Constructional Elements of Salient Pole Synchro-
energy to mechanical energy or vice-versa. The energy
nous Machine
conversion takes place through magnetic field. Every
Stator Rotor
rotating machine have the following three quantities.
The presence of any two quantities, will produce the • Frame • Field pole,
third quantity. • Armat ure core • Pole shoe.
► Magnetic Field - (Field) • Armature winding • Field winding.
► Magnetic field - (Armature) • Damper winding
► Mechanical force 3. Constructional Elements of Cylindrical Rotor Syn-
• In generator. the armature is rotated by a mechanical chronous Machine
force inside a magnetic field or the magnetic field is Stator Rotor
rotated by keeping armature stationary. By Faraday's • Frame • Solid rotor
law of induction, an emf is induced in the armature.
• Armature core • Field conductors or
When the generator is loaded, the armature current
flows, which produce another magnetic field (armature bars
magnetic field). Hence in a generator, by the presence • Armature winding
of a magnetic field and mechanical force, an another 4. Constructional Elements of Squirrel Cage Induc-
magnetic field is produced. tion Motor
• The mechanical force developed by the motor is due
Stator
to the reaction of two magnetic fields, A current Rotor
carrying conductor has a magnetic field around it. • Frame • Rotor core
When it is placed •in another magnetic field it
• Stator core • Rotor bars
experiences a mechanical force due to the reaction of
two magnetic field. Hence in a motor by the presence • Stator winding • End ri~g
of two magnetic fields a mechanical force is 5. Constructional Elements of Slip-Ring Induction
developed. Motor
• From the above discussion it is clear that any rotating
Stator
machine requires two magnetic field and one of the Rotor
field is rotating. Hence a rotating machine will have a • Frame • Rotor core
stationary and rotating electromagnet, each consisting • Stator core • Rotor winding
of a core and winding. The stationary electromagnet is
• Stator winding • Slip ring
 ~N. OF ELEC. MACHINE DESIGN (BATU ELECT.) (1.3) PRINOPLES AND DESIGN OF ELECTRICAL MACHINE

1.6 BASIC STRUCTURE OF ELECTRICAL !1.7 CLASSIRCATION OF DESIGN PROBLEMS /

MACHINE ..
• The design of an electrical machine involves solution of
The Basic Structure of Electrical Machines many complex and engineering problems.
It consists of following parts • The Design Problems may be Classified under the
1. Magnetic circuits Following Four Headings.
2. Electric circuits 1. Electromagnetic design
3. Dielectric circuits 2. Mechanical design
4. Thermal circuits 3. Thermal design
5. Mechanical parts 4. Dielectric design
Cores Frames Each problem may be solved Separately and the results
are combined to give overall solution. Each of these
three major problems may be further divided into
simple problems and solutions of individual problem
are combined to give the solution of a major design
problem.
1. Electromagnetic Design
The electromagnetic design problem in rotating
machines involves the design of stator & rotor core
Rotor dimensions, stator & rotor teeth dimensions, air-gap
length, stator and rotor windings. In transformer, it is
Stator--a1-----t..i the problem of designing the core and the windings.
2. Mechanical Design
Fig. 1.1: Basic constructional details o{ r~tating machines
The mechanical design in rotating machine involves
1. Magnetic Circuits : It provides the path for the the design of frame (enclosure), shaft and bearings. In
magnetic flux and consists of air g·ap, s.tator and rotor transformer, it is -the design of tank (i.e., housing for
teeth and stator and rotor cores (yokes) core and winding assembly).
2. Electric Circuits : It consists .of stator and rotor 3. Thermal Design
winding, winding of transformer. It shoold ensure that
The thermal design in rotating machine involves the
required emf is induced with no comp!~xity in winding design of cooling ducts in core and cooling fans. In
arrangement. case of large machines coolants like air or hydrogen
3. Dielectric Circuits : The dielectric. circuit consists of may be forced to circulate in the ducts and air-gap. In
insulation required to isolate one -conductor from transformer, it involves the design of cooling tubes or
another and also the windings frq112_ the core. The radiators.
insulating materials are essentially- non-metallic and 4. Dielectric Design
may be organic or inorganic ,natur~ o~ synthetic. Another important design problem, that may require
4. Thermal Circuits : The thermal circuits is concerned great attention is the design of insulations (Dielectric
with mode and media for dissipat1on qJ heat produced design). Dielectric materials are used to insulate one
inside the machine on account of losses. conductor from other and also the windings from the
s. Main Mechanical Parts core. The dielectric materials are designed to withstand
high voltage stresses. The breakdown of dielectric
• Frame
materials may lead to failure of machine.
• Bearings
!1.8 LIMITATIONS IN DESIGN
• Shaft
Following are the limitations for design :
A successful design brings out_~n economic
compromise for space occupied by iron, copp~r 1. Saturation
(aluminum) insulation and coolan~ _(wffich may be air, 2. Current Density
hydrogen, water or oil) 3. Temperature rise
 . PRINCIPLES AND DESIGN OF ELECTRICAL MACHINE
(1.4)
PRIN. OF ELEC. MACHINE DESIGN (BATU ELECT.)
9. Consumer's Specif ication s : The specifi cations as laid
4 . Insulat ion down in the consum er's order have to be met and the
5. Efficiency design evolved should be such that it satisfie d all the
and also the econo mic constra ints
6. Mechanical parts spec,·t·1cat·ions
7. Comm utation imposed on the manufa cturer.
8. Power factor 10. Standard Specifications : This specifi cations are the
9. Consumers specifications bigges t strain on the design because both the
10. Standard specifications manufa cturer as well as the consum er canno t get away
Electromagnetic machines use from them withou t satisfyi ng them.
1. Saturation
ferromagnetic materials .The maxim um allowab le flux 11.9 STANDARD SPECIFICATIONS
density to be used is determined by the saturation • Every country has standard organiz ation to fix standa rd
level of the ferromagnetic material used. A high value specifications for the manufacturer. The specific ation is
of flux density results in increased core loss and guidelines for the manufacturers to produc e econo mic
increased excitation resulting in higher cost for the products withou t compro mising quality .
field system. It also introduces harmonics. • The manufacturer who are compil ing with standa rds
2. Current Density : Higher current density reduces the and will be issued a certific ation for their produc ts. The
volume of copper but increases the losses and quality of the certifie d produc ts will be period ically
temper ature. monito red by the standard organi zation.
3. Temperature Rise : The most import ant parts of the 1.9.1 Stand ardiza tion and Stand ards
machine is insulation. The operating life of a machine
• Standardization and standa rd specifi cations play an
depends upon the types of insulating materials used in
import ant part in the choice, design manuf acture and
its construction .The life of insulating materials in turn
operation of any apparatus.
depends upon the temperature rise of the machine.
Proper cooling and ventilation techniques are required • Every countr y has a standards organi zation to fix
to keep the temperature rise within safe limits. stan~ard specifications for the manufa cturers .
4. Insulation : The insulation materials used in a machine • The specification are guideli nes for the manufacturers
should be able to withstand the electrical ,mechanical to produc e econom ic produc ts withou t compr omising
and thermal stresses which are produced in the quality.
machines. The type of insulation is decided by the· • The manufacturers who are compi ling with the
maximum operating temperature of the machine parts standards will be issued a certific ation for their
where it is put. And also the size of the insulation is products.
decided by maximum voltage stress and mechan ical • The quality of the certifie d produc ts will be periodically
stresses produced. monito red by the standa rd organi zation.
5. Efficiency : The efficiency of the machine should be as Advantages of Standardization
high as possible to reduce the operating costs. • To manufa cturer
6. Mechanical Parts : The construction of mechanical
► Reduction in cost as econo my results when
parts should be as simple as possi ble and also it is
numbe r of objects are built at the same time.
techno logical ly good. The design of mechanical part is
► Easy to produc tion plannin g.
particu larly import ant in case of high speed machines.
► Stream -lining the a produc tion line.
7. Commutation : The problem of commu tation is
• To user
import ant in the case of commu tator machines.
Comm utation conditi on limit the maximum output that ► Standardization means interch angeab ility of

