Looking for a reliable charger for your e-bike, off-grid marine facility, RV, golf cart, or other applications? A 48V lithium battery charger offers maximum efficiency and convenience, catering to various devices. In this guide, we’ll delve into its usage principles and maintenance tips for optimal performance.
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 …
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 the ship is working, and also consider the requirements of the cabin layout, external structure size, weight and other requirements.
The basic requirements for a propulsion motor are as follows:
1. High reliability. The requirement for high motor reliability is a comprehensive concept that determines many structural features of the motor. The high reliability of the motor can usually be understood as follows:
(1) The motor can work uninterrupted for a long time, and only needs to stop working in a short time during maintenance, that is, to replace the parts that need to be repaired, such as the brush of the DC propulsion motor, the power module of the mechatronic propulsion motor, and the bearing lubricating oil, and it will not stop working for a long time when the dust on the motor is removed;
(2) The propulsion motor is installed on the ship, requiring all the most important structural components to be easily maintained regularly;
(3) Moisture resistance and water resistance of winding insulation. Under the action of normal marine air humidity, the motor can still maintain good insulation performance. Even if the motor immersed in seawater is washed with fresh water after the seawater is discharged from the tank, and after short-term baking and drying, it can still work for a short period of time;
(4) Mechanical strength of the motor. It has the ability to withstand the mechanical stress generated during normal operation and when the ship performs tasks, and can withstand large shocks and vibrations;
(5) Ensure that the motor can work reliably under heeling, pitching and rolling;
(6) The independence of the motor from the auxiliary machinery, that is, within a limited time interval, the motor does not lose its ability to work independently and vitality due to the abnormality of auxiliary machinery (such as fans, oil pumps for bearing oil and other auxiliary machinery);
(7) Ensure the safe operation of the motor cooling system.
2. It is best to use forced ventilation for the propulsion motor to reduce the volume and weight and avoid overheating when the ship is braking or reversing. Water cooling can also be used for motors with excessive power.
3. Depending on the conditions of the ship, either a separate ventilation system or a general ventilation system can be used, but it is better to use a separate ventilation system.
4. Motors with forced ventilation should be able to withstand low load operation without forced ventilation.
5. In order to prevent the internal condensation of water after the motor is stopped, and to improve the insulation of the motor, an electric heater should be installed inside the motor to keep the temperature in the motor 2~3°C higher than the temperature of the surrounding medium. For this purpose it is also permissible to use the field winding of the electric motor as a heater.
6. The insulation resistance of the propulsion motor, before and after the withstand voltage test, should be corrected to the insulation resistance value at 25°C (corrected by doubling the insulation resistance for every 15°C decrease in temperature), which should not be lower than the following values :
Field winding (B, F, H class insulation): 50MΩ;
Armature circuit (B, F, H class insulation): 25MΩ;
Electric heater: 25MΩ;
7. The motor should be able to withstand the test of overspeed operation. The motor should be able to withstand 125% of the rated speed and run at no-load overspeed for 5 minutes without damage or harmful deformation. When 125% of the rated speed is less than 120% of the maximum working speed, the test should be carried out at 120% of the maximum working speed.
8. REVERSE. For reversible motors, they should be able to operate normally under the reversing conditions specified in the product technical specifications.
9. Effective measures should be taken for the motor to prevent the shaft current from harming the bearing. Generally, the peak-to-peak value of the shaft voltage should not exceed 1V. Higher shaft voltages are permissible if the bearings are electrically insulated and the shaft current circuit is disconnected.
When taking bearing insulation measures, at least the non-drive end bearing of the motor should be electrically insulated, and the metal oil pipes, metal cooling pipes and other conductive connectors connected to the insulated bearings should be electrically insulated.
10. Bearings and lubrication of the propulsion motor. Within the allowable inclination range of the motor, it should be ensured that the bearings can be well lubricated and work normally. Lubricating oil (grease) cannot leak or overflow from the bearing.
11. The propulsion motor should use sliding bearings as much as possible.
12. When the propulsion motor adopts rolling bearings, it should be considered:
(1) The rolling bearings can be replaced smoothly on the ship;
(2) Set the bearing oiling cup (hole) and set the oil discharge channel; correctly select the lubricating oil to ensure the bearing is well lubricated; 3) The service life of the rolling bearing should be no less than 20000h.
