How Do I Control the Torque on a Wind Turbine Generator

Not to mention that the wind will almost NEVER blow at a speed," that generate exactly the calculated power at the calculated speed, thereby having exactly the calculated torque, but doing this is practical impossible"

Use a governor to control the speed when it's too high. Pray, if it's to low.

Ha ha. you'r Right. I didn't mean it literally. Nevertheless, i might as well try to get the best preconditions, for the final power output. :)

I would have designed the blades for variable pitch angle. That way software could vary the pitch of the blades. To give optimal torque and speed depending on wind speed.

The torque that the alternator presents to the blades is proportional to the current coming out the terminals.

The current coming out the terminals is a function of the alternator voltage.

Presuming it is being rectified and charging batteries, the voltage at the terminals is a function of the battery voltage and the level of charge in the battery.

So if the battery voltage is low, the current will be high, and the speed of the shaft will be reduced. If the battery voltage is high then the current will be low and the shaft speed will increase.

In summary, if charging batteries, the batteries will control the speed of the alternator, however in high wind speeds it might be worth inmtroducing either a mechanical brake, or, by switching some resistance in parallel with the terminals, an electric brake; as the wind speed becomes excessive, start reducing the shunt resistance.

It sounds as though there is plenty of room for on-site experimentation!

I calculated the optimal design of the blades for a specific wind- and rotor speed, and the output power at the same conditions.

My problem is to select a specific alternator, that can induce the before mentioned torque, at the exact same rpm, so that i will get the power calculated, so that the turbine will be optimal for its design.

If wind speed is fairly constant, find an alternator that has a max output of what you need. RPM can be adjusted to what you need, within a certain set of parameters, by using a belt and pulleys.

It's possible that you could also modify the shifter and gears from a 10 speed bicycle, used with chain between rotor and alternator. That way you could manually adjust rpm for varying wind speeds.

Well it all depends on what kind of an alternator you have and what kind of a mechanical linkage you have.

First, you must realize that the angular velocity of the wind turbine must drop in speed when a significant electric load gets applied to the alternator. This is the actual effect of the transfer of mechanical energy to electrical energy. That is unless there is some way to change the pitch of the blade to bite into more wind.

Now you actually have two regulating problems to consider here. First in my book is the output voltage from the alternator. The speed of the alternator rotor will linearly vary the output voltage of the alternator but only if the rotor's magnetic field remains constant. So just as in a car's voltage regulator of its alternator, if the rotor spins faster the magnetic field in the rotor decreases and thus the peak output voltage can be regulated.

Second, the mechanical drag (torque) that the alternator will impose on whatever style transmission you have between alternator and turbine will therefore change with the electric load, speed of the rotor and the magnetic field magnitude in the rotor. Now the earlier mentioned voltage regulator will minimize the rotor field as the speed increases. This will mean that as wind speed picks up and the turbine increases its speed the drag from the alternator will decrease with a constant electric load applied. The only safety issue I would consider in your transmission as far as torque will be adding a safety peak drag kind of a clutch in case something suddenly seizes. Belt slippage may suffice here.

You might find some good info hear. http://www.otherpower.com/ and hear http://www.fieldlines.com/board/

Best answer, guest. When in doubt, go to the source.

As far as I know, alternators are excited by battery voltage, which gets increased somehow by alternator magic. If you want to control the torque, control the exciter voltage.

Besides the links above, consider the new regenerative braking. I expect it works by controlling the (torque) load on the generative ability of the electric drive.

34point5 wrote: "He would be better off using a tail furl."

I'd just like to amplify that a little bit--I've done some reading / research into building a wind turbine--many of them are intended to produce electricity in winds as low as possible (it seems it often takes a minimum of 7 mph to get a turbine to turn--note there surely is hysteresis involved) up to about 22 mph.

Above 22 mph, they need to use schemes like the tail furl to avoid damage to the turbine. Ideally, the tail furl is designed to face the turbine at an angle to the wind so as to maintain an effective 22 mph through the turbine even in much higher speed winds (as opposed to shutting down and generating no power in high winds).

You may easily change any permanent magnet device into a regulated one by magnetically short-circuiting the magnets. (Partially only!)

Between the magnets there is in most designs some space to introduce iron or steel pieces that would divert the magnetic flux so up to 70% would be forced to flow through these flux-short-circuits.

RHABE

You are right; decreasing the drag on a permanent magnet rotor would allow curve matching of an 'over powered' alternator to the wind curve in a 'chord' working envelope.

_{Stolen from link below;}

However the more mechanical stuff you put up a pole, the more likely spider webs will bring it undone (like some pitch control designs out there)

But say we are talking 'medium size' machines and a maintenance schedule - what advantage do you see it having over manipulating the current to a wound rotor?

