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Windmill Generator - why won't a small brush-less motor work as a windmill generator? Answered

I bought a small brush-less motor that was designed as a motor for a small RC plane. I tested it by spinning it with a drill. I was surprised to find that the motor did not generate much in the way of voltage. Why not?

It's too bad - many small brush-less motors are so nice. No brushes, ball bearings, and low cost. I would have thought that forcing a brush-less motor to spin would have generated power.

Thanks for any insight.


yes im currently building a project i use brusless boot motor 2kw generator


1 year ago

I'm currently building a project that will use a brushless motor as a generator and have already bought the motor, which is a 300KV motor from a quad-copter.

Though I have just found this, which is actually designed for the job, please check the link I have supplied..


Also from the same website was this one, which is for a wind powered setup..


Did you measure the voltage in AC or DC? Because it only shows on the meter as AC.

i have a gasoline engine that runs up to 12k rpm i want to make a generator what brushless motor should i use please give me links if possible thx

Would this work underwater?

KV. it wants to be very low, like in the 100kv or less range. KV is related to RPM's and wind turbines run at ~1000rpm. If you turn a 12v motor into a generator for the same RPM you will get in the range of 6v out. Really you want the turbine to make usable power (say charge 12v batteries) at maybe 300 rpm. At a guess you are looking for a 25kv or less outrunner (lots of thin wire turns). Next thing is the diameter, make it big, its a volumetric equation (hint).. You will probably have to rewind for this... Nice thing about outrunners are good magnets and LRK magnet/stator ratio. Attach one to a battery drill with some rectifiers/diodes and see what it does. This is all 'seat of my pants' & at a guess. Thinking about it myself, I have played with normal (12/12 ratio) BLDC but the magnets suck. Gd luck.

Noted. Concerning your comment about stepper, I tried just yesterday a NEMA17 (42X42X47 mm - 350 gr) connected to my drill, @3000 rpm gave 5,5V - 1,15 A under small load (20W bulb).
Not bad for my purpose, as to charge a phone I just need 5V -1 A. Anyway to stay on the safe side, I purchased an outrunner (KV=50) and some wire 0.2 mm.
By the way, do you have an idea how the geometrical dimensions of a motor affect the performance? I noticed in the market there are motors very flat (like pancakes) with large diameter and motors with smaller diameter but taller. Considering them motors, the shape probably affects the torque, but considering them generators this could affect the number of wire turns (id est the voltage). Any suggestion? Thx

Hi! I have a spare out runner motor designed for drones (KV 900) and want to give it a try to charge my phone (5V - 500 mA). KV 900 means that theoretically by giving 1 volt through electronic board it should spin 900 rpm. Is vice versa true? I calculated that a bicycle running @25 Km/hr (a performance even I could exhibit) the "motor" - "generator" would spin at 5000 rpm,which theoretically means around 5 volts. Downstream the BLDC there's a diode bridge, with some losses. Does this reasoning sound good? Any tip/experience appreciated. Manlio

Hi. I tried the stepper motor setup and yes it does produce voltage but you NOT get any usefull current from this due to the deign of a stepper..

Throw it away and use a BLDC.

RE kv, the lower the KV the slower it turns as a motor, if you use it as a generator then you will get about 1/2 the voltage out at the same rpm. Low KV is what you want for a generator.

Say 50kv motor at 10v = 500rpm means as a generator at 1000rpm you should see 10v open circuit back.
You are suggesting 5000rpm, so 50v open circuit is what you will expect.

The voltage will drop under load, so as a ball park estimate this may work ok.

Re windings, space is limited in BLDC and there is a trade off between gauge and number of turns. More turns = higher output voltage and heavier gauge wire = more cuttent. You want lots of turns of thin wire to get output voltage up. Output current may only be 1amp or so for your setup.

Yeah it could work, I guess you are talking of running it as a Dynamo off the tyre, this will definately help keep the revs up. Have a search on instructables to see if its been done already and maybe commercial products as I have seen it before. I recently say some BLCD designed for gimball use, they are flat pancake style with a larger diameter, interesting thing is they were 48kv. As the key thing is to get the voltage up first going to low kv is the trick, once the voltage is up its easy to control the charge rate.


Thanks, I tried with a stepper NEMA17 and it works! Too much may be, connected to a drill @3000 rpm provides 1.2 A - 5,6 V. Open circuit guest sky! Had a glance to gimbal, actually they have very low KV. For my personal background, do you know a web site where to read something about the relation between rpm, turns of wire, wire size, etc, in case I have to re-wind something? Thx a lot for the advice.

The problem with a stepper is that they have a lot of no load torque. In other words, try turning the shaft with no electrical load. You can feel each step of the motor fighting you.

