"Shifting gears" to regulate wind turbine output

[trailb4u]

The concept of shifting gear ratios would only apply if you use a

wind turbine design that has a means of stepping up rpm from that

of the turbine shaft with a gearbox, pulleys, sprockets, etc...

I think this meathod would be identified as a stall regulated

system since by switching to a higher gear ratio, the turbine

blades would be rotating at a slower rpm which would reduce

the turbines ability to extract energy from the wind.

The wind turbine design I'm currently considering would already

have the ability to change between 2 generators.  One smaller

one to take better advantage of ligher winds and a larger one

to handle the heavier winds.  It wouldn't take much more effort

to add the option of another ratio for higher winds on the

larger generator input.

Is this concept used on any of the commercial wind turbines?

Thanks,

Clint

[birdhouse]

Hey trail-

in the 80's danish turbine designers used two generators, one samll for light winds and one large for high winds.  it seemed to work well!  as far as gearing/belts/pulleys/tensioners...  they typically have been proven to be unreliable and require more maint.  i always wondered about a system where a big and small genny could be used as stated above, yet both could be engaged for really high winds??

good luck!

[scottsAI]

trailb4u,

Many things can be made to work, yet the question is it a good choice?

Complexity of transmission in the generator path is questionable.

How to shift gears while running?

People have used Star to delta switching works OK, yet from Delta to star is a problem.

Variable pitch is simpler and offers more benefit as compared to other solutions.

Have fun,

Scott.

[trailb4u]

Hey birdhouse and Scott, I have heard of wind turbines which use two generators,

but I don't know any more about how they are set up.  For example, what are the

respective gear ratios for the small and large generators?  Since I live

in a marginal wind area, my goal is to make the best use of lighter winds

while still being able to capture energy from larger winds without frying

my generator.

I could simply take an existing design like the breezy 5.5 and fit it with

a larger set of blades and a matching gearbox.  The problem with that

scenario is that unless I set up a mechanism for regulating power input,

I'd quickly overpower and overheat my generator in higher winds.  Some options

discussed on this board are furling tail, variable pitch blades, blade mounted

drag flaps, etc.  I'm not saying I won't consider any or all of those options,

but what I'd like to know is how effective changing ratios on a wind turbine

fitted with an asynchronous generator would be at regulating power input from

the blades.  With this type of generator the blades run at a more or less

constant speed when generating power varying only a few rpm between low

and high output modes.  

The easiest way I can think of to change gears remotely would be to use

a belt final drive and operate the tensioners with pneumatic cylinders

controlled with electric air valves.  Unless I was kicking in a second

generator set at the same gear ratio as the first as birdhouse suggested,

I wouldn't change gears while running.  I would first stop the turbine

using a brake attached to the output shaft of the gearbox, change pulley

ratios and then re-release the brake and engage the generator when it

reaches cut in speed.  I'm looking at using a set of blades that are

about 30 feet in diameter.

If changing speeds doesn't seem practical, I'll try to match the

blades and generator(s) for max output and shut it down when the

windspeed becomes too severe.

Any ideas how changing prop rpm would affect the power curve?

Thanks again for your input.

Trail

[birdhouse]

trail-

designing a mill that can run at almost constant speed via two different gennys is quite a difficult task. i'm not saying it can't be done, but to do it in a way to ensure durability and reliability is a whole different task. as mentioned above, the danish did it, but they had teams of highly educated individuals all collaborating on one project, not to mention having resources that make our shops look like childs play.  

now add a 30 foot diameter blade...  wow.  good luck!  i'm not positive, but i think the largest mills built by folks on this site are around the 20 foot range.  

not meaning to be pesimistic, just doesn't sound like a project i'd want to touch with a ten foot pole..  

best of luck!

[scottsAI]

Clint,

Since you mention breezy 5.5, you understand how it works?

Blade is shaped to support constant RPM in varying wind speeds to drive induction generator (motor).

Nice idea, yet, efficiency suffers... yet that is how it can work. The focus was on the lower wind speeds.

Designed a 24 foot induction motor wind turbine couple years ago.

Area has a few hours each day of 14mph wind, with 8 mph and below for rest of the day. The few hours at 14mph delivers more power then the lower winds. 5:1

Power Profile of a blade based on RPM is a hill. Peak is when operating at designed TSR. As the blade speed is above or below efficiency drops, to zero at 2x speed, trails off to zero at zero RPM:)

Variable pitch fixes the TSR efficiency issue over a wide range of RPM.

Complexity of variable pitch is on par with transmission (I felt). Extra benefit is no furling is required, place blades in line with wind to stop them. Felt system could handle 70mph wind and keep going, kept the blade pitch speed fast to deal with gusting winds. Used active servo for turbine direction, as a last resort could use it to furl system.

Looked into CVT transmission, interesting, did not get the warm and fuzzes from the design.

Good luck,

Scott.

[trailb4u]

Hey Scott, I do understand the breezy fundamentals and if I lived in an area

with higher average wind speed, I'd build one to spec and want nothing more.