can be taken from a machine. equipm ents and spares.

• To design er
8. Power Factor : Poor power factor results in larger
values of current for the same power and therefore ► It means Rigidit y

large conduc tor sizes have to be used. Power factor Indian standards these are issued by Bureau of Indian
problem is particularly import ant in case of inducti on Standards_ (BIS), Manak Bhavan, Bahad ur Shah Zafar Road,
New Delhi
motor.
PRIN. OF ELEC. MACHINE DESIGN (BATU ELECT.) (1.5) PRINOPLES AND DESIGN OF ELECTRICAL MACHINE

• The Standard Specifications Issued for Electrical • IS 4722 - 1992 : Specifications for rotating electrical
Machines: machines.
► Standard ratings of machines • IS 12802 - 1989 : Temperatute rise measurement of
► Types of enclosure rotating electrical machines.
► stand ard dimensions of conductors to be used • IS 4889 -· 1968 Method of determination of efficiency
► Meth0 d of marking ratings and name plate details. of rotating electrical machines.
► Performance specifications to be used • IS 13555 - 1993 : Guide for selection and application of
► T_ypes of insulation and permissible temperature three phase induction for different types of driven
nse equipment.
► Permissible losses and range of efficiency • IS 7132 -1973 : Guide for testing Synchronous
► Procedure for testing of machine parts and machines.
machines • IS 5422 - 1996 : Turbine type generators.
► Auxiliary equipments to be provided • IS 7572 - 1974 : Guide for testing single-phase ac and
► Cooling methods to be adopted universal motors.
• In India, the Bureau of Indian standards (BIS) has laid • IS 8789 - 1996 : Values of performance characteristics
down their specification (ISI) for various products. The for three phase induction motors.
standards will be amended time to time, in order to • IS 12066 - 1986 : Three phase induction motors for
include the latest developments in technology. machine tools.
• The name plate of the rotating machine has to bear • IS 1180 - 1989 : Specifications for outdoor 3-phase
the following details as per ISi specifications. distribution transformer 100 kVA. [Sealed and Non-
sealed type)
► KW or KVA rating of machine
• IS 2026 - 1994 : Specifications of power transformers.
► Rated working voltage
• IS 11171 - 1985 :_ Dry type power transformers.
► Operating speed • IS 5142 - 1969 : Continuously variable voltage auto
► Full load current transformers
► Class of insulation • IS 10028 -1985 : Code of practice for selection,
► Frame size installation and maintenance of transformers.
► Manufacturers name • IS 10561 - 1983 : Application guide for power
transformers.
► Serial number of the product
• IS 13956 - 1994 : Testing transformers.
1.9.2 Some of the Indian Standard Specifications
• IS 9678 - 1980 : Methods of measuring temperature
Numbers along with Year of Issue for rise of electrical equipment.
Electrical Machines are Listed Here • IS 12063 - 1987 : Classification of degree of protections
• IS 325 - 1996 : Specifications for three phase induction provided by enclosures of electrical equipment.
motor. • IS 3855 - 1966 : Standard dimensions of rectangular
• IS 1231 - 1974 : Specifications for foot mounted enamelled Copper conductor.
induction motor. • IS 449 - 1962, : Standard dimensions of enamelled
• IS 4029 - 1967 : Guide for testing three phase round Copper conductor (oleo resinous enaniel).
• IS95 - 1960 : Standard dimensions of enamelled rounc
induction motor.
copper conductor (synthetic enamel).
• IS 996 - 1979 : Specifications for single phase cc and
• IS 1897 - 1962: Standard dimensions of bore coppe
universal meter.
strip.
• IS 1885 - 1993 : Specifications for electric and magnetic
• IS 1666 - 1961 : Standard dimensions of paper covere
circuits. rectangular copper conductor for transformer windin~
• IS 9499 - 1980 : Conventions concerning electric and • IS 2068 - 1962 : Standard dimensions of cottc
magnetic circuits. covered rectangular copper conductor for transforrr
• IS 7538 - 1996 : Specifications of three -phase induction windings.
motor centrifugal pumps and agricultur~I applications. • IS 3454 - 1966 : Standard dimensions of paper cove1
• IS 12615 - 1986 : Specifications for energy efficient round conductors used for transformer windings.
induction motor. • IS 450 - 1964 : Standard dimensions of cotton cove
• IS 9320 - 1979 Guide for testing de machines. round conductors for transformer windings.
 PRIN. OF ELEC. MACHINE DESIGN (BATU ELECT.) (1.6)
PRINOPLES AND DESIGN OF ELECTRICAL MAC~
p
!1.10 GENERAL DESIGN PROCEDURE Total flux linkages
T
q, =
• In general any electrical machine has two windings. 2
n
The transformer has primary and secondary winding. ... (1.1) T
The de machine and synchronous machine has k=l
ir
armature and field winding. The induction machine has C
Nk = The number of turns which link with flux <l>k
stator and rotor winding.
• The basic principle of operation of all electrical In this case there .1s a change m · the value of the flux
. .
linkages of the coil,an induced emf is produced is given by,
machine is governed by Faraday's law of induction.
Also in every electrical machine the energy is - d'I' V It ... (1.2)
e - - dt o
transferred through the magnetic field. Hence a
general design procedure can be developed for the (-) sign indicates that the direction of the ind uced emf
design of elelctrical machines. 1
The Change in Flux Linkages can be Caused in Three
• The general design procedure is to relate the main C
Ways:
dimentions of the machine to its rated power output. a
1. The coil is stationary with respect to flux and the flux
An electrical machine is designed to deliver a certain
varies in magnitude with respect to time.
amount of power called rated power. The rated power
output of a machine is defined as the maximum power 2. The flux is constant with respect to time and is
that can be delivered by machine safely. In de machine stationary and the coil moves through it.
