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mikemill757
02-05-2012, 11:07 PM
Does anyone know how to wire a brushless motor to be a generator? Lots of info on alternators, but hardly anything on 3-pole motors.
Thanks, Mike

CNY_Dave
02-06-2012, 12:40 AM
6 diodes if you want dc.

Not sure if you do anything different for star (shown) or delta.

http://www.opamp-electronics.com/tutorials/images/experiments/05019.png

kyleservicetech
02-06-2012, 06:33 AM
Does anyone know how to wire a brushless motor to be a generator? Lots of info on alternators, but hardly anything on 3-pole motors.
Thanks, Mike

Both those automotive alternators and our brushless motors are three wire, three phase phase units. As the previous post shows, you vary the output voltage of the auto alternator by varying the current flowing through the alternators rotating field windings. That way you can produce 14 Volts DC from both while the engine is idling, and while its turning at 4000 RPM or more. Years ago, I did rewind an automotive alternator to put out 60 VDC at 2000 RPM, and about 5 Amps. It's a lot of work, and not recommended unless you've rewound stuff before, and have access to a good supply of magnet wire.

Making a generator out of a brushless permanent magnet motor doesn't give you that option. The output voltage of a brushless motor is fixed by the RPM on the motors shaft, and nothing else.

Unless you turn over your motor at perhaps 8000 RPM, you're not going to get much useable voltage out of it. As an example, if you've got a brushless motor that at no load turns the prop at 6000 RPM on a two cell LiPo, you're only going to get about 3 1/2 Volts AC out of it at 3000 RPM. Add to that the voltage drop on the three phase DC rectifier bridge, and you've not much voltage left.

Going to a $$$$ 1KW motor with a high KV rating would work, but it's still got to turn over at 6000 RPM+ to be useable. To handle the varying voltage coming out of this type of setup, you'd have to use something like the Castle Creations uBEC to provide a constant voltage output. Places like www.mpja.com (http://www.mpja.com) also sell DC in, DC output switching power supplies.

(Been there, tried that.)

On the other hand, if you're just trying to light some LED's or similar, wiring up that three phase bridge in the previous posting would do the job. Still got to turn that motor over at a pretty good RPM though.

CNY_Dave
02-06-2012, 02:17 PM
Yes, I should have mentioned, the field current on a permanent magnet alternator is 'fixed'.

This is actually the only time the KV rating is precise...

If you had a 1000kv motor, you would need 12000 rpm to get 12v (peak), that is, 1000rpm per volt.


Dave

mikemill757
02-09-2012, 04:13 AM
Thanks, Dave
That's exactly what I was looking for! I want to build a generator w/o brushes - the rpm will vary slightly, but within reason. Will just have to increase the RPM of the driver.

CNY_Dave
02-09-2012, 03:11 PM
C'mon, you have to tell us what it's for!

mikemill757
02-17-2012, 05:33 AM
OK, What I'm thinking is a pedal powered generator to run and recharge (along with solar panels) a battery powered bike. This way I can charge the batteries at a red light. I know there'll be efficiency losses, but it'll cut down on the number of gears in the transmission.

Mike

kyleservicetech
02-17-2012, 05:47 AM
OK, What I'm thinking is a pedal powered generator to run and recharge (along with solar panels) a battery powered bike. This way I can charge the batteries at a red light. I know there'll be efficiency losses, but it'll cut down on the number of gears in the transmission.

Mike


Sounds interesting.

One thing to be aware of is the efficiency of converting perhaps 120 RPM of the bike crank to about 8000 RPM required for the generator. That's about a 60/1 step up in RPM. Problem is, that's likely going to need about three step up gear drives or belt drives. With perhaps 85% efficiency on each stage, that's going to be .85 raised to the third power. Something on the order of 40% of your energy will be just making the gears hot.

CNY_Dave
02-17-2012, 03:38 PM
Sounds interesting.

One thing to be aware of is the efficiency of converting perhaps 120 RPM of the bike crank to about 8000 RPM required for the generator. That's about a 60/1 step up in RPM. Problem is, that's likely going to need about three step up gear drives or belt drives. With perhaps 85% efficiency on each stage, that's going to be .85 raised to the third power. Something on the order of 40% of your energy will be just making the gears hot.

You'd get better efficiency by driving a brushed generator that drives a brushed motor that drives the brushless motor!

I think there are some methods to get a high mechanical step-up fairly efficiently, I'll have to see if I can recall any of 'em.

Dave

mikemill757
02-23-2012, 03:53 PM
The lowest K motor I've found was a 200K/V. How hard would it be to rewire to 50K/V? Would that be replacing the existing turns with finer or thicker wire?

kyleservicetech
02-23-2012, 04:40 PM
The lowest K motor I've found was a 200K/V. How hard would it be to rewire to 50K/V? Would that be replacing the existing turns with finer or thicker wire?

