29. State concept about electric driving mode, advantage & disadvantage?

1- Electric Motor

2- Excited coil of the motor

3-Excited coil of the generator

4- Generator

5- Main engine

6- Rheostat

a. Feature

-There is not mechanical connection between the main engine and the propeller.

-There are two times of energy transformation.

Principle

-Generator (4) is driven by the main engine (5) to supply a direct current to the direct current motor (1).

-The excited coil (2) of the motor is connected to the direct current grid. The excited coil (3) of the generator is connected to the direct current grid through a rheostat (6) therefore the value of the current can be changed.

-To change revolution of the motor, the magnetic flux of the excited coil (2) must be changed. Besides this, there can change the voltage of the motor to change its revolution. This can be carried out by the rheostat.

-To change direction of the motor, the direction of excited current must be changed.

-In fact the system is very complex and an alternative current electric driving system can be used.

b. Advantages

-Main engine is usually a high or medium-speed therefore, the dimension and weight of the propulsion plant is small and thus reduces the dimensions of the engine room.

-It is not necessary to reverse the main engine when altering moving direction of the ship.

-Revolution of the propeller can be changed while the main engine is still working at a constant speed.

-It is not necessary to have a long shafting and many intermediate bearings, so the main engine can be located in the most suitable position in the engine room.

-When the ship is stable sailing on the sea, the main generator can satisfy a part of the electric energy requirements of the ship.

c. Disadvantages

-Transmission efficiency is much lower than the two kinds of driving mode listed above.

-Overload ability of the electric motor is less than diesel.

-It is complex, uneconomic and expensive.

-Longevity is shorter than mechanical connection.

-Electric driving mode is more suitable for naval ships, fishing ships...