the power rating is expressed in kW and in ac machine 3. Both the changes mentioned above occur together
in kVA. In case of motor the output power is expressed (ie)the coil moves through a time varying field.
in HP.
Method 1 : Where the coil is stationary and the flux is time
• In electrical machines the cor~ and winding of the varying, an emf called transformer or pulsational emf is
machine are together called active part. (Because the
produced. No motion is involved. There is no energy
energy conversion takes place only in all active part of
conversion. The process that really takes place is energy
the machine).
transference. This principles used in transformers.
• A general output equation can be developed for de
Method 2 : The flux cutting rule can be employed to
machine which relates the power output to volume of
illustrate the emf generated in a conductor moving in a
active part (D2L), speed, magnetic and electric loading.
constant stationary field. The emf generated in a conductor
• Similarly a general output equation can be developed of length moving at right angles to a uniform, stationary,
for ac machine which relates kVA rating to volume of time invarying magnetic field.
active part (D2L), speed, magnetic and e·lectric loading.
e = Blv Volt
11.11 BASIC PRINCIPLES I B - flux density ,wb/m2 (T)
The action of electromagnetic machines can be related to
I = length of conductor ,(m)
three basic principles which are,
v - linear velocity of conductors (m/s)
1. Induction
The generated emf in this case is called a "motioned emf'.
2. Interaction
Emf generated due to motion of conductor ,since motion is
· 3. Alignment
involved in' the production of this emf,the process involves
1. Faraday's Law of Electromagnetic Induction electromechanical energy conversion.
This law states that emf induced in a closed electric
This principle is utilize in rotating machines like DC.
circuit is equal to the rate of change of flux linkages.
induction machines, synchronous machines.
Flux linkages 'I' = N 4>
Method 3 : A conductor or coil is moving across a
N _ the number of turns in a coil
stationary time varying magnetic field (flux)and therefore
~ _ flux linking with all of them
both transformer as well as motional emf are produced in
In most cases, the flux ~ does not link with all the turns or the conductor or coil. This process involves bot~
alternatively all the turns do not link with the same flux. transformer and energy conversion.
 pRJN. OF ELEC. MACHINE DESIGN (BATU ELECT.) (1.7) PRINCIPLES AND DESIGN OF ELECTRICAL MACHINE
This principle is utilized in the commutator machines. produced remains unaltered. Fig. 1.2 (c) shows the
2. Interaction Principle (Biot-Savart's Law) effect of reversing the current when the direction of
This law gives the value of force produced on account of the field is unchanged. It is clear that under these
interaction between a magnetic field and a current carrying conditions the direction of force produced is reversed.
conductor . • Biot Savart's law can be applied to determine force
F = B l I sina Newton ... (1.3) between two current carrying conductors. Fig. 1.3
where, shows two parallel current carrying conductors of l
B - Flux density, wb/m 2 (T) separated by a distance D and situated in a medium of
l - length of conductor permeability µ. The two currents are 11 and 12. In Fig. 1.3
- current carried by conductor, A (a) the two currents flow in the same direction while in
a - Angle between the direction of current Fig. 1.3 (b) they flow in the opposite direction. The
and the direction of magnetic field resultant magnetic fields are also shown. It is clear that
The direction of force produced is perpendicular to both when conductors carry currents in the same direction,
current and magnetic field. Conductor and magnetic field there is a force of attraction between them, while there
=
are perpendicular to each other, thus a 90°,sin 90° 1 = is a force of repulsion between them if they carry
F = B l I Newton currents in the opposite directions.
I I I I I I / I I The value of the flux density, at the position of
I I I I I I I I I conductor carrying current I2.
I I I I I J I I I Due to current I is :

I I I \ ~ ( \~\ taII Flux density p = µH

..1L
I .....,__~~ I I \ I ' I I µ = 27tD

i~~ ~i~ ~\~ !~ ~ Electromagnetic force

F = BIL
(a) (b)
I I I I I ..1L
F = µ 27tD l2/
I I I \ I I
I
I
U@'I ~/
I j
ta
I
I
.J!£...
F = 2'tD IiI2 Newton
r
I I I
I I I
/( I
/ I I
D

,l
---+i

''' ,'' (c) i L

•
Fig. 1.2 : Force on a current carrying conductor situated
perpendicular to a magnetic field (Interaction law)
In Fig. 1.2 (a), B represents the flux density of an
undisturbed (original) magnetic field. The introduction
J
of a current carrying conductor introduces new
magnetic field. The original field and the field due to
conductor combine to produce a resultant field as
shown in Fig. 1.2 (b). The resultant field is distorted in
the neighbourhood of the conductor, the resultant flux (a) Attraction (b) Repulsion
density being greater on one side and l~sser on the Fig. 1.3 : Forces between current carrying conductor
other and this results in production of an 3. Alignment
electromagnetic force in the direction . in~icated. In
If a magnetic field exists in a low permeability medium like
case the increase in flux density on one side is equal t_o
air and if piece of high permeability material is placed in
the reduction on the other side, the electromagnetic
this field, the latter experiences a force which tries to align
force is given by Eq. 1.3.
it with the direction of field in such away that it occupies a
• When either the direction of the current or the
position of minimum reluctance. The principle of
direction of the magnetic field is reversed. However, if
production of force due to alignment is used in reluctance
the directions of both the current as well as the
motors.
magnetic .field are reversed, the direction of the force
PRIN. OF ELEC. MACHINE DESIGN (BATU ELECT.) (1.8) PRINOPLES AND DESIGN OF ELECTRICAL M ~