No good ideas for web sites to visit. But I would avoid winding your own. I've got a motor in my junk box cause I tried to wind one myself. Just look for a gimbal motor with a low kv rating. There are a bunch on aliexpress.

Good Luck,


Well, I finally mustered the never and cut into my motor windings. I removed all the old copper windings and replaced with brand new colored magnet wire from GoBrushless.com. The new magnet wire cost me $10 bucks, by the way. However, I now have a ton of magnet wire (120' of red, green, and gold). 

The old windings were wound in a delta pattern using multi-strand bundle of 5 wires with 6 turns on each tooth. So really, there were 30 turns of wire on each tooth.

The new windings were wound as a star, or wye, pattern using 25 turns on each tooth. 

The old winds produced 0.25 volts rms across one phase, open circuit. The new windings produced 2.0 volts rms, open circuit. Both results were achieved using a drill motor to spin the motor at 3600 rpm.

So, going from 6 turns delta to 25 turns wye gave an eight fold increase in voltage, at the same rpm.

With more practice, I think I could get all 30 turns back onto each tooth. That might boost the voltage to maybe 2.5 volts rms.

So finally, to answer my own question, the reason small brushless motors are poor windmill generators is because they are too small. Even rewinding the motor into a wye pattern and using the maximum number of turns the motor still needs to be spun at over 3000 rpm to light an LED. And gearing is not so good because the motor already has a lot of cogging. Startup torque would be awful if there was a 5:1 ratio in front of the motor. 

Anyway, it's been fun. Nothing like rewinding your own motor to make you fell like a true EE!

Thanks for all the comments,


Interesting! Being more realistic, I'm going to adapt a spare drone BLDC as a generator to charge my phone while riding my bicycle: to do this, I just need 5 VDC - 500 mAmp. Did you take any measure of mAmp after rewinding your motor? Which were the results? Thx. Manlio

Take care with this, you can get electric shocks, rewound smart drive turbines will kill you.

I don't know if has been said yet, but if you check out how to turn a brushless computer fan into a generator, you may realize that the motor you are using needs a similar modification to work as a generator.

Outrunners do make great generators just watch this you doubting Thomas's


if this is simply a science project that you want to have practical use this is a useful link, https://www.instructables.com/id/DIY-1000-watt-wind...

if you want a turbine to live off grid with no annoying problems that go with building it,


as creating something from an rc car motor, yes it is possible, however what you need to do may vary slightly from one motor to another. i would advise either getting the supplies and building yourself a large one from scratch or simply purchasing one. i know the fun of modifying things to suit your needs, but i also understand that the amount of effort you will put into making one would be better spent either going from scratch or purchasing one. you will probably get much more power and if created correctly probably get something more reliable. good luck

-tom ╦


6 years ago

I know this is an old thread, but in case anyone is interested, the problem is simply RPM
KV is the number of RPM the motor develops per volt, and conversely indicates the number of RPM you need to spin the motor at to produce a volt.

If you're wanting to use a small BLDC motor as a generator, you'll need to find a way to spin it really fast, or alternatively find a much lower KV motor. Some very large RC motors can be found in the 100-200 KV range, and would be much more appropriate for use as a wind generator. (they're quite a bit larger and more expensive though)

They're going to be much larger, stronger motors though, so you'll probably need to run a bigger prop to get over the cogging. The upside is that a bigger prop will be collecting a lot more energy.


7 years ago

Well, brushless doesn't mean a motor without brushes to transfer electricity to a inner drive. To me it seams that it uses triple phase AC. This means that it has 3 sets of coils that run with electricity in high speed switching directions. You may be able use a bridge rectifier to make it into usable DC.

A brushed motor has a rotor inside with a magnet ring on the outside, this allows DC current to be made because of the way the magnets rotate relative to the coil. In a brushed motor, coils are wound and mounted parallel to the rotor, this means that the coil is stimulated whenever passed, in a brushed motor the coil is wrapped like lights on a tree, around the center. This allows a constant flow by pulling energy around and around. It is like pulling a magnet around a table with one underneath. If you move the bottom one left to right and the top one is only allowed to go up and down it will be stimulated a different direction with every pass, if the top magnet is allowed to go left to right then it will fallow the bottom in one long stream.

I am doing a science fair project that involves a solar panel, connected to a charge controller, connected to a 12V battery, connected to a switch system that can either go to a flywheel with a 12V DC motor or straight to a DC to AC inverter which leads to light standard light bulbs. The system worked (light bulbs lit) when I connected the panel to the controller to the battery to the inverter. However, when I had the battery power the flywheel and then have the flywheel output its energy to the inverter, the inverter shut down and would not work (red alarm went off). The flywheel is producing more than 10V (starts at about 11V) for at least 30 seconds, and the inverter's low voltage shut down in 10.5 plus or minus 5 volts. So why doesn't it work? Is the inverter messed up or is the flywheel outputing AC power? Also, when I connected the solar panel to the charge controller to the flywheel, the charge controller completely shut off. Are charge controllers not able to be directly linked to motors? And if I tried to directly connect the solar panel (which has 20V) to the flywheel (with a 12V motor), without a charge controller in between, what would happen?

