Friday, August 2, 2019

Micro Wind Genny

Who didn't play with whirlygigs as a child?  I sure had hours of fun with paper and wood twirly whirlies, but I frequently wondered about making something bigger, that can produce useful power.

Fifty odd years later...

Little Alternator with 6 LEDs

A small three phase alternator that can be had for the princely sum of about $5 from aliexpress.com, can produce a few Watts of power.  Twirl it by hand to light up 6 LEDs.

I took it to our production manager Siegfried Losch, who is always willing to play with a new toy and we stuck it into a drill press.  With no load, it generated 14V AC rms when running at 3000 rpm.  So it can charge a 12V SLA motor cycle battery when hooked to a high speed engine, but it will not do much with a wind rotor that will only turn at about 200 rpm or less.

If you are content with 6 or maybe 12 LEDs to light up a little tree in your backyard, then you can skip all the electronics below, get some balsa and go carve a propellor.

If you are like me, then you can invest/waste (depending on your point of view!) some more money and time and build a boost power supply to charge a battery in your radio shack, or power a SatNOGS ground station with it https://satnogs.org/

Boost PSU
Assuming that the alternator can be spun up to generate at least 5V DC, you can build a little boost PSU using a triple five timer and a coil.  One can buy a fancy switcher from Linear Technology, but next time you want to do the same, the IC you used before, is guaranteed to be obsolete.  I like learning new things, but I don't like having to reinvent the wheel all the time.  The venerable 555 timer however, has developed a life of its own - similar to the 741 op amp - somebody will always make them.


NE555 Boost PSU Circuit

With this simple circuit, one can get almost any output voltage to power a toy, from 14V to charge a battery, to 160V for a Nixie tube.  Just change the 18k resistor and use higher voltage output capacitors.  If you play with high voltages and thermionic valves https://www.aeronetworks.ca/2015/02/cool-amplifier.html, then I can assume that you know what you are doing and don't need to look at this circuit - but those who don't know what they are doing yet, may find it a shocking experience...

Before you start building this, please put a rubber carpet on your workshop floor.  You won't regret it.

The components are not critical, but you need to take care to make the design efficient, otherwise you will not get much power from it - if any.  The most important parts are the MOSFET and the Diode.  Both of those are typical parts of switch mode PSUs - ultrafast, high voltage devices.  The electrolytic capacitors should be at least 25V rated, preferably high ripple current types.  The coil is 100uH 5A, or you can wind 30 turns on a ferrite bobbin and see how it goes.

Boost Switch Mode Battery Charger

Note that a two transistor astable will do the trick also, but its output waveform is not very square, which will cause the MOSFET switch to turn on slowly and heat up.  You would need to add a push-pull driver pair to it, making it four transistors.  Here is an alternative improved circuit which adds a few diodes to square things up: https://arxiv.org/ftp/arxiv/papers/1201/1201.1819.pdf It would also need a push pull pair - so it quickly becomes more of a chore than a 555 timer.

The parts in the schematic are seriously overspecced at 5A.  This should ensure long life, but if you short the output, the transistor will immediately blow up and adding a fuse won't help.  So do try not to do that and when you buy parts, get extras - you may need them.

To say again: Before you start building this, please put a rubber carpet on your workshop floor.

How it works
The 555 timer runs as an astable oscillator which turns the MOSFET on/off.  When the MOSFET turns off, the coil generates a high voltage which pumps through the high speed diode and charges the output capacitor.

The ideal switching speed should be fast enough to avoid saturating the coil, but not so fast that the diode/FET becomes inefficient.  All depends on the input/output voltage and current.  I expect having a 5V input and 14V output with a current of maybe 100 mA.  What I'll actually get is anyone's guess.  The 555 timer is tuned for 10 kHz to begin with - a nice round number - and round numbers are always wrong...  Well, it turned out to be good enough.