1.12 MAIN DIMENSIONS OF ROTATING General Symbols Used for Designing of Inductj°'1
MACHINE Motor
D = Inner diamete r of stator or stator
• In rotating machines the active part is cylindrical in
bore, m
shape. The volume of the cylinder is given by the
L = Stator core length, m
product of area of cross section and length. If D is the
n = Speed, rps;
diamete r and L is the length of cylinder, then the
ns = synchronous speed, rps;
volume is given by nD 2l/4. Therefore D and L are
specified as main dimensions. p = No. of poles;
z = Total no. of armatur e or stator
conductors
T = Pole pitch, m;
Tph = Turns per phase
lz = Current in each conduct or, Amp
Kw = Winding factor;
Ip11 = Current per phase, Amp;
Eph = Induced EMF per phase, Volt;

Fig. 1.4 : Main dimensions of rotating machines Q = kYA rating of machine

• In case of de machine, D represent the diameter of 1.13 DIMENSIONS AND RATING OF THE
armature and L represent the length of armature. In MACHINE
case of ac machine, D represent the inner diameter of The power rating of rotating machines is related to its main
stator and L represent the length of stator core. The dimensions, namely the armature diameter and armature
Fig. 1.5 shows the main dimensions of rotating length. A few general equations are developed are
machines applicable to all types of rotating machines like DC,
Here, D, = Diameter of rotor induction and synchronous machines. However it must be
l9 = Length of air-gap mentioned that the design process of different machines
cannot be demonstrated with a set of few general
equations.
. t ,Lg
11.14 TOTAL LOADINGS
t
i-------- • Total Magnetic Loading
~
L
The Total flux around the stator periphery at the air
gap is called the total magnetic loading.
Total magnetic load = pq,
• Total Electric Loading :
(a) Main dimensions of DC machines
L The t otal number of ampere conducto rs around the
stator periphery is called the total electric loading.
Total electric loading = IzZ
• Specific Magnetic Loading
The Average flux density over the air gap of a machine
----L---..i
is known as specific magneti c loading and it is also
--- --- defined as the ratio of total flux around the air gap and
the area of flux path at the air gap.
(b) Main dimensions of AC machines B = Total flux around the air gap _ ..£t_
av Area of flux path at the air gap - 1tDL
Fig. 1.5 : Main dimensions of rotating machines
PRIN. OF ELEC. MACHINE DESIGN (BATU ELECT.) (1.9) PRINOPLES AND DESIGN OF ELECTRICAL MACHINE

• Specific Electric Loading • . Alternating Current Machines

It is defined as the ratio of total number of ampere Output Equation:"Expressed in terms of its main
conductors and the armature periphery at the air gap. dimensions, specific loadings and speed."
Total number of ampere conductors - ~
ac = ---:---_;;;..~:.:...::.~~~:.:.:.:::.~~ IZ Consider, m = number of phases and having 1 circuit
Armature periphery at the air gap - 1tD (parallel path) per phase
The typical values of specific magnetic loading and kVA rating of machine:
electric loading for various types of rotating machines Q = no. of phases x o/p voltage / phase x
are listed in Table 1.1 3
current/ phase x 10·
Table 1.1 3
= mEphlph x 10·
Ma~hine , ___Specific.Magn~tic · · Specific Electric Terminal voltage/phase may be taken as equal to induced
loadl~g Bav In ~~ingacin . EMF/phase.
'

-·wb/mL-.
• ., ❖:
"
,, lmf);Cond./m We have,
DC machine 0.4 to 0.8 15000 to 50000 Eph = 4.44f(j)TphKw,
3
Q = m x 4.44f(j)TphKw x Iph x 10·
Induction motor 0.3 to 0.6 5000 to 45000
But, f = p.nJ2
Synchronous machine 0.52 to 0.65 20000 to 40000
Turbo-alternator 0.52 to 0.65 50000 to 75000 Q = m x 4.44 (E:.!11
2
) (j)TphKJph x 10-
3

!1.15 OUTPUT EQUATION ... ·- - ..I = 1.11 kw (p(j) 2m Iph Tph) ns X 10-
3

nExpressed in terms of its main dimensions, specific Current in each conductor Iz = Iph (as only one
loadings and speed.'' circuit/phase)
Total no. of armature conductors
• Direct Current Machines
Z = no. of phases x (2 x turns per phase)
Power level developed by armature in kW
= 2mTph
Pa = generated EMF x armature current
3 Total electric loading= IzZ = 2miphTph
X 10- 3
Hence, Q = 1.11 Kw(p(j))(lzZ}nsxl0-
= E.la X 10-3
= 1.llKw(total magnetic loading) (total
E = <!> · Z-nEa electric loading) (synchronous speed)
But,
x10·3
E -3 But, p(j) = 1tOL-Bav and IzZ = 1tO-ac
Pa = (j) · Z · n a Ia X 10 3
Q = 1.11 Kw (1tOL·Bavl (1tO-ac)ns X 10-
= (p . (j)) (: · z) n x 10-
3
(since Iz = :)
2
3 2
= (l.ll1t2 BavacKw x 10- )0 Lns
= (11 BavacKw X 10-3)0 Lns
= (P . (j)) (lz . Z) n x 10-3 2
= C0 0 Lns
3
Hence, Pa = (total magnetic loading) (total electric where, C0 = liBavacKw x 10-
3
loading) (speed in rps) x 10- · =~ fficient
p. g, The capacity of motor is usually given either in horse
Specific magnetic loading Bav = 1t . o . L or
power (h.p) or in Kw.But this has to be changed in kVA
p . <I> = 1t . D . L . Bav kW
Input kVa, Q = . cos(j)
By substituting these values we get, 11
I ·Z The rating of induction motor is given in Horse Power
Specific electric loading ac = ~ or Iz · Z = 1t · O · ac HP x 0.746
3 Q = ,, . cos <I>
Pa = (1t . D . L . Bav) (1t · D · ac) n x 10-
2 Full load p.f. usually lies 0.82 to 0.92
= C0 D Ln
3
High p.f. is generally obtained with high speed motor.
2
where, Co = 1t . Bav . ac X 10- The full load efficiency is usually varies between 0.82 to
c 0
= Output co-efficient 0.93
 PRIN. OF ELEC. MACHINE DESIGN (BATU (1.10) pRJNOPLES AND DESIGN OF ELECTRICAL MAt1-tt,.
ELECT.) ~
• Factors Affecting Size of a Rotating Mac
hine: value of flux d en sity in the teet.h is not exceeded . ll.
. . ·•~
Speed: maximum va1ue o f flux density in the teeth 1s betwe
2 eri
According to the outp ut equation of a rotat 1.7 to 2.2 Wb/ m ·
ing machine
with a given value of kVA rating
2. Magnetizing Current f h" . d
Q = CoD2Lns The magnetiz . . current O a mac me 1s irert,.
• ing . . · ·1
⇒ Represent the volume of machine na 1 to m mf· The mm f 1s directly proportional
2 •
D L = _Q_C proport1o
ons
to spec,"fi1c magnetic loading. Hence . a large value 01
Speed, 1
ns ex D2L spec1.f1c
.
magne 1ct· load ing resu lts in increased values 01
.
magnet 1zin . g mm f and magnetizing current.

