The composition and classification of electric propulsion system
The ship’s electric propulsion system is generally composed of propellers, electric motors, generators, prime movers and control equipment. Its structure is shown in Figure 1.
Among them, the mechanical energy of the prime mover Y is transformed into electrical energy by the generator G, which is transmitted to the propulsion motor M, and the electrical energy is transformed into mechanical energy by the motor, which is transferred to the spiral diesel J to promote the movement of the ship. Due to the large power required by the propeller, the propulsion motor Y-primary mover, G-generator, M-propulsion motor, K-digging equipment, and J-propeller cannot be powered by the general daily power grid, and a separate generator must be installed. (Group) or more powerful power source, so the electric propulsion ship can set up two independent power stations, or a comprehensive power station.
The prime mover for electric propulsion can be diesel engine, steam turbine or gas turbine. At present, high-speed or medium-high-speed diesel engines are generally used, and steam turbines or gas turbines are used for high power.
The generator can be a DC separately excited generator, an AC synchronous generator or an AC rectifier generator.
The motor can be a DC separately excited motor, AC synchronous motor, asynchronous motor or permanent magnet motor. Ship propellers generally use fixed pitch propellers because of their high efficiency and small size.
The electric propulsion device uses a rotating electric motor to transfer power from the prime mover to the propeller. The generator driven by the prime mover supplies power to the propulsion motor directly or through a solid-state rectifier or frequency converter, and the propulsion motor is coupled to the propeller directly or through a reduction gear device. Due to the use of solid-state converters, the electric energy generated by the generator for the propulsion motor does not need to be of the same type, and can be alternating current or direct current. Therefore, the marine electric propulsion system can be classified according to the type of prime mover used, the type of main circuit current and the function of the device.
- Classified by prime mover type
(1) Diesel engine electric propulsion
Diesel engines are currently the most widely used prime mover in ship electric propulsion, especially for small and medium-sized ships. Diesel engines are more economical than steam turbines. In order to reduce weight and volume, electric propulsion generally uses high and medium speed diesel generator sets.
(2) Steam turbine electric propulsion
Generally suitable for high-power electric propulsion and ships that require a large amount of steam consumption. Steam turbines can use low-grade and cheap fuels to reduce the cost of ship operation. The disadvantage of steam turbines is that they require a steam boiler, which makes the power plant bulky and heavy.
(3) Gas turbine electric propulsion
The gas turbine has high power, small size, light weight, simple structure and fast starting, and is the prime mover that can be considered for electric propulsion.
(4) Electric propulsion of nuclear reactor installation
The thermal energy generated in the nuclear reactor is passed through the heat exchanger to heat the steam or inert gas, and then the steam turbine is used to generate electricity. As a ship’s electric propulsion, it can sail for a long time without fuel storage, so it is especially suitable for large and medium-sized ships such as icebreakers, submarines, and ocean-going ships.
(5) Fuel cell electric propulsion
A fuel cell is a chemical cell that directly or indirectly uses the free energy of fuel oxidation. It is different from a normal battery in that it can continuously generate electricity as long as it is continuously supplied with fuel. In addition, it is reliable, noise-free, and can be connected in series or parallel according to needs. These advantages make fuel cells have broad development prospects in the application of electric propulsion.
- Classified by current type
According to the type of main circuit current, it can be divided into DC, AC, AC and DC and DC system electric propulsion.
(1) DC electric propulsion
According to the principle of system regulation, it can be divided into constant voltage electric propulsion, simple G-M electric propulsion, constant power electric propulsion and constant current electric propulsion.
(2) AC electric propulsion
The propulsion motor adopts AC motors, including asynchronous motors, synchronous motors, and permanent magnet motors. The speed regulation methods mainly adopt AC-AC variable-frequency speed regulation, AC-DC-AC variable speed regulation, etc. At present, most ships use this type of propulsion.
(3) AC and DC electric propulsion
AC and DC systems use power electronics technology to combine AC power and DC motors into one system.
(4) Direct AC electric propulsion
The direct-ac system uses power electronic technology to combine the direct-current power supply and the alternating-current motor into one system.
- Classified by propulsion function
(1) Independent electric propulsion
The propeller is driven exclusively by the propulsion motor, the main engine
In addition to supplying power to the propulsion motor, the electric motor can supply part of the electric power to the ship’s power grid.
(2) Joint electric propulsion
Joint electric propulsion, this kind of propulsion is shown in the figure2 shown,
It can have four working conditions:
1) The propeller is driven by the propulsion motor (the main engine is separated from the propeller at this time) for low-speed sailing;
2) The propeller is directly driven by the main engine (primary mover) (at this time the propulsion motor is disconnected from the propeller);
3 The propeller is driven by the main engine (primary mover) and the propulsion motor for high-speed sailing;
4) When sailing, the propulsion motor is driven by the main shaft to generate electricity and feed the electric energy back to the grid (equivalent to a shaft generator).
(3) Special electric propulsion
Special electric propulsion includes side thrust electric propulsion, azimuth pod electric propulsion, superconducting electric propulsion, magnetic fluid electric propulsion and pump-jet electric propulsion.
(4) Integrated electric propulsion
Power system integration technology is used to realize the generation, transmission, transformation, distribution and utilization of ship electric energy to meet the needs of ship propulsion, daily loads, high-power pulse loads, etc. It integrates ship power generation and propulsion power, and shipboard equipment power use in a unified system, so as to achieve unified dispatch and centralized control of power generation, power distribution, power propulsion power and other equipment power use.