Characteristics of ship electric propulsion
Compared with direct or geared propulsion, the main advantages of electric propulsion are:
(1) Flexible layout and installation. The prime mover of large ships is almost without exception installed in the lower space of the stern, and at the same time, a longer drive shaft is required to connect the propeller. Electric propulsion motors are usually very close to the propellers, eliminating the need for transmission shafts and correspondingly saving space. The power generation equipment can be reasonably arranged according to the configuration of the whole ship, and is not restricted by the propulsion motor and the propeller, and can be arranged three-dimensionally in the entire space of the engine room, which is convenient and flexible, and makes full use of the engine room capacity. If considering fire protection and safety, the generators can also be divided into several groups and arranged in different compartments.
(2) It is easy to obtain ideal drag characteristics and improve the technical and economic performance of ships.
1) Low speed characteristics. The speed regulation ratio of diesel engines is generally 1:3, so it is not easy to obtain low speeds when using direct propulsion (when the rated speed is 250 ~ 300r/min, the stable low speed cannot be lower than 90 ~ 120r/min). The speed adjustment ratio of the motor can reach 1:10 or even one to several tens. Therefore, the propeller can obtain a very low speed ((below 4r/min) when using electric propulsion, which is conducive to the realization of maneuvering of the ship, such as stable low-speed approaching to the target, Wait from the dock.
2) The rapidity of the parking of high-speed trains. The starting, stopping and reversing of the electric motor is faster than that of the diesel engine, so the parking and reversing speed of the propeller motor car is very fast, which is beneficial to improve the maneuverability of the ship.
3) Constant power characteristics. When a ship is sailing, due to the influence of wind and waves and other factors, the resistance often changes. The use of electric propulsion devices can always maintain the power equipment (diesel engine, generator or electric motor) in constant power operation under the conditions of frequent changes in resistance, so that the efficiency of the power equipment can be maintained at a high level, so as to make the best use of the power equipment. Efficiency (that is, make full use of the device power of the device).
4) Constant current characteristics. The main circuit current of the electric propulsion device can be adjusted to keep a certain value unchanged, which makes it possible to connect several motors in series in the main circuit. These motors can be adjusted independently without affecting each other. This feature It is especially suitable for some engineering ships. These ships have several loads that are similar in capacity but not used at the same time. Connecting their drag motor armatures in series in a main circuit and powered by a common generator set can greatly reduce the device capacity of the generator set. When the constant current system is adopted, it also has the characteristics of faster motor transition process, reliable work, flexible operation, and no overload hazard of the system.
5) Locked-rotor characteristics. When the propeller is jammed by ropes, ice, etc., due to the use of electric propulsion, the system has “lock-rotor characteristics”. There is no need to disconnect the motor in a short time. After the cause of the jam is eliminated, the propeller will quickly resume normal operation. , Eliminates the system’s frequent “off-on” shortcomings.
(3) The medium and high speed non-reversing prime mover can be used, which reduces the weight and volume of the equipment. The speed of the propeller should not be too high, usually below 300r/min, otherwise its efficiency will be reduced. Therefore, in direct propulsion, if the prime mover is a diesel engine, its speed cannot be higher, and only the so-called heavy-duty can be used. Low-speed diesel engine. It is characterized by high power and low speed, so it is large in weight and large in size. If the electric propulsion device is adopted, light and small medium and high-speed diesel generator sets can be used, and the diesel engine does not need to be reversible, which can greatly reduce the weight and volume of the prime mover.
The medium and high speed diesel engine is light in weight and small in size, which is convenient for cabin layout. The non-reverse diesel engine has simple structure, reliable operation and long life. When other conditions are the same, the life of a non-reversing diesel engine is much longer than that of a reversible diesel engine. According to existing data, the wear of a diesel engine for each reversal is equivalent to the wear for 16 hours of operation. Since the prime mover does not have to be reversed, the electric propulsion device also creates good conditions for the wide application of gas turbines.
(4) Flexible operation and good maneuverability. Electric propulsion is easy to realize the maneuvering of the ship directly from the bridge, which makes the maneuvering of the ship very flexible. For direct propulsion, generally, the operator’s cab transmits the main engine operating instructions to the engine room through the car clock, and the main engine operator controls the diesel engine according to the instructions, and then returns the order to the cab through the car clock. This will not only slow the oncoming traffic, but also easily cause misoperation. If electric propulsion is adopted, the driver only needs to manipulate the speed regulating device of the driving propulsion motor in the cab to control the ship, which greatly reduces the possibility of misoperation.
The maneuvering transition process of electric propulsion is much shorter than that of direct propulsion (coming cars are faster), so it has a stronger ability to cope with emergencies, which greatly increases the safety of navigation.
(5) High reliability. Multiple generator sets and motors can be used to ensure greater reliability, and the loss of one device will not cause total loss of power. At the same time, repairing multiple small-capacity devices is more efficient and easier than repairing a single large-capacity device.
(6) The vibration is small. Abandoning the transmission shaft system and the speed-regulating gear box, less propeller vibration is transmitted to the prime mover.
(7) Strong applicability. In addition to providing propulsion power, generator sets can also be used to supply power to other daily loads.
(8) Fuel economy. Since some power generation devices can be completely shut down, and other power generation devices are operating at close to full load and high efficiency, the fuel economy is excellent when the power is reduced. By using a solid-state converter to control the motor speed, the generator runs at its optimal speed, so that the prime mover obtains the highest efficiency.
Compared with direct or geared propulsion, the main disadvantages of electric propulsion are:
(1) The overall efficiency at the highest speed is usually low.
(2) The use of electrical equipment may lead to additional hazards that need to be avoided, such as possible fires in electrical equipment, disturbances caused by faults (flashover, short circuit and grounding), personal injuries caused by electric shocks, etc.
(3) Electric propulsion devices require well-trained and highly skilled operators. (4) A wide variety of spare parts are required.