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Module 2
Synchronous Converters
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AC Generator
Alternator (AC Generator) or Synchronous
Generator it is an electrical machine converting
mechanical energy to ac electrical energy.
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Principle of Operation
The principle of operation of the alternator is exactly the same as that ofthe dc generator that is there is induced emf in the stator conductors when
they cut the magnetic flux produced in its magnetic field poles. However in
an alternator it is not essential for the armature to rotate or and either the
armature or field can be made to rotate while the other is kept constant.
Nowadays the armature is kept stationary and the field rotates around it.
Rotating armature alternator Rotating field alternator
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dvantages of Revolving
Field Stationary rmature
The armature winding is more complex than the field and can
be constructed more easily on stationary armature.
The armature winding can be braced more securely in rigid
frame. It is easier to insulate and protect the high-voltage armature
winding common to alternators.
The armature winding is cooled more because the stator
case core can be made large enough and with many airpassages or air ducts for force air circulation.
The low voltage field can be constructed for efficient high-
speed operation.
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Alternator Construction
The main parts of the alternator are the following:
1. Stator. It is built up with stampings which are
insulated with paper varnish. The stator is housed in
frame which is fabricated from electrically weldedsteel plates. Slots are cut around the inner surface
which accommodates the windings.
2. Rotor. It has rotating magnetic field poles as in dc
generators which are separately excited from a dcsource known as exciter.
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Types of Rotor Construction
Two Types of RotorConstruction:
1. Salient (or projecting)
poles. It is made of cast
iron steel of good
magnetic quality. These
types of rotors are used in
low speed and medium
speed machines. These
machines have short axial
length and large
diameters. Hydropower
alternators and diesel
engine alternators are of
this type.
Types of rotors used in alternator.
(A) Cylindrical type; (B) Salient-pole type
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Types of Rotor Construction
2. Smooth-cylindrical type.
It is made of a solid steel
piece and slots and made
on the circumference of the
rotor to hold the fieldwindings. This type of rotor
is suitable for high speed
turbo alternators. This type
of rotor is cylindrical and
has a large axial lengthand small diameter
Types of rotors used in alternator.
(A) Cylindrical type; (B) Salient-pole type
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Alternator Construction (cont.)
2. Alternator Construction
a. Stationary field revolving armature
b. Revolving field stationary armature
3. Damper Windings (Squirrel Cage Windings)
Functions of Damper Windings
Useful in preventing hunting (momentary speed
fluctuations)
Provides the starting torque needed in synchronous
motor
Tends to maintain balanced 3-phase voltage under
unbalanced load conditions
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General Types of Alternator
1. Synchronous Generator. It is a generator which is driven
at constant speed (synchronous speed) and it is used in
almost all types of applications.
2. Induction Generator. It is an induction motor which runs
as a generator with a speed above synchronous speed. Its
p.f. is normally leading and usually connected in parallel
with a synchronous generator in order to supply power for
lighting loads.
3. Induction Alternator. It generates voltage at higher
frequencies (500 Hz to 10 kHz). It is used to supply power
to induction furnace in order to heat and melt the metal.
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Prime Movers for Alternator
The following are the various prime movers used foralternators:
For large AC generator
a. Steam turbine
b. Gas turbine
c. Hydraulic turbine
d. Internal combustion engine
For small AC generator
a. Internal combustion engine
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Generated Voltage in an lternator
Where:
f = frequency in Hertz
p = number of poles
nS = synchronous speed in rpm
=
= . = .
Where: E = rms voltage generated per phase
m = maximum flux per pole
N = number of turns per phaseZ = number of conductors in series per phase
kp = pitch or chording or coil span factor
kb = breadth or distribution or winding or spread factor
Frequency of Generated EMF
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Pitch FactorPitch Factor or Chording Factor or Coil Span Factorit may be
defined as the ratio of the vector sum of the induced emfsper coilto the arithmetic sum of the induced emfsper coil. It is may also be
defined as the ratio of the emfsof short pitch coil to emfsof full-
pitch coil. By formula, it is given as
=
= [ ]
=
.
Where: kp= pitch factor
q = qth harmonic (1 for fundamental or first harmonic, 3
for third harmonic, 5 for fifth harmonic and so on.
= span of coil (coil pitch) in electrical degrees
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Pitch Factor
Short-pitched winding or chorded windinga winding issaid to be short-pitched or fractional-pitched if its coils are
placed less than one pitch (180 electrical degrees) apart.
Reasons for using Short-pitched winding They save copper of end connections.
They improved the waveform of the generated emf by
reducing or totally eliminating distorting harmonics
They reduced the eddy current and hysteresis loss byeliminating high frequency harmonics.
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Breadth Factor
Breadth or distribution or winding or spread factor it isdefined as the ratio of emfsof distributed winding to emfsof
concentrated winding.
=
=
Where:kb = breadth factor
n = number of slots per pole per phase
= number of electrical degrees between adjacent slots
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Sample Problems
1. Calculate the pitch factor for the given windings:a. 36 stator slots, 4-poles, coil span = 1 to 8
b. 72 stator slots, 6-poles, coil span = 1 to 10
c. 96 stator slots, 6-poles, coil span = 1 to 12
2. Calculate the breadth factor for a 36-slots, 4 pole,
single-layer, three phase winding.
3. A 3-, 16-pole alternator has a star-connected windingwith 144 slots and 10 conductors per slot. The flux per
pole is 0.03 Weber sinusoidally distributed and the
speed is 375 rpm. Find the frequency and the phase and
line emf. Assume full-pitched coil.
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Sample Problems
4. The stator of a 3-,16-pole alternator has 144 slots andthere are 4 conductors per slot connected in two layers
and the conductors of each phase are connected in
series. If the speed of the alternator is 375 rpm,
calculate the emf per phase. Resultant flux in the air-
gap is 5 x 10-2Weber sinusoidally distributed. Assume
the coil span as 150 electrical.
5. An alternator on open circuit generates 360 V at 60 Hz
when the field current is 3.6 A. Neglecting saturation,
determine the open-circuit emf when the frequency is
40 Hz and the field current is 2.4 A.
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Sample Problems
6. A 4-pole, 3-, 50 Hz, star-connected, alternator has 60slots, with 4 conductors per slot. Coils are short-pitched
by 3 slots. If the phase spread is 60, find the line
voltage induced for a flux per pole of 0.943 Wb
distributed sinusoidally in space. All the turns per phase
are in series.
7. A 4-pole, 50-Hz, star-connected alternator has 15 slots
per pole and each slot has 10 conductors. All the
conductors of each phase are connected in series. The
winding factor being 0.95. When running on no-load fora certain flux per pole, the terminal emf was 1825 volts.
If the windings are lap-connected as in a dc machine, what
would be emf between brushes for the same speed and
the same flux/pole. Assume sinusoidal distribution of flux.
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Synchronous Motors
Synchronous Motor it is an ac machine that operates at synchronous speed
and converts electrical energy into mechanical energy using field magnets
excited with direct current. It is also called as synchronous condenser.
Characteristics of a Synchronous Motor:
It runs either at synchronous speed or not at all. It is not inherently self-starting.
It is capable of being operated under a wide range of power factors, both
lagging and leading.