It is clear from outp ut equation that the • The vaIue of magn etizing current is not usually a
volume of active . us d es1g
. n consideration in de machines. But in
parts is inversely proportional to the spee serio
d. Increase in
speed means less volume (smaller size) that induction motors an increased value of
is low cost. magnetizing
Out put Co-Efficient: current results in low pow er factor. Henc
e specific
According to the output equation of a magnetic loading in induction moto rs is
rotating machine lower than in
with a given value of speed de machines. For synchronous mac
hiens the
Q = C0 D2 Lns magnetizing current is not so critical and
the value of
Output Co-efficient, specific magnetic loading is intermediate
between that
1 of de and induction machines.
Co ex D2L
3. Core Loss
It is clear from outp ut equation that the volu
me of active • The core loss in any part of the magnetic
parts is inversely proportional to the circuit is
value of output directly proportional to flux density for
coefficient C0 • speed. So Increase in value which it is
of Co results in
reduction in size and cost of the machine going to be designed. The flux dens
.. ity is directly
C0 = Output co-efficient proportional to the specific magnetic load
ing. Hence
11.16 CHOICE OF SPECIFIC LOADINGS ,, . the core loss in a machine varies directly
~/ · as the specific
magnetic loading. Thus a large valu
1.16 .1 Choice of Specific Magnetic Loadings e of specific
magnetic loading results in increased core
The choice if specific magnetic loading loss and
is influence by consequently a decreased efficiency and
certain factors. Some are general in nature an increased
and apply to all temperature rise.
types of machines and some are spec
ific and apply to
individual machines. • With a given specific magnetic loading,
the core los5
Some general factors are: increases as the frequency of flux reversals
is increased
1. Maximum flux density in iron parts of mac This is because the hystersis loss
hine is direct!~
2. proportional to the frequency and eddy
Magnetizing current curr ent loss i~
proportional to the square of the frequency
3. Core losses . lt follo w
that for high speed de machines, or high
1. Max imu m Flux Den sity in Iron frequency a
machines, specific magnetic load ings mus
t be reduce
• The max imum flux density in any iron part
of machine in order to achieve lower iron loss.
mus t be belo w a certain limit ing value.
The maximum
flux dens ity occurs in the teeth of the 1.16.2 Choice of Specific Electric Loading
armature (or
stat or core). [Teeth are the port ion 1. Copper Loss and Tem p Rise: larg e valu
of the core in e of ac, nee<
betw een slots]. greater amo unt of copper, results in
high er copp
• The flux dens ity in the teet h is dire ctly
prop ortio nal to losses and large temp erat ure rise
spec ific mag neti c load ing. Hence the choi 2. Voltage: for high volta ge machines, less
ce of specific
value of
load ing shou ld be such that the maximum
value of flux should be chosen, because it needs larg
den sity in the teet h is not exceeded e space 1
. The max imum insulation.
Pt\lN, OP Ii.IC, M~CHINI DISIGN (BATU ILICT.) _ _ (1.11:L
) _ __!P~Rl~N~C!!:
IP!:!
LE~S~A~N~D~D~ES~IG~N!.:O
~F
:..!E
::L::
ECT
~ Rl:C=A
:. :.;L
:...M
_A
_C_H_IN_E
Let w~ • Width of tho slot M'aximum allowable specific electric loading,
d, iii Oopth of the slot .JL " ' (1.6)
ac = P6 c
S, • Slot space factor
It can be inferred that the heat dissipated per unit area
Y, • Slot pitch
of armature is proportional to specific electric loading.
\) • Current density
From equation (1.6) it is clear that allowable specific
Tht spaclfi<' @lectrlc loading can be related to the electric loading is fixed by allowable temperature rise
8bove tern,s by the equation,
and the cooling coefficient. A high value of ac can be
ac • d, (wJy,) os, " ' (1.4) used in a machine when a high temperature rise is
From tq llMi011 (l.4) it is cleor that the specific loading allowed. The maximum allowable temperature rise if a
Is directly proportional to slot space factor s,. In high machine is determined by the type of insulating
volt1g1 rnftchines, greater insulation thickness is materials used in it. When better quality insulating
required and therefore the space factor for these materials which can withstand high temperature rises
machines Is lower. Hence an increase In voltage will, in are used in the machines, increased values of specific
general. necessitate a reduction in specific electric electric loading can be used. This results in reduction in
loading oc.
the size of the machine.
3. Ov•rload Capacity: larger value of ac , results i~ large
A high value of electric loading may be used if the
number of turns per phase. Which in turn increase the
cooling coefficient of the machine is small. The value of
leakage reactance of the machine, reduces the
overlot1d capacity of the machine. cooling coefficient depends upon the ventilation
conditions in the machine. High speed machines will
4. P•nnlsslble Temperature Rise
have better ventilation and so higher value of ac can
Let. 8 = Temperature rise
be used.
S = Dissipating surface
5. Size of Machine
Q = Loss dissipated
c = Cooling Coefficient From the equation (1.6) it is clear that ac depends on
the dimension of the slot. For large machines the
~ = Current density
depth of the slot will be greater and so higher values
p = Resistivity
of ac can be used. Actually if the current density and
Loss di~sipated per} (current)' x No. of conductors x Resistance
u111t 11rea of
the slot space factor are assumed constant, then
q• Surface area of armature
ormature surface specific electric loading is proportional to the diameter
2
11 X Z X p L /81 as slot depth usually depends upon the diameter.
= itDL
6. Current Density
IZ I
= ~ x .!L. x p = ac 6p " ' (1.5)
From the equation, q = ac 6 p it is clear that a higher
itD az
value of specific electric loading can be used in a
where, 6 = IJa1
machine which employs lower current density in its
Also, q = Q,/S
conductors. (because ac = q/6 p).
The temperature rise, 8 = ~ = qc
5 Typical values of current density are in the range of 2
q = 8/c to 5 A/mm 2. The temperature rise is usually 40°(
(above ambient) for normal applications and cooling
ac 6 p = !!C
coefficient is between 0.02 and 0.035°( W-m2.
 _~_!._.DESIGN
l~_~_!!_l!._~_(BATU
~_~_!Y._!1.y
_~_!_l.____.::.::_-.::.::~~ 2
PRIN.
- -OP- ILIC.
--- ~ACHINI
~- ~- ~_ ... ELECT.) (l.l ~-
) -~P~RI~N~C~IP~L~f)~A~N_::LI_:..,:...:'"="-
-· --·-·----- - - - -
-- ~

