1
This chapter provides principles of design, types of electrical machines, limitations in design and recent trends in electrical machine design. A chapter-wise detail outline of the text has also been presented.
There are different specific tasks to be performed by the electrical machines. The design of electrical machines/equipments for the specific applications is based upon the application of theoretical scientific concepts, technology and related inventions. The suitable design depends upon the proper adjustment of iron portion, copper, air gap, insulation, ventilation and cooling of machine. The selection of conducting, magnetic and insulating materials has important role in machine design. We have to explore the availability of the each type of material, its characteristics, which should be according to the specifications, the performance limitations and its cost.
When a machine is to be designed for a specific application then it is preferred that the designed machine should fulfill the required specifications. But it is very difficult to design a machine which meets all the specifications and requirements. The machine should be cheaper, durable, reliable and should perform according to requirements. The manufacturing costs, operating and running costs of the machine are considered. Design of a machine means that the cost should be minimum. If the cost is minimized, it may not be a durable machine. So the important criteria for good design is to get the minimum losses for given cost. If only cost is considered and we try to minimize it, this may result in a machine which have higher operating and maintenance cost. The losses will be more. If we try to minimize the losses in the machine, it may become more costly. So there should be a balance between cost and the losses to have an optimum design of the machine for a particular application.
An electric machine is an electro-mechanical energy conversion device, which converts mechanical energy into electrical energy and vice versa. When the machine converts mechanical energy into electrical energy it is called as generator. When the machine converts electrical energy into mechanical energy it is called as motor. A part of energy is converted to heat. This energy is lost and cannot be recovered.
An electrical machine can be designed to operate either as a generator or as a motor. Faraday’s law of electromagnetic induction states that e.m.f. induced in a closed electric circuit is equal to the rate of change flux linkages

4 DESIGN OF ELECTRICAL MACHINES
Flux linkages,
Ψ
= N
φ where, N = number of turns of coil
φ
= flux linking with all the turns
∴
The e.m.f. induced in the coil e dt or e
= −
N d
φ dt
The negative sign in above expression indicates that the current produced due to induced e.m.f.
always opposes the change in flux linkages. The change in the flux linkages can be achieved by
( i ) a stationary coil with respect to flux but magnitude of flux varying with respect to time.
( ii ) the coils moving through a flux which is constant with respect to time.
( iii ) the coils moving through a flux with varying magnitude with respect to time.
When a conductor moving at right angles to a uniform, stationary and constant magnetic field an e.m.f. is induced and is expressed by e = B l v volts where, B = flux density of magnetic fields (Wb/m 2 ) l = length of conductor perpendicular to the magnetic field (m) v = velocity of conductor (m/sec)
The force produced by the interaction of magnetic field and current carrying conductor is given by the Biot-Savart’s law and can be expressed by
F = B l I where, B = flux density of magnetic fields (Wb/m 2 ) l = length of conductor perpendicular to magnetic field (m)
I = currents (Amp)
When a conductor is placed on a rotor with radius r and the torque produced by the current in the conductor is given by
Torque = F × r
Torque, T = B l I
r Newton-metre
This is an electro-magnetic torque.
Power = Torque × Angular velocity
= T
ω
It is, therefore, concluded that the design and construction of generators and motors are based upon the facts given below:
( i ) An e.m.f. is induced in a conductor or a set of conductors subjected to a magnetic field in such a manner that either conductor moves and cuts the stationary lines of flux or it (stationary conductors) is cut by the varying lines of flux. In both the cases there will be a relative motion between the conductor and the magnetic field. This is the characteristics of the generator action.

