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AC Generators – how they work
current (ac) generators (often called alternators). The basic principle of an
ac generator is very simple. Pairs of magnetic poles are driven (by the prime
mover) past fixed coils of wire.
which has a sinusoidal waveform, is induced into each coil:
flux ((D) and the rate at which this flux cuts the coils –
emf generated is the number of waveform cycles per second. Above figure shows 1
and frequency dictate how to control the voltage and frequency output of a
practically constant by the generator’s prime mover (governor) which fixes the
output frequency normally 60 Hz. The
constant speed then allows the size of generated emf (440V RMS) to be controlled by the adjusting
the strength of magnetic field (excitation).
generators form the heart of the electrical design and their correct sizing is
the key to a safe, workable and economical system. Main generator electrical
power ratings range from, typically, 100kW to 2MW at 440V, 60Hz. Capacity of
generators depends up on the nature of the load. In addition to satisfying load
requirements under different conditions e.g.
- Cargo operation
- Navigation at sea
generator often works on its own and is subjected to large system load swings
during starting of equipment like air compressors, winches, ballast pumps,
cargo machinery, and air-conditioning.
International maritime regulations (e.g. SOLAS Chapter II-1,), require at least
two generators for a ship’s main electrical power system. The generators are
normally driven from their own dedicated diesel engine but this can be
expensive, taking up additional space that could be used for other purposes.
For ships engaged on long sea voyages, it can be economical to drive the
generators from the main propulsion plant
(shaft generator) or from waste heat (Turbo alternator).
maritime regulations also require at least one electrical generator to be
independent of the speed and rotation of the main propellers and associated
shafting and accordingly at least one generator must have its own prime mover.
If a minimum of two generators is provided, one of which is driven from
the propeller shaft, failure of one of the generators could make the ship
non-compliant with the International regulations.
this reason many owners opt to provide three generators. One is used for the
normal sea load (e.g. the shaft generator), leaving two available to meet any
unusually high loads or to provide security when manoeuvring. Alternately, the
third is retained as a standby set able to provide power should one set fail in
service or require specific maintenance work.
elements of a marine distribution system are the main and emergency
switchboards, power panel boards, motor controllers, lighting and small power
panel boards. The system is generally designed such that under all normal
conditions of operation, power is distributed from the main switchboard.
The main switchboard is generally located
in the main engine room or machinery control room. These locations are
normally below the ship’s waterline or below the uppermost continuous deck of
the ship i.e. the bulkhead or main deck. Consequently, in the event of a
fire or flooding it is likely that the main generators and switchboard
would be disabled.
ensure that electrical supplies are available to emergency and safety systems,
an emergency generator and associated emergency switchboard will be
located above the main deck in a separate space, completely isolated from the
main machinery spaces.
is often convenient to group motor driven auxiliaries according to their
function, e.g. fuel and lubrication oil services, accommodation ventilation
systems, machinery ventilation systems, and domestic service systems. This can
often simplify the machinery control functions and required protection systems.
systems are provided to shut down all ventilation systems and all
fuel oil systems in the event of fire from a remote location.
Describe the use of circuit-breakers and fuses
the power supply to an electrical circuit. In case of short circuit when
current flow exceeds the safe limit, circuit breaker trips and interrupts the
power supply. Circuit breaker can be reset and used again. The fuse on other
hand burns out and needs to be replaced. Before carrying out maintenance, the
power supply to the circuit is cut off by switching off and locking the main
circuit from damage when excessive current flows. Fuses are designed to blow
rapidly before excessive current can damage the circuit they protect. When a
fuse blows it must be replaced by a fuse of same type and same rating.
is dead, if there is need to work on it.
WHY EMERGENCY SOURCE
OF ELECTRICAL POWER IS PROVIDED?
switchboard is smaller than the main switchboard. It is provided so that in the
event of an emergency involving a total power failure, a supply will still be
available for emergency lighting, alarms, communications, watertight doors and
other services necessary to maintain safety and to permit safe evacuation of
the ship by lifeboats.
switchboard is generally arranged to be above the bulkhead deck and readily
accessible from open deck.
POWER SUPPLY FOR THE EMERGENCY SWITCH BOARD
- Emergency switchboard is normally supplied power from main
switchboard through BUS TIE CIRCUIT BREAKER.
- When the mains power fails, the emergency switchboard is
supplied by its own independent power source (emergency generator).The
emergency source of power must be self contained and not dependent upon
any other engine room power supply. It should be capable of functioning
when the ship is inclined by 22.50 and/or trim of 100
Starting arrangement for emergency generator
- Emergency generating sets shall be capable of being readily
started in their cold condition at a temperature of 00C.
- Each emergency generating set arranged to be automatically
started with compressed air, hydraulic or electric means. The stored
energy for starting should be sufficient for 3 consecutive starts.
- Additional energy for 3 consecutive starts with in 30 mins is
- The energy storage device (electric battery or compressed air)
is always kept charged.
- Some generators may have manual starting by hydraulic
accumulator or hand cranking.
Sources of Emergency
BATTERY capable of carrying load, with not more than
Automatically supplying power to emergency switchboard in case of mains power
automatically from cold within
half an hour is provided along with emergency diesel generator
power during momentary blackout before emergency generator comes on load within
45 seconds, so that passengers don’t panic.
selected power, if the emergency generator is hand starting type.
INTERLOCKS FOR PROTECTION
supplied from only one source at a time to the emergency switchboard i.e. mains
supply or emergency generator, so that the circuit breakers of the two supplies
cannot be closed together.
system, without synchronising the phase, will cause short circuit and damage to
List the essential
services to be supplied from the emergency generator
following essential services.
service NO.1 & 2 for half an hour)
SERVICE No 1.EMERGENCY LIGHTS
compartment, emergency fire pump, storage for fireman’s outfit, emergency bilge
lights, not under command lights.
SERVICE NO 2.
COMMUNICATION AND ALARMS
communications as required in an emergency, radio,
detection and alarm system.
operation of day light signalling lamp, ship’s
manual fire alarms.
direction finder,Gyrocompass, Echo sounder,Radio equipment,Rudder angle
indicator,speed log etc.
vessel above 10,000 tons.
Supplementary emergency lighting for ro-ro passenger ships
are provided with supplementary electric lighting that can operate for
at least 3 h when all other sources of electrical power have failed and under
any condition of heel.
approach to the means of escape can be readily seen.
lighting consists of accumulator batteries located within the
lighting units that are continuously charged, where practicable, from the
emergency switchboard. These batteries are replaced at regular interval as per
lamp is provided in every crew space
alleyway, recreational space and every working space which is normally occupied
unless supplementary emergency lighting.
fixed arrangement, because all ship’s differ in some respect. This gives the
basic concept. Some ships may have a diesel driven emergency fire pump. Others
may have only batteries as an emergency power source, which supply limited
power to few essential services. This depends upon the type of ship and safety
regulations applicable to it.
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