Unit I design of electrical machines

EE6604: DESIGN OF ELECTRICAL MACHINES
Unit – I
Introduction:
Design factors; Limitations; Modern trends; Electrical
Engineering Materials; Space factor; Choice of Specific
Electric and Magnetic loadings; Thermal Considerations;
Heat flow; Temperature rise; Insulating Materials; Properties;
Rating of Machines; Various Standard Specifications ;

UNIT-II
Design of D.C. Machines
Output Equation; Main dimensions; Separation of D & L;
Choice of Electric and Magnetic Loadings; Magnetic
circuit calculations; Carter’s Coefficient; Net length of
Iron; Real and Apparent flux densities; Selection of No. of
poles; Design of Armature; Design of Commutator and
brushes; Performance prediction using design values.

UNIT - III
Design of Transformers:
Output equations; Main Dimensions; kVA output for 1 & 3
phase transformers; Window space factor; Design of core
and winding; Overall dimensions; Operating
characteristics; No-load current; Temperature rise in
Transformers; Design of Tank; Methods of cooling of
Transformers.

UNIT- IV
Design of 3 phase Induction Motors:
Output equation of Induction motor; Main dimensions;
Separation of D and L; Choice of Average flux density;
length of air gap; Design of Stator core; Rules for selecting
rotor slots of squirrel cage machines; Design of rotor bars
and slots; Design of end rings; Design of wound rotor;
Magnetic leakage calculations; Leakage reactance of
polyphase machines; Magnetizing current; Short circuit
current; Operating characteristics; Losses and Efficiency.

UNIT - V
Design of Synchronous Machines
Output equation; Choice of Magnetic and Electric
loadings; Main dimensions; Separation of D & L for
Salient pole and Turbo m/c. Types; Design of Salient pole
machines; Short circuit ratio; shape of pole face; Armature
design; Armature parameters; Estimation of air gap length;
Design of Rotor; Design of Damper windings;
Determination of full load mmf; Design of field windings;
Design of Turbo alternators; Rotor design.

BOOKS
• A.K.SAWHNEY – Dhanpat Rai & Sons
• R.K. AGARWAL – Esskay Publications
• M.V. DESHPANDE – Wheeler publications.
• S.K. SEN – Oxford and IBH Publishing Co. Pvt. Ltd

WHAT IS DESIGN?
Design is defined as a creative physical realization of theoretical concepts.
Engineering Design is application of science, technology and invention to produce
machines to perform specified tasks with optimum economy and efficiency.

M A J O R
C O N S I D E R AT I O N S
I N D E S I G N

 RELIABILITY / DURABILITY
 PERFORMANCE
 COST
 COMPLIANCE WITH SPECIFICATIONS

IM : 20 TO 30 Years - Low Initial Cost
SM & Trs : Designed with Reliability and Durability
Less emphasis on Initial Cost
Electrical Machines
Static Machines - Transformers
Rotating Machines- Generators & Motors
Conversion in any electrical M/C takes place through magnetic field.
Magnetic Field produced by an EM which require core and winding.

BASIC STRUCTURE OF EM ROTATING
ELECTRICAL M/C.
1. Magnetic Circuit
2. Electric Circuit
3. Dielectric Circuit
4. Thermal Circuit
5. Mechanical Parts

Magnetic Circuit
Provide path for magnetic flux
Air gap
Stator and Rotor Teeth
Stator and Rotor Core(yokes)

Electric Circuit
Stator and Rotor Windings.
Winding of a Transformer, Rotating machine conveys energy and is concerned with
Production of emf and Development of mmf.
Windings are formed from suitably insulated conductors.
Dielectric Circuit
To isolate one conductor from another conductor and also the windings from the
core.
Insulating Materials are Non –Metallic, Organic, inorganic, natural or Synthetic.
Thermal Circuit
Mode and Media for dissipation of heat produced inside the machine on account of
losses.

Mechanical Parts
Frame, Bearings and Shaft
A successful design brings out an economic compromise for space occupied by iron,
copper, (aluminium), insulation and coolant(which may be air, hydrogen, water or
oil).

Saturation:
Electromagnetic machines use ferro-magnetic materials.
The maximum allowable flux density to be used is determined by the saturation
level of the material used.
A high value of flux density results in increased excitation resulting in higher
cost for the field system.
Saturation is the state reached when an increase in applied external magnetic field
H cannot increase the magnetization of the material further, so the total magnetic
flux density B more or less levels off.

Ferromagnetic materials (like iron) are composed of microscopic regions called
magnetic domains, that act like tiny permanent magnets that can change their
direction of magnetization.
Before an external magnetic field is applied to the material, the domains' magnetic
fields are oriented randomly, effectively cancelling each other out, and so its
magnetic field is negligibly small.
When an external magnetizing field H is applied to the material, it penetrates the
material and aligns the domains, causing their tiny magnetic fields to turn and align
parallel to the external field, adding together to create a large magnetic field B which
extends out from the material.
This is called magnetization.

The stronger the external magnetic field H, the more the domains align yielding a
higher magnetic flux density B.
Saturation occurs when practically all the domains are lined up, so further increases in
H can't increase B beyond the increment that would be caused in a nonmagnetic
material, or in other words cannot cause further alignment of the domains.

A magnetic domain is a region within a magnetic material in which the
magnetization is in a uniform direction.
This means that the individual magnetic moments of the atoms are aligned with one
another and they point in the same direction.

