Matching the alternator to the power curve

[winston]

A basic problem with gennies as I see it is that the wind energy goes up as the cube of the wind speed but the generator out put is fairly linear with respect to rpm (I'm sure someone will correct me on this if I am wrong)

So matching the alternator to the rotor is a compromise at best-- (It has an easy start up but doesn't capture high power or vice versa)

Has anyone experimented with turning on and off coils within the stator to match the power output from the alternator to the power curve of the wind mill

that is turn on all the coils for the full rated power out put in high winds and have maybe 10 or 20% 'turned on' in low wind conditions for some generation even in 8 to 10 mph breezes with easy start up

then if the wind increases to say 20 mph increase the number of coils as the speed and potential power ramps up

with solid state devices this should not be difficult (monitor frequency and increase the number of coils turned on as a function of increased freq)

I am guessing somebody has already tried this...

I would be interested in knowing if it worked and was worth the effort

I have another idea too but I will start another thread for that

Winston we have a rule here about posting more than one story a day so why don't you confine your thoughts to this one or slow down a little rome was not built in a day.
Kurt

[Nando]

Winston:

Your idea, basically, is not sound, because, though the power harvested is to the cube of the wind speed the, RPM does not follow such factor.

Do you know the RPM factor ?.

Turning ON or OFF a number of coils makes the mill too complicated electronically and this group seems to have an aversion to electronic components.

This aversion could be caused by many factors, from not understanding how the Electrons run in the wire, or lack of experience, or what ever one may think.

The simplest the mill is best it is, I think, it is the general point of view.

Same reason why no one in this group really wants to make a Pitch Controlled hub, ( one or two exceptions).

Nando

[Ungrounded Lightning Rod]

With different wind speed comes different RPM and thus different generated voltages.  Getting this to work right would require all sorts of stuff - various interconnects of coils, taps on coils, complicated switching.  It would also involve only having some coils driving - and thus heating - much of the time, creating uneven thermal stress in a cast stator.

For a simple setup that does a decent approximation, search the board for "delta wye switch"  and "delta Y switch".  Two "gears" lets the mill stay in the rough neighborhood of ideal efficiency from a low cutin speed to the point where you have to furl due to an embarrassment of available power overheating the mill.  This covers the bulk of your operating time in some sites (depending on wind conditions at the site).  And delta has an output current of almost twice Y at the alternator heating limit for a given winding.

One downside to this approach, though, is a sudden and prolonged torque and high-current transient when it "downshifts" from delta to Y.

A max power point controller does the job right and gets the most out of the mill (minus its own losses).  But it requires more (and higher power) electronics.

[WXYZCIENCE]

Winston, Nando has it correct. Most of the time good design is a long trial and testing procedure. Electronics work is HARD. Too many people think that they can fix a badly designed alternator with a quick fix electronic circuit. It is very easy to think of a device that can handle unlimited volts and amps and be worth nothing. But in the real world it does not exist. Maybe someday there will be a simple design posted here on this site that will do it all. Then the world will beat a path to the door. There is a lot of experience on this site, and people willing to share it. Welcome on board.
Joseph.

[(unknown)]

Tapping the coils at some point is an interesting idea, monitor voltage on both taps and use the middle tap as the voltage rises to some point. Or monitor current and do the same, higher the current goes switch out part of the coil to lower the resistance. It would take some experimenting to get it right.

[Flux]

All tap changing ideas are a compromise. As others have said there is some advantage in using a star /delta change but there are limitations.

More complex tap changing arrangements will most likely be little improvement. Not only must you match the voltage for good results, you must also match the power of the alternator to the power of the blades and still maintain high alternator efficiency.

The switching becomes complex, relays are not really adequate for other than rough matching. When you start to use electronics you suddenly find that the switching is actually more complex than the far better approach of using a buck or boost converter to match the load and provide what is effectively a electronic tap changing method that maintains the whole winding with optimum efficiency over the whole speed range.

I have looked at many options over the years and I posted my findings in a diary entry about matching the load some time ago.

Flux

[winston]

Thank you all for your thoughts and inputs

I'll check out the delta Y

Thanks Flux I'll check out your diary

I could not find the rule about more than one post per day

and how come this got switched to 'storage' ?

[Ungrounded Lightning Rod]

Tapping also has the disadvantage that it leaves some of the copper acting as a non-participant.  This means the energy you ARE generating produces a higher current density in the copper that is still active, leading to a higher percentage of resistive losses.

The same number of turns would have been with thicker wire and had less resistance and resistive losses if they could have used the volume dedicated to the inactive copper.  You'd switch to a tap when the RPM goes up, in order to bring the voltage down to what you need, which means you go to less copper at the same time your power (and thus your current) goes up.  Resistive losses go with the square of the current, so using taps makes a bad situation even worse.  Net result is a drop in both your efficiency and your maximum power at the heating-must-furl limit.

[finnsawyer]

What you are proposing is a "piece-wise linear" approximation of the wind power curve.  This is a valid concept and to a certain extent an alternator can do it naturally.  With a standard Y-connected three phase, when say the voltage between leads a and b is such that current flows into the battery from a and out through b, it is possible with increasing RPM for the voltage between lead c and lead b to rise enough for current to flow from lead c changing the relationship from there on between the power output of the alternator and the RPM or windspeed.  Unfortunately, the mill is likely to furl before this effect becomes important.  In my diary concerning an alternate alternator design I discuss what I call "a center tapped" configuration for the alternator output (single phase), which could conceivably result in a three piece piecewise approximation to the wind power curve.  The first piece is the one for no current at low RPMs.

[partsman]

What about variable air gap.

Thais could be done with a spring.

The harder the wind blows the closer the magnets get.

[Capt Slog]

Ed Lenz experimented with this idea.

http://www.windstuffnow.com/main/test_page.htm

As you can see, he is undecided as to whether it was worth the effort.