I think you may want to try your original experiment with the lights again to see if your inverter is messed up, also your inverter may have a higher wattage than your motor can produce, this may send it into the same thing low voltage would. You may need a bigger DC motor. If your solar panels are 12V DC and your motor is to I don't see any need for a charge controller, I don't think you need one in the first place, most people use them only for dealing with a battery bank.

I've never had any good luck with power inverters. I recently tried to run a 1/4 hoarse motor off a 1000 watt inverter. Should work in theory - in the end it just let the smoke out of the inverter.

My guess is that the inverter draws a surge from the batteries at start up. A solar panel would not be able to supply that needed surge.

In terms of the motor / solar panel - you gotta match the solar panel to the DC motor. A 15 watt panel is not gonna start a 1/4 hoarse DC motor. The solar panel will have a sticker on the back that specs. the amount of current. Try running your motor on a variable power supply at that same current. If the motor does not run then it sure won't run on the solar panel either.

Same goes for the fly-wheel. Gotta match the fly wheel output to the load. But again, the fly-wheel is not able to supply the surge the inverter needs.

Hope this helps,


get some ideas here, easy stuff

making my own out of neodenium magnets, no power needed to spin magnetic motor, but i have to buy 216 1"X1/2"X1" dowel magnets to embed them into acrylic rotors,
if you use the brushless motors you will have to add a lead and this involves separating the armature and adding another post then having a control or trigger, i could use a stepper motor using a small trigger device to keep it going thus allowing power generation with magnets over a coil, kinda like the Bendini Motor circuit, wish me luck cause tomorrow im going to turn the plates down

Good Luck!

Post some pictures when you get something working.


How many R.P.Ms does it give under a certain voltage/current? It should generate quite a bit less than that when spun at the same speed because these are not designed to be used as generators, and are much more efficient being used as a motor. Not that it won't work. I recommend going to some auto repair shop and asking for an alternator, I've gotten a couple for free and some for 10-15 bucks. Just wire it in with a diode and windmill away, make sure to use gears on the windmill to help it spin faster.


7 years ago

Maybe this thing will do the trick. http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14427


Use a stepper motor from a printer or scanner they have six wires but can be pinned out with a LED. After you have found the two wires you can attatch them to the ~~ side of a diode bridge and hook what ever you want to power to the + and - sides and then use that as a windmill I know this because I have tried and this has worked. For blades use a desk fan blades attatched to the shaft of the stepper. Hope this helps :-)

Try a network of diodes like this.

brushless to rectifier.JPG

I don't think the diodes are going to work. And here is why - I just spun the motor again while connecting my voltmeter between one phase. On the AC scale, I only measured about 0.25 volts at 1000+ RPM. Not enough voltage to be able to use diodes.

I also tried using the current scale on my voltmeter. Again, on the AC scale, I measured 200mA of current. So, with a dead short across one phase, I can get 200mA.

So the results are, open circuit, one phase produces 250mV @ 0mA. With short circuit, we get 200mA @ 0V.

I think maybe the problem is, the motor is designed to produce torque. The magnets are not that powerful. The coils produce the vast majority of the magnetism. The magnets just get pulled around by the coils. By back driving the motor, the weak magnets don't induce that much power into the coils.

I'm guessing, to make the motor good as a generator, the weak magnets would need to be replaced with very strong magnets. Also, I worry the coils don't have enough windings. Again, the coils are designed to produce magnetism. Instead, the coils need to be designed to accept a rotating magnet field and induce a current.

I'm just guessing on most of these points. What I can tell you is, back driving a brush-less toy motor does not produce much usable power.

No. That's a 350 W motor, it has a motor constant of 3500 KV, and its designed operating voltage is from 4 to 14.8 V.

That means that the thing can run between 14000 and 49,000 RPM, unloaded. Its not TERRIBLY surprising you can't get more than 0.25 volts off it at 1000RPM.

You really need to read up on motor operation ! A motor is a generator. Its even a generator whilst its being a motor. You DON'T need to change anything, you just need to run it the way it needs to be run.


OK, OK, I get it.

My question, after all, "Can one of these little brush-less motors be used as a generator in a windmill application?" The answer appears to be no.

Also, have a look at this:

Now, this guy has done some homework. It's gonna take me some time to digest this.