Rewinding a brushless motor would generally be a nightmare. I've done it many times. One problem is obtaining the proper magnet wire that is suitable for the smaller motors we use in our projects, at least as compared to those multihorsepower units. And smaller motors indicates smaller radius of turns on the magnet wire while winding the motor. The varnish can flake off at those sharp turns, which is likely why the motors such as Hacker and similar use multiple parallel connected strands of fine wire for each winding.

For what's involved, check out www.gobrushless.com (http://www.gobrushless.com), where they sell kits to wind your own motors.

And:

http://www.gobrushless.com/shop/index.php?app=ccp0&ns=catshow&ref=diy+brushless+kits

http://www.bavaria-direct.co.za/models/motor_info.htm

mikemill757
02-24-2012, 05:12 AM
The motors I found were in the 50-65mm dia range. Seems like these would be easier easier to rewind. Do I need to go to a finer wire size & more winds to get a lower K/V?

kyleservicetech
02-24-2012, 05:52 AM
The motors I found were in the 50-65mm dia range. Seems like these would be easier easier to rewind. Do I need to go to a finer wire size & more winds to get a lower K/V?


The "KV" rating of a brushless motor is inversely proportional to the number of turns of each winding in the motor. So, to go from say 500KV to 1000KV, you'd need one half the number of turns in each winding of that motor. One half the number of turns indicates that the wire size for 1000KV must have about twice the area of the 500KV windings, in order to fit in the winding slots.

Or, the VOLTAGE rating of a brushless motor is directly proportional to the number of turns of each winding. Double the number of turns in a winding doubles the voltage rating of that motor. That's what those 120/240 VAC motors are doing when changing over from 120 to 240 VAC. They have two 120 VAC windings. For running at 120 VAC, those windings are in parallel. For 240 VAC, those two windings are reconnected in series.

CNY_Dave
02-24-2012, 05:52 PM
But to lower the KV, fewer windings.

The current will not go up, though.

So what he's looking for would be multiple parallel windings where there is now a 'single' winding?

Dave

kyleservicetech
02-24-2012, 06:10 PM
But to lower the KV, fewer windings.

The current will not go up, though.

So what he's looking for would be multiple parallel windings where there is now a 'single' winding?

Dave


OOOOOPS :oops: :oops: :oops:
I've got to think about that a little bit. (I changed my text in posting #13) :eek: The motors KV rating is KV=RPM/Volts. So, if the motor is 500KV that would be 1000 RPM/2 Volts, or 4000 RPM/8 volts and so on.

Now, go to 1500 KV. That would be 3000 RPM/2 Volts. Since the motor is turning over at a faster RPM, that is 3000 RPM versus 1000 RPM on the 1500KV versus 500KV, the motor must have less turns, of heavier wire.

As a generator or alterator in this case, KV=RPM/Volts translates to Volts=RPM/KV. So to get more AC volts out of this motor operating as an alternator, you need to have a lower KV rating, or more turns of smaller diameter wire.

quorneng
02-25-2012, 07:30 PM
Before I started worrying about the technology of doing it I would work out just how much power you could put back whilst pedalling hard for the average wait at a set of traffic lights.
I suspect the answer is not alot compared to that which you used used getting there.

kyleservicetech
02-25-2012, 10:02 PM
Before I started worrying about the technology of doing it I would work out just how much power you could put back whilst pedalling hard for the average wait at a set of traffic lights.
I suspect the answer is not alot compared to that which you used used getting there.

Good point.

I've got a 3 Hp Briggs engine driving a 65 Amp Automotive Alternator set up for a 12 VDC power source for recharging several 12S2P A123 battery packs. It takes 20 minutes for that setup to finish charging. That Briggs engine is running wide open throttle the entire time.

Since a person in good shape can put out about 1/10 HP, it would take over a half day to do the same thing.

http://www.wattflyer.com/forums/showthread.php?t=63918

FYI, Harbor Freight is going to have a sale on their 212 cc gasoline engine in two weeks, for $99. I'll be reworking my alternator setup to run with that engine when its available.

mikemill757
03-03-2012, 06:35 PM
OK...now I'm thoroughly confused ! What I want is a SLOW turning motor that will get me about 28VDC, the slower the better. With my pedal drive, I can put out 300 rpm @ 60 - 75W. I can gear up my RPMs to match the motor/gen. How many winds and what size wire?
With a recumbent 3 wheeler with a canopy/aeroshell, I'm thinking I can cruise at 50W, but acceleration/hills will need the help from the battery. If I start out with charged batteries, this scheme should get me to school (5-6 miles one way) without being exhausted by the time I get there. Hmmm...maybe ALL electric or ALL pedaling would be a better choice after all the complications/expense/inefficiencies are considered, but it's an idea I'd like to explore.
Or, maybe a pneumatic system with a pedal driven compressor (less 12V motor)/tank/air tool motor.
Arugh! Maybe I just have too much time on my hands