[
i""'
- - SOLVED EXAMPLES ] Hence, flux per pole, E v soo
Example l.l : A 350 kW, 500 v, 450 rpm, 6-pole, de
~ =Zn == Zn =660 X 450/60
generator ls built with an armature diameter of 0.87 m and = 0.101 Wb
core length of 0.32 m. The lap wound armature has 660
Specific magnetic loading,
conductors. Calculate the specific electric and magnetic _£!_ _ 6 X 0.101
loadings. Bav = 1tDL - 7t X 0.87 X 0.32
Solution : Givan : 2
- 0.6929 Wb/m
p = 350 kW
jl.17 ELECTRICAL ENGINEERING MATE~[
n = 450/60 rps
machines are classified intc
Materials used in Electrical
D = 0.87 m
three types:
Z = 660
1. Conducting;
V = 500 V
2. Magnetic
p = 6
3. Insulating
L = 0.32 m
Design of electrical machines depends mainly on quality oi
Lap wound materials used. If low quality materials are used, t~
Specific electric loading, machine will be less efficient, more bulky, higher weight
12 and higher cost. Operational running cost will also be
ac = ~ higher. A designer should have perfect knowledge o/
Specific magnetic loading, properties and cost of these materials so that the design
can be both efficient and cost-effective.
BI V -- ..et.
1tDL 1.17.1 Conducting Materials·
The power output of the generator, Conducting Materials are of Following Categories
P = VI x 10· 3 in kW 1. High conductivity materials

full load current, 2. High resistivity materials

P 350 3. Electrical carbon materials

I = V x 10· 3 = 500 x 10·3 =700 amps 4. Super conducting materials
Neglecting field current, 1. High Conductivity Materials (low Resistance): Used
11 ,.. I for windings of electrical machines and equipments.
Material with lowest resistance should be selected so
current through}
armature current 1, that it contributes lowest ohmic losses to enhance
each armature lz = N f th =
conductor o. o para 11 e1 pa s a efficiency and to reduce temp-rise.

700 Requirements of high conductive materials:

= 6 =116.67 amps • Highest Possible conductivity (least Resist ance)
(a = p in lap wound) • Least possible temperature coefficient of resistance
specific electric loading, • Adequate resistance to corrosion
.!if 116.67 X 660 • Adequate mechanical strength and high tensile
ac = 1tD = 1t x 0.87 strength
= 28173 amp.cond./m • Suitable for jointing by brazing/ soldering/welding
ac = 28173 amp.cond./m so that the joints are highly reliable contributing
lowest resistance.
Induced emf in de generator,
• Suitable for roll ability, draw ability, so that
E = cp Z n; = cp Z n, for lap winding conductors of required shape (wire/strip) are easi~
manufactured.
(-: p = a)
 PRIN. OF ELEC. MACHINE DESIGN (BATU ELECT.)
(1.13)
PRINCIPLES AND DESIGN OF ELECTRICAL MACHINE
Material: Copper, Aluminium, Iron steel, Alloys of copper 2.
Material of Low Conductivity (High Resistivity): It is
C~pper : Relative immunity from oxidation and corrosion,
usually called high resistance conductors as resistors,
highly malleable & ductile metal - it can be cast, forged, resistance coils, resistance elements or heating
rolled, drawn, machined, easily soldered. Annealed high elements- are used to dissipate electrical energy as
conductivity copper and h d d . heat i.e., in starting and regulating devices for motors.
. . ar rawn copper wires are used
for windings of electrical machines Used for heating devices, thermo couples, resistance
etc
Aluminium : The existing copper deposits are fast
Categories According to their Purposes
exhausting and the price of copper fluctuates widely,
I Group : Materials used for Precision work- for making
therefore it replaces copper in many applications. standard resistances and resistance boxes.
Aluminium when used in small transformer, decreases Properties
overall cost (comparatively lesser than copper) and weight • Stability of resistance over the period of time and
(approximately 3.3 times lighter than copper) Aluminium during fluctuations of temperature.
when used in large transformer, increases overall cost ( its • Low temperature co-efficient.
resistivity 1.62 times higher than copper and size ( its • Minimum thermo electric effect at contact of
material -does not introduce errors into
volume approximately 2.04 times greater than copper)
measurements.
(while designing due account should be taken of their
Material : Manganin (Cu 86%, Mn 12%and Ni 2%)
differences in resitivity, cost, weight, conductivity, temp. D Group : Materials used for making rheostats
etc.,)
Properties
Iron and Steel : Steel alloyed with chromium and • Should have large thermo-emf
Aluminium (robustness with good heat dissipation) - for • Large resistance temperature co-efficient.
Special Requirement :
making starter rheostats. Cast iron - resistance grids to be
• High permissible working temperature and low
used in starters of large motors.
cost
Alloys of Copper : Bronze (copper base alloys containing Material : Constantan (Cu 65% and 40 to 35% Ni)
tin, cadmium & berryllium) possesses higher resistivity and Sometimes small amounts of manganese and iron also
mechanical strength. Beryllium copper - current carrying included.