13. When the propulsion motor adopts sliding bearing, it should be considered;
(1) Bearing temperature measurement, oil level observation and over-temperature alarm devices should be installed in obvious parts;
(2) If pressure lubrication is used, sufficient oil pressure or oil level must be maintained to ensure continuous oil. If splash-type sliding bearings are used, bearing oiling cups (holes) should be provided to replenish lubricating oil;
(3) The propulsion motor with sliding bearing must be equipped with a rotor lifting device to facilitate the replacement of bearing bushes.
14. The diameter of the rotor of the propulsion motor should be as small as possible to reduce the moment of inertia, reduce the transition time during reversal or speed regulation, and improve maneuverability.
15. Where necessary, overspeed protection of the propulsion motor shall be installed to prevent exceeding the speed limit for which the motor is designed under maneuvering and fault conditions.
16. In the specified operating mode and emergency control mode, the regenerative power shall not cause any alarm of the propulsion system.
17. The propulsion motor excitation circuit protection shall not cause an open circuit unless the armature circuit is simultaneously disconnected.
18. For motors with one field winding or two armature windings, the failure of one armature circuit shall not cause an open circuit of the field circuit.
19. There should be a device to detect the internal temperature of the motor and the lubrication of the bearing, and the signal should be reflected on the control panel.
20. For propulsion motors powered by static frequency converters, the influence of harmonics in the power supply should be considered in the design.
21. The export of heat from the motor in the cabin. The cabin space of the ship is very limited, and the heat discharged by the large-capacity propulsion motor has a great impact on the living conditions on the ship and the normal working conditions of the maintenance personnel. Therefore, the heat dissipation of the motor in the cabin is very important. Marine propulsion motors generally use a closed circulation ventilation cooling system with a water-cooled air cooler or a water-cooled system.
22. Meet the requirements for marine use. Marine propulsion motors must be moisture-proof, mildew-proof and salt-fog proof.
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 …
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 …
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 1. Myj and Md are the torque of the prime mover and the propeller, respectively. The positive value shown in the figure is the forward rotation speed of the propeller, and the negative value is the reverse rotation speed of the propeller.
When discussing the reversal characteristics of the propeller, the reversal of the propeller itself must be distinguished from the reverse of the ship. In order to reverse the ship, the propeller must be reversed first, and the propeller reversal time is very short, measured in s. However, the time required for the ship to reverse (for example, from full-speed forward to full-speed backward) is very long, which is calculated in minutes. Therefore, it can be considered that during a certain period of time when the ship is in reverse, the ship still continues to sail at almost full speed, although the propeller is already reversed at this time.
During reversal, there is a very specific change in the propeller drag torque due to this condition. If the propeller is reversed when the speed of the ship is zero (for example, when the ship is at berth), its reversal characteristic curve is a symmetrical curve. At this time, the reversal characteristics and the mooring characteristics will coincide as one, as shown in curve 1 in Figure 1.
If the propeller is reversed at the speed of other ships (such as when the ship is moving forward or backward), its reversal characteristic curve is a mutually asymmetrical curve, and these mutually asymmetrical curves will have such characteristics. That is, when the speed of the propeller is maintained at a positive value, negative torque will appear on their individual line segments, as shown in the BCD segment of curve 3 in Figure 1 (from full speed forward to full speed backward), and a maximum value will appear at a certain speed, such as point C.
The appearance of negative braking torque, ie, negative propeller torque, indicates that the propeller will try to maintain the original direction of rotation under the action of water pressure as the ship continues to move forward. At this time, the screw firewood no longer works as a propeller, but starts to work as a hydraulic motor.
When reversing, the magnitude of the propeller braking torque is related to the forward speed of the ship. If the forward speed of the ship was originally high, the negative braking torque of the propeller would also be high, as can be seen by comparing curves 2 and 3. Curve 2 corresponds to the ship speed V = 0.6Ve, and curve 3 corresponds to the full ship speed, ie V = 1.0Ve. Ve is the rated boat speed. The change of the torque of the propeller when it reverses can be analyzed by the force diagram of the propeller blade element. The details are as follows: The so-called propeller blade element is the propeller flake at the radius r. The force on the entire propeller can be understood by analyzing the force on the blade element.