What should be done is to select an alternator that can produce usable voltage at a low RPM. Why? Because you never select an alternator for wind applications based on a fixed speed. You select an efficient low rpm alternator and electronically regulate and rectify the output voltage. You simply get more current with more rpm. If the alternator is putting out 20v, the regulator puts 14 to the batteries. If there is a windstorm and the alternator is putting out 50 volts, the regulator still puts out 14v but with more current. You simply never have to worry about rpm then, of course unless there is no wind!

With a 1.2 meter diameter turbine - you have nothing to worry about . . . The power output will be only a few watts, . . . enough to get your transistor radio going . . . or perhaps a few efficient LED lights. Your problem is to get an efficient "step-up" gear ratio to increase the generator RPM. Oleh

Do you remember how the old windmill pumps were designed? They looked something like this:-

The wind vane had two jobs, depending upon whether pumping was needed or not:-

1) Keeping the propeller blades pointed into the wind when pumping needed

2) To turn the propeller blades 90° to the wind when no pumping required.

Look here, the vane is now stopping the wind from turning the blades:-

Now if you had a simple rev counter that altered the relationship between the vane and the blades, you should be able to keep the speed of the blades stable when the wind is in fact stronger than needed for the load.

In fact in this example, it could be that someone has already done just that:-

Though the picture is not clear enough to be certain......but a variable length arm, that could furl the vane, would be just perfect!!!

Hopefully you feel stimulated to try something along those lines....

Hi again.

Thanks for all the comments! It certainly helped me a lot, and gave me some keywords, of what to search for. I think i'l go for 35point5's method.

The alternator is a ACPM. I all ready have the power curve, so i only need to find power curves for an appropriate alternator.

Found this site, which talks about the same subject, and comes up with an method to raise the efficiency a bit:

http://s4wsbox.com/?q=node/32

Regards, J.

Yes, star delta switching has some gains - just be aware that wind power is a 'hobby' and can become very expensive for little return.

Spring load the blade angle adjuster.

What is your Avatar, can you post a "bigger" version for me to look at please?

The idea is good, I also thought about mounting the whole aparatus on a spring loaded easy sliding "sledge", which when blown back by stong winds, it also turns slightly away from the wind, via a curved track or just adjusting the wind vane, to reduce the speed of the propeller by increasing their angle away from the wind....

I personally would use mechatronics, not a sledge, to do it as I understand the concepts well, but maybe a mechanical design would also be possible for those who do not list mechatronics as a DIY hobby that they like.........

I'd design it to generate as much power as possible and to have a variable load, by switching the power in and out, to the windings... or sort of like applying onely ONE bottle generator to a bicycle wheel when the wind was weak, and as it rose in velocity, then just keep switching more and more little generators in, until you have about 10 of them in the circuit.

It can be either mechanical or electrical switching, or both.

But unless the wind is doing 300kmh, you really don't want to furl it, as that is just dumping or wasting potential power or the opportunity to harvest it.

Issues such as aerofoil design and stall over the blades in high wind with high load from the generator also come into it....

There are lots of really stupid people that say a 1.2 meter wind generator won't do much, blah blah blah....

But in a good windy spot, with really clever design - well just reverse engineer how much power would be required to drive a 1.2 meter fan driving that much air at that speed?

If your really clever about the whole thing, you would be able to make heaps of power from the unit.

Also if your in an area that you can't stick some monster up on a huge pole, consider a gang of them along the roof line or fence line - if you have wide open fields and fairly constant prevailing winds...

I thought a lot about this problem, and i still have a few questions, that someone might be able to answer me.

i would like to use a 12 V battery, a PM alternator and a charge controller with a drop load:

I have the power curve vs speed of my turbine, thereby allowing me to select an cut in speed, simply by looking at different alternator specs(Minimum charging voltage/alternator volts pr rpm).

The battery will start charging at about 9V(if its near empty) to 14V(if its near full) volts. I know how many amps that i can draw, without slowing down the turbine blades, since i know my max allowable power at certain rpm's. Since the alternator power curve(charging power), is straight, and the starting point is known, one more point should be enough to choose the alternator.

My max allowable torque is 3 nm at 800 rpm, which gives me roughly 250 watts available. charging at 9 volt(assuming the battery is empty), would mean a charging current of 27,7 amps!

Am i totally lost in the woods here?

1) so i know How much i can draw(with out slowing down the blades), but how do i know how many amps the charge controller actually draws? or how can i control it?

All this is still theoretic, so i cant measure anything!

