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 a 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.
What is a Synchronous Generator?
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 the 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.
Generation of Emf in a synchronous machine:
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. 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 volts
where
φ = flux in Webers
t = time in seconds
e = average induced emf in volts
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| Generation of EMF |
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.
Construction of a Synchronous Generator:
Synchronous generators
require a prime mover that imparts linear or reciprocating motion to the conductor. Most commercial prime movers provide rotary motion in 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) known as the armature. The armature is supported at both ends using 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 1 to 1.5 cm. 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
concerning 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 close to
a sine wave as possible by suitable shaping (chamfering as it is technically known) of the
pole shoe. The conductors are housed in slots cut in the armature
structure.
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| 2 Pole Synchronous Machine |