Why do wind turbines have to rotate at a constant speed?

Question

I am aware that the blades of a wind turbine are capable of 'feathering' or rotating on their own axis so as to adjust the angle of attack from the wind and thereby regulate the rotation of the turbine as a whole. The faster the wind speed the slighter the angle of the blades to the wind and therefore the lower the turning force.

But what is the importance of maintaining a constant rotational speed? Do I understand this correctly? Is the feathering simply to keep the rotational speed of the turbine within broadly acceptable limits or is it very important to maintain a constant rotaional speed with minimal variation?

People have often commented that large wind turbines always seem to be going at the same speed regardless of the wind speed.

Answer 1

They dont rotate at a constant speed, however they may appear to do so because they ar generally positioned where there is a constant wind!

In reality the blades move about their axis to make them as efficient at harnessing the wind as possible, and also to prevent damage to the turbine. In high winds the propellors shed the wind and stop to prevent damage.

Inside there are rectifiers and transformers and all sorts of trickery that changes the power produced to a form that can be exported and used (te correct frequency, phase, voltage etc), and this makes the speed of the rotor irrelevant.

You often see the turbines stopped as they turn to face a new wind direction and they do this by altering the blade angle rather than with mechanical brakes!

Answer 2

Controller:
The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.
Gear box:
Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes.
Generator:
Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.
High-speed shaft:
Drives the generator.
Low-speed shaft:
The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
Nacelle:
The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.
Pitch:
Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.
Rotor:
The blades and the hub together are called the rotor.

Answer 3

The speed of any AC generator is controlled by the frequency of the grid it supplies. If a generator has two poles, it will need to rotate at 3000 revs/ minute in the UK to give a frequency of 50Hz (50 cycles per second) If the generator has four poles, it will rotate at 1500 revs/ min and so on. So, if they all have the same number of poles, they will rotate at the same speed. There are governors on all generators - all designed to the same parameters - which will prevent over-speeding. The grid is massive compared to a single generator and any attempt to increase speed by any generator will only result in an increase in output on that generator. Trying to keep this answer simple, hope this helps.

Answer 4

Wind turbines don't rotate at a constant speed. They generate variable frequency AC, which is turned into DC, and then back to 50Hz AC. The unit is called an invertor

Answer 5

Needed from the angle of construction of the windmill and also to provide a fairlfy constant torque / electricity if used for power generation.

Answer 6

Latest technlogy does not require for blades to rotate at a constant speed now with synchronous motors.

have a look at
www.pages.drexel.edu/~rcr32/htw/windmills.pdf

Answer 7

This site gives you a lot of info on wind turbine generators:

http://www.windpower.org/en/core.htm

Answer 8

within limits. Torque issues. That much weight rotating freely will rip apart the housing with vibrations..