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Does anyone know how to convert the three phase output from a brushless motor/generator into straight DC voltage? Answered

I would like to pedal the brushless motor/generator to drive a propeller powered by the resulting voltage

Thank You?


You will need to be able to get the alternator up to around 3000RPM.

You are capable of outputting about 1/2 hp this is around 340 watts and you wil have difficulty in maintaining this for any length of time so your not going to generate as much electricity as you may think.


Thanks for the reply
Allow me to add more detail. I would like to produce up to 200 walts, but only for a about 2 minutes.. The goal is to used the generated powered to turn a large brushless RC motor turning a propeller. which is used just to extend glide of a human powered aircraft.

Pedals to generator to converter to Electronic Speed Controller to Brushless motor

Your not going to generate enough power.

To get your HPP airborne your going to need around 200 - 250 watts this doesn't leave enough over to drive your generator.

You can only get out of a system what you put in so if you maximum power input is 1/2 hp that's all you can take out (less any electrical/mechanical losses)

Your best bet is to use a battery pack BUT Human powered flight is ALL about weight. If you add weight your putting up the power requirements so the battery/motor and generator give you more disadvantage then advantage.

the cct I supplied is a standard car alternator charging cct.

Again, thank you for taking the time.

I don't understand your comment 'doesn't leave enough over to drive your generator.’ If I use a large brushless motor design for the RC community, that runs at about ¼ horsepower and ‘pedal it, putting in the ¼ horsepower that humans can produce why should I be able to get that power I put in(less mechanical and electrical losses).

The goal isn’t to get airborne, but to extend glide. The aircraft will be launched from 30 feet. I assume that just gliding it will go 200 to 300 feet(many other factors involved). The goal of the power system is to extend that glide.
I am not sure what comment I make to imply a ‘car alternator’ , but that was never the plan. I tried to clear that up.

What I don’t know how to do is convert the three phase power that comes from a brushless motor into DC power, which is what the ESC, and propeller brushless motor is design to use as input. Is there a simple way to do this? I don't understand the diagram you attached well enough to modifiy it.


OK I anticipated your HP plane was directly peddle powered and the Motor was ancillary.

I don't think this is going to work out for you As I understand your going to mount a prop on a human sized glider presumably very light weight. Then drive that motor from a peddled similar motor use as a generator?

Your going to encounter losses in both motor and generator and so will have less power available than if you drive the prop directly from the peddles.

My leave enough over assumed you were driving the prop directly

Modern gliders may have a glide ratio of 50:1 your only looking at 10:1 Oddly the Gossamer Albatross had a poor glide ratio as like most HPP it was designed to have a very low sink rate which isn't the same as the glide ratio.

The power required to drive a HPP directly is about 1/2 hp. (not 1/4hp as you suggest) This will JUST keep you airborne and if your fit is attainable for a while.

The Gossamer Albatross that crossed the channel in around 49 mins had an air speed of 18 mph to stay aloft. Allan the pilot was a good track cyclist, didn't weight a lot and was a skilled pilot. In still air it required 300 watts to fly.

I used the circuit from a car alternator - simplified version below - because that is a 3 phase generator that produces DC. I was intimating you need to think on a wider range sometimes to find what you need. You will have to sort out the connections from your motor.

As Steve says start thinking 1000,s of RPM for an RC motor for any sort of output and a considerable mechanical load as well.

I don't know about your flying experience but a fall from 30 feet will at best be a shock and uncomfortable and at worst will kill you if (when) things go wrong.

Dumping a glider on the floor hard fro 10feet is a serious bump - I know!

You may find this informative


Jem has a degree in aeronautics.

I think at best your going to need to look at direct drive and forget the fancy generator/motor link attractive though it may look.

As always thank you for your time.

The University of Michigan is using the electric generator to drive motor system on their Human Powered Helicopter project.


My only change is trying to use off the shelf parts

Since you seem interested in the technical issue of the aircraft:

The best on-line primer is: The University of Adelaide paper ‘Human Powered Flight’


Chris Roper's page is very good, but not updated


While the books form my library below were publish some time ago, the basic issue haven’t changed. I have listed from most technical to least

‘The History of Man-Powered Flight’ D.A. Reay
‘Man Powered Flight’ Keith Sherwin
‘To Fly Like a Bird’ Keith Sherwin
‘The Compleat Birdman’ Peter Haining
‘Pedal Powered Planes’ Keith Sherwin
‘More with Less’ Paul Ciotii
‘Gossamer Odyssey’ Morton Grosser
‘Birdman’ Alan Stewart

Thanks anyways, and good luck

There is something weird about their calculations - Not that I suggest they are wrong but they as yet as far as I can see have not built and flown this design.

If the motor spec says 3000RPM for 1400watts then if you consider everything to be linear (it isn't in fact it will be worse) the 80 RPM = 37 watts (3000/80=37:1 - wattage at the same ration=1400 /37=37 watts)

37 watts isn't close to the minimum of 1/2 hp they need to fly.

Something odd there.

An electric motor gets hot in use as does a generator - This heat represents losses in the system. As a builder of electric cars using 24 volt 250 watts motors run a 200% I can honestly say they get VERY hot.

I can't believe that a motor generator is more efficient than a good gear box or chain system.

Please keep us updated with your efforts, maybe you have something the only real way to find out is try.

You can try this at a small scale to get an idea of efficiency without spending a lot on high spec motors.

Pace Rick, and using his drawing, converting BLDC to volts is not going to be easy. Consider the voltage/speed characteristic of the motor - 18,000 RPM = 6V on the motor windings. So, you'll have to spin it at 18K to get 6V out.

There are better methods !

If you connected the BLDC motor to your generating BLDC motor, AND geared up the cranking, it should work directly - but that's one hell of a step up gear.


Thank for taking the time.
I have connected two brushless motor directly together and turned one as a generator. I can get the propeller motor spinning. The problem is that any variation in generator speed and they are 'out of phase(/) and the propeller motor stops working.

Hmm, yes, I suppose I should have thought of that. Many, many moons ago when we did the Electrical machines lab at university we studied synchronous AC machines....which is what you have ! Its a tragedy though to WASTE the energy you will in converting to DC and then chop it all back up again ! At these low voltages, the diodes are going to be piddling away quite a lot of power, unless you use synchronous rectifiers. You could try germanium diodes like the 1N34 (??), which have a much lower voltage drop than silicon diodes, instead.

Take Rick's circuit, and cross out ANYTHING not connected from the motor coils and diodes and you have a BLDC to DC generator.