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Basic principles of marine DC motors

Basic principles of marine DC motors

1. Electromagnetic torque equation Figure 1 is a schematic diagram of a shunt motor. It can be seen from the figure that when the power is turned on, the current If passes through the armature, while the current Ia passes through the shunt winding in parallel with 

Requirements for marine propulsion motors

Requirements for marine propulsion motors

The requirements for the propulsion motor are not only determined by the characteristics of the marine propulsion motor, but also the environmental conditions such as seawater, salt spray, mold, etc., and the tactical and technical conditions such as the influence of tilt, sway and impact, etc. when 

Features of marine propulsion motors

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.

Read more: Requirements for propellers on mechanical properties of propulsion motors

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.

Requirements for propellers on mechanical properties of propulsion motors

Requirements for propellers on mechanical properties of propulsion motors

The propeller is the working object of the propulsion motor, and the characteristics of the propulsion motor must be adapted to the working characteristics of the propeller, so that they can work well with each other. The following takes DC electric propulsion as an example to 

Propeller reversal characteristics

Propeller reversal characteristics

When the speed is constant, the relationship curve Mt=f(n) between the resistance torque and the rotational speed during the propeller reversal process is called the propeller reversal characteristic curve. The reverse characteristic curve of the propeller has a very peculiar shape, as shown in Figure 

Free sailing characteristics and mooring (anchoring) characteristics of propellers

Free sailing characteristics and mooring (anchoring) characteristics of propellers

Propeller characteristics refer to the relationship curve between propeller torque, power and rotational speed, that is, M=f(n), P=f(n) curves. The most commonly used are the following three typical characteristic curves:

(1) Free navigation characteristics My=f(n), Py=f(n);

(2) Mooring characteristics or anchoring characteristics Mz=f(n), Pz=f(n);

(3) Inversion characteristic Mf=f(n).

Free sailing characteristics

The relationship curve between the propeller resistance torque (or power) and its rotational speed obtained when a fully loaded ship sails in still water is called the free-navigation characteristic.

The torque-speed characteristic is an approximate quadratic curve, and its expression can be written as:

My= Kyn2     (1-1)

The power-speed characteristic is an approximate cubic curve, and its expression can be written as

Py= Ky‘n3   (1-2)

In the formula, My is the torque (N m); Py is the power (kW); n is the rotational speed (r/min); Ky and Ky‘ are constants.

Figure 1 shows the free-navigation characteristic curve. The ship speed is approximately proportional to the propeller speed (Van). Therefore, each propeller speed on this characteristic curve corresponds to a certain ship speed. The whole characteristic curve corresponds to many different speeds.

 Free sailing characteristics and mooring (anchoring) characteristics of propellers
Figure 1 – Free sailing characteristic curve

To make the propeller run stably at a certain speed, the resistance torque of the propeller at this speed must be overcome, and there must be a corresponding prime mover torque for this purpose. For example, to run at the speed of n1 or n2, the torque of the prime mover must be equal in magnitude and opposite in direction to the propeller resistance torque My1 or My2.

Mooring (anchoring) characteristics

The relationship curve Mz=f(x) or Pz=(fn) of the propeller resistance torque Mz (or power Pz) and its rotational speed obtained by the fully loaded ship when the speed is equal to zero is called the mooring characteristic or throwing characteristic. When doing the test, the ship was moored, hence the name. The curve is shown in Figure 1.

The tethering characteristic expression is

Mz= Kzn2     (1-3)

Pz= Kz‘n3     (1-4)

It must be noted that in the free-navigation characteristic, each speed n of the propeller corresponds to a different speed, while in the mooring characteristic, the speed is always zero, that is, V=0. When sailing upwind in strong winds and waves, there is a lot of resistance and it is possible to approach this situation. When the propeller is started when the ship is stationary, the relationship between the propeller resistance torque and the rotational speed is a mooring characteristic. Therefore, when studying the co-working characteristics of the prime mover (or motor) and the propeller during starting, the mooring characteristics should be used.

If the ship has a dragging load (such as a tugboat), its propeller torque-speed curve is Mt=f(n), which is between the free-navigation characteristics and the mooring characteristics.

If the ship sails under the condition of light load or downwind, the resistance of the ship is small, and the propeller characteristics will be below the free sailing characteristics, as shown in Ms=f(n) in Figure 1.

Between Mx=f(n) and Ms=f(n), there are actually many similar characteristics, which vary with the ship’s load conditions and resistance conditions. When a ship is sailing in stormy weather, ship resistance can vary widely. Sometimes the propeller may also fall off, be damaged, or come out of water, etc., so that the propeller resistance moment is reduced to nearly zero. At this time, under the action of the torque of the prime mover, the propeller may even produce a “flying car” phenomenon, which makes the speed reach an unacceptable level.

Interaction of propeller and hull

Interaction of propeller and hull

The open-water properties of a propeller refer to the hydrodynamic performance of an isolated propeller in a uniform flow field. The ship resistance is generally considered the resistance of the isolated hull alone. The actual propeller works at the stern of the ship. The ship 

The working characteristics of the propeller and the resistance of the ship

The working characteristics of the propeller and the resistance of the ship

The working characteristics of the propeller For a propeller with a certain geometry, its thrust coefficient KP, drag torque coefficient KM and efficiency ηP are only related to the advance speed ratio J, and the relationship between KP, KM, ηP and J is called the propeller characteristic curve. 