kyleservicetech
03-03-2012, 07:28 PM
OK...now I'm thoroughly confused ! What I want is a SLOW turning motor that will get me about 28VDC, the slower the better. With my pedal drive, I can put out 300 rpm @ 60 - 75W. I can gear up my RPMs to match the motor/gen. How many winds and what size wire?
With a recumbent 3 wheeler with a canopy/aeroshell, I'm thinking I can cruise at 50W, but acceleration/hills will need the help from the battery. If I start out with charged batteries, this scheme should get me to school (5-6 miles one way) without being exhausted by the time I get there. Hmmm...maybe ALL electric or ALL pedaling would be a better choice after all the complications/expense/inefficiencies are considered, but it's an idea I'd like to explore.
Or, maybe a pneumatic system with a pedal driven compressor (less 12V motor)/tank/air tool motor.
Arugh! Maybe I just have too much time on my hands

Don't think you will be able to find a "Slow" turning alternator that can put out 28 VDC. Ibe way to get one would be to rewind a standard automotive alternator with a lot of turns on the windings, along with a custom built voltage regulator and so on. (Yup, I've done it. Not an easy job at all.)

Also, an air compression system is not very efficient. Just place your hand on the air pipe coming out of a big air compressor. That is pure heat, that will be lost energy when using the compressed air to run an air motor.

mikemill757
03-04-2012, 04:46 PM
Dennis, What I had in mind was to rewire a med, size hobby brushless motor to give me a low KV rating ( look at the start of this thread ).
Thanks for the info on the air sys.

Mike

Wildflyer
03-04-2012, 06:17 PM
I think step 1, should be getting the recumbent bike to move properly with electric power.
I have a feeling it will take much more than 50w, that is less than the power of a Cox .049 Baby Bee. My EFlite Jenny J-N4 flys with a 50w motor, and it only weighs 9oz, an .049 would be far too much.

After you get the bike working OK, then you will have an idea how much power you need to recharge. The best solution I can think of is to make the battery unit removable, with a charger built in, then just take it inside the school and plug it in.

When I was young and crazy, I fastened a piece of plywood to my bicycle, with 2 large 60 size motors on it. They stuck out over the front wheel. My father started the motors, and I started pedaling, pretty soon, I was going over 50 mph. The combined power of those 2 motors was somewhere around 1500 -2000 watts. I had tried 1 McCoy 35, but it seemed to have no effect on the bike.

kyleservicetech
03-04-2012, 06:29 PM
Dennis, What I had in mind was to rewire a med, size hobby brushless motor to give me a low KV rating ( look at the start of this thread ).
Thanks for the info on the air sys.

Mike

Problem with rewinding a medium size hobby brushless motor to provide 28 VDC at fairly low RPM's will require a LOT of turns on the windings.

My "Gobrushless" 150 watt motors required 16 turns on each of the three poles per three phase winding to provide about 8 VDC at 9000 RPM. That would be 160 turns at 800 RPM at the same 8 VDC. Make that 160 times 28/8 or 560 turns at 28 VDC output.

Problem is, that would require wire size something in the area of #30 to #36 or so. The winding resistance of 560 turns of #36 wire will be considerable, limiting the amount of watts you can get out of the rewound motor operating as an alternator. It won't be much unfortunately.

kyleservicetech
03-04-2012, 07:07 PM
I think step 1, should be getting the recumbent bike to move properly with electric power.
I have a feeling it will take much more than 50w, that is less than the power of a Cox .049 Baby Bee. My EFlite Jenny J-N4 flys with a 50w motor, and it only weighs 9oz, an .049 would be far too much..

My wife has two electric scooters, both run on 24 Volts DC. The smaller one pulls about 3 - 5 Amps or so running around in a store on flat surface. Climbing a small slope on our asphalt driveway, that increases to 10-12 Amps. The big scooter runs on a pair of 24 Ampere Hour 12 VDC deep cycle batteries, and has enough power to drive through a farmers plowed field! That can pull 20 amps on a steep grade.

We also had one of those electric scooters years ago, that had a brush type DC motor in it. Its motor is identified as "MY1016" rated at 24 VDC, 15.4 Amps, 280 watts at 2750 RPM. That would move the scooter at 15 MPH for a mile or three off of a pair of 12 VDC 8 Amp hour lead acid batteries.

The motor is heavy though, it weighs in at 3.8 pounds.

Take a look at one Ebay listing:
http://www.ebay.com/itm/24V-250Watt-Electric-Scooter-Motor-Unite-Brush-MY-1016-/150759136440?pt=Motorcycles_Parts_Accessories&hash=item2319f1dcb8&vxp=mtr

To that you could add a switching power supply to boost its output voltage to 28 VDC to charge your batteries. You'd have to design you own though, even though some places that sell the Integrated Circuits for those switching power supplies also have on line design software for designing your own power switching power supply. Nice thing about switching power supplies, their output voltage is regulated to what ever you set it for.

Take a look:
http://www.national.com/en/webench/power.html