wind brake?

how do you brake a wind turbine if the wind is going to be really strong ie hurricane force... its cool to make a turbine - but it has to have speed limits or it will explode?

There is a design that uses a spring loaded tail that turns the turbine out of the wind when it is strong enough to over come the spring pressure. I think that would be the best option, but you could also make a centrifugal brake or electrical dynamic brake system but I doubt either would last through a hurricane.

As in many cases, this depends, and mostly on size.

Small Scale:
The preferred method would be the use of a tilt down tower which allows you to use a pivot point at the base and basically lay down the entire tower, turbine and all out of the wind. This is great for events like hurricanes you know are approaching and have time to tilt down. For wind gusts of high speeds I think Barry's got the right idea. Also VAWT (vertical) are a bit more sturdy for fast winds from what the research says.

Large Scale:
For utility scale wind turbines the Pitch and Yaw systems are the main mechanical means of wind control. The Pitch system turns the blades in and out of the wind to catch more or less wind as needed. The Yaw system will rotate the Turbine nacelle (big box the blades are attached to) on the tower so the blades face the wind. Typically for big storms you pitch the blades to catch the least amount wind and won't even need to set the break. The angle of attack makes all the difference.

And yes, they all have their limits.

Rather than using a spring, my turbines use gravity. The tail is weighted to adjust the furling speed, and is mounted on an offset angled hinge pin. The turbine rotor and thrust centerline is also mounted off-center from the yaw so rotor thrust acts in one direction while the tail forces act in the opposite direction. When rotor thrust exceeds tail steering forces the machine turns out of the wind and reduces the amount of swept area exposed to the wind, as well as increasing blade angle of attack, reducing rotor efficiency. These machines furl and limit power to 2,500-3,000 watts in 80 mph winds with no problem. If it exceeds 144V input to the controller, there is also electro-mechanical braking that applies in the form of a three-phase voltage clipper applying additional load to the generator, which in turn more heavily increases load on the rotor, which in turn causes the airfoils to run at higher angle of attack and be less efficient. The voltage clipper will actually fully stall the airfoils if fully applied, stopping the turbine even in very strong thunderstorm winds.

This is a 3.5 meter (11.5 foot) diameter machine:



This is a closer view of the machine with the tower laid down during its annual service:



And a static photo showing how the furling geometry interacts. You can see wind direction from the directional indicator on the anemometer, as compared to the direction the rotor is facing when the machine is furled - the rotor is almost 90 degrees to the true wind direction, which drastically reduces its effective swept area and power output. This photo was taken at a wind speed of about 60 mph




And a photo standing at the base of the tower so you can see the geometry of the tail offset when the machine is in the normal "flying" position

Back in the old days cowboys used to have to climb the towers to trip the tail back in on the old aero motor wind / water pumps after a storm .