When cooled below a temperature called the Curie temperature, the magnetization
of a piece of ferromagnetic material spontaneously divides into many small regions
called magnetic domains.
Magnetic domain structure is responsible for the magnetic behaviour of
ferromagnetic materials like iron, nickel, cobalt and their alloys, and ferrimagnetic
materials like ferrite.
This includes the formation of permanent magnets and the attraction of
ferromagnetic materials to a magnetic field.
The regions separating magnetic domains are called domain walls, where the
magnetization rotates coherently from the direction in one domain to that in the next
domain.
The study of magnetic domains is called micromagnetics.

Temperature rise:
The most vulnerable part of the machine is its insulation.
The operating life of the machine depends upon the type of the insulating material
used in its construction.
The life of the insulating materials in turn depends upon the temperature rise of the
machine.
If an insulating material is operated beyond its maximum allowable temperature,
its life is drastically reduced.
Insulation:
The insulating materials used in a machine should be able to withstand the
electrical, mechanical and thermal stresses produced in the machine.
The mechanical strength of the insulation is particularly important in the case of
transformers

Large axial and radial forces are produced when the secondary winding of a
transformer is short circuited with the primary on.
Therefore, while designing the insulation of a transformer, due consideration must be
given to the capability of the insulation to withstand large mechanical stresses that
are produced under short circuit conditions as apart from electrical and thermal
breakdown considerations.
The type of insulation is decided by the maximum operating temperature of the
machine.
The size of insulation is not only decided by the maximum voltage stress but also
by the mechanical stresses.
For example, for the same operating voltage, thicker insulation has to be used for
large sized conductors than for a smaller sized ones.

Efficiency:
The efficiency of a machine should be as high as possible in order to reduce the
operating costs.
To design a high efficient machine, the magnetic and electric loadings used should
be small and this requires the use of large amount of materials. (both iron as well as
copper/aluminium).
Therefore, the capital cost of the machine designed for high efficiency is high while
its running cost is low.
Mechanical parts:
The construction should be as simple and economical means with as little labour as
possible.
The design of mechanical parts is very important in the case of high speed machines.

For example, while designing a turbo – alternator, the rotor slot dimensions are so
selected that the mechanical stresses at the bottom of the rotor teeth do not exceed the
maximum allowable limit.
In induction motors, the length of the airgap is kept as small as possible in order to
have high power factor.
The size of the shaft should be such that it does not give rise to excessive unbalanced
magnetic pull (U.M.P.) when deflected.
The shaft of the should be short and stiff so there is no significant deflection of the
shaft and hence the unbalanced magnetic pull is small and is within the manageable
proportions.
In large machines, the size of the shaft is decided by considering the critical speed
which depends on deflection of the shaft.

Bearings of the machines are subjected to the action of rotor weight, external
loads, inertia forces due to unbalanced rotors and forces on account of
unbalanced magnetic pull.
The type of bearings to be used in a machine are decided by considering the above
mentioned forces and also the type of the construction whether the machine is
horizontally or vertical mounted
Commutation:
The problem of commutation is important in the case of commutator machines as
commutation conditions limit the maximum output that can be taken from a
machine.
For example, the maximum power of a single unit d.c. machine is approximately
10MW and this limitation is solely on account of commutation difficulty.

Power Factor:
Poor power factor results in larger values of current for the same power and,
therefore, larger conductor sizes have to be used.
This problem of power factor is particularly important in the case of induction motors.
The size and hence cost of induction motor can be reduced by using a high value of
flux density in the airgap, but this results in saturation in iron parts of machine and
consequently a poor power factor .
Thus the value of flux density used depends upon the power factor and hence power
factor becomes a limiting factor.
In fact, the length of the air gap to be provided in an induction motor is primarily
determined by the power factor considerations.

Consumer’s specifications:
The limitations imposed by consumer’s specifications on the design of electric
machines are evident.
The specifications as laid down in the consumer’s order have to be met and the design
evolved should be such that it satisfies all the specifications and also the economic
constraints imposed on the manufacturer.
Standard Specifications:
These specifications are the biggest strain on the design because both the
manufacturer as well as the consumer cannot get away from them without satisfying
them.

MODERN MACHINE MANUFACTURING
TECHNIQUES
The modern machines are characterized by wide range of power output.
The power range varies from a fraction of a watt to several hundreds of megawatt
in a single unit.
The ratio of power output of the smallest machine to that of the largest machine is
1: 1010
Small size Machines – Machines having power output of 750watts.
Medium size Machines – Machines having power output of few KW to 250KW.
Large size Machines – Machines having power output of 250KW to 5000KW.
Larger size Machines – Machines having power output as high as hundreds of
megawatt.

The second important feature is the trend to build the machines which are smaller in
size and therefore involve the use of lesser material and at the same time have the
same efficiency and overload capacity
The increase in power ratings using smaller size coupled with good overall
performance has been possible only due to the following technological advancements :
There has been considerable development and refinement in the techniques
relating to construction and arrangement of conductors and some other parts of the
machine and this results in drastic reduction in stray load losses.
There has been vast development in the cooling and ventilation systems for
machines.
The new methods are much more effective for dissipation of heat generated
inside the machine.

The third important factor in the manufacture of modern machines is the use of
magnetic materials having high permeability, low iron loss and high mechanical
strength.
The materials permit the use of the high values of flux density and therefore result in
the reduction in the reduction in the size of the machine and promote the extension
of power output.
There has been a significant improvement in the insulating materials and newer
materials are increasingly being used in the present day machines.
These insulating materials are able to withstand much higher temperature.
Since, the rating of the machines mainly depends upon the insulating materials used in
it, greater outputs are possible with the use of these insulating materials.
In other words, the use of better class of insulating materials allows the machine sizes
to be used for the same output power ratings.