springs, brush holders, sliding contacts and knife switch m Group : Materials used for making heating devices in
electric furnaces and loading rheostats
blades. Cadmium copper- contact wires and commutator
Special Requirements
segments. Brass (66% Cu and 34% Zinc) has greater
• High permissible working temperature, low cost
mechanical strength ,wear resistance and lower and should have non-corrosive
conductivity - widely used in current carrying materials . * Material : Nichrome (Nickel, chromium and iron)
Copper silver alloy (99.10% and 0.06% to 0.1 %) has (optimum working temperature-900°-to 1000°)
. t ance
res1s to thermal shortening and creep (turbo- 3. Electrical Carbon Materials : Electrical carbon
materials are made from graphite and other forms of
alternators)
carbon coal.
Best conducing material is silver. Next _best is copp~r and Properties
then aluminum. Properties of these are compared in the • Negative temp. Co-efficient (contact voltage drop
following table. decreases with increasing temp)
• Low wear and tear (due to self lubricant property)
.
Table 12
.,. .,
Matena • I : Carb o n, carbon graphite graphite electro
I I

Unft Sliver Copper ~lumlnlun graphite, metal graphite- used for making brushes for
Sr. -No: · · Property ·
electrical machines.
0.975 0.585
1 Conductivity - 1.0
4. Super Conducting Materials : Materials exhibiting
1.46 1.777 2.826 zero value of resistivity are known as Super conductors.
2 Resistivity µ.Q-cm
A large number of metals become super-conducting
0.337 0.393 0.4
3 Temp-coeff. % per°C below a particular temperature characteristic of the
Medium Low particular metal. This temperature is known as the
4 Cost - Prohibitively
transition temperature.
high
 ~~~~~~~
PRIN. OF ELEC. MACHINE DESIGN (BATU ELECT.)
~~~~- -~~-_____:~-==-
(1.14) PRINOPLES AND DESIGN OF ELECTRICAL MACHINE
==-=-=~~--------.:...::::..:_
For Example: Alluminium - Trans.temp.- 1.18 K Uranium - whose hysteresis loops are more or less narrow (see
0.80 K It is interesting to note that copper, silver, gold etc., Figs. 1.6 (a) and (b)]. (Silicon steel, nickel- iron alloys
.are very good conductors at room temp., but do not etc.)
exhibit superconducting properties. ( vice versa for other These materials are called soft magnetic materials. Soft
metals and alloys) Application of superconductor It can be magnetic materials are used in the manufacture of
used for the transformers and rotating electrical machines, electrical machines, transformers and many kinds of
depending upon the comparative gain in the reduction of electrical apparatus, instruments and devices.
full load copper losses against the cost for provision of • Classifications:
cryogenic conditions
► Solid core materials
1.17.2 Magnetic Materials
► Laminated core material for pulsating fluxes
• All magnetic materials possess magnetic properties to
► Electrical sheet and strip
a greater or a lesser degree. The magnetic properties
of materials are characterized by their relative ► Special purpose alloys
permeability. In accordance with the value of relative • Solid Core Materials : For steady fluxes. These
permeability, materials may be divided into three materials are normally used for parts of magnetic
broad classes. circuits carrying steady flux as cores of d.c .
1. Ferromagnetic Materials The relative Electromagnets, relays and field frame (i.e., yoke) of
permeabilities of these materials are much greater d.c. machines. Examples: cast iron, cast steel and Ferro-
than unity and these permeability values are cobalt
dependent upon the magnetizing force • Laminated Core Material for Pulsating Fluxes :
Relative permeability -µr > > 1 (Nickel, cobalt, iron, These materials are normally used for parts of
steel, silicon steel etc.,) magnetic circuits carrying pulsating flux as cores of d.c.
2. Paramagnetic Materials : These materials have armature, stator and rotor of ac machines
their relative permeabilities only slightly greater • Electrical Steel Sheets Dynamo Grade Steels : Low
than unity. The value of susceptibility, is thus silicon content are used in rotating electrical machines
positive for these materials.
Relative permeability -µr > 1 (Air, Alluminium,
• Transformer Grade Steels : High silicon content are
used in transformers. Cold rolled grain oriented steel
palladium etc.,)
(CRGO) - is suitable for use in large transformers and
3. Diamagnetic Materials : These materials have
turbo alternators
their relative permeabilities slightly less than unity.
In both Paramagnetic and Diamagnetic materials • Special Purpose Alloys : Are used in instrument
the value of permeability is independent of the transformers, induction coils and choke. Example :
magnetizing force. permalloys, superpermalloy, perminvar etc.,
Relative permeability - µr < 1 (Bismuth, silver, lead, 2. Hard Magnetic Materials :
copper, water etc.,) • Materials with broad hysteresis loops [Fig 1.6 (c)] are
Ferromagnetic materials are very useful for electrical called hard magnetic materials. These materials are
engineering applications. Why? used in certain types of electrical machines. rating, and
When a ferromagnetic material is placed in a magnetic in all kinds of instruments and devices requiring
field, there is considerable distortion and, therefore, the permanent magnets which set up magnetic fields of
force exerted is very large. This property makes ferro- their own.
magnetic materials very useful for electrical engineering B
B
applications.
Example: Iron, nickel, cobalt, and many of their alloys are
ferromagnetic.
Types of Magnetic Materials
l. Soft Magnetic Material
H
2. Hard Magnetic Materials
l. Soft Magnetic Material
• The hysteresis loss depends upon . the area of
hysteresis loop. For this reason, magnette cores use~ in
alternating magnetic fields are made from materials (a)
(b)

JI
PRIN. OF ELEC. MACHINE DESIGN (BATU ELECT.) PRINOPLES AND DESIGN OF ELECTRICAL MACHINE
(1.15)

• Should not consume any power or should have a low

dielectric loss angle d
• Should withstand stresses due to centrifugal forces (as
in rotating machines), electro dynamic or mechanical
forces ( as in transformers)
• Should withstand vibration, abrasion, bending
• Should not absorb moisture
• Should be flexible and cheap
Factors Affecting the Electrical Properties
• Dimensions of test piece
(c) • r.m.s. value, wave form and frequency of impressed
Fig. 1.6 : Hysteresis loops voltage.
• Hard or permanent magnetic materials have large size • Temperature and moisture content of test piece
hysteresis loop (obviously hysteresis loss is more) and • Mechanical pressure on test piece
gradually rising magnetization curve. Applications of Insulating Materials Employed for the
insulation of
Ex.: carbon steel, tungsten steal, cobalt steel, alnico,
hard ferrite etc. • Wires for magnet coils and windings of machine
• Laminations for Machines and transformers
• Properties of Good Magnetic Material :
Insulating Material Classification
The some of the properties that a good magnetic • Insulating materials can be classified as
material should possess are listed below. ► Solid
► Low reluctance or should be highly permeable or ► Liquid
should have a high value of relative permeability ► Gas
µr. ► Vacuum.

