Alternator [Video Lesson]
Shipboard power is generated using a prime mover and an alternator working together. For this, an alternating current generator is used on board.
The alternator 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 a magnetic field. This field cuts across the conductor, generating an induced EMF or electromagnetic 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. For ports, shipyards, and structures located inland, marine electricity is utilized from the electricity supply of the land-based power generation plants.
Unlike land, the ship’s generator has insulated neutral points i.e. its neutral is not grounded or connected to the ship’s hull. This is done to ensure all the essential machinery is up and running even if there is an earth fault.
The ships plying in international waters generally have 3 phase D.C. supply with a 440v insulated neutral system. Ships like RORO, passengers,s, etc. having large electrical load requirements are installed with high voltage operating gensets in the range of 3KV to 11KV.
On land, the frequency of the power supplied can be 50 or 60 Hz depending on different parts of the world. On ships, 60hz frequency is adopted as standard practice which helps hundreds of motors on a ship run at higher speeds even if they are of smaller size.
The supply which is at 440v is stepped down using a transformer to 220V or 110V for lights and low-power signal equipment.
All the electrical equipment on board ships is similar to land however, they are upgraded to withstand the rigorous atmosphere of the sea and moving ship to withstand humid surroundings, high temperatures, salty and corrosive atmosphere, vibration, etc.
The Power generated by these marine generators is transported to the Main switchboard using Busbars. There are no electrical wire connections inside the main and emergency switchboards on ships for connecting the Power supply from generators to these switchboards. All high voltage and high current systems are connected by bus bars.