Modern machine building is marked with the use of higher electro magnetic
loadings for active parts and increased mechanical loadings for construction
materials.
In order to advance the process of machine manufacture at reduced cost, different
and refined manufacturing techniques are used for individual machine parts.
Modern machines have a wide field of applications.
They are used in varied environments and under different operating conditions.
The design of the machine and its manufacture should be such that the machine
operates satisfactorily under the desired environmental conditions.

ELECTRICAL ENGINEERING MATERIALS
Electrical Engineering
Materials
High Conductivity
Materials
High Resistivity
Materials

HighConductivitymaterials
Used for making all types
of windings
All types of apparatus and
devices
Used for transmission and
Distribution of electric
energy
Least possible resistivity

HighResistivitymaterials
Used for making
resistances
Used for making heating
devices

HIGH CONDUCTIVITY MATERIALS
Fundamental Requirements to be met are
1. Highest possible conductivity
2. Least possible temperature co-efficient of resistance
3. Mechanical strength
4. High tensile strength and absence of brittleness
5. Rollability and Drawability
6. Weldability and Solderability
7. Adequate resistance to corrosion.

Resistivity
Specific weight
Density
Resistance temperature co-efficient
Co-efficient of thermal expansion
 thermal conductivity
Specific heat
Tensile strength
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COPPER
Properties:
High electrical conductivity.
Excellent Mechanical Properties.
Immunity from oxidation and corrosion.
Highly malleable.
Ductile metal.
Can be forged, rolled, drawn, machined.
Most electrical machines employ windings of annealed high conductivity copper.
Hard drawn copper wire – used in electrical machines as wires.

ALUMINIUM
Aluminium is available in abundance on earth’s surface.
Softer than Copper
Can not be drawn into fine wires due to low mechanical strength
Machines have to be redesigned for larger slots to accommodate aluminium wires.
For induction motors with power outputs upto 100 kW – Aluminium used as bars
and Squirrel cage.
Super enamelled aluminium wires - used as Stator Windings of small induction
motors.
Aluminium used as Transformer tank because of its light weight.

FOIL TYPE WINDING
A new development in the transformer manufacture is the use of foil type windings.
This is because aluminium can be rolled to thinner and more flexible sheet than
copper.
Foil type winding are often used for low voltage windings of small and medium
rated transformers.

IRON AND STEEL
Iron and Steel
Steel alloyed with chromium and aluminium is used for making
starter rheostats.
Cast iron is used in the manufacture of resistance grids to be used in
the starters of large motors.

ALLOYS OF COPPER
Bronze
Copper based alloys containing tin, cadmium , beryllium and other
metals are called bronze.
Used as high conductivity materials.
Possess high mechanical strength as
compared with copper, but have higher
resistivities.

Beryllium Copper
Used for carrying springs, brush holders, sliding contacts and knife switch blades
Resistivity 3 to 6 times that of copper

Cadmium Copper :
Copper alloys containing 1.1 percent cadmium give wires which are stiffer, harder
and of high tensile strength than hard-drawn copper.
Used for making contact wires and commutator segments.
It is also used for cage windings.

Brass
It contains 66% of copper and 34% of zinc.
High mechanical strength
Wear resistance.
Lower conductivity than copper.
Easily shaped by press forming methods.
Good weldability and solderability.
Fairly resistance to corrosion..
Used in the manufacture of electrical
apparatus as current carrying and
structural materials.
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Copper silver alloy :
This alloy contains 99.10% copper and 0.06 to 0.1 per cent silver.
It has a resistivity of 0.01814 × 106 Ωm.
Silver bearing copper is used in turbo alternators because of its resistance to
thermal shortening and creep.

Materials of High Resistivity:
Used to dissipate electrical energy as heat i.e., in starting and
regulating devices for motors.
Materials of high resistivity are primarily alloys of different metals.
Nickel, silver and iron.
52

Materials of High
Resistivity
First group Second group Third group

FirstGroup
Used in precision
measuring instruments
Used in making standard
resistances
Used in making resistance
boxes

SecondGroup
Used for making resistance
elements for
Rheostats
Similar control devices

ThirdGroup
Used for making high
temperature elements for
Electric furnaces
Heating devices
Loading rheostats

Materials used for precision work
Important requirement –stability of resistance over the period of time and during
fluctuations of temperature
Magnanin:
Low resistance temperature co-efficient
Composition – Cu 86% Mn 12% Ni 2%.
Nickel serves to reduce thermal emf of contact with copper to a very low value of
about 1.0µV/0C.
Resistivity of 0.43×10-6Ωm
Resistance temperature co-efficient of the order of 1×10-5/0C
57

Materials used for Rheostats:
Large thermo-emf
Large resistance temperature co-efficient
Low cost
Constantan are used where large changes in resistances are allowed.
Cu 60 to 65% Ni 35 to 40%.
Soft constantan wire has resistivity 0.46 to 0.53×10-6Ωm
Hard constantan wire has resistivity 0.46 to 0.53×10-6Ωm
The resistant temperature coefficient is near zero.
thermo emf of 39µV/0C w.r.t. Cu.
Safely used upto a temperature of 500oC
59

Materials used for heating devices
High working temperature
High melting point
Platinum is an incorrodible material with a high melting point of 1710oC with a
resistivity of 0.117×10-6Ωm.
It is also used in laboratory electric furnaces with a working temperature of
1300oC.
The most extensively used high working temperature resistance materials are alloys
of nickel, chromium and iron called Nichrome and alloy of aluminium, iron and
chromium.
The resistivity of Nichrome varies from 1.1 to 1.27×10-6Ωm
Working temperature of Nichrome is 900oC to 1000oC.
60

Electrical Carbon Materials
It is manufactured from graphite and other form of carbon coal etc.
The conductivity of carbon used is slightly less than metals and alloys.
It is used for making brushes for electrical machines.
62

Superconductivity:
Electromechanical devices are built using only current carrying conductors and no
iron to act as magnetic circuit because of the relatively small forces or torques that
can be produced per unit of machine volume
This is because of weak magnetic fields owing to the absence of iron.
While there is no limit on the value of flux density that can exit in air or space but in
order to produce a strong field in air, the conductors have to carry a large value of
current.
The high value of current can be obtained by:
Adopting a large conductor area and thus using a small value of current density and
vice versa.