A sketch of the propeller and its blade elements is drawn on Figure 2. In the figure, the hollow cylinder A is the hub; B is the blade; the shaded C is the cross-sectional schematic diagram of the blade element.
When the propeller rotates at the speed n, the circumferential linear velocity of the blade element is 2πrn, so the relative velocity of the water flow to it is also 2πrn, so it can be considered that the blade element does not move, and the water flow rushes to the blade element at a speed of 2πrn. Driven by the propeller, the ship moves at a certain speed, so the current also rushes towards the blade element from the opposite direction of the ship at a relative speed vp. Combining these two velocities as vectors, the vector W is obtained. The water flow rushes towards the leaf element at this synthetic speed, see Figure 3.
Under the impact of water flow, lift dY (direction perpendicular to W) and resistance dX (direction consistent with W) are generated on the blade element. By projecting their combined force dR to the vertical and horizontal directions, the thrust dP and the rotational force dQ can be obtained.
At point A of the reversal curve, the thrust dP is in the positive direction, pushing the ship forward. The rotational force dQ is also in the positive direction, and a resistance torque is generated to prevent the propeller from rotating. Therefore, in order to make the propeller run at the speed n, the prime mover must issue the same torque to overcome this resistance torque. In Figure 1, the schematic diagram of the direction of the propeller torque, the rotational speed and the torque of the prime mover is drawn. At point A, it is considered that M (propeller torque), Myd (prime mover torque), and n (propeller speed) are all positive.
If the prime mover torque is reduced so that it is always smaller than the propeller resistance torque, the propeller resistance torque will have excess after offsetting the prime mover torque. Under the action of this residual torque, the propeller will decelerate, and at the same time, its resistance torque will also decrease due to the reduction of the rotational speed, and it will change according to the characteristic AB section.
When the propeller speed is low to point B, its resistance torque becomes zero, that is, the propeller rotation is not hindered. This can be seen in Figure 3b. It can be seen from the figure that the resultant force dR of dX and dY coincides with the vertical direction, its projection dP (thrust) in the vertical direction is itself, and the projection dQ (rotation force) in the horizontal direction is zero. If the prime mover torque is also zero, the propeller will run stably at this point. At this time, the rotation of the propeller is not due to the drive of the prime mover, but due to the inertia of the propeller itself.
To further reduce the propeller speed, the prime mover must give the propeller a braking torque, that is, the prime mover torque should become negative and opposite to the original direction. Under the action of the prime mover torque, the propeller decelerates and enters the BC section. In this section, although the propeller is still rotating in the original direction, its torque becomes negative, because the direction of its turning force is reversed. As shown in Figure 1, the propeller steering remains unchanged and the torque becomes negative, so this torque becomes the active torque that pushes the propeller to rotate, and the prime mover torque is the resistance torque. If Myd=Mj at a certain speed n in the BCD segment, the propeller is in torque balance, and it is like a hydraulic turbine, which overcomes the resistance torque under the impact of the water flow and rotates continuously and stably at the speed n. Only when the absolute value of the prime mover torque is greater than the absolute value of the propeller torque, that is, |Myd|>|Mj|, the propeller will continue to decelerate.
In the BC segment, the included angle between dR and dQ decreases as n decreases (Fig. 3b), and the included angle is 90°, while the included angle corresponding to Fig. 3c is much smaller than 90°. Therefore dQ increases as n decreases. At point C, dQ reaches the maximum value, and Mj also reaches the maximum value. When n further decreases, since W decreases, dR also decreases, therefore, dQ decreases, and Mj also decreases. In this way, a peak torque appears at point C, forming a peculiar shape of the propeller reversal characteristic.
