Features of marine propulsion motors
The ship’s speed and propulsion shaft power vary widely, and the ship’s requirements for propulsion motors determine the characteristics of propulsion motors. It is a multi-working-condition motor with high reliability, large capacity, low speed, high torque, high power ratio, and a wide range of power and speed changes.
1. High reliability
The propulsion motor is the main and even the only power source of the ship’s electric propulsion system, and its reliability is directly related to the safe navigation of the ship, especially the submarine propulsion motor. Early submarines used diesel engines and propulsion motors for coaxial propulsion, and there were multiple main propulsion motors and multiple warp propulsion motors. But now there is only one main propulsion motor, and the propulsion motor has become the only propulsion power for the submarine to dive, float, and sail. It must have very high reliability to ensure the vitality of the ship.
2. Large capacity
The maximum power of the ship’s propulsion motor depends on the maximum speed, displacement, motion resistance and the characteristics of the propulsion propeller required by the ship’s navigation, and is generally determined during the overall design of the ship. The power of the propulsion motor is roughly proportional to the cube of the propeller speed, that is, P=k·n3 (k is a constant). Therefore, increasing the speed of the ship requires a significant increase in the power of the propulsion motor.
As the displacement of the ship increases and the speed increases, the required propulsion power becomes larger and larger, resulting in a gradual increase in the capacity of the propulsion motor. Modern ships have been developed from double-propeller propulsion to single-propeller propulsion, and the capacity of a single propulsion motor has been doubled.
3. low speed, high torque
The early ship propellers were small-diameter high-speed propellers. In order to improve propeller propulsion efficiency and reduce noise, low-speed large-diameter propellers are now used. In general, the propulsion motor and the propeller are coaxially connected, and there is no gear reduction, so the speed of the propulsion motor develops to a low speed. The capacity of a single propulsion motor is greatly increased, and the torque of the propulsion motor is also greatly improved.
4. High power ratio
The cabin space and displacement of the ship are limited. It is hoped that the volume and weight of the equipment should be as light as possible. The propulsion motor is one of the large equipment on the ship, especially the submarine propulsion motor is generally placed at the tail of the ship. In order to improve the hydrodynamic and acoustic performance of modern submarines, they tend to adopt a drop-shaped tail shrinkage, which leads to a smaller volume of the cabin, so the propulsion motor is required to be small in size, light in weight and high in power density.
The requirement for a propulsion motor to have a minimum size and weight contradicts the ease of maintenance of the motor and the ease of access to certain components, but for marine propulsion motors, the minimum size and weight becomes a design priority.
5. Wide speed regulation range and speed regulation performance
The propulsion motor must be able to meet the full speed requirements of the ship’s navigation, and its speed is generally tens to hundreds of revolutions per minute. In the case of the same output power, compared with the general constant speed motor and the speed regulating motor with the speed interruption area, the actual shared power, electrical load and magnetic load are much larger.
6. High efficiency
The efficiency of the electric motor has a large impact on the fuel consumption, main generator size and weight of the vessel. The low efficiency of the propulsion motor and the increase of the total power consumption will significantly increase the capacity, size, weight and fuel consumption of the main generator.
The efficiency of the ship’s propulsion motor directly affects the ship’s endurance and combat radius.
7. Low vibration and noise
Marine propulsion motors are usually installed in very small cabins, where long-term workers will be troubled by the constant noise of the motor. Therefore, in order to improve the living and working conditions of the staff, it is necessary to reduce the noise of the motor.
For special ships such as surveying ships and marine research ships, the vibration and noise of the propulsion motor can also interfere with the accuracy of the measurement.
In addition, with the development of anti-submarine technology such as sonar, it is necessary to research and develop quiet submarines. When the submarine sails at low speed, the propulsion motor is the main source of vibration and noise, which has a great influence on the concealment of the submarine, especially the tactical and technical performance indicators of the submarine. Therefore, according to the limit of the sound energy value emitted by the ship into the water, the requirements for the vibration and noise of the submarine propulsion motor are getting higher and higher.
It must be pointed out that the requirement of low vibration and noise is in direct contradiction to the limitation of external size and weight, because the effective material utilization of the motor is higher. That is, the higher the electromagnetic load and speed of the motor, the greater the noise of the motor. To obtain a motor with low noise, the electromagnetic load and speed of the motor must be reduced, which will increase the overall size and weight. Therefore, in the actual design, it is usually necessary to take into account the influence of the above factors, and finally obtain an optimal solution.
8. Multi-condition operation
The propulsion motor should meet the operating requirements of various working conditions to push the ship forward and backward at different speeds. Submarine propulsion motors generally have three working systems: basic, short-term and continuous. The basic working system is the short-term working state of underwater navigation, and it is also the rated working condition of the motor, generally 1h; the short-time working system can make the submarine more flexible and maneuverable, which is conducive to approaching the enemy, occupying favorable positions and avoiding various unfavorable situations, generally 10min working conditions; the continuous working system can meet various speeds of the submarine under the surface, underwater, snorkel sailing, reversing and other conditions.
9. Multi-voltage power supply mode
Submarine propulsion motors are generally powered by battery voltage. Due to the discharge characteristics of the battery, the power supply voltage fluctuates in a large range. On the other hand, in order to facilitate speed regulation, the series-parallel structure of the battery pack is often adjusted during operation. When the propulsion motor has the same output power, compared with the general motor, the electric load and the magnetic load are larger.