Will the controller somehow sense how many amps it can draw, without overloading the alternator? or is it somehow preset?

34point5 was talking about the alternators power curve, which i guess must be the charging power curve. I read that one can lower the slope of the this curve, by adding more line resistance, in relation to Ohms law. Again, this indicates that a certain amount of amps is drawn.

2) My original idea was, that the charge controller should apply a steady charge rate, thereby making the power curve a straight line, by applying excess power to the drop load(not wasting power, as the drop load should be used to heat up water). Does any one know how this works in real life? how does the charge controller know how much power to send to the drop load?

i mean, the whole picture changes, once the battery charging voltage changes!

I can't seem to find any info on charge rate, on different controllers on the web.

3) Most alternators on the web only gives open voltage and shorted current at different rpm. I know i can use the voltage information, but does this information tell me anything about the actual power curve of the alternator? and how?

Anyone that can answer a few of these questions? would be a great help! sorry if its a mess, but its a bit late here.

I'd suggest you read "Windpower workshop: building your own wind turbine" by Hugh Piggot. See http://books.google.com/books?id=jLCZXvSGYs4C&pg=PA4&lpg=PP1&dq=Windpower+Workshop.

I arranged to borrow it from my local library via ILL (Inter Library Loan). I think it is still available for sale in England at a reasonable price (like 10 pounds + 30% S&H). (Some booksellers over here (the US) are trying to charge an outrageous price.)

I'd read the whole book, there is a lot of stuff that is non-electrical. IIRC, Piggot's recommendation is to build your own generator (or, at least, he gives instructions on a variety of ways to do that).

Taking 3 first. if 250 W is your max design output and 12 V is the nominal voltage then you are looking for a ~20A alternator. As said the power (watts/rpm) is virtually a straight line.

The is no 'cut in speed' in wind.

As wind energy rises, the blades turn as fast as they like, until the alternator voltage rises to 'run into' the load.

If there is insufficient wind energy to cause current to flow, it will just sit there spinning making a matching voltage to the load.

If the wind energy rises, it will cause current to flow, or force it's way up the alternator curve (until it wins over the max alternator drag/torque/output, and 'runs away')

Maybe have a look at a few Chinese sites - in general they are way ahead in this field including 'matching small turbines to PM alternators'

Here is a link

Note the graph;

I.e. you might want to look more closely at the blades as a factor in 'curve matching'.

Something with good low speed lift and monstrous drag at high speed.

GA for a good common sense post.

I think you may need to also use the generated frequency as input as this will let you know the speed of rotation of the alternator. This could be detected and used to furl the tail vane as a form of speed control that would function no matter what the load was from 0 to max......

Changing the wound rotor out for a permanent magnet will mean that the output is linear to speed = but no voltage control. But this could be useful.

Also, you will find that usable output starts at a far lower speed.....look on YouTube for further help.....the slip rings and voltage control can be removed as it will now not do anything at all.....

But if you leave the voltage control and the wound rotor in place, you will not have quite such a problem with overcharging. Voltage will be kept at around 14.4volts max, which will only slightly overcharge the battery at worst.....minor modification of the rotor current (resistor?) could drop the max voltage to say 13.2 volts!!! Which would be perfect!!

With a PM rotor, if your battery bank is large enough, the high voltage developed at high revs will allow a larger charging current to flow, recharging quicker of course....but watch out for overcharging.....fatal!!

Do not forget that regulating a PM alternator is impossible........that means the charge controller must handle the full output current!!!!

The next decision is what do you do when the batteries are full? Just removing the load may allow the assembly to over speed in strong winds. Although I have never personally done it, it is possible to have installed electronics which take the nominal 12 volts and change it to either 110 or 220 VAC and feed it back into the grid.

In some countries you will be recompensed for all the power made......

Batteries

You should have more battery capacity than you will ever need, leisure batteries are best, though more expensive, but will accept deep discharge and slight over charging better than normal car batteries.....though over charging brings nothing useful except electrolyte loss.....water. And is thought by many to reduce battery life. I am one of those.

Charging to a max of 13.2 volts means that the battery will only be around 70% full, but will probably double or even triple its life.

Never allowing the charge to drop below 12.6 volts for very long will dramatically prolong the life of car batteries. It is not a bad parameter for Leisure batteries either!! Though they are far less sensitive in this area....

There are alternators around on some cars like VW TDI engines that can achieve up to 120 amps of output. There are also 24 volt alternators on many trucks with large outputs as well. Not using the full output will mean that the alternator will have a long useful life.....

Alternators from a breakers yard are very cheap to buy......

Maybe easier to just build this...

Hmmm.....global brain freeze?