Magnetic Materials:
The magnetic properties of materials are characterized by their relative
permeability.
Classification:
Ferromagnetic materials - relative permeability much greater than unity and
dependent upon the magnetizing force
Paramagnetic materials - relative permeability slightly greater than unity and
independent of magnetizing force.
value of susceptibility is positive.
Diamagnetic materials - relative permeability slightly less than unity.
64

CLASSIFICATION
Classified according to µr :
• Dia , Para, Ferro, Ferri, Anti ferri
Classified according to Area of Hysteresis loop:
• Soft - Narrow loop – meant for electrical machines / instruments.
• Hard – Broad loop – meant for low power rating m/c & device requires
residual magnetism

SOFT MAGNETIC MATERIALS
• Solid Core materials:
– Iron, Low carbon, Silicon steel
– Cast Iron
– Gray cast iron
– Cast steel
– Soft steel
– Ferro-Cobalt

SHEET STEELS / STRIPS
• Non - Oriented sheet steels:
• Dynamo Grade, Transformer Grade, High resistance.
• Grain Oriented steels:
• Cold rolled – High permeability, Less MMF, Less Iron loss, costly
• Hot rolled - Low permeability, High MMF, High Iron loss, Cheaper

SPECIAL PURPOSE ALLOYS
• Used to obtain high flux densities in weak magnetic fields. Nickel, Molybdenum
and Chromium at suitable proportion can be added to get high permeabilities and
low iron loss.
• Eg: Mumetal, Permalloys, Supermalloy, Perminvar, Permendur
• Application: Instrument Transformers.

APPLICATIONS
• Armature core
• Transformer core
• Pole body
• Pole Shoe
• Yoke
• Rotor core
• M/C outer frames (Tank / Covers etc.,)

MAGNETIC AND ELECTRIC LOADINGS
Total Magnetic Loading :
The total flux around the armature (or stator) periphery at the air gap is called the
total magnetic loading.
Total magnetic loading = ρФ
Total Electric loading:
The total number of ampere conductors around the armature (or stator) periphery
is called the total electric loading.
Total electric loading = Iz Z

Specific Magnetic Loading :
The average flux density over the air gap of a machine is known as specific
magnetic loading
Specific magnetic loading
Bav = Total flux around the air gap = ρФ = Ф
Area of flux path at the air gap πDL τL
Specific Electric loading:
The number of armature (or stator) ampere conductors per metre of armature
(or stator) periphery at the air gap is called the specific electric loading.
Specific electric loading
ac = Total armature ampere conductors = Iz Z
Armature periphery at the air gap πD

C H O I C E
O F
S P E C I F I C
M A G N E T I C
L O A D I N G

Maximum flux density in iron:
The maximum flux density in any iron part of magnetic circuit of the machine must be
definitely below a certain limiting value depending on the material used.
The flux density in iron parts is directly proportional to the average flux density in
the air gap i.e., specific magnetic loading
In a well designed machine the maximum flux density occurs in the teeth of the
machine
Relation between flux density in the teeth to the average flux density in air gap :

Let us consider a non – salient pole machine having S armature slots.
Flux over one slot pitch = pФ = p. Bav πDL . 1 = Bav πD L = Bav ys L
S p S S
Where ys = slot pitch = πD/S
If we neglect saturation the entire flux over a slot pitch is carried by the tooth
Flux density in teeth Bt = flux in each tooth = Bav . ys L = Bav . ys
Area of each tooth Wt L Wt
In a salient pole machine, the flux is concentrated over the pole arc and therefore
the teeth which are under the pole arc carry whole of the flux and hardly any flux is
carried by the teeth lying outside the pole arc

Flux density in teeth of the salient pole machine is Bt = Bav . ys = Bg . ys
ψ Wt Wt
where Bg = maximum flux density in the air gap, and
ψ = ratio of pole arc to pole pitch.
The flux density in the teeth is directly proportional to the specific magnetic
loading
Machines using parallel sided slots have tapered teeth and therefore the tooth
width is not the same over the entire height of the tooth.
This gives different values of flux density in teeth at different heights.
The maximum value of flux density in teeth occurs where the width of the tooth is
smallest i.e., at the root of the teeth in case of d.c. machines and at a section near the
air gap for synchronous machines.

In big machines which have large diameters, taper of the teeth is not significant
and therefore the width of the teeth is almost the same over entire height.
However in small machines which have smaller diameters, the taper of teeth is
very pronounced and consequently the ratio Bt/Bav is very large at the section where
the teeth have the smallest width and hence for a given maximum value of Bt it
follows that Bav must be reduced.
In general, therefore, small machines have lower specific magnetic loadings.

