What is the problem with overspeeding?

[oppy]

What is the problem with overspeeding?  Is it just over stressing the blades from the centripetal force, or is the alternator or circuitry at risk?

[hiker]

i would say all of the above,,,

     speed kills

[Flux]

You ask this in such a general way that it is difficult to answer.

What do you mean by overspeeding I suppose is the vital bit. Do you mean runaway with no load or do you mean having an alternator too small to fully load the prop in a high wind.

Runaway is certainly bad in most respects, the centripetal force can become high enough to shed blades in the worst case. Possibly as damaging is the violent gyroscopic forces during yaw which may cause a tower strike, snap blades or bend shafts.

The noise becomes diabolical and frightening and will most likely frighten most people from the area before destruction occurs.

Alternator damage may occur if the design is marginal, you may shed magnets or the voltage may rise higher than the insulation can stand. You may in some cases see about 6 times the design voltage, so insulation and construction intended for 12 or 24v may be inadequate. If applied to rectifiers you may exceed the piv and blow them.

If the overspeed occurs as a result of the alternator being incapable of loading the blades then other factors may apply. Reactance limited slotted cored machines may survive with no ill effect. Most air gap machines will burn out if the overspeed continues for a significant time.

Flux

[neilho]

 Flux lays out most of the problems very factually and accurately.

In addition, thrust forces on the tower increase exponentially in overspeed. If the machine hangs together long enough at very hi RPM, the tower may break, bend, fall over, pull guy anchors out, break other tower hardware, etc. If the machine sheds a blade at hi RPM, the resulting imbalance can also cause tower failure. In the biz, it's called catastrophic failure, and it's a drag. Machines and towers destroyed and laying on the ground get expensive quickly, result in lots of bad PR for windpower, are bad for business and dangerous to humans.

Uncontrolled overspeed is the factor (or should be) in the back of every designer and maintainer's mind, through every step of every design and maintenance process.  Extracting power from the wind loads the wind rotor, restricting it from spinning as fast as it would like to. The load must be maintained to avoid overspeed. Maintaining the load requires that almost all electrical components function properly at all times. Clearly unlikely, so wind machines in general are an overspeed (substitute the word accident here as appropriate) waiting to happen. Everyone involved should do their best to ensure that it doesn't.

neil

[Usman]

Hello again Flux,

Still stuck in with my confusions on the way a battery loads a wind turbine! I was wondering if using a certain charge controller would make that algorithm slightly different? i.e. it would let the rotor spin at low load or in other words, alter the load intelligently as compared to direct hook-up with a battery-pack?

Secondly, how matching is the rotor size to a certain generator is the key element, but the phenomenon becomes much more complicated if the system has gearing or speed increasers of some sort. Smaller generator would cause the rotor to over-speed and a larger generator may cause the rotor to stall, so how to calculate that match 'numerically'? or is it just more of an Imagineering than Engineering, particularly in DIY type projects?

Thanks.

[TomW]

usman;

Why not include this in one of your regular questions you seem to post?

It is not nice to blatantly divert a thread like this. The thread is about blade over speed and you divert it to battery loading.

Feels like you sit around thinking up questions to post despite the fact that most have been covered previously. I guess it is nice to have a free world class research department.

Maybe its me but there you go.

[Warrior]

I'd say a speeding turbine is extremly dangerous.

I've seen videos of a broken Air-X blade going through the tin roof of a shed and with enough energy left to stick itself into a wooden door. Imagine if it hit a person.

By all means, a run away turbine should always be avoided.

Warrior

[TomW]

Warrior;



By all means, a run away turbine should always be avoided.

I would say if you see a run away turbine run away

I seem to be in a nutty mood today.

I had my garbogen shuck a blade and it came off the tower stub in a most spectacular manner. My fault for not thinking over the prop blade mounting properly.

My smaller Zubbly conversion had a threaded waterpipe floor flange yaw mount failure that destroyed it and I have never found one of the blades. They were white and we have lots of land around the house so you would think I would have spotted it in the 2 years since.

Sometimes "good enough" isn't.

Cheers.

TomW

[Warrior]

"I would say if you see a run away turbine run away"

Exactly!!!

When I first started playing with wind power, we built a 7 foot 2 blader, chain geared to a motor conversion. It was mounted on the roof of my shop on a small test tower.

On a few occasions it was left unloaded...oh boy, did we run after hearing the noise that prop made!!! Quite scary!!!

[boB]

Can't overspeeding be hard on bearings as well ??  I know it can with hydro.

boB

[Ungrounded Lightning Rod]

The gyroscopic forces on the mill when it yaws, which can make blades bend toward the tower, strike it, and shatter, go up with the product of the rotation rate and the yaw rate.  So letting the prop speed up in a high wind (which can also yaw it more quickly when it shifts direction) increases the chances of such a smashup.

