This should be due to the change in its frequency when it deviates from its rated speed.
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What is the working principle of a propeller boat, i.e. what device is used to generate power?
The earth rotates once per day and the moon continues to revolve around the Earth. Due to the gravitational pull of the Moon, the water in Earth's oceans will form bulging parts that rise around the Earth. This is what we think of as the rise and fall of the tides. (Among them, the gravitational pull of the sun also plays a role, accounting for about a quarter of the total pull on seawater.) A tidal power plant consists of a seawall-like spillway gate and is usually constructed at the outlet of a river and other places with large tidal fluctuations. The spillway gate is equipped with gates and turbine blades. As long as the water level difference between the inside and outside of the spillway gate exceeds 3 m, the water will pass tothrough the blades, driving the turbine to produce electricity. The door automatically opens and closes at the right time to achieve the highest water level difference and water pressure. This type of bidirectional tidal power plant can drive the turbine to produce electricity regardless of the rise and fall of the tide. However, the turbine blades rotate in the opposite direction as the water rises and falls.
Due to limitations in tidal fluctuations, tidal power plants can only produce electricity for about 8 hours per day. Additionally, its construction is expensive, it affects water flow and wildlife along the coast, and it creates obstacles for fish and aquatic animals. Currently, the world's only large-scale tidal power plant is located in northern France, on the Reims River, near Saint-Malo. Since 1966, it has produced more than 240 megawattss of electricity.
As the propeller rotates, the blades continually push a large amount of water backward, generating a forward force on the blades, i.e. propulsion. In general, propellers have a forward speed in addition to rotation. If you look at a small section of a blade, it looks like a small section of an airfoil. Its relative water speed is a combination of forward speed and rotation speed. The component of the hydrodynamic force exerted on the blade towards the front constitutes the traction force. The component in the plane of rotation forms a moment that prevents the propeller from rotating and is balanced by the torque of the motor. The angle between the blade section chord (equivalent to the wing chord) and the plane of rotation is called the blade installation angle. When the propeller turns once, the distance it advances forward, guided by the blade installation angle, is callednot the propeller. In fact, the speed of advancement of each section on the blade is the same, but the circumferential speed is proportional to the distance (radius) of the section to the axis of rotation, therefore the angle between the relative flow of each section and plan. of rotation increases with the distance from the axis of rotation. Gradually increases and decreases. In order to keep each blade section and relative airflow within a favorable angle of attack range, the installation angle of each section also increases with distance. the axis of rotation.increase and decrease. This is why each blade has a twist.