JIM: After reading some of the remarks and the replies, I see the basic problem that is not addressed properly. The Motor designed to operate at extreme RPM for small airplanes in the areas of 15,000 to 20,000 RPM with 4 to 12 volts or so -- with a constant of 3500 RPM/Volt is indicating that the motor to operate as a generator has to be spinned above 8000 RPM to produce enough voltage to light an LED, this is 1.8 to 3.8 votls depending on the LED type from RED to Ultraviolet ( white).

A quick glance and that instructable looks great.  Lots of good info there.  For any motor to be an efficient generator, it should spin at or near it's operating speed.

See the difference ? Its all to do with operating RPM. The hub motor is NOT designed to spin at many K rpm !


Spin it faster!

 You can make a generator out of any motor. See my instructible "mini dynamo". It shows you how to turn a motor from a walkman and an LED into a quick hand crank light.

Yep, I've done that before too (connect a LED to a toy brushed motor). I recently showed a bunch of Cub Scouts and they were all amazed. Being boys, they all had to compete and try and see who could get the LED the brightest.

However, what I was shooting for was a brushless motor. I loved the idea that it would last a really long time since no brushes were used. Plus, the brushless motor seem to be better made (mine uses ball bearings).

But sadly, the toy brushless motors made for remote control (RC) stuff are just all wrong. They are small in diameter, have delta windings, and have cogging problem. Just no good for a small windmill application. Note, there are larger brushless motors available for RC stuff. But the price goes way up.

My next idea is going to use a power steering motor. Delphi make an electric power steering system that uses a large brushless motor. The Delphi motor is 3 1/2 inches in diameter and four inches tall. There are millions of these motors out there in junk yards! A bit large for my tastes, but since I already have one I guess I'll make do.

Thanks for the comment,

Brush-less DC motors are like a stepper motor but have internal electronics.
You could tap and rectify each coil.

Not sure if i have already posted on this question if not here goes, I have a 24 volt DC brushless motor that will produce voltage but It has to be spun at a very high RPM inorder to do it when spun at approximately 30,000 rpms as with a dremel tool on high it will produce approximately 96 volts which is enough to make a  110 volt AC light bulb light up fairly good!

I'm pretty sure brushless motors don't work period. It has to have a brush for it to have the same configuration as a generator. Brushless motors work differently than othe motors.

 LOL :D   sry no offence but the "I'm pretty sure brushless motors don't work period." sounded funny to a guy skimming through the ible site. :)

Have a look a this guy's instructable:

I learned a bunch from reading about his project.

One of the neat ideas from reading his work, a motor is really a transducer. It converts between torque and electricity. The transducer works in both directions too. This is what makes regenerative braking work on hybrids. In other words, the hybrid uses the motor to push the car forward. But, when braking, the motor operates as a generator and converts torque back into electricity that is pumped back into the batteries.

In terms of using a brushless motor as a good generator, the jury is still out. My toy BLDC (brushless DC) motor was not wound well for use as a generator. The windings were wound in a delta pattern using five parallel strands for a total of six turns on each tooth.

I've recently ripped all the windings off my motor. My plan was to reuse the copper to rewind the motor. However, I kinda messed up the strands by pulling them off the stator. So, I've ordered new (multi colored) magnet wire. I'm expecting the new wire any day now.

My plan is to rewind the motor in a star (Wye) pattern using a bunch of turns. I expect to get at least 30+ turns on each tooth. This should make a big difference in the output of the motor / generator. Once rewound I'll back-drive the motor again and see what we get.


 What doy uo mean by  brushless motor?

Do a google search on "BLDC motor". The first link to Wikipedia is good. 

The best part of BLDC is, of coarse, there are no brushes. The down side is, there are no brushes. So, kinda good and kinda bad. 

With no brushes the motor will last a very long time. There is only one moving part. So, as long as the bearings are good the motor will work. With no brushes, there is no friction either.

However, without brushes, the motor must be driven with a microprocessor. The micro switches the windings on, and off, to pull the magnets around. By controlling the switching rate the speed of the motor can be controlled. Many folks call the micro control box an ESC (electronic speed control).

It's easier to deal with the BLDC motor as a generator. The motor, when forced to turn, outputs a nice three phase waveform. You would need a scope with at least three channels to see this waveform. Without a scope, just use good-old google on "three phase waveform". Again, Wikipedia is the first link and they have some good info.


Your brushless DC motor works in a way more similar to a synchronous three-phase AC motor than like a single-phase brushed DC motor.  It is still fed by a DC power source, but notice that in order to operate it requires a different type of ESC. The waveform of the brushless ESC is very similar to a three-phase AC, but with a lot of harmonics (more square wave, sorry if I am getting too technical).  When used in generator mode, expect the output to be similar to a three-phase synchronous generator. There are many ways to play with the output voltage of the generator. The easiest way is to connect the windings in Wye rather than in Delta.