Matching Axial Flux Generator to Power Requirements

[Empower Playgrounds]

Empower Playgrounds is a non-profit developing power generating playground equipment to be used in rural African schools. Objectives include children having fun (rural African children have no toys) and providing a science lab. We have a prototype merry-go-round with gears and a generator. Electrical output is controlled by a PWM circuit so most of the work is used to make the merry-go-round spin. We want to investigate a design that uses an axial flux generator/alternator integrated into the floor platform of the merry-go-round. The rpm will be only 5-12, but the diameter is 10'. The input work available is 4-6 children pushing (600-900 watts). Is there a good reference designing an axial flux generator for such an application?

Thanks

[Flux]

"Electrical output is controlled by a PWM circuit so most of the work is used to make the merry-go-round spin. "

Don't follow that, are you motoring it or are you just trying to prevent it stalling the kids.

I have heard of this scheme being used to pump water in Africa, seems a good idea to harness a free source of power. What do you intend to do with the power, that does affect the answer quite a lot.

It is going to be costly in magnets at such low speed but it seems a perfectly practical idea. I just wonder if it may not be far cheaper to use a modest size axial alternator and a timing belt speed increase. It would be little different in efficiency unless you made the direct alternator a monster, in which case direct connection would do better. You will need a load of big magnets, initial cost will be high, I don't know how that affects the considerations.

Flux

[pepa]

probably the the best way would be to mount a large plywood disk under the floor  and centered at the pivot point. mount a bike chain eposied in a dado to the outer rim of the disk and have it drive several pm motors with sprokets on them. sinse the motors turn freely until charging speed, the kids won't even know the motors are there. see my last diary post for an experiment i am working on. i have horses and have been wanting make some use of the energy they put out as they turn a exercise walker. it would work something like the old horse drawn sugar cane grinders we used a while back. pepa.

[Empower Playgrounds]

In order for the playground equipment to be fun, most of the input work has to stay in the playground.  We are using pulse width modulation to turn the generation on only 25% of the time.  I am very new to wind power terminology, but I believe we are motoring in a sense.

We are charging batteries then using the power for lighting classrooms.  

Your last paragraph is right on target.  I need to learn how to design the options so we can compare cost and performance estimates.

Any help is most welcome.

Ben

[Flux]

Ok I understand the pwm bit, your generator is probably too powerful for the available kid power.

I have had a quick look and with 120 poles ( 240 magnets ) 3 x 1 x 1/2" you could in theory get what you want at about 70% efficiency. I am not sure you could maintain the clearances with that large diameter without very good engineering. You would probably need thicker magnets to work with a bigger gap. There is going to be a lot of copper as well so not a cheap machine. Not sure how much first cost matters if you can get free power in the long run.

If you took the alternator from Dan's 20 ft machine and geared it about 10/1 you would have a bit better efficiency. That uses 20 poles (40 magnets) 3 x 1.5 x .75"

Flux

[Empower Playgrounds]

Flux, thanks for the info.  I would like to avoid gearing to keep things simple, but as you mention, I am worried about maintaining clearance. Is there any kind of design guideline?  Ben

[wdyasq]

Ben,

I don't want to rain on a parade but you should take a realistic look and maybe make some true take-offs of power from your 'merry-go-round' before you build something not so merry that won't be used to go round.

World class athletes have trouble maintaining 300 Watts of power. I don't think a batch of kids will be able to give that output and you harvest energy both. A simple, adjustable brake might give you an idea how much power is available for harvest. This same number will also dictate a lot of the design of the proposed alternator.

I suspect one of the most practical drive systems will be a 'serpentine' style belt as used on modern automobiles to drive accessories. One could roll the drive pulley of steel or turn it with hand tools of made of wood. The driven and slack take-up pulley would be sourced from a salvage yard. This 'start' would give one the opportunity to gather output information.

My first inclination for the power end would be a permanent magnet conversion of a small 3 Phase motor. There would be several voltage connections available. The belt-pulley ratio could be easily changed for desired voltage. Possibly, a controller could harvest variable amounts of power when it was possible.

Good luck,

Ron

[hvirtane]

You might do something similar as WindstuffEd's 'minigen' using a car alternator converted into a PMG. The advantage is that you would easily get high voltages at low RPM. Please see the design of the minigen here:

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

- Hannu

[Empower Playgrounds]

Ron, thanks for you constructive comments.  I have worked the available power issue you mention pretty hard.  We have three designs for playground equipment that 'yoke; 4-6 kids together so total sustainable output is enough to be able to bleed off 25% to a generator and and still let the kids have fun.

What is the best way to take 3 phase star connected AC to DC to charge a battery?

Ben

[wdyasq]

Generate the proper voltage and use a bridge rectifier.

Ron

[Empower Playgrounds]

Can only one of the 3 phases be rectified?  That would take 33% of the power out of my merry-go-round and avoid using a PWM circuit.

Thanks,

Ben