Magnetising current :
The magnetising current of a machine is directly proportional to the mmf required to
force the flux through the air gap and iron parts.
The mmf required for air gap is directly proportional to the gap flux density i.e.,
specific magnetic loading.
As far as the iron part are considered, the value of flux density in them depends upon
the value of specific magnetic loading.

If a small value of specific magnetic loading is chosen, the flux density in the iron
parts is low and therefore these parts are worked on the linear or knee portion of the
B -H curve.
This requires a small or even negligible amount of mmf for iron parts, as H, the mmf
per metre length is very small for flux densities on the linear and knee portions of the
curve.
However, if a large value of specific magnetic loading is assumed, the flux density in
iron parts (especially teeth) maybe such as to work these parts in the saturation
region of the B – H curve.
If the iron parts are worked in the saturation region, the mmf per metre length and
consequently the mmf required for iron parts is excessively large.
Thus a large value of specific magnetic loading results in increased values of
magnetising mmf and hence more magnetising current.

The value of magnetising current is not usually a serious design consideration in d.c.
machines as there is ample space on salient poles to accommodate the required number
of field windings.
In induction motors, the consideration of magnetising current is very important as an
increased value of magnetising current means a low operating power factor.
Therefore the specific magnetic loading in the case of induction motors is lower than
that in d.c. machines.
For synchronous machines the magnetising current is not so critical and the value of
specific magnetic loading intermediate between d.c. and induction machines may be
used.

Core losses :
The core loss in any part of the magnetic circuit is directly proportional to the flux
density for which it is going to be designed.
Since the flux density in any part of the magnetic circuit is proportional to the specific
magnetic loading
Therefore, the core loss in a machine varies directly as the specific magnetic loading.
Thus a large value of specific magnetic loading indicates an increased core loss and
consequently a decreased efficiency and an increased temperature rise.

C H O I C E
O F
S P E C I F I C
E L E C T R I C
L O A D I N G

Permissible temperature rise:
An armature of a rotating machine is shown in the figure below :
For this machine, let
Z = total number of armature conductors
S = number of armature slots
az = area of each conductor
ρ = resistivity of the conductor material
δ = current density
Therefore if we consider the slot pitch, ampere conductors per metre for this portion
are
ac = Iz Z = Iz Z /S = IzZs
πD πD / S ys
where Zs = Z/S = number of conductors per slot.

Resistance of slot portion of each conductor = ρL
az
I2 R loss in slot portion of each conductor = Iz
2 ρL
az
I2 R loss in each slot = Zz . Iz
2 ρL
az
Heat produced in a slot is dissipated over the surface over one slot pitch. Considering
only the cylindrical surface,
heat dissipating surface S = ys .L
Loss dissipated per unit area of armature surface q = loss = Zz Iz
2 ρL = IzZs . Iz . ρ
surface az ys L ys az
Now ac = Iz Zz / ys and δ = Iz / az
q = ac δ ρ

From the above equation, heat dissipated per unit area of armature surface is
proportional to specific electric loading.
Temperature rise θ = Qc/S
where Q – the loss to be dissipated
S – the dissipation surface
c – the cooling co-efficient
Loss dissipated per unit area q = Q/S.
Hence temperature rise θ = qc = ac ρ δ c
Specific electric loading = ac = θ / ρ δ c
The limiting value of specific electric loading ac is fixed by maximum allowable
temperature rise θ and the cooling co-efficient c.

(i) Temperature rise:
From the above equation, a high value of specific electric loading can be used in a
machine where a high temperature rise is allowed.
The maximum allowable temperature rise of a machine is determined by the type of
insulating material used in it.
For example, organic materials, like cotton, paper and many varnishes may be worked
upto a maximum temperature (not temperature rise) 105○C while inorganic materials
like mica, asbestos, and glass fibre bounded with silicone can withstand a temperature
of 180○C without deterioration.
Hence when better quality insulating materials, which can withstand high temperature
rises, are used in machines, increased values of specific electric loading can be used
resulting in reduction in the size of the machine.

ii) Cooling coefficient:
If the cooling co-efficient of the machine is small, a high value of specific electric
loading may be used in the machine.
The value of cooling co-efficient, c, depends upon the ventilation conditions in the
machine.
A machine with better ventilation has a lower value of cooling co-efficient and
therefore, a high value of specific electric loading may be used in it.
So for the same reason in a high speed machines a high value of ac may be used as
due to high speed the ventilation conditions in the machine are improved owing to
natural fanning action of the motor.

Size of the machine:
It can be assumed that depth of slot, ratio of width of the slot to slot pitch, current
density and slot space factor are the same for all machines, the specific electric
loading is constant.
In practice, of course, these assumptions are not quite valid and slightly varying
values must be must be used throughout a range of sizes.
For example, the assumption of slot depth is no true.
The larger the machine the greater the slot depth and greater the specific electric
loading.
Actually if the current density and the slot space factor are assumed constant, then
specific electric loading is proportional to the diameter as slot depth usually depends
upon the diameter.

Current density:
A higher value of specific electric loading can be used in a machine which employs
lower current density in its conductors.
Typical values of current density are in the range of 2 -5 A/mm2
The temperature rise is usually 40○C for normal applications and cooling co-efficient
is between 0.02 to 0.035○C W-m2

T H E R M A L
C O N S I D E R AT I O N S

CONDUCTION
This mode of dissipation of heat is important in the case of solid parts of machine like
copper iron and insulation.
Conduction is the transfer of energy through matter from particle to particle.
It is the transfer and distribution of heat energy from atom to atom within a substance.
For example, a spoon in a cup of hot soup becomes warmer because the heat from the
soup is conducted along the spoon.
Conduction is most effective in solids-but it can happen in fluids.