INTRODUCTION 5
( ii ) When a conductor or a set of conductors are placed in a magnetic field and an electric current is supplied through the conductor then conductor observes a mechanical force. This is the characteristics of the motor action.
( iii ) The armature of an electrical machine carries the conductors in which the e.m.f. is induced.
The field portion of the machine produces the magnetic field. The armature may be rotating with stationary field or the armature may be stationary with rotating field. Usually in large machines the armature is stationary (called stator) and the field is rotating (rotor). Relative advantages and disadvantages are discussed in detail in 5.2 Chapter 5.
There are two different types of machines based on the nature of e.m.f. and current produced:
( a ) a.c. machines.
( b ) d.c. machines.
The a.c. machines can be further classified as
( i ) Synchronous machines.
( ii ) Induction machines.
A synchronous machine has normally a field winding on the rotor and three-phase winding on the stator. The field winding is supplied by a separate d.c. source. The field winding produces a field and due to rotation of rotor, the field moves in space at the speed of the rotor. This rotating field links with the three-phase stator conductors and hence voltage is induced in them.
The magnitude of the voltage will depend upon the strength of field (magnetic field), number of turns and frequency (corresponding to poles and r.p.m.) for an alternator. For a synchronous motor three-phase supply is given to the stator. Since the three-phase winding is distributed in space in the stator at 120° apart and the currents in three-phases are varying with time so a rotating field
(at a synchronous speed, N s
=
120 f
) is produced in the air gap of the machine. In the rotating field
P fictitious North and South poles are produced rotating at synchronous speed ( N s
) and behave as North and South poles were being rotated at synchronous speed. The field produced by the rotor conductor current reacts with the stator field. Initially the stator poles interact with rotor poles and since rotor is at standstill and stator poles are rotating at synchronous speed. So if North pole of stator field is in contact with South pole of rotor at any instant then attractive force will be observed by rotor and it will try to move in the direction of stator rotating field. But soon after that South pole of stator field will come in contact of South pole of rotor and repulsive force will act upon rotor. Hence a pulsating force acts upon a stationary rotor and so it does not move. That is why a three-phase synchronous motor is not self starting.
We can rotate the rotor of the motor by some other means at or reacts the synch. speed so that
North and South poles of stator and rotor fields (opposite poles) are coupled (magnetically locked) with each other. The motor keeps on rotating at synchronous speed ( N s
). The other way to make it self starting may be to provide the damper winding on the rotor of the motor. Damper winding on the rotor behaves like the cage rotor of three-phase induction motor. So three-phase synchronous motor with damper winding is started as induction motor. It picks up the speed near to synchronous speed and

6 DESIGN OF ELECTRICAL MACHINES when opposite poles of stator fields and rotor fields come in contact (polar region), the speed of rotor suddenly jumps from induction motor speed to synchronous speed and poles are coupled (magnetically locked) with each other and rotor rotates at synchronous speed. The three-phase synchronous motor runs only at synchronous speed or not at all.
An induction machine has polyphase winding on the stator as well as on the rotor. The three-phase supply to the stator winding produces a rotating magnetic field (r.m.f.) in the air gap. This field links with the rotor conductors and due to change in time an e.m.f. is induced in the rotor winding and hence a current is produced in the rotor winding. The field produced by the rotor current reacts with the stator field and tries to oppose the cause by which it is being produced. The cause due to which e.m.f. is induced in rotor conductor is relative speed between rotor and stator field. So induction motor starts running in the direction of r.m.f. to minimize the cause (relative speed).
A d.c. machine has stationary poles on the stationary part. The field winding is provided for poles which produces a stationary field in the air gap of the machine. The armature winding is placed on the rotor of the machine.
When a machine has to be designed and constructed, the choice of suitable materials and manufacturing technology becomes important. The following considerations are required which impose the limitations on the machine design:
( i ) Saturation of the Magnetic Circuit: The saturation of the magnetic circuit disturbs the straight line characteristics of magnetisation ( B H ) curve resulting in increased excitation required and hence higher cost for the field system.
( ii ) The Temperature Rise Over the Ambient Temperature: The excessive temperature rise may cause insulation failure. The life of the machine depends upon the life of the insulation. If the machine is continuously operated above the specified temperature limit, the life of the insulation and hence the life of the machine will be reduced. By providing proper ventilation and cooling, the temperature rise can be kept within the safe limit.
( iii ) Insulation: The insulating properties and the strength of the insulating materials are considered on account of breakdown due to excessive voltage gradients set up in the machine.
( iv ) Mechanical Strength: The machine should have the ability to withstand centrifugal forces and other stresses.
( v ) Efficiency: The efficiency of the machine should be high for low running cost. The specific magnetic and electric loading should be low to achieve high efficiency. With low value of magnetic and electric loadings, the size of machine will be larger and hence more capital cost
(initial investment).
( vi ) Type of commutation desired whether ideal, under or over commutation.
( vii ) Power Factor: Power factor required in a.c. machines whether it is low or high.
The best design would be one which attains the maximum advantage of better performance and high efficiency in the least possible cost. Sometimes the life, heating, rating and even efficiency of the machine have to be changed to bring down the cost of design and manufacturing.

