i don’t understand why a wind turbine doesn’t begin to produce power as soon as the blades move but only after a certain “cut in speed” it begins to produce energy. and why is start up speed different from cut in speed?
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{ 3 comments… read them below or add one }
I am not acquainted with the term “cut in speed” but it is correct that a wind turbine needs some time before it can produce electricity.
The reason is: once the generator is connected, it gives a resistance and before it can happen, the rotors must have a certain torque moment.
I am not a specialist in wind turbines but I am a pilot and I fly my own experimental aircraft that I have built myself and installed engine and propeller.
Propellers or rotors of any type are subject to a complex set of factors. For example the gyroscopic effect is not negligible.
WInd turbines are controlled by a computer that direct them and control the revolutions according to a computed profile. For example, as you certainly have noticed, contrary to an old fashion windmill, a wind turbine always spins at the same speed. That is because it is found to be the optimal speed. When the wind speed increases, the effect of the generator increases too and it produces more electrical energy. But the rotor is kept at the same speed.
What few people know is that e.g. the tip of my propeller at maximum RPM is moving at a speed slightly under the speed of the sound. That is when it is most effective. But if it was to exceed the speed of the sound, the vibrations would be such as the propeller would most probably disintegrate. You an imagine what would be the consequences if that happened to a wind turbine blade that is perhaps ten meters long!
A turbine generator of any kind works on the principle that a conductor moving through a magnetic field has a current induced onto it. And, that conductor must be moving at a certain speed through the magnetic field to reach a certain current level which, when applied to the load, results in a certain output voltage. Also, the speed of the turbine generator determines the frequency of the generated alternating current. So, a turbine generator must get up to a certain speed to generate a certain voltage and frequency of operation.
Also, when you load a generator electrically (hook it up to a circuit), you are putting a counter-electromotive force on the generator. That is, the current that flows makes the generator act like a motor to oppose the motion of the generator that caused the current. So, if you have a wind turbine running unloaded and then close a circuit breaker to have it supply a load, this will cause the turbine to slow down a bit from its unloaded speed.
Perhaps this is what you’re referring to.
The blades start rotating as soon as the wind blows fast enough to move the massive machinery (usually around 3 miles per hour, depending on the size of the turbine.) That speed is the ‘start-up speed,’ and it differs by manufacturer, design, and equipment. But the blades rarely directly drive the motors (they do sometimes on small systems, but that isn’t what you are asking about.) The blades end up turning at about 3 revolutions per minute. However, the electric generator doesn’t begin producing electricity until it is turning at 7,500 rpm (revolutions per minute.) At that speed, called the “cut in speed,” the blades have sped up, and through a gear box, have turned the rotational speed into power. That happens at about a 7-8 miles per hour speed. The turbine reaches optimal speed, usually, around about 17 mph. At that point, the generator is turning at 15,000 rpm, and the blades at about 4 rpm. (The gear box changes the power from the blades into the speed the motor needs.)
All that said there is a type of turbine that DOES produce speed as soon as the blades turn. It is called a “Direct Drive” type wind turbine. However, it is usually used on very small (household and smaller) type systems. It isn’t nearly as efficient.