From the above table, we have ρ = 20 for air and ρ = 7.5 for paper.
Air has greater thermal resistivity than paper and thus the presence of air pockets in the
insulation of the machine would have disastrous effects on heat dissipation, resulting
in large temperature rises.

RADIATION
Radiation is the emission or transmission of energy in the form
of waves or particles through space or through a material medium.
Electromagnetic waves that directly transport energy through space.
Sunlight is a form of radiation that is radiated through space to our planet without the
aid of fluids or solids.
The energy travels through nothingness.
The sun transfers heat through space.
Because there are no solids between the sun and our planet, conduction is not
responsible for bringing heat to Earth.
Since there are no fluids in space, convection is not responsible for transferring the
heat.
Thus, radiation brings heat to our planet.

From the above table the value of coefficient of emissivity for dull metallic paint is
0.9 while for polished metal it is 0.15.
This means that the specific heat dissipation due to radiation for surfaces painted with
dull metallic paints ( usually grey in colour) is large.
Hence all the electrical machines are painted with dull metallic paints in order to have
a large heat dissipation due to radiation.

CONVECTION
Convection is heat transfer by mass motion of a fluid such as air or water when the
heated fluid is caused to move away from the source of heat, carrying energy with it.
Convection above a hot surface occurs because hot air expands, becomes less dense,
and rises .
Hot water is likewise less dense than cold water and rises, causing convection
currents which transport energy.

CONVECTION
CELLS
Convection cells are visible in
the heated cooking oil in the
pot at left.
Heating the oil produces
changes in the index of
refraction of the oil, making the
cell boundaries visible.
Circulation patterns form, and
presumably the wall-like
structures visible are the
boundaries between the
circulation patterns.

CLASSES OF MOTOR DUTY
Duty cycle of a motor :
Relationship between the active (operating) time and the inactive (resting
time) of an equipment or machine.
In other words, it is expressed as the ratio of active time (operating) to the total
time period.
Electric motors, for example, are rated on the basis of continuous duty (non-
stop operation lasting an hour or more) or intermittent duty (alternate period of rest
and operation lasting, 5, 30, or 60 minutes).
Duty cycle = Active (operating) time period (or) ON time
Total time period (ON time + OFF time)

TYPES OF MOTOR DUTY
The duty cycles of the motor can be classified into Eight categories as
follows:
i. Continuous duty
ii. Short time duty
iii. Intermittent periodic duty
iv. Intermittent periodic duty with starting
v. Intermittent periodic duty with starting and braking
vi. Continuous duty with intermittent periodic loading
vii. Continuous duty with starting and braking
viii. Continuous duty with periodic speed changes

CONTINUOUS DUTY
It denotes the motor operation at constant load torque for long duration of
time.
As a result the temperature of the motor reaches steady state value.
This duty is characterized by a constant motor loss.
Here N indicates the duration of the operation.
θmax indicates maximum temperature rise.
Duty Cycle = N = 1
N

EXAMPLES
Motors used for :
1. Compressors.
2. Fans.
3. Centrifugal pumps.
4. Paper mill drives.
5. Conveyors.
Oilfield centrifugal pump Vacuum pump in paper mills

SHORT TIME DUTY
This denotes the operation at constant load during a given time, followed
by a rest of sufficient duration.
In this, time of drive operation is considerably less than the heating time
constant and machine is allowed to cool off to ambient temperature before
the motor is required to operate again.
N indicates the duration of operation.
R indicates the period of rest.
Duty Cycle = N
N+R

EXAMPLES
Motors used for :
1.Domestic appliances like mixer.
2. Battery charging units.
3. Lock gates.
4. Bridges.

INTERMITTENT DUTY
It consists of periodic duty cycles, each consisting of a period of running at
a constant load and a rest period.
Neither the duration of running period is sufficient to raise the temperature
to a steady-state value, nor the rest period is long enough for the machine
to cool off to ambient temperature.
In this duty, heating of machine during starting and braking operations is
negligible.
N indicates duration of the operation.
Duty Cycle = N
N+R

EXAMPLES
Motors used for :
1. Hoist.
2. Lift.
3. Traction motors.
4. Trolley buses.
Gearless machine for elevators (lifts)

Components of gearless machine Geared machine for elevators (lift)

INTERMITTENT PERIODIC DUTY WITH
STARTING
This is intermittent periodic duty where heat losses during starting cannot be
ignored.
Thus, it consists of a period of starting, a period of operation at a constant load and a
rest period.
The operating and rest periods being too short for the respective steady state
temperature to be attained.
In this duty, heating of machine during braking is considered to be negligible, because
mechanical brakes are used for stopping or motor is allowed to stop due to its own
friction.

Duty Cycle = S + N
S+ N+R
S indicates starting period.

Examples :
Motors used for
1. Machine tools
2. Metal cutting lathes.

INTERMITTENT PERIODIC DUTY WITH
STARTING AND BRAKING
This is the intermittent periodic duty where heat losses during starting and braking
cannot be ignored.
Thus, it consists of a period of starting, a period of operation with a constant load, a
braking period with electrical braking and a rest period; with operating and rest
periods being too short for the respective steady state temperatures to be attained.
Duty Cycle = S + N + B
S+ N+ B+R

B indicates the period of braking.
Examples :
Motors used for
1. Suburban electric trains.
2. Billet mill drive.

