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Файл:English for marine electro-technical officers. Supplementary book. Учебное пособие
.pdf
31
Answer the questions:
1. Will the electrical power demand aboard ship vary
according to the ship typy?
2. How many generators are used to meet the power
demand?
3. What are the main generator power ratings?
4. An emergency generator will be diesel driven,
won’t it?
Words and expressions (2.2, 2.3)
to be assembled from
быть собранным из
to be housed in
быть размещенным в
end-winding region
концевая обмотка
terminal box
распределительная коробка
salient pole
явно выраженный полюс
permanent magnet
постоянный магнит
utilize
использовать
rotary pilot exciter
роторный вспомогательный
возбудитель
output
выход, вывод
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2.2. Generator construction and cooling
Construction
The two main parts of any rotating a.c. machine are its
stator and rotor.
The fabricated steel stator frame supports the stator core
and its three phase windings.
The stator core is assembled from laminated steel with
the windings housed in slots around the inner periphery of the
cylindrical core.
The stator coils are interconnected (in the end-winding
regions) to form three separate phase windings with six ends.
These phase ends are found in the stator terminal box. In some
cases only three terminals are available in the terminal box. In
this case, the neutral or star point connection is an internal part
of the stator winding arrangement.
The main outgoing cables connected to these terminals
conduct the generator's electric power to its circuit-breaker at
the main switchboard.
The rotor of a main a.c. generator provides the field excitation from its electromagnetic poles.
Two constructional forms of rotor are available.
- salient pole type;
- cylindrical type.
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2.3. Excitation methods
Broadly, the excitation methods are either rotary or static. A rotary method utilises an a.c. or d.c. exciter which is
shaft-mounted and rotates with the main generator rotor. Traditionally, rotary exciters were d.c. generators with stationary
field poles, rotating armature, commutator and brushgear. Now
the most common arrangement is to use a shaft mounted a.c.
exciter.
In some applications, a small additional rotary pilot exciter may be used to supply current to the main exciter field.
A pilot exciter is a small permanent magnet a.c. generator
which is driven from the generator shaft. Its output voltage is
generally at a high frequency (e.g. 1 000 Hz) but this is rectified to d.c. before being fed into the main exciter field.
Answer the questions:
1. The two main parts of any rotating a.c. machine are
its stator and rotor, aren’t they?
2. What does the fabricated steel stator frame support?
3. How many terminals are available in the terminal
box?
34
4. Does the rotor of the main a.c. generator provide the
field excitation from its electromagnetic poles?
5. The excitation methods are either rotary or static,
aren’t they?
6. What does rotary method utilize?
7. What is a pilot exciter?
Words and expressions (2.4, 2.5)
reasonably
разумно, достаточно
to be omitted
исключить
interconnector
соединитель
run-up
период подготовки, ход
running
ход, работа
to be shut down
остановлен, выключен
auto-start circuit
цепь автозапуска
frayed insulation
изношенная изоляция
contamination
загрязнение
lint-free cloth
безворсовая ткань
to dislodge
вытеснить, вымыть
nozzle
насадка
abrasive damage
износ, вызываемый трением
slipring
контактное кольцо
pull-type spring balance
прицепная пружина баланса
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2.4. Emergency generators
The power rating of an emergency generator is determined by the size and role of the ship. On some small vessels a
few kW will suffice for emergency lighting only. Larger and
more complicated vessels, e.g. LPG carriers, passenger liners,
etc., may require hundreds of kW for emergency lighting, restarting of the main engine auxiliaries and to supply firefighting pumps.
The construction and operation of an emergency generator is similar to that of a main generator. Excitation supplies,
either static or rotary, will usually be governed by an automatic
voltage regulator. In some cases where a static compounded
exciter provides a reasonably constant generator voltage, the
AVR may be omitted.
Generally, the emergency generator output voltage is at
the same level as that of the main generators, e.g. 440 V,
60 Hz, 3-phase a.c. In an HV / LV system e.g. 6,6 kV / 440 V,
the emergency generator will usually operate at 440 V and the
emergency switchboard will be interconnected with the Engine
Room 440 V switchboard in normal operation.
However, smaller emergency generator sets may deliver
power at 220 V 3-phase a.c. or even single-phase a.c. for lighting and essential navigation aids only. An emergency generator
is connected to its own emergency switchboard and they are
located together in a compartment above the water-line, e.g. on
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the boat deck. In normal operation the emergency board is supplied from the main board by a cable called the interconnector.