NE555 Oscillator - 10 kHz

The feedback circuit is optional.  I added it to limit the output voltage when there is no load, to avoid blowing up the output capacitor - set it to something between 15 and 20 Volt.  You could do the same with a 15V 5W zener diode crowbar.

The NPN transistor will turn on hard, once its base to emitter voltage reaches about 0.65V and pull the Control input down, the timing capacitor will Discharge and the Output will drop, turning the MOSFET off.  Tweak the little 1k trimmer to limit the maximum output voltage to something above 14.4V that won't blow the 25V output capacitor.  When you hook up a small 12V Sealed Lead Acid battery, the voltage will drop and will eventually stabilize at about 14.4V when fully charged.

Since this is a micro power system with a very small wind genny, it will at best only trickle charge a SLA. One could therefore hook it permanently to a SLA, in which case the output capacitor and feedback circuit are both not required.  The chemistry of the SLA will limit the voltage to about 14.4V and it can stay connected forever.  However, if one would disconnect it, then a high voltage will result, so it is best to develop the PSU as a standalone module that can operate without sparks and smoke, on its own.

Boost Switcher Test
Testing the circuit is tricky, since without a load, it doesn't run (only runs for a few cycles, then stop) and putting a big load on it right off the bat is not a good idea either.  So keep the 555 timer output open circuit, till you are sure it runs, then keep a scope on the 555 output, connect it to the MOSFET and observe that it stops oscillating almost immediately.  Try different loads and tweak the trimmer, till it stabilizes at about 15V.

Note that your oscilloscope should be floating.  Cut the earth wire in the power lead and put a sticker on the plug to identify it as such. I always get annoyed when people steal/borrow my scope power lead. The culprit then has a suicide cable - Justice be done I guess...

With strip board, take care that you don't make a short circuit somewhere. A thorough cleaning with alcohol and an inspection with a magnifier is recommended before you turn it on.

Hooked to a bench PSU set to 5V and current limit set to 200 mA initially, with a SLA as a load, I pushed the Vero Board circuit to 2A (24 Watt) and the MOSFET hardly got warm, so this circuit is very efficient.  The input voltage stabilizes at the minimum voltage that the 555 can operate at, which is about 4.5V, below that, nothing will happen.

Once it is working properly, clean it with alcohol and then lightly spray it with conformal coating (e.g V66) to keep it working.

This circuit seems to be efficient enough that you could use it with a better motor cycle alternator, but don't hook it to a 1 kW generator, unless you are a masochist...

Rectifier
The output of the alternator needs a full wave rectifier, which can be made from 6 diodes.  Again, in the interest of efficiency, don't use garden variety rectifier diodes.  You need 6 Schottky diodes, which have a much lower forward voltage drop than the ubiquitous 1N4001. MOSFET Ideal Diodes would be even better, but too expensive for a toy maybe.

Schottky Diode Rectifier

I have an amazing collection of parts in my workshop, but I am fresh out of Shottkys,  which is why I started with the PSU, while I wait for mouser.com to deliver my diodes.  The SB5100T https://eu.mouser.com/ProductDetail/Diodes-Incorporated/SB5100-T, will do.  I don't expect to get much current out of this little genny, so a 5A diode should not be stressed and last for a long time.

One thing I learned over the years with hobby electronics, is to always seriously overrate the parts.  For a a little toy, I don't want to spend much time designing and usually just use a first order thumb suck.  Therefore it is easy to overlook something and end up with disappointing smoke signals.  However, if one overrates the parts by 5 or 10 times, then it either works or not, but usually doesn't go pop.

This circuit will work fine in normal weather, but if there is a storm, the alternator could possibly generate more than 18V and blow up the 555 timer.  If you are worried about losing 40 cents, then for another 30 cents, you could add a big fat 15V, 5W zener diode or two as a crowbar, to clamp the output and prevent sparks and smoke in the PSU: https://eu.mouser.com/ProductDetail/ON-Semiconductor/1N5352BRLG

Slip Rings
A wind genny doesn't need slip rings.  The wind normally doesn't go round and round and if you ever get caught up in a tornado, then you will have more to worry about than a few twisted wires on your toy wind genny.  Just run the wires loosely down the post (or inside the post if it is a pipe) and let it be.  It will not wind up hundreds of times.