DC Series motor
Synchronous motor

CONTINUOUS DUTY WITH
INTERMITTENT PERIODIC LOADING
It consists of periodic duty cycles, each consisting of a period of running at a
constant load and a period of running at no load, with normal voltage across the
excitation winding.
Again the load period and no load period being too short for the respective
temperatures to be attained.
This duty is distinguished from the intermittent periodic duty by the fact that a
period of running at a constant load is followed by a period of running at no load
instead of rest.

N indicates duration of the operation.
V indicates operation on no load condition.
Duty Cycle = N
N + V

EXAMPLES
Motors used for :
1. Pressing.
2. Cutting.
3. Shearing and
4. Drilling machine drives.

CONTINUOUS DUTY WITH STARTING AND
BRAKING
Consists of periodic duty cycle, each having
a period of starting,
a period of running at a constant load and
a period of electrical braking
there is no period of rest.
The duty cycle for this class is 1. Duty Cycle = S + N + B
S + N + B

B indicates the period of braking.

EXAMPLES
Blooming mill
Motor room which houses the
motor that droves the heavy mill
bloom rolls

CONTINUOUS DUTY WITH PERIODIC
CHANGES IN SPEED
This class indicates a sequence identical duty cycle, each having a
period of running at one load and speed, and another period of
running at different speed and load.
Again both operating periods are too short for respective steady-state
temperatures to be attained.
Further there is no period of rest.

N1, N2 and N3 indicates operation at three different motor speeds.
B1, B2 is the duration of electric braking.
S is the duration of starting.
Duty Cycle = S + N1 = B1 + N2 = B2 + N3
X X X
where X = S + N1 + B1 + N2 + B2 + N3

MAGNETIC MATERIALS
PROPERTIES:
• Saturation density; Wb / m 2
• Coercive force, A/Mt.
• Permeability
• Specific iron loss
• Density; Kg / m 3
• Retentivity (retaining magnetic property)
• Ease of machinability

I N S U L AT I N G
M A T E R I A L S

PROPERTIES
High dielectric strength, sustained at elevated temperature
High resistivity or specific resistance
Good thermal conductivity
High degree of thermal stability
Low dielectric hysteresis,
Good mechanical properties
Ability to withstand moisture, chemical attack, heat etc.,

Class Temperature (o C) Limit
Y 90
A 105
E 120
B 130
F 155
H 180
C > 180

Class Y:
This insulation consists of materials, or combinations of materials, such as cotton,
silk, paper without impregnation.
Other materials or combinations of materials can be included in this class, if by
experience or accepted tests they can be shown to be capable of operating at class Y
temperatures.
Examples:
Cotton, Silk, Paper, Cellulose, Wood etc., neither impregnated nor immersed in oil.
Materials of class Y are unsuitable for electrical machines and apparatus as they
deteriorate rapidly and are extremely hygroscopic

Class A:
This insulation consists of materials, or combinations of materials, such as cotton,
silk, paper when suitably impregnated or coated when immersed in a dielectric
liquid such as oil.
Other materials or combinations of materials can be included in this class, if by
experience or accepted tests they can be shown to be capable of operating at class A
temperatures.
Examples:
Materials of class Y impregnated with natural resins cellulose esters, insulating oils,
etc. Also included in this class are laminated wood, varnished paper.

Class E:
This insulation consists of materials, or combinations of materials, which by
experience or accepted tests they can be shown to be capable of operating at class E
temperatures.(materials possessing a degree of thermal stability allowing them to be
operated at a temperature 15○C higher than class A materials)
Examples:
Synthetic resin enamels, cotton and paper laminates with formaldehyde bonding, etc.

Class B:
This insulation consists of materials, or combinations of materials, such as mica, glass
fibre, asbestos etc. with suitable bonding substances. Other materials or
combination of materials, not necessarily organic, may be included in this class, if by
experience or accepted tests they can be shown to be capable of operating at class B
temperatures.
Examples:
Mica, glass fibre, asbestos with suitable bonding substances; built up mica, glass fibre,
and asbestos laminates.

Class F:
This insulation consists of materials, or combinations of materials, such as mica, glass
fibre, asbestos etc. with suitable bonding substances as well as other materials or
combinations of materials, not necessarily inorganic, which by experience or accepted
tests they can be shown to be capable of operation at class F temperatures.(materials
possessing a degree of thermal stability allowing them to be operated at a temperature
25○C higher than class B materials)
Examples:
Materials of class B with bonding materials of higher thermal stability.

Class H:
This insulation consists of materials such as Silicon elastomer and combinations of
materials such as mica, glass fibre, asbestos etc. with suitable bonding substances,
such as appropriate silicon resins. Other materials or combinations of materials may
be included in this class, if by experience or accepted tests they can be shown to be
capable of operation at class H temperatures.
Examples:
Glass fibre and asbestos materials and built up mica, with silicon resins.

Class C:
This insulation consists of materials, or combinations of materials, such as mica,
porcelain, glass and quartz with or without an inorganic binder. Other materials or
combinations of materials, maybe included in this class, if by experience or accepted
tests they can be shown to be capable of operation at temperatures above the class H
limit. Specific materials or combinations of materials in this class will have a
temperature limit which is dependent upon their physical, chemical and electrical
properties.
Examples:
Mica, ceramics, glass, quartz, without binders or with silicon resins of higher thermal
stability.

I N S U L AT I N G
M AT E R I A L S
U S E D
I N
M O D E R N
E L E C T R I C
M A C H I N E S

MICA
Mica used in its natural form or sheet state is difficult to work.
Therefore, it is used in the form of splittings with shellac, bitumen or polyester
binding.

