# 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 analyze the requirements of the propeller for the characteristics of the DC motor, and why there are these requirements. These requirements include:

(1) The ideal no-load speed n of the motor must be automatically limited, and it is not allowed to be too high.

The general design is n0≤120% ~ 140% ne. If the no-load speed of the motor is much greater than the rated speed, when the propeller comes out of water (the ship is often bumpy in strong winds and waves) or falls off, the load of the motor will be greatly reduced, and the speed of the motor will be greatly increased, causing the motor to suffer great mechanical damage.

(2) The “locked-rotor” torque of the motor must be able to be automatically controlled, and it is not allowed to be too high.

The general design is Mdz= 150% ~250% Me. This is because propellers, especially those of port operation ships and icebreakers, are easily stuck by steel cables, ice cubes, etc., causing the motor to “lock up”. That is, the speed of the motor decreases sharply to zero, the current flowing through the armature of the motor will increase rapidly, and the electromagnetic torque M=CmΦI of the motor becomes very large. The increase in current and electromagnetic torque makes the motor armature overheat and the motor and shafting mechanical stress is too large, so the motor “lock torque” should be limited.

The characteristic 1 of Figure 1 is the mechanical characteristic of the separately excited motor, which is a hard characteristic. If the ideal no-load speed is designed to meet the requirements, the value of the stall torque will be extremely large, and its position cannot be found on the graph. If the stall torque is designed to meet the requirements, the no-load speed will be greatly increased. It can be seen that the characteristics of the separately excited motor cannot meet the requirements of the no-load point and the stall point at the same time.

(3) When the load changes, the power of the motor should be fully utilized.

During the sailing of the ship, especially in the wind and waves, the load of the motor often changes, and the characteristics of the propeller often change on both sides of the free sailing characteristic curve. At this time, it is impossible to keep the motor working stably at a certain point (such as point a, see Figure 2). When the load increases (the limit is that the propeller characteristics change from free sailing characteristics to mooring characteristics), the motor will transition to point a, and point b is not only on the mechanical characteristics of the motor, but also on the characteristics of the propeller tether cable. It can be seen that at point b, the rotational speed is similar to that of point a, but the torque is much larger than Ma (=Me). This will overload the electric motor and cause overloading of the generator and diesel engine. When the load is reduced, the motor will transition to work at point c, the motor will be seriously underloaded, and the power will not be fully utilized.

It can be seen that the characteristics of the separately excited motor can not meet the requirements of making full use of the power of the unit when the load changes.

In order to better meet the above requirements, the mechanical properties of the motor must have the shape shown in Figure 3. In the acb section of this characteristic curve dacbe, the curve is hyperbolic, and each point in the ab section always has the relationship of M·n=constant.

And we know that the output shaft power of the motor is:

P=M·n/975

Each unit of measure in the formula: P (kW); M (kg m); n (r/min).

Therefore, at each point on this hyperbola, the output power of the motor is equal, and this state of the motor is called the “constant power” state. Point a is on the propeller free-navigation characteristic, and point b is on the propeller tethering characteristic. This ensures that when the load of the motor changes between 1 and 2, the motor will automatically maintain “constant power” operation, and the motor power will not be overloaded, so the generator and diesel engine power will not be overloaded. Although the torque is greater than the rated value, it is within the allowable torque overload range of the motor and will not cause electrical and mechanical damage.

Point d of this characteristic limits the no-load speed of the motor, and point e limits the stall torque of the motor. Their values can be designed within the allowable range.

Generally speaking, it is difficult to obtain the ideal constant power characteristic of the acb segment, but it is easier to obtain the so-called “convex characteristic”. The convexity characteristic is shown in the curve dafbe of Figure 3. When the load changes and the motor transitions to work at point f, although this point is higher than the constant power curve, the motor will be slightly overloaded (Pf>Pe), but the overload is not large, so the application is still relatively common. Generally, we call it the constant power characteristic like the ideal constant power characteristic.

The constant power characteristic is suitable for the application of tugboats, trawlers, minesweepers, icebreakers and other ships. On some ships, such as fireboats, ferries, dredgers and other ships, in addition to the propeller, there are some large-capacity auxiliary engines with the same capacity as the propeller. One of their characteristics is that they have a large capacity, and the other is that their peak load is often staggered from the propeller in time. At this time, their motor and the propeller motor are often connected in series on the same main circuit and powered by a common generator set. In order to ensure that the speed of each motor can be adjusted independently without affecting each other, a so-called “constant current” system has been developed.

(4) In the constant current system, the main circuit current is required to remain unchanged, and it is not affected by the speed change of any motor.

The characteristic shape is shown as the curve dabe in Figure 4. The abscissa in the figure is represented by the current I. The resulting characteristic is the n=f(I) characteristic. When the motor Φ=constant, M∝I, this characteristic also represents the mechanical characteristic n=f(M). This characteristic is more interesting than the characteristic dafbe in Figure 3, whose torque is almost constant below point a. When the propeller characteristic changes from 1 to 2, the motor operating point changes from α to b. Obviously, the power at point b is smaller than that at point a, that is, Pb<Pa. The power utilization of this motor is insufficient, but since the load peaks of each motor do not appear at the same time, the load of the entire diesel generator set may be relatively uniform, and its power utilization may be relatively sufficient.

(5) Motor speed regulation should be more convenient.

During the operation of the ship, the propellers are often required to rotate forward and reverse and obtain different low speeds. This requires the motor to have different speeds, and the speed regulation is relatively simple. In terms of speed regulation performance, the DC electric propulsion system is superior to the AC electric propulsion system.