unit -Ipptx Introductionof Electrical Machine Apparatus

EE-2355 Design of Electrical
Machines – UNIT -I
7/16/2024 1
St.Joseph's College of Engineering

Major consideration in design
• Design of machines:
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.
The problem of design and manufacture of
electric machinery is to build, as economically as
possible , a machine which fulfils a certain set of
specifications and guarantees.
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Contd..,
• The major design considerations to evolve a good design are as
follows:
• Design Base: Matching the existing experience with R & D; bringing
in the latest material technology; limitations in design; convenience
in production line and transportation; working safety and reliability;
maintenance and repair; environmental conditions ; cost economy;
optimization.
• Specification: Meeting with the customer’s requirements,
guarantees; satisfy the national and international standards.
• Design Transfer: Transfer of design to factory foreman i.e.
drawings, processes, instructions, job-flow, meeting the delivery
schedule.
• Information updating: Technical journals, R and D papers and
reports, interaction in meetings and seminars.
7/16/2024 3

Contd.,
• It is impossible to design a machine which is
cheap and is also durable at the same time.
• Long life span – high quality materials and
advanced manufacturing techniques → high
cost
• Good design – machine has reasonable
operating life (20-30 years) and reasonable
initial cost.
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Areas of design
• Main areas of design:
Knowing the characteristics and specifications that a
machine has to satisfy, the main areas of design are:
1. The magnetic circuit; core, yoke, airgap etc.,
2. The electric circuit; the windings
3. The insulation (Dielectric circuit)
4. Heating and cooling circuit (Thermal circuit)
5. The mechanical construction
The designer work lies in suitably allocating the space to
frame, core, airgap, winding, insulation and cooling
medium in the machine.
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Limitations
• Limitations:
1. Saturation of magnetic parts: Increased core losses and excitation at higher flux
density.
2. Temperature rise: increased temperature rise under higher output deteriorates
the insulation and affects the life of the machine.
3. Insulation: Breakdown due to high voltage gradient; temperature rise
limitations, mechanical damage.
4. Mechanical strength: specially in turbo-machine due to large size and high
speed.
5. Efficiency: If high efficiency is the aim, the machine becomes costly, for lower
efficiency higher running cost and temperature rise with associated problems.
6. Customer’s specifications: Impose limitations to identify criterion for best
design.
7. Commutation: In d.c. machine output is limited because of commutation
problem
8. Power Factor: P.F imposes a limitation specially in case of 3-ph induction motor.
Because of p.f. limitation higher flux density cannot be taken in airgap which
would have otherwise resulted in more compact size motor.
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The design of electric machines- science and art.
The process of design of a single machine may
be divided into three major design problems,
such as:
i) Electromagnetic design
ii) Mechanical design
iii) Thermal design
7/16/2024 St.Joseph's College of Engineering 7

Various Principles of Machine
• Basic principles:
All electric machines are electromagnetic
devices. Among them the rotating machines are
electromechanical energy conversion devices
but transformer is a static machine where
energy conversion does not take place, only the
level of voltage changes. All electrical machines
are based upon three principles:
1)Induction 2)Interaction 3) Alignment.

Contd.,
• Faraday’s Law of electromagnetic Induction:
Emf induced in a electric circuit is equal to the rate of change of flux linkages
Flux linkage Ψ = NΦ, e= -dΨ/dt
The change in flux linkages can be caused in three ways:
i) The coil is stationary w.r.t flux and the flux varies in magnitude w.r.t time
ii) The flux is constant w.r.t time and is stationary and the coil moves
through it.
iii) Both the changes mentioned above occur together i.e., the coil moves
through a time varying field.
In method (i) transformer or pulsational emf is produced . No energy
conversion only energy transference – Transformer
In (ii)method motional emf due to motion of conductor- electromechanical
energy conversion - D.C.Machines, induction and synchronous machines.
In method (iii) Tranformer and motional emf takes place. Both transfer and
conversion of energy takes place – Commutator motors.

RATING OF MOTORS
• The rating of the motor selected should be in
accordance with the mech. work required by the
driving machine.
• Motor selected is must not have excess
temperature rise when it is fully loaded(damage
the insulation).
• It should drive the load satisfactorily both under
steady state as well as transient operation.
• Motor must be capable of withstanding short
time overloads and must have enough starting
torque to accelerate the motor to desired speed
in given time.

Power loss and heating of motors
• Power losses are copper loss, core loss, mechanical losses.
• Copper loss – armature and field copper loss
• Core loss or iron loss – eddy current loss and hysteresis loss.
• Mech loss – windage and friction loss
• This power loss cause localised heating and responsible for
temperature rise.
• Losses occurring in a machine are,
• W = Wc + X2 Wv
Constant loss – independent of current
Variable loss – square of the load current
X – load on the motor expressed as a function of rated load.

Heating and cooling curve
The size and rating of a motor to be used as
a drive element in a particular appln
depends upon
• Heating effects in the motors
• Loading conditions and classes of duty
• Load inertia or inertias
• Environment conditions

Heating effects
• When a motor operates, heat is produced due to losses
inside the machine and its temp rises.
• As the temperature increases beyond(exceeds) ambient
value, a portion of heat produced, flows out to the
surrounding medium.
• The amount of outflow of heat is a function of temp rise
of motor above the ambient value.
• As motor temp rises, the heat outflow increases and
equilibrium ultimately sets in when the heat generated
becomes equal to heat dissipated to surrounding
medium.
• Steady state temperature depends on power loss, which
in turn depends on the output power of the machine.
• Steady state temperature is not same at various parts of
the machine.(winding – high – loss density is high,
dissipation is slow.

• Too complex to predict the accuracy of heat flow and
temperature distribution.
• Direction of heat flow does not remain same at all
load conditions.
• At no-load and lightly loaded condition ,heat flows
from iron parts to windings due to temp gradient.
• When load increases, heat generation in the winding is
greater than iron core.
Heating and cooling curves

simplifications
• Homogeneous body – uniform temp gradient.
• At all the points at which heat is generated have the
same temp.
• At all the points at which heat is dissipated to
cooling medium have the same temp.
• Heat dissipation is directly proportional to the
difference of temp of the body and surrounding
medium. No heat is radiated.
• Rate of dissipation of heat is constant at all temp.

CLASSES OF DUTY
• The rating of motor selected from the viewpoint of
heating depends on the load conditions to which it
is subjected.
• There are eight types of classes of duty.
• REF – S.K PILLAI

CONTINUOUS DUTY
• Operation at constant load of sufficient duration of thermal
equilibrium to be reached. E.g.. Fan, conveyor etc..

INTERMITTENT PERODIC DUTY WITH
STARTING

INTERMITTENT PERODIC DUTY WITH
STARTING AND BRAKING

CONTINUOUS DUTY WITH
INTERMITTENT PERODIC LOADING

CONTINUOUS DUTY WITH STARTING
AND BRAKING

CONTINUOUS DUTY WITH PERODIC
SPEED CHANGES

unit -Ipptx Introductionof Electrical Machine Apparatus

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