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Introduction
With the development of the technology and the way in which human labour is getting
minimized and the comforts increasing tremendously the use of electrical energy is ever
increasing. Basically electric power is the main source of energy for carrying out many functions,
as it is a clean and efficient energy source, which can be easily transmitted over long
distances. With the availability of Transformer for changing the voltage levels to a very high
value (of say 132kV to 400kV) the use of AC power has increased rapidly and the DC power
is used only at remote places where AC power cannot be supplied through power lines or
cables or for a few social purposes.
A synchronous generator is an electrical machine producing alternating emf (Electromotive
force or voltage) of constant frequency. In our country the standard commercial
frequency of AC supply is 50 Hz. In U.S.A. and a few other countries the frequency is 60
Hz. The AC voltages generated may be single phase or 3-phase depending on the power
supplied. For low power applications single phase generators are preferable. The basic principles
involved in the production of emf and the constructional details of the generators are
discussed below.
1.1 Generation of emf
In 1831 Faraday discovered that an emf can be induced (or generated) due to relative
motion between a magnetic field and a conductor of electricity. This voltage was termed
as the induced emf since the emf is produced only due to motion between the conductor
and the magnetic field without actual physical contact between them. The principle of
electromagnetic induction is best understood by referring to Fig. 1. The magnetic field is
produced by the two fixed poles one being the north pole from which the magnetic flux
lines emerge and enter into the other pole known as the south pole. It was found that the
magnitude of the voltage induced in the conductor is proportional to the rate of change of
flux lines linking the conductor.
Mathematically it is given as
e =
dφ
dt
≈
φ
t
volts
The direction of the induced emf is given by Fleming’s Right Hand Rule which
states: If the thuMb, First finger and the seCond finger of the right hand are stretched out
and held in three mutually perpendicular directions such that the First finger is held pointing
in the direction of the magnetic field and the thuMb pointing in the direction of motion,
then the seCond finger will be pointing in the direction of the induced emf such that the
current flows in that direction. As shown in Fig. 4 the induced emf is in a direction so as to
circulate current in the direction shown by the middle finger. Schematically we indicate the
direction of the emf by a dot as shown in Fig. 5(a) to represent an emf so as to send current
in a direction perpendicular to the plane of the paper and out of it. A cross will indicate the
emf of opposite polarity, see Fig. 5(b). Although the Right Hand Rule assumes the magnetic
filed to be stationary, we can also apply this rule to the case of a stationary conductor and
moving magnetic field, by assuming that the conductor is moving in the opposite direction.
For example, as shown in Fig. 4 the direction of the induced emf will be the same if the poles
producing the field had been moved upwards.
1.3 Electromagnetic Force
The motion of the conductor in a magnetic field can be imparted by the application
of an external mechanical force to the conductor. In such a case the mechanical work
done in moving the conductor is converted to an electric energy in agreement with the law
of conservation of energy. The electric energy is not produced by the magnetic field since
the field is not changed or destroyed in the process. The name electro mechanical energy
conversion is given to the process of converting energy from mechanical form obtained from
a prime mover, such as an IC engine, water/steam turbine etc, into electric energy.
The emf induced in the conductor will circulate a current through it if a closed circuit
is formed by an external connection. The direction of the current flowing in the conductor
will be such as to oppose the cause of it as stated by Lenz’s Law. A current carrying
conductor located in a magnetic field will experience a force given by Biot-savart’s law:
f = Bli (5)
In other words, whenever a change in flux linkages occur, an emf is induced that
tends to set up a current in such a direction as to produce a magnetic flux that opposes the
cause of it. Thus if a current carrying conductor is placed in a magnetic field as shown in
Fig. 5 the current tends to produce a magnetic field in the direction shown by the dotted
circles.
The direction of the flux lines around the current carrying conductor can be easily
determined by Corkscrew Rule - which states that the flux lines will be in the same direction
as the rotation of a right threaded screw so as to advance in the direction of flow of current.
As a result the magnetic field, for the case shown in Fig. 5(a), is strengthened at the top and
weakened at the bottom of the conductor, thereby setting up a force to move the conductor
downwards. For the case of a Generator, the conductor must be moved up against this
counter force or the opposing force. Similarly the current is to be supplied to the conductor
against the emf generated (known as the counter emf or back emf) in the conductor as it
moves due to the motor action. Thus, the same machine can be operated as a generator
or a motor, depending on whether we supply mechanical power or electrical power to it,
respectively.
The generators shown in Fig. 1 and Fig. 4 and discussed in the earlier sections
are clearly impractical for a number of reasons. The main reason is that such generators
require a prime mover that imparts linear or reciprocating motion to the conductor. Most
of the commercial prime movers provide rotary motion in the commercial generators. The
conductors of most commercial generators are rotated about a central axis of a shaft. The
conductors are housed in slots cut in a cylindrical structure (made of magnetic material)
8
Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao
Indian Institute of Technology Madras
known as the armature. The armature is supported at both ends by means of bearings
attached to the shaft that goes through the center of the armature. The armature is rotated
inside the field structure by providing a small gap between these two members. This gap is
known as the air gap and is usually of the order of 1 to 1.5 cms. If the air gap is maintained
constant throughout the spread of the pole arc, we have a fairly constant flux density under
it in a plane perpendicular to the plane of the conductor’s motion. i.e. in a radial direction
with respect to the field and armature structure. Since the emf is also proportional to B,
the flux density in the air gap of AC generators is arranged to be distributed as closely to
a sine wave as possible by suitable shaping (chamfering as it is technically known) of the
pole shoe. Since the relative motion between the conductors and the magnetic flux lines is
responsible for the production of emf, it is immaterial whether the conductors are rotated
or the magnetic flux producing poles are rotated. In most of the alternators it is the field
that is rotated rather than the conductors. In an alternator the external connection to the
load can be taken directly from the conductors since there is no need for any rectification
as in a DC generator. In a DC generator the rectification of the emf is achieved through a
mechanical rectifier—- the commutator and brush arrangement. Moreover the load current
supplied by the alternator can be easily supplied from stationary coils without any difficulty
as there will be no sparking and wear and tear of the brushes and slip rings. Where as the
low values of D.C excitation current to the field coils can be easily sent through the slip
rings and brush arrangement. Thus the usual arrangement in an elementary synchronous
generator is as shown in Fig. 6. The conductors are housed in slots cut in the armature
structure. Only a single coil of N turns, indicated in its cross-section by the two coil sides
a and -a placed in diametrically opposite slots on the inner periphery of the stator (i.e. the
armature, as it is a stationary member here) is shown in Fig. 6.
The conductors forming these coil sides are parallel to the shaft of the machine and
are connected in series by end connections (not shown in the figure ). The coils are actually
formed by taking a continuous copper wire of suitable cross section round a diamond shaped
bobbin. The completed coil is shown in Fig. 7. The copper wire is usually of fine linen
covered, cotton covered or enamel covered so as to have sufficient insulation between the
conductors of the same coil. The actual layout and interconnection of various coils so as to
obtain the required voltage from the synchronous machine (alternator) is presented in the
following section.