If you put the rectifier diodes at the genny, then you only need two wires - a copper saving of 33%.

Carving a Prop
Most of the fun is carving the prop.  If you have never done it before, it is very easy.  You get a piece of soft wood and carve it, till it looks sort of like a prop - that's it.  For good measure, balance it with a small piece of solder.

Just bear in mind that a wind prop is opposite from a model aircraft prop.  You want the leading edge to lead, the trailing edge to trail and the wind to blow onto the flat underside of the wing.  The curved side should be leeward.

If you want to go seriously scientific, then you can design a fancy aerodynamic wind prop, thanks to the Berlin Institute of Technology http://www.q-blade.org/

Q-Blade Wind Prop Designer

If you are like me, then you can wing it, but it will help if you keep a few things in mind:
  • Any blade shape will work, since the wind is free, but a proper aerodynamic shape will not make noise.
  • For a small rotor, the optimal pitch is 4 to 9 degrees angle of attack, so I use 6 - which is about what a helicopter uses for take-off. 
  • The efficiency of a blade depends very much on the trailing edge, which must be thin and sharp.
  • As for the profile, if it more or less looks like a wing, then it will work.  I use the NACA-TS (thumb suck) profile.
How big should the blade be?  Well, that is why you have two thumbs, one for the profile and one for the size, but here is a calculator to make it easier: https://rechneronline.de/wind-power/ and here is a study of the optimum pitch angle for small rotors: http://www.jgsee.kmutt.ac.th/see1/cd/file/B-034.pdf

I decided to make a test blade 300mm by 50mm and slightly S-shaped, cut from a Japanese Daiso store bread board.  After about an hour of carving, it looked like this:


Poplar Test Rotor

A power tool will make quick work of it, but with a lot of dust.  Living in the desert is bad enough, I don't want to breathe wood dust also, so I used a carving knife.  A practical rotor that will generate about 40 Watts in a soft breeze of 5 m/s, would need to be about 600 mm in diameter - twice the size of my amazing carving skillz test blade.  So I have to make a bigger balsa wood laminate and try again.

There are many prop carving tutorials on the wild wild web, but all you need to do is glue a few sheets of balsa together with white glue, drill a hole for the shaft - glue two washers over the hole, draw some guide lines, get your jig saw and sharpen your knife:
http://www.gryffinaero.com/models/ffpages/tips/propcarve.htm

The poplar test prop proved to be too heavy - too much momentum.  So, get some balsa and go for it!

Postscript - Automotive Alternators
I have on occasion, wondered why people don't use low cost 2nd hand automotive alternators for wind generators.  The usual excuse is that they don't work well at low speed and therefore needs a gear/pulley system to increase the speed, which is a mechanical complication.  Others point out that they need a field current, which make them inefficient, compared to a permanent magnet alternator.

Some people will go to a lot of trouble to modify an automotive alternator, to fit permanent magnets to the rotor and rewind the stator, to make it more efficient.

However, the wind is free and old alternators are almost free, so efficiency doesn't matter much!

The real problem is that at very low speed, the amount of current used for the field, may exceed the current that can be generated by the stator, plus the rectifier diode losses, so that at low speed it may not generate any net power.  That is true for a simple control circuit of the type that one finds built into the back of an automotive alternator.

If however, one would replace the prehistoric built-in rectifier/controller with a system that monitors the rotational speed and when above a minimum speed, boosts and pulse width modulates the field current at the optimum level (about 2A for a 50A alternator), in order to extract the maximum amount of energy out of the wind, without slowing the rotor down causing it to stall, then an automotive alternator can be made to work at low speed, without resorting to the use of rare earth magnets.

I will eventually get back to this, just to prove the point.
;)


Herman


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