Since I live in a marginal wind zone, I'm interested in tweaking the design

to take more advantage of lower wind speeds.  To get more power from a

breezy design at lower wind speeds would be as simple as fitting it with

a larger set of blades and an appropriate ratio gearbox.  This solves

one problem and creates another, since the blades would collect more wind

energy than the generator could handle in moderate winds requiring the brake

to be set shutting down the turbine where it would otherwise keep on producing

power.....bummer.  Then it occured to me that shifting between different

gear ratios could be a solution worth considering.  Basically the faster the

wind blows, the slower the turbine blades would turn, regulating power input

from the blades.  Then I figured if I were to go to that much more effort,

it wouldn't take much more to switch between 2 different generators.  One for

lighter winds, and one for heavier winds allowing much more electricity to be

generated when the winds are generous.  To keep the design in my comfort zone

I need it to be as simple and cheap to build as possible while still meeting

my minimal requirements.  So for the shifting between ratios,(and generators)

I'd prefer to go with multiple v-belts and belt tensioners actuated

by compressed air cylinders.  Yup, more complex, but since I'd likely be

building my own PLC controller anyway, it wouldn't take much more effort to

program it to control the extra features.

I looked here on the board to see if you had posted a story about your

24 foot induction motor wind turbine and couldn't find one.  Did you

ever post information about it?  I'd be interested to learn more about it.

I'd be especially interested in your design for the active servo for turbine

direction feature.

If you ever check back on this thread, let me know.

BTW do you live in Michigan also?

Clint

[zeusmorg]

 I can see several ways to design and build a windmill to increase output in higher winds, yet allow for a lower startup speed.

 Several schemes have been tried and used. A variable pitch prop was commonly used in them, and the jacobs design still incorporates this.

 Gear changing has also been done, and if you used syncromesh gears it could be done under speed, although it would be hard to design and implement in a low output mill.(braking would also have to be incorporated)

 Any design of this nature will complicate the overall build, and costs involved, so it is a matter of cost, and reliability over the additional output gained.

 If i were to design a multi-speed ratio system, I'd most likely use variable pitch pulleys, however you do have the friction losses to overcome, and the same can be said for gearing, so is the gain enough to sacrifice the friction losses?

 One system I have thought about, and have never seen implemented which would be possible with the axial flux design is a system that would change the air gap on the magnets. Have one set with the ability to slide on a keyed shaft, with centrifugal weights that would close the air gap at higher speeds. We all know if there is less flux, the windmill will start up easier, and if the flux increases that the output is higher. So I could see this system as practical and solving several problems without friction losses involved. The only issue is a more complicated build and designing the system to be reliable. It may even make MPPT more practical.

 Anybody willing to build one?

[RandomJoe]

Sticking my nose into something I know little about, but what the heck...  Maybe I'll learn something!

What kind of generators are you planning to use?  If the axial types described here, seems it would be simpler to build two - one lower-power earlier-cutin the other higher-power and mount them on a common shaft.  I'm not sure how open-circuiting an alternator affects things though.  Would the low-power one survive if spinning way overspeed but open-circuit?  I'm assuming it's too much current that actually burns up the coils (unless you hit breakdown voltage of the insulation).  Or would the high-power one still induce drag at low speeds when open-circuit?

Switching could be tricky, might want something to "soft-load" between the two...

All the large (power company sized) turbines around me use variable-pitch on the blades.  Amazing they can generate what they do for the (apparent) slow speed they turn!

[trailb4u]

Hey RandomJoe,  the generators I plan to use would be 3-phase induction motors.

These are only useable if hooked directly to the grid, they won't work as a

stand-alone generator off grid somewhere since they require the grid to

energize and sychronize the generator to provide grid quality electricity

without the need for an inverter.  

You are right, switching could be tricky if one were to try to switch from

one to the other while the blades were turning.  I figure the simplest way to

shift from one to another ratio or generator would be to stop the turbine

by engaging the brake and shutting down the generator before making the change.

From what I gather regarding the axial flux permanent magnet alternators, it is

preferred to avoid using a gearbox alltogether.  These direct drive alternators

have been built with a 20 foot dia blade set, but with larger turbines it

may eventually be desirable to use a gearbox to increase output shaft speed.

Thanks for the input,

Clint

[scottsAI]

Trailb4u,

Just noticed your reply. We will see if you notice mine!

Yep Induction motor Design located in my file space: http://www.otherpower.com/images/scimages/3276/10kw_system_design9.pdf

Focus is on lower wind speeds, yet willing to use higher.

Live in Brighton, MI. Send me email if you have questions about design. Requires passing IQ test:-)

Design based on: Keep It Simple Scott. Cost is king.

Fixed pitch blades (or fixed TSR) from cut in (5mph) to 20mph, RPM changes by 4x.

Gear box must contend with at least 2x to get good power at low speeds and mid speeds, loosing extra power at higher speeds.

Many ways to do a thing, some better than others, yet important to stay within comfort zone.

Induction stand alone. Not the best generator, yet can be made to work. Above 10hp are a cheap generator.

I work with electrical models in my job (engineer), create missing models.

Working for 2 years on PMA wind turbine model. (very close)

Much miss information on wind turbines out there, lots of good stuff too.

Have fun,

Scott.