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The
alternators are of salient pole
/ cylindrical type with armature
windings in the stator and field
windings on the rotor. The armature
is made up of low loss Silicon
Steel laminations, stacked under
hydraulic pressure. Stator slots
are skewed to reduce slot harmonics
in induced voltage. Slots are
insulated with good quality insulating
materials. The armature windings
are well distributed in the slots
and are chorded to reduce harmonics.
These windings are normally double-lap
winding with super quality enameled
copper conductors. |
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| The pole stacks
are made up of pole steel laminations
and are fitted over the shaft
by means of high tensile pole
screws or made with integral pole
stampings, using silicon steel
laminations. The pole shoes are
profiled to give sinusoidal voltage.
Connected Damper bars which are
housed in the slots of the pole
shoes, give effective damping
during parallel running. The field
bobbins are insulated and wound
with enameled copper wires or
strips. Both stator and rotor
windings are vacuum and pressure
(VPI) impregnated to protect the
windings from moisture. |
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| The alternator is
provided with a centrifugal fan
at the driving end, which axially
draws cool air in from the rear
side and radially throws hot air
out at the front side. This gives
good ventilation and keeps the
temperature of the windings within
limits. The inside openings and
air passages are properly designed
to get optimum ventilation. |
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| The body and covers
are of cast iron or mild steel
fabricated construction. Antifriction
ball and / or roller bearings
are fitted to give long &
smooth running. Each rotor assembly
is dynamically balanced to high
accuracy. A balancing disk is
incorporated for this purpose.
Prototype rotors are over speed
tested as per specification requirement. |
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| The alternators
are screen protected and drip
proof, closed-air-circuit- air-cooled
or water tight, up to bottom of
shaft level. Normally there are
two bearings - a roller bearing
in the driving end side and a
ball/roller bearing in the non-driving
end side. |
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| The alternators
are coupled to the prime mover
with a flexible coupling. Some
alternators are made with single
ball/roller bearing, fitted in
the non-driving end side. These
are rigidly coupled to the drive. |
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| The alternators
are normally foot mounted type.
Raised feet construction is also
given, on request. |
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| Self-Excited
Self-Regulated (SESR) type Alternators
(Using CT/ Choke) |
| |
| These alternators
are self-excited and self-regulated
by an excitation unit mounted
on the top of the alternator.
This unit consists of choke, current
transformer, diodes and condensers.
Voltage is maintained within +/-2.5%
of the nominal voltage from no-load
to full load at rated power factor
by this unit. For schematic diagram
refer drawing no 4EK
-3196. |
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| The choke is meant
for adjusting the terminal voltage
at no load. The gap of the choke
filled with insulating sheets,
can be set to achieve the desired
no-load voltage. The air gap in
all the three limbs of the choke
should be equal and the top yoke
is to be tightened after setting
for the desired no-load voltage. |
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| The Current Transformer
(CT) Is meant for adjusting the
voltage at load conditions. This
is factory-set to give good regulation
at rated load. The terminal voltage
on load can be increased or decreased
by changing the CT tap connections
as indicated in the connection
diagram supplied with the machine. |
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| The alternator is
self-excited due to residual flux.
Once the alternator is run to
rated speed at no-load, the residual
voltage appearing across the terminals.
gets rectified by a 3 phase, full-
wave bridge rectifier and is fed
to the field windings through
brushes and slip-rings. Thus the
alternator builds voltage by itself
and reaches to steady value depending
upon the choke air gap. |
| |
| When the alternator
is loaded, the CT injects extra
field current to the field windings
to compensate for the armature
reaction. This helps in maintaining
almost constant voltage across
the terminals. The condensers
help in building up the voltage
during starting. |
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| With the help of
Manual voltage Regulator (MVR),
voltage can be adjusted when the
machine is running, with or without
load. This will be very much useful
for paralleling with other alternators.
This is an optional item. |
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| Brush-less
Alternators |
| |
| Brush-less alternator
embodies two alternators, connected
electrically to one another, mounted
on the same shaft. One is the
main alternator and the other
is the A.C. exciter. Main alternator
is of salient-pole type with rotating
field and stationary armature.
The exciter is of rotating armature
and stationery field. The exciter
is overhung with the main alternator.
The armature of the exciter is
fitted on the shaft extension
at non-driving end. The poles
of the exciter are fitted on a
steel frame which in turn is fitted
to the cover of the main alternator.
A diode wheel consisting of six
diodes and a selenium surge suppressor
is fitted to the shaft at the
non-driving end side. An automatic
Voltage Regulator (AVR) is mounted
on the top of the alternator.
This is a thyristor controlled
electronic unit. Main alternator
construction is same as that of
CT-Choke type alternator. The
other components of the brush-less
machine are explained below : |
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| A - Exciter
- The exciter armature
is also made up of low-loss silicon
laminations and is wound with
enameled copper conductors. The
stator is housing field system
with pole steel lamination and
copper conductors. The number
of poles in the exciter are so
chosen as to get better performance
during transient loads. Moreover,
exciter is oversized to cater
to the requirement of overloads,
transient load conditions and
heavy motor starting duties. |
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| B - Diode
Wheel - This incorporates
three phase full-wave bridge rectifier
circuit formed by three of normal-polarity
diodes and three of reverse- polarity
diodes. The diode ratings are
amply chosen to get reliable operation.
The circuit is protected against
surge voltages by a selenium surge
suppressor. All diodes are fitted
with aluminum heat-sinks, which
act as d. c. bus bars. |
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| C - AVR
The automatic voltage regulator
(AVR) senses the alternator terminal
voltage and maintains this voltage
constant within ± 1.0%
/ ± 0.5% of the rated voltage.
The power is directly taken from
alternator terminals and is rectified
by a suitably designed thyristor
converter circuit to feed the
excitation power, to the exciter
field windings. Circuits for voltage
adjustability and voltage-droop
adjustability are also provided. |