Blade profile and blade area

Blade profile and blade area

The profile of the blade can be represented by the front and side views of the propeller. The front view of the propeller is seen from the back of the ship to the bow, and the side view is seen from the side of the ship. As shown in Figure 1, a common propeller diagram is shown, and the names and terms of each part of the propeller are indicated on the diagram. The shape and name of the propellers have been described in detail earlier.

Blade profile and blade area
Figure 1 – Outline of the blade

In order to correctly express the relationship between the front view and the side view, a generatrix in the middle of the blade surface is taken as the reference line for drawing, which is called the blade reference line or the blade surface reference line, such as the straight line OU in the figure. If the propeller blade surface is a positive helical surface, the reference line OU is perpendicular to the axis on the side view. If it is an oblique helix, the reference line and the vertical line of the axis form a certain angle ε, which is called the longitudinal oblique angle. The projected length of the reference line OU on the axis is called the vertical slope and is represented by zR. The pitch propellers are generally inclined backward, and the purpose is to increase the clearance between the blades and the stern frame or the hull to reduce the hull vibration induced by the propeller, but the pitch should not be too large (generally ε<15°, otherwise the centrifugal force of the propeller will increase the bending stress at the blade root during operation, which is unfavorable to the blade strength).

The projection of the blade on the plane perpendicular to the axis of the blade is called the orthographic projection, and its outline is called the projection profile. The sum of the projected areas of all the blades of the propeller is called the projected area of the propeller, expressed as Ap. The ratio of the projected area Ap to the disk area A0 is called the projected surface ratio, namely:

Projection surface ratio=Ap/A0

Projected profiles that are symmetrical to the reference line are called symmetrical lobes. If its shape is not symmetrical to the reference line, it is an asymmetrical lobe. The distance xs between the tip of the asymmetric blade and the reference line is called the side slope, and the corresponding angle θs. is the side slope angle. The skew direction of the blade is generally opposite to the steering of the propeller. Reasonable selection of the skew of the blade can significantly reduce the hull vibration induced by the propeller.

The projection of the blade on a plane parallel to the containing axis and radial reference line is called the side projection. In addition to drawing the outline of the blade and the position of the reference line OU, the maximum thickness line needs to be drawn. The axial distance t between the maximum thickness line and the reference line OU represents the maximum thickness of the leaf section at that radius. It only represents the radial distribution of the maximum thickness of the cut surface at different radii, and does not indicate the position of the maximum thickness along the chord direction of the cut surface. The maximum thickness of the cut surface connected to the hub is called the thickness of the blade root (excluding the fillets on both sides). The ratio t0/D between the distance t0 of the intersection of the radiation reference line and the extension line of the maximum thickness line on the axis and the diameter D is called the leaf thickness fraction. In the process, the thickness of the blade at the blade tip is often thinned into an arc shape. In order to obtain the blade tip thickness, the maximum thickness line of the blade needs to be extended to the tip diameter, as shown in Figure 1a.

The shape of the propeller hub is generally a cone, and it can be seen from the side projection that the diameters are not equal everywhere. The diameter of the propeller hub (referred to as the hub diameter for short) refers to twice the distance from the intersection of the radiation reference line and the surface of the propeller hub (ignoring the corner filler at the blade root) to the axis, and is represented by d (see Figure 1a). The ratio d/D of the hub diameter d to the propeller diameter D is called the hub diameter ratio.

After the tangential planes intersecting the coaxial cylindrical surface at each radius and the blade are developed into a plane, the chord length is placed on the horizontal line of the corresponding radius, and the contour obtained by connecting the endpoints is called the stretch contour, as shown in Figure 1c. The sum of the areas contained in the extension profile of each blade of the propeller is called the extension area, which is represented by AE. The ratio of the stretched area AE to the disk area A0 is called the stretched surface ratio, that is

Stretch surface ratio = AE/A0

The wheel obtained by approximately spreading the blade surface on the plane is called the unfolding profile, as shown in Figure 1b. The sum of the areas included in the development contour of each blade is called the development area, which is represented by AD. The ratio of the expanded area AD to the disk area A0 is called the expanded surface ratio, that is,

Expanded area ratio=AD/A0

The expansion area of the propeller blade is very close to the expansion area, so it can be called the blade area, and the expansion surface ratio and the expansion surface ratio can be called the disk surface ratio or the blade surface ratio. The size of the disk-to-surface ratio essentially represents the width of the blade. Under the same number of blades, the larger the disk-to-surface ratio, the wider the blade.

In addition, the average width bm of the blade can also be used to represent the width of the blade, and its value can be calculated as follows:

Blade profile and blade area
(1-1)

where AE is the extension area of the propeller; d is the hub diameter; Z. is the number of blades.

Or expressed by the average width ratio, that is

Blade profile and blade area
(1-2)
Surface pitch and blade section of the propeller

Surface pitch and blade section of the propeller

surface pitch of the propeller The blade surface of the propeller blade is a part of the helical surface (see Figure 1a), so the intersection of any cylindrical surface that is coaxial with the propeller and the blade surface is a segment of the helix,