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How is rotor overspeed contolled on a Vertical Axis Wind Turbine? Answered

I know the rotor overspeed can be controlled with the tail on an HAWT, but how is it accomplished on a VAWT


I saw an ingenious way to regulate windmill and/or turbine speed on a windmill in Malaysia. The windmill was mounted on a swivel.It had a horizontal tail fin inclined upward with the blades perpendicular to the ground In gentle winds. However, as the wind speed increased, the tail dropped down, angling the blades off vertical so they didn't "catch" the wind as well. In very, very strong winds, the tail lifted and the blades would be angled as much as 45 degrees off vertical thereby reducing the rotational speed. This could be applied to either a windmill or turbine. in the case of a turbine. rather than angle the turbine, how about making a "wind break" in front of the turbine. If you were to take pivot a "board or any other stiff material on a pivot on top of the turbine. Something sort of like a crooked letter L, and angle the back portion downward, with the front portion protruding out over the front of the turbine, as the wind blew hard, it would raise the tail, lowering the "wind break" in front of the turbine, slowing it down. The wind break would not have to be so large as to catch the wind itself and get blown down over the front, it only has to be large enough to set up turbulence or to divert some of the wind away from the "incoming" side of the turbine. In fact, the front "wind break" doesn't even have flat, A piece of lightweight PVC pipe or a narrow Vee shape could cause enough turbulence to slow down the rotor. A bit of testing could yield some interesting results.

There are a few methods:  Lemonie's link has tonnes of info.

Centrifugal braking - weights mounted on the axis move outwards as speed increases - if it gets to a certain speed the weights apply pressure on brake pads.  Pros: Somewhat simple. Cons: Failure prone, heat/waste, expendable parts, mechanical failure a problem.

Electro-magnetic braking - A computer senses the current required by the load and disconnects the load while short circuiting the coils in the rotor - the shorted coils basically cancel the torque applied since one magnet makes a field in a coil that pushes against another magnet - thus slowing the rotor.  Easy way to look at it - Torque required to turn the rotor = electrical load on the system + friction loss.  Short circuit = infinite load, thus torque required goes WAY up, slowing the rotor.  Pros: No mechanical parts to worry about, Can be set up with only one switch.  Cons:  Can be as complex as you want it - the controller might already have the brains to do it automatically.

I'd rate the mechanical brake as more reliable than something which needed electrical power to decide whether to run it, and you are assuming your generator hasn't failed open circuit, which they have a distressing habit of doing when you don't want them to...

The centrifugal brake isn't wasting anything, underspeed its doing nothing, so there is nothing wearing out.

This is very true! 

It does take 'some' work to move the mechanical brake components although not much...In continued overspeed condition you'd wear through the brake pads pretty quick...

Failing open circuit would definitely be bad as well :)

Thankfully the controllers are usually hooked up to a battery bank and pull power from the turbine to a dumping load in a high-power situation.

Good points though - keeping me in check!

In an overspeed, everything else has failed anyway, you could just drop the pads in and WEDGE the system !

Very good thought!  preservation through stasis :D

Classic method would be a centrifugal break.