It is not normally possible to synchronize the emergency and main generators. Special interlocks in the control circuits of the circuit-breakers, at each end of the interconnector,
prevent parallel running.
Starting of the emergency generator prime mover is
generally automatic. The run-up is initiated by an electrical relay which monitors the normal voltage supply (e.g. 440 V).
2.5. Generator maintenance
Regular inspection and the correct maintenance of generators and their associated control gear is essential to prevent
failure and inefficient operation.
CAUTION: Always ensure that the generator primemover is shut down and locked off before you begin any
maintenance. Also ensure that the generator circuit breaker is
locked off, auto-start circuits are disabled and electric heaters
are switched off and isolated.
All wiring to the generator should be inspected for
damage or frayed insulation and tightness of terminal connections. Particularly check for signs of oil and water contamination of cable insulation within terminal boxes.
37
Check that the cooling air intake and exhaust openings
are not blocked and are free of dirt and dust.
Inspect and clean the generator rotor and stator windings by removing dust with a dry lint-free cloth. Low pressure,
dry compressed air may be used to dislodge heavier dirt but be
careful not to drive the dirt deeper into the windings. An industrial type vacuum cleaner is very effective for removing dirt
from the windings. Use a rubber or plastic-coated nozzle on the
vacuum cleaner tube to prevent abrasive damage to the sensitive winding insulation. Oil on the surface of winding insulation will reduce the insulation resistance and shorten its life.
The oily deposits can be removed by washing the windings
with special degreasing liquids. Minor abrasions to winding
insulation can be repaired, after cleaning, by the application of
a suitable air-drying varnish.
Rotor sliprings must be checked for uniform (even)
wear and that the carbon brushes have free movement in their
boxes. Correct brush pressure can be checked using a pull-type
spring balance and compared with the manufacturer's instructions.
Answer the questions:
1. What is the power rating of an emergency generator
determined by?
2. The construction and operation of an emergency gen-
erator is similar to that of a main generator, isn’t it?
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3. What is the level of the emergency generator output
voltage?
4. Is starting of the emergency generator prime mover
generally automatic?
5. What prevents parallel running?
6. Should you ensure that the generator prime-mover is
shut down and locked off before you begin any maintenance?
7. Should you inspect all wiring to the generator for
damage?
8. What should you use to clean the generator rotor and
stator windings?
9. What must rotor sliprings be checked for?
10. How can correct brush pressure be checked?
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3. MOTORS AND STARTERS
Words and expressions (3.0, 3.1)
сage-rotor motor
электродвигатель
с короткозамкнутым ротором
tough
жесткий, тяжелый
direct-on-line
непосредственный, прямой
apart
отдельно
сage winding
короткозамкнутая обмотка
robust
прочный, надежный
troublesome
трудный, проблемный
3.0. Introduction
The drive power for compressors, pumps and fans
aboard ship comes from electric motors. By far the most common type of motor is the 3-phase a.c, cage-rotor induction motor. It is popular because it is simple, tough and requires very
little attention. Another advantage is that starting and stopping
these motors can be done with simple and reliable direct-online contactor starters. Three phase induction motors are usually supplied at 440 V, 60 Hz, but 3.3 kV and 6.6 kV, 60 Hz are
40
sometimes used for very large drives such as bow thrusters,
cargo pumps, air compressors and gas compressors.
Special types of motor can also be found on board
ships. DC commutator motors are sometimes used for driving
deck machinery where speed control is important. Single-phase
a.c. motors are used in low power drives such as galley equipment and domestic tools.
High power synchronous a.c. motors are frequently
used for electric propulsion drives.
3.1. Motor construction
The induction motor has two main components, the stator and the rotor. The stator carries three separate insulated
phase windings which are spaced 120° (electrical) apart and
lying in slots cut into a laminated steel magnetic core. This
type of stator winding is similar to the construction used for an
a.c. generator.
The rotor consists of copper or aluminium conductor
bars which are connected together at their ends by shortcircuiting rings to form a cage winding. The conductor bars are
set in a laminated steel magnetic core. The essential reliability
of the induction motor comes from having this type of simple,
robust rotor which usually has no insulation on the conductor
bars and does not have any troublesome rotary contacts like
brushes, commutator or sliprings.
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