1.1.1 Prime movers including main propulsion plant
Definition of Prime Mover
The actual meaning of prime mover is a primary source of power. It means all the machinery that provide power for performing different mechanical work.Technically, it is a group of machines that transform energy from thermal, electrical, or pressure into mechanical form for use in different sources for some mechanical work. Engines and turbines are examples.
Power generation On board
Shipboard power is generated using a prime mover and an alternator working together. For this an alternating current generator is used on board. The generator works on the principle that when a magnetic field around a conductor varies, a current is induced in the conductor.The generator consists of a stationary set of conductors wound in coils on an iron core. This is known as the stator. A rotating magnet called the rotor turns inside this stator producing magnetic field. This field cuts across the conductor, generating an induced EMF or electro-magnetic force as the mechanical input causes the rotor to turn.The magnetic field is generated by induction (in a brushless alternator) and by a rotor winding energized by DC current through slip rings and brushes. Few points to be noted about power on board are :
- AC, 3 phase power is preferred over DC as it gives more power for the same size.
- 3 phases is preferred over single phase as it draws more power and in the event of failure of one phase, other 2 can still work.
Power Distribution on board
The Power Distributed on board a ship needs to be supplied efficiently throughout the ship. For this the power distribution system of the ship is used.
A shipboard distribution system consists of different component for distribution and safe operation of the system. They are:
- Ship Generator consisting of prime mover and alternator.
- Main switch board which is a metal enclosure taking power from the diesel generator and supplying it to different machinery.
- Bus Bars which acts as a carrier and allow transfer of load from one point to another. Circuit breakers which act as a switch and in unsafe condition can be tripped to avoid breakdown and accidents. Fuses as safety device for machinery.
- Transformers to step up or step down the voltage. When supply is to be given to the lighting system a step down transformer is used in the distribution system.
- In a power distribution system, the voltage at which the system works is usually 440v.
- Power is supplied through circuit breakers to large auxiliary machinery at high voltage.
- For smaller supply fuse and miniature circuit breakers are used.
- The distribution system is three wires and can be neutrally insulated or earthed.
- Insulated system is more preferred as compare to earthed system because during an earth fault essential machinery such as steering gear can be lost.
Emergency Power
In case of the failure of the main power generation system on the ship, an emergency power system or a standby system is also present. The emergency power supply ensures that the essential machinery and system continues to operate the ship.
Emergency power can be supplied by batteries or an emergency generator or even both systems can be used.
Rating of the emergency power supply should be made in such a way that it provides supply to the essential systems of the ship such as
a) Steering gear system
b) Emergency bilge and fire p/p
c) Watertight doors.
d) Fire fighting system.
e) Ships navigation lights and emergency lights.
f) Communication and alarm system.
Emergency generator is normally located outside the machinery space of the ship. This is done mainly to avoid those emergency situations wherein access to the engine room is not possible. A switch board in the emergency generator room supplies power to different essential machinery.
Ship automation and control system
On a ship there are many parameters that needs to be controlled or monitored including: temperatures, pressure, level, viscosity, flow control, position of vessel, speed, torque control, voltage, current, machinery status (on/ off), and equipment status (open/ closed).
The morden ships are operated with a complex automatic control and monitoring system, that enabled unattended operation of machinery spaces. Vessels capable of safe operation at any period of time qualify as UMS (Unattended Machinery Space) ships.
A modern automation and control system is a fully integrated systems covering many aspects of the ship operation that includes the propulsion plant operation, power management operation on the auxiliary engines, auxiliary machinery operation, cargo on-and-off-loading operation, navigation and administration of maintenance and purchasing of spares.
Propulsion (Main Engine) and Power (AuxiliaryEngines) Monitoring & Control
Monitoring and control of the ships propulsion and power is essential for its efficiency and safety and there are many systems and parameters to consider like: fuel consumption, combustion temperature, engine temperature, diesel engine safety and start/stop, generator voltage and frequency control, generator load in KW and %, load control, torque, heavy consumers logic, control of diesel electric propulsion, thrusters monitoring and control etc.
Auxiliary Machinery Monitoring and Control
Auxiliary machinery monitoring and control covers several systems like: main sea & fresh water cooling system – pumps, system pressure, temp, potable and fresh water control, Air purifier, Bilge & sludge control – Tank level, pumps, Fuel oil system – Tank levels, temp., viscosity, flow, purifiers, heaters etc., Other cooling systems, Boiler/steam system – pumps, valves, pressure temp. etc., Air Conditioning, ballast water treatment, exhaust gas treatment equipment.
Condition based monitoring
In order to further improve the ships efficiency many equipment manufacturers are looking into feeding the main control and monitoring system with opportunities for condition based monitoring. This would further improve the possibilities of preventing breakdowns on board.
Machinery Space Alarm
The ship’s engine room is fitted with an alarm system which continuously monitors the operation parameters of machinery.
If any machinery malfunctions, a common engine room alarm is operated, and the problem can be seen in the control room alarm panel, which will display the cause for the alarm.
Action to be taken by the crew once the engine room alarm is sounded
Engine room of a ship.
Earthed neutral and insulated neutral systems
Insulated Neutral System:
When the neutral of the generator is completely insulated from earth or ship`s hull it is called insulated neutral system. It is very advantageous because it does not pose any danger to human life even if there is a single earth fault because the whole ship will be at same potential leading to no damage to machinery and life.No tripping of electrical machinery and the main engine is available all the time.
Earthed Neutral System:
When the generators neutral is connected to the earth or ship`s hull it is called earthed neutral system. A single fault can cause very large current to flow causing damage to the machinery.High voltage ships usually use earthed neutral system so that when an earth fault occurs, the circuit breaker trips immediately avoiding thermal damage to the equipment.
Sequential start and interlocking in electrical circuits
When the power is to be restored by connecting the first generator after a power failure onboard a ship ,the air circuit breaker will be repeatedly tripping due to overloading.This is because all the pumps which were operational at the time of power failure will start together overloading the generator.To avoid this timers are used within starter control circuits so that they all start with a time gap but not together.The most important pumps start on power recovery and all other pumps start a with time difference to avoid overloading generator.
Interlocking in electrical control circuits
When an electrical circuit has two contactors and only one of them needs to be turned on at a time then the interlocking system is used. The interlocking system ensures that only one contactor can be on while another one will be in off condition even if we try to manually on.An interlocking system is very useful for motor starter and power circuits. In a three-phase star delta motor starter, two contactors are used. One provides star connection while another provides delta connection. During starting time the Star-connected contactor will on and during the running condition delta connected contactor will on. If both of them are turned on at the same time, there will be a short circuit. So the interlocking system in a star-delta starter prevents the short circuit.The interlocking system in a changeover circuit helps to connect the load with one source of power supply and isolated it from another source of power supply.
Here you can see the control diagram for contactor interlocking.
As per the below mentioned control diagram, you can see the coil of contactor 1 is connected through the NC points of contactor 2 and the coil of contactor 2 is connected through the NC points of contactor 1. So when contactor 1 is in ON condition, its NC contacts become open so contactor 2 can not be turned on even we push the switch S2 to turned on contactor 2 manually.When contactor 2 is in ON condition, its NC contacts become open so contactor 1 can not be turned on even we push the switch S1 to turned on contactor 1 manually.
Interlocking in a control circuit
Sequential start
Starters
Direct on line starter
Star/Delta starter
Auto transformer starter
Soft starter