► High saturation induction (to minimize weight and • The term insulting material is sometimes used in a
volume of iron parts) broader sense to designate also insulating liquids, gas
and vacuum.
► High electrical resistivity so that the eddy emf and
Solid: Used with field, armature, transformer windings etc.
the hence eddy current loss is less
The examples are:
► Narrow hysteresis loop or low coercivity so that
• Fibrous or inorganic animal or plant origin, natural or
hysteresis loss is less and efficiency of operation is
synthetic paper, wood, card board, cotton, jute, silk
high etc., rayon, nylon, terelane, asbestos, fiber glass etc.,
► A high curie point. (Above Curie point or • Plastic or resins. Natural resins-lac, amber, shellac etc.,
tempera ture the material loses the magnetic Synthetic resins-phenol formaldehyde, melamine,
property or becomes paramagnetic, that is polyesters, epoxy, silicon resins, bakelite, Teflon, PVC
effectively non-mag netic) etc
► Should have a high value of energy product • Rubber : natural rubber, synthetic rubber-butadiene,
(expressed in joules/ m3). silicone rubber, hypalon, etc.,
• Mineral : mica, marble, slate, talc chloride etc.,
lt.18 INSULATING MATERIALS
• Ceramic : porcelain, steatite, alumina etc.,
To avoid any electrical activity between parts at differe_ nt • Glass : soda lime glass, silica glass, lead glass,
potentials, insulatio n is used. An ideal insulating material
borosilicate glass
should possess the followin g properties.
• Non-resinous : mineral waxes, asphalt, bitumen,
• Should have high dielectri c strength. chlorinated naphthalene, enamel etc.,
• Should with stand high temperature. Uquid: Used in transformers, circuit breakers, reactors,
• Should have good thermal conduct ivity rheostats, cables, capacitors etc., & for impregnation. The
• Should not undergo thermal oxidatio n examples are:
• Should not deterior ate due to higher tempera ture and • Mineral oil (petrole um by product)

repeated heat cycle • Synthetic oil askarels, pyranols etc.,

• Should have high value of resistivit y (like 1018 Wern) • Varnish, French polish, lacquer epoxy resin etc.,
 6)_ _ _PlltlNCIPLIS AN ~I ~
OF IUCTRJCAl. MACH1._. ,
ES~l:=G (B~A
~N..l ~ T~U ~E~U ~ C~T~.)~ - - -(,_ 1._ 1....;
ELE
__. _O_F..::.
_PIUN .M
= C:..: N::.
:..::l::.:
AC.::::H
:.:.::..: E...:D:.:
pcro tu re 1s the
• The maximum opcr3tlng tem
Gaseous: The examples are: during oper1.,tion
sion and temperature the Insulation con rcoch
rs, transmis
• Air used in switches, air condense and is the sum of standt1rdized amb
ient temperature
distribution lines etc., temperoture nse
gas pres sure cabl es etc., I.e. 40 degree cen tigrade, permissible
• Nitrogen use in capacitors, HV and allowance tolerance for hot spo
t in winding For
a diele ctric , generally
• Hydrogen thou gh not used as
example. the maximum tempera
ture of class a
used as a coolant 40 + allowable
s neo n, argo n, mer cury and sodium vapors Insulation is (ambient tem pera ture
• Iner t gase e 10) • 130°(.
generally used for neon sign lamps. temperature rise 80 + hot spot toleranc
er high pressure in against hea t and is a
• Halogens like fluorine, used und • Insulation is the weakest element
life of electrica l
cables critical factor in deciding the
satisfies all the g temperatures
No insulating material in practice equipment. The maximum operatin
mat eria l whic h lation are for a
desirable properties. Therefore a prescribed for different class of insu
erties mus t be rs. The height
satisfies most of the desirable prop healthy lifetime of 20,000 hou
parts is usually
selected.
rma l temperature permitted for the machine
ls Bas ed on The Exceeding the
Classification of Insulating Materia about 2000C at the maximum.
Consideratio n affect the life of
latio n class ) for maximum operating temperature will
• The insulation system (also calle d insu the lifetime of the
mot ors tran sformers and the insulation. As a rule of thumb,
wires used in gen erat ors, by half for every
l com pon ents is divid ed into winding insulation will be reduced
othe r wire-wound elec trica day trend is to
perature that they 10°C rise in temperature. The present
diffe ren t classes according the tem lation for class B
design the machine using class F insu
can safely withstand.
l evaluation and temperature rise.
• As per Indian Standard •(Therma IS. No. 1271, 1985 ,
classification of Electrical Insulation, EXERCSE
inte rnat iona l stan dard of an
first revision) and other 1. Name the basic structural parts
es A, E, B, F, H
insulation is classified by letter grad electromagnetic rotating machine.
(previous Y, A. E, B, F, H, q . t the design of a
2. What are the factors those limi
Table 1.3
TyplcalMarllll
machine?
Sr, lnlulatlon Muimum the view poin t of
C1aA Operating 3. Classify electrical machines from
No.
Temperature manufacturing process.
agnetic induction
ln -C 4. How the Faraday's law of electrom
Cotton , silk, paper, wood, cellulose, fiber of an electrical
1 y 90 principle is applied in design
etc. without impreQllation or imme rsed
machine?
105 The material of class Y impregnated with as applicable in
2 A
naklral resils, c:elulose esters, inaulalilg
5. Briefly explain Biot-Savart's law
oils elc., and also laminated wood, electric machine design.
machine design ?
varnished oaoer etc. 6. What are the main dimensions in
Synthetic resin enamels of vinyl acetate or ing ?
3 E 120 7. What do you mean by specific load
nylon tapes, cotton and paper lamin ates
8. Develop the out put equ atio n of
wrlh formaldehvde bondinQ etc.
(i) d.c. machine
s f30 Mica, glau filer, asbestos etc.. with
4
suable bonding subllances, built up mica. (ii) a.c. machine
?
alass bran d asbettos lamilates. 9. What is out put coefficient of machine
The malenals of Class Bwith more thermal size of rota tiii
5 F 155 10. What are the factors thos e affect the
resiulc:e bondino materials. machines?
180 Gia fiber #Id asbeR>s materials and bultl irem ents of h,;
11. Wh at are fund ame ntal requ
6 H
up mica wilh aoomonate silicone resins.
con duc tivit y material?
C > 180 Mica. cerarnics, glass, quartz and asbestos
7 material.
with biidett or resins ol super thermal 12. Wri te sho rt not e on mag neti c
slabil:ty.
I