More significantly, the centrifugal force (the real reaction force exerted by the mass of the blades against the centripetal force pulling their motion into a circle) goes up with the square of the speed.  If your rotor speeds up by a factor of 10, the forces trying to tear the blades off and let them fly away at (wind speed * TSR)/2 or tear the tips off and let them fly away at (wind speed * TSR) go up by a factor of 100.

Additionally, a lift-type mill loaded for maximum power may be turning about half the speed it would turn if it were unloaded at the same wind speed (and angle to it).  If it's not furled, removing the load will double its speed and quadruple the tear-apart forces in it.  But the actual speedup may be much more, for several reasons:

 - The load of a simple alternator/rectifier/battery system rises more with higher wind that what is necessary to track the maximum-power point.  It will be designed to run at max-power-point for a wind speed on the low side of the typical wind distribution.  This means the speedup may be much more than a factor of two in typical-to-high winds if the load is lost or the wind overcomes the generator drag.

 - The furling force in an offset-rotor, pivoted-tail mill is a result of wind drag on the rotor, and most of that is the result of lift on the blades from the alternator load.  If the mill is furled and the load is lost, the furling force is lost.  The mill will turn full-on to the wind as if there were no furling system.

 - Current goes up with the speed above cutin and heating from it goes up with the square of current.  (Available energy goes up with the cube of the wind speed so there's no lack of it to drive this failure mode.)  So if your mill didn't furl adequately in a high wind and the genny doesn't load it enough to drive the rotor into aerodynamic stall, the genny heating will rise greatly and the genny wiring will melt (and perhaps the genny will catch fire and/or drop molten metal on the weeds below) or the diodes will fail open, after which it provides no load.  Then the prop runs away as above.

If you lose all or part of a blade due to centrifugal tear-apart or impact with the mast due to gyroscopic forces on yaw (or impact with the tail), the remaining rotor will be massively imbalanced.  So it will shake the mill and tower mightily until enough stuff fails (probably including other flying pieces of blades) that the rotation stops.  This may cause shaft bending, rotor-stator rubbing, bearing failures, guy or tower failure, or even make the tail jump off its pivot or the whole mill jump off the mast.

[Ungrounded Lightning Rod]

To get a handle on this, consider a mill with operating TSR of 6 optimized for max power point at 10 MPH, in a storm building up to a mere 50 MPH gust lasting a few seconds.  One failure scenario:

 - Furling system was inadequate.

 - Genny went into overcurrent and burned out, unloading the rotor.

 - Rotor speed is 2 * 10 MPH RMP * wind speed / 10 MPH.

 - Without loading the mill completely unfurls and faces the wind regardless of wind speed.

 - During the 100 MPH sustained gust the rotor gets up to 10 times the RMP it had when loaded in a 10 MPH wind.

 - Centrifugal forces on the rotor are 100 times what they were at loaded 10 MPH.

 - Mill throws a blade.

 - Tip speed is 50 * 6 * 2 = 600 MPH - nearly the speed of sound.

 - Center of mass speed of the thrown blade is about 300 MPH.

Note that storms can easily have sustained gusts in the 100 MPH range.  The rotor, however, is unlikely to get to a tip speed significantly over the speed of sound, due to the massively increased drag near the "sonic barrier".  However I think you can understand the forces involved in making something massive moving near the speed of sound travele in, say, a ten-foot circle rather than a straight line.

[ghurd]

I would like to add something.

All this stuff is still important with tiny windmills.

I had 18" diameter blades I wouldn't get near when they were Not overspeeding.  Wish I could have seen it when they blew up.

I used to think 500RPM was extreme and 1 or 2 thousand RPM was impossible.  Completely wrong.

G-

[vawtman]

Tom

 I remember awhile back when vawts were the killers, busters, blowupers what have you"s

 Any low solidity high tsr turbine has a much greater potential for failures.Pretty much common sense.

 Ill look around here to see if i can spot your blade.

[ULR]

The tip speed of a rotor is wind speed times TSR, regardless of the radius.  So the speed of the moving blade masses, and the energy per unit of mass, is the same regardless of radius.  The little ones just turn at higher RPM.

Since the tips of the little ones are moving the same speed in a smaller circle, the forces on the material in them are actually greater than in the big ones - though the square/cube law makes that even out when you're looking at the strength of the material holding them together.

[Ungrounded Lightning Rod]

They both have high energy per unit mass.

The big issue with VAWTs vs HAWTs is that lift-type HAWTs have a strong load that is pretty constant through a revolution while lift-type VAWTs (i.e. Darrieus types) have a strong load whose direction reverses twice per revolution.  Nasty opportunity for metal fatigue.

Of course it didn't help that a lot of the experiments were done with aluminum, which is subject to metal fatigue at a small fraction of its yield strength, rather than steel, which can handle flexing at most of its yield strength before serious metal fatigue sets in, or wood, which is also good that way.  (That's why they're good for springs.  B-)  )

I'm kind of partial to mostly-drag type VAWTs myself.  They may have to be maybe half-again bigger for a given amount of power.  But their geometries are stronger and they're easy to scale up.