synch generator introduction

1. SYNCHRONOUS
GENERATOR
Introduction & Construction
NOORNABI SHAIKH
DEPARTMENT OF ELECTRICAL ENGINEERING
MUET

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INTRODUCTION
Synchronous generators (also called Alternators or AC generators) are
synchronous machines used to convert mechanical power to ac electric
power.
 Synchronous generators are the primary source of all the electrical
energy we consume.
 These machines are the largest energy converters in the world. They
range in size from a fraction of a KVA to 1500MVA. [1991 Charles I.
Hubert]
 Commonly used to convert the mechanical power output of steam
turbines, gas turbines, engines, hydro turbines into electrical power for
the grid.
 The bulk of electric power is generated with high-speed steam
turbines driving cylindrical (Nonsalient) rotor machines, and low speed
hydraulic turbines driving salient pole machines.
 Synchronous generators are built with either a stationary or a
rotating dc magnetic field.

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BASIC WORKING PRINCIPLE
 A DC current is applied to the rotor winding, which produces a rotor
magnetic field.
 The rotor of the generator is then turned by a prime mover, producing
a rotating magnetic field within the machine.
 The rotating magnetic field induces a 3 – phase set of voltages within
the stator windings of the generator.
 The electrical frequency of the 3-phase output depends upon the
mechanical speed and the number of poles
120
Pn
f m
e =

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Synchronous Generator Rotor
It is essentially a large electromagnet. The magnetic poles on the rotor
can be either:
 Salient
 Nonsalient
Salient means “sticking out” and a salient pole is a magnetic pole that
sticks out from the surface of the rotor. These type of rotors are used
with 4 or more poles.
A nonsalient pole is a magnetic pole constructed flush with the surface
of the rotor. These types of rotors are normally used for 2 and 4 – pole
rotors.
NOTE
Due to changing magnetic fields, rotor is constructed of thin laminations
to reduce eddy current losses.

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A salient 6 – pole
rotor
A nonsalient
2 – pole rotor

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A dc current must be supplied to the field circuit on the rotor. Since rotor
is rotating, a special arrangement is required to get the dc power to its
field windings.
There are two common approaches to supply this dc power.
1. Separately Excited System
2. Self (brushless) Excited System

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In this system dc power is supplied from an external dc source to the
rotor by means of slip rings and brushes.
Slip rings are metal rings completely encircling the shaft of a machine
but insulated from it.
One end of the dc rotor winding is tied to each of the 2 – slip rings, and
a stationary brush (graphite like carbon compound that conducts
electricity) rides on each slip ring.
1. Separately Excited System:
NOTE
Slip rings and brushes increase the amount of maintenance required on the machine, since brushes must be checked for wear
regularly.
Brush voltage drop can be the cause of significant power losses on machines with larger field currents.

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Slip Rings
They are metal rings completely encircling the shaft of a machine but insulated from it. One end of the dc
rotor winding is tied to each of the two slip rings, and a stationary brush rides on each slip ring.
Brush
A “brush” is a block of graphite like carbon compound that conducts electricity freely but has very low friction,
so that it doesn’t wear down the slip ring. If the positive end of a dc voltage source is connected to one brush
and negative end is connected to the other, then the same dc voltage will be applied to the field winding at all
times regardless of the angular position or speed of the rotor.

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Self (brushless) Excited System:
a brushless exciter is a small ac generator with its field circuit mounted
on the stator and armature circuit mounted on the rotor shaft. The 3 –
phase output of the exciter generator is rectified by 3-phase rectifier also
mounted on the shaft of the generator, and is then fed into the main dc
field circuit.

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By controlling the small dc field current of the exciter generator
(located on the stator), it is possible to adjust the field current on the
main machine without slip rings and brushes. Refer figure shown
above.

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To make the excitation of a generator completely independent of any
external power sources, a small pilot exciter generator is often included
in the system.
Many generators include brushless exciters and slip rings, so that
auxiliary source of dc field current is available in emergencies.

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Synchronous Generator stator
Stator of a synchronous generators are normally made of preformed
stator coils in a double layer winding. The winding is itself is distributed
and chorded in order to reduce the harmonic content of the output
voltages and currents.

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