It is a wrapping consisting of mica splittings which are to be paper and air dried.
It may be moulded directly on conductors, then rolled and compressed between
heated plates to solidify the material and to expel air.
MICAFOLIUM

FIBROUS GLASS
It is made from material which is free alkali metal oxides which may form a surface
coating that may attack the glass silicates.
Glass does not absorb moisture volumetrically, but may attract it by capillary action
between fine filaments.
Tapes and clothes woven from continuous filament yarns of glass have a high
resistivity, thermal conductivity, and tensile strength and form a good class
insulation.
The space factor of this insulating material is good but the material is prone to
abrasive damage.
These glass silk coverings are used for wires of field coils or much windings of
induction motor.

ASBESTOS
This material is mechanically weak, even when woven with cotton fibres, and is a
poorer insulating material as compared with fibre glass.
Laminates of asbestos with synthetic resins have good mechanical strength and thermal
resistivity
Asbestos in the form of wire and strip coverings have resilience and abrasion
resistance.
Space factor is low.

COTTON FIBRE
Fibre cotton woven from acetylates cotton, recently developed, have remarkable
resistant to heat, “tendering”.
They are much less hygroscopic than ordinary cotton materials.

POLYAMIDES
Polyamides in the form of nylon tapes have high mechanical strength and have a good
space factor on account of their thinness.
Nylon film is one of the few plastic films having adequate resistance to temperature
and can withstand tearing.

SYNTHETIC – RESIN ENAMELS
These enamels of the vinyl – acetate or nylon tapes have an excellent smooth finish and
have been used for much windings, with considerable improvement in winding times
and length of mean turn.
They also give good binding to windings.

SLOT LINING MATERIALS
These materials in the past have been mica composites.
However, the mica content is easily damaged in the forming.
In small motors a two ply varnished cotton cloth bended to press board is found
satisfactory.
On the other hand three ply material may serve for heavier windings.

WOOD
Wood, the form of synthetic resin impregnates compresses laminations, has proven
itself to be a robust and accurate materials for packing blocks, coil supports and
spacers.

SILICONES
Silicones are semi-organic materials with a basic structure of alternate silicon and
oxygen atoms.
They are extremely resistant to heats.
They act as binders in Class H insulation and permit their continuous operations at
180◦C.
Even when they disintegrate by excessive temperatures, the residue is the insulator
silica.
Silicones are water-repellent and anti-corrosive.
They are used in dry (oil-less) transformers, traction motors, mill motors and miniature
aircraft machines operating over a winding temperature of 200 to -40 ◦C.
They have a high thermal conductivity, improved heat transfer co-efficient, which
facilitates heat dissipation from conductors.

EPOXIDE THERMOSETTING RESINS
These materials have assumed importance in casting potting, laminating-adhesive and
varnishing applications and in the encapsulation of small transformers.

SYNTHETIC RESIN
Bonded paper, cotton and glass – fibre with synthetic resin laminates have good
electrical and mechanical properties as sheets in large cylinders and tubes.

PETROLEUM BASED MINERAL OIL
They are extensively used in the cooling and insulation of immersed transformers.
The characteristics of importance are chemical stability, expansion coefficient,
resistance to sludging by oxidation, and viscosity .
Their electric strength is good when they are clean and moisture-proof.

ASKARELS
They are synthetic non-flammable insulating liquids which, when decomposed by an
electric arc, evolve only as non-explosive gases.
The commonest askarel is a 60/40 mixture of hexachlorodiphenyl trichlorovenzine
giving a low pour point and a satisfactory viscosity/temperature characteristics.

APPLICATIONS
• Between core and windings
• Between turns in a coil
• Between coils
• Between stampings in a laminated core
• Between commutator segments
• Between sliprings

SELECTION OF MATERIALS
• Cost
• M/C rating (Voltage / Power)
• Size of M/C
• Duty cycle
• User requirement (application)
• Life
• Proper specifications

COOLING OF ROTATING M/CS
• TYPES:
– Natural
– Fan
– Separate :
Open circuit, Surface, Closed circuit, Liquid

TYPES OF ENCLOSURES
• Totally enclosed M/C
• Totally Enclosed Fan cooled M/C
• Totally Enclose Separately Air Cooled
• Totally Enclosed Water / Liquid Cooled
• Weather Proof
• Watertight / Submersible
• Flame Proof
• Drip proof / Splash Proof

COOLING SYSTEM
• AIR:
– Radial Ventilation
– Axial Ventilation
• LIQUID / GAS
– Hydrogen
– Water
– SF6
– High Grade Transformer oil

SPECIFICATIONS
• STANDARDS:IEC; IS ; BEE; NEMA; IEEMA
• SPECIFICATIONS:
– Name plate details: KW/HP/ KVA; Phase; Volts, Frequency, Rated current; Speed; Connection
( Y/Δ ) Insulation class; Temperature; Duty cycle
– What else?

SPECIFICATIONS – CONTD…
• Type of cooling
• Frame size (Shaft length, Mounting holes, Height from ground to shaft
centre)
• Standards
• Type of enclosure - flame proof / water proof / dust proof
• Type of mounting (surface / vertical)
• Bearing details (DE / NDE)

CLASSIFCATION OF M/CS
•According to Size:
– Small Size : up to 750 Watts
– Medium Size : up to 250 KW
– Larger Size : 250 KW to 5000KW
•According to speed:
– Low speed : < 500 r.p.m.
– Medium speed : 500 to 1000 r.p.m.
– High speed / Turbo: > 1000 r.p.m.

MANUFACTURERS
• SIEMENS
• ABB
• KIRLOSKAR
• BHARATH BIJLEE
• GEC
• BHEL
• INDO TECH

Unit I design of electrical machines

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