When the speed of the propeller is reduced to zero (point D), the torque of the propeller is not equal to zero, as can be seen from Figure 3d, the torque dQ has a certain value at this time, which means that under the impact of water flow, an active torque is generated on the propeller, trying to maintain the original rotation direction. To keep the propeller stationary at this point, there must be an equal amount of drag torque from the prime mover. In order to accelerate the propeller in the opposite direction, the negative torque of the prime mover must be further increased. When the propeller accelerates in the opposite direction, the relationship between its torque and rotational speed is shown in the DE section. In this section, even if the speed is very low, the torque that the prime mover should send is quite high, otherwise it is not enough to overcome the propeller torque. Therefore, in the process of propeller reversal, the prime mover works very heavy.
If the propeller is reversed at lower ship speeds, its reversal characteristic is above that of full ship speed. For example, in Figure 1, characteristic 2 (corresponding to ship speed V=0.6Ve) is above characteristic 3 (corresponding to ship speed V=Ve, that is, full ship speed). When reversing from zero ship speed (this is reverse starting), the characteristic takes the shape of curve 1. This characteristic curve is the reverse anchoring characteristic, which is completely symmetrical with the anchoring characteristic during forward starting.
During the reverse rotation of the propeller, the speed of the boat does not actually remain constant, but decreases continuously. Therefore, the propeller torque does not change according to a certain characteristic curve, but continuously transitions from the lower characteristic curve to the upper characteristic curve, corresponding to the ship speed at each moment.
In the following discussion, we will generally ignore the friction loss of the shaft and consider the propeller power and torque as the motor power and torque.
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
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. And because the performance of the propeller in question does not consider the influence of the hull, the curve is also called the open-water characteristic curve of the propeller, as shown in Figure 1. In the figure, because the KM value is too small, a curve of 10 KM is usually given. Neither KP nor KM is a straight line, and both are monotonically decreasing as J increases. It can generally be considered that KP and KM are approximately parabolas, and are expressed as
The coefficients K0, K1, K2, etc. in the formula can be determined by curve fitting. If it is a given propeller, they are all constant coefficients.
From the operating characteristic curve in Figure 1, we can see that the KP~J curve and the KM~J curve are very similar in the first quadrant. In fact, the KP~J curve and the KM~J curve are relatively similar in the entire coordinate system. Figure 2 shows the KP~J curve when J takes a wide range of values.
The curve from the upper left to the lower right in Figure 2 is the characteristic when the propeller speed n is forward rotation. The first quadrant corresponds to the state in which the propeller propels the ship forward, and the second quadrant corresponds to the state in which the ship turns to reverse operation, and the fourth quadrant part is imagined that a tugboat is dragging the ship forward, and the propeller becomes the working state of the water turbine. The curve from the upper right to the lower left in the figure is the characteristic when the propeller speed n is reversed. The fourth quadrant corresponds to the state in which the propeller pushes the ship to reverse, and the third quadrant corresponds to the state that the ship changes from reverse to forward, and the first quadrant corresponds to the working state in which it is assumed that a tugboat drags the ship backward, and the propeller becomes the working state of the turbine.
When the ship is actually sailing, it will be affected by various external factors, and its advance speed ratio J will change, so that the propeller thrust and drag torque associated with it will also change. It can be seen that starting from the working characteristics of the propeller, the working characteristics of the propeller in any working state of the ship can be simulated.
Resistance of the ship
The ship will be resisted when sailing in the water, and the thrust generated by the propeller is used to overcome the resistance of the ship, so as to ensure its normal navigation. When the hull moves in the actual fluid, it will be subjected to the pressure perpendicular to the surface of the hull. This pressure is caused by waves and vortices; at the same time, the hull is subjected to the action of the tangential force of the water point along the surface of the hull, that is, the frictional resistance of the water. Therefore, the resistance of the ship when sailing includes vortex resistance, wave-making resistance and frictional resistance, and their sizes all increase with the increase of the ship’s speed. The relationship between the resistance and the speed is:
In the formula, R is the hull resistance; V is the speed; Kr is the resistance coefficient. When the operating conditions are constant, Kr is a constant, and when the operating conditions change, Kr also changes.
Usually, the actual ship sailing resistance curve is used, the curve value is stored in the database, and the resistance value corresponding to the ship speed at a certain moment is obtained by numerical interpolation algorithm.