INTRODUCTION 7
The electrical machine design depends upon several factors like mechanical stresses, magnetic forces, temperature rise and cooling medium adopted for the machine.
Taking into consideration the different parameters and operating conditions, the designer has to design a machine which should be best suitable. It should not merely fulfil the specifications. The machine designed should be rugged, simple, efficient, economic and safe. Designers should have sufficient technical knowledge and should be able to visualize the simultaneous inter-relationships of all the parameters and their effect. The visualization can be based on wide practical experiences.
Sometime machine may have to operate in isolation but sometime it is used in a system with so many machines. The machine which has to operate in such system cannot be designed in isolation but the design of machine depends upon the optimization of the system performance.
For a particular application, several set of design of machine may need to be done to find the optimum designed machine. In finding the optimum design of the machine many iterations may be required to incorporate the changes in parameters till the satisfactory performance in the machine is obtained. These calculations with indefinite iterations are manually not possible. These calculations can be easily done by means of a digital computers. By using computer the data can be easily varied several times to get the optimum design. So the computer has become a powerful and important tool in designing the electrical machines.
Chapter 1 highlights the principle of design, limitations in design and recent trends in design. Important features like effect of fringing of flux, types of damper windings, different type of stator leakage flux, calculation of slot leakage reactance for different type of slots, calculation of pole leakage flux, different types of stator and rotor slots are described in chapter 2. Chapter 3 starts with the description of magnetic conducting and insulating materials. These materials are separately discussed with their specific applications. In chapter 4 different cooling systems, heat dissipation by radiation, conduction and convection, volume of air and liquid required, temperature rise time curve, types of enclosures for rotating electrical machines, different methods of ventilation, rating of electrical machines, methods of measurement of temperature rise etc. are explained.
Chapter 5 is devoted to the description of construction, principle and design aspects of three-phase synchronous machine. Derivation of output equation, estimation of main dimensions, effective length, design of stator teeth and slots, winding design, rotor design with damper winding design, determination of open circuit characteristics (OCC), temperature rise in alternator and rotor design of turbo-alternator are discussed in this chapter.
Chapter 6 provides the details of construction of induction machines. This chapter also provides the output equation, estimation of main dimensions, effective length of machine, stator teeth and slot design, winding design, outer diameter, rotor design, efficiency, flattened flux density, no load current, estimation of performance of induction motor, construction of circle diagram from design data, stator temperature rise etc.
Chapter 7 describes the construction and some features of d.c. machines. Output equation and its derivation, design of field system, design of commutator, design of interpoles, compensating winding and losses and efficiency are also given in this chapter.

8 DESIGN OF ELECTRICAL MACHINES
Chapter 8 elaborates different types of transformers, stepped core and yoke, output equation, window space factor, e.m.f. per turn and different dimensions of transformers. The steps to design a transformer, estimation of no loud current, leakage reactance and regulation, design of tank etc. are also provided in this chapter.
In chapter 9, general principles of computer aided design are discussed. The different approaches for computer aided design, optimization and standardization of design are also discussed in this chapter.
Chapter 10 provides the computer aided design of transformer, three-phase alternator, three-phase induction machines and d.c. machines. Different design problems for each machines are solved with the help of computer programme in C. The related flow chart for these problems are given. The flow chart for overall design of the machines are also provided in this chapter.
Some tables which are required in the design of different machines are given in appendices.