Finding the right generator

[irethedo]

I am helping my son with a science project and attempting to build a very efficient small wind generator.

We have a very efficient rotor design which can spin (without load) in 1.5 MPH wind but now we need  to
find a very inexpensive generator which can be spun easily and with as powerful of an output as possible.

I have read aboout people using tape drive motors but from what I have read, these are typically yield lower output
levels less than 10 W.

I have heard of people that use car alternators but these require too high of an RPM for substantial
power. Someone also mentioned to me that the older police cars and emergency vehicles had higher output
alternators which had a full output at idle but I have not been able to find much on these.

We would like to rectify the output and pass it through an inverter for ~ 120V AC @ 60Hz and perhaps we are dreaming but
we would also like to get 100 W if possible...

I was playing around with a flywheel off a briggs and the coil but didn't get very far...Not sure if there are any overunity configurations
out there that might be retorfitted for this task or not but thought I might ask on this board to see if anyone had any suggestions...

Can anyone recomend a relatively inexpensive generator that we might use for this project?

thanks in advance for any helpful ideas

[capthook]

irethedo -

The power you can generate is directly related to the wind resource.  There are a number of formulas you can look up to give the potential energy related to wind speed.

As such - trying to extract 100W from 1.5 MPH is impossible.  You could generate a tiny amount of power given such low wind speeds - but it's not practical.  A min. wind speed of 7mph, while still quite low, is kind of a min. rule of thumb for practical (tiny) generation.

As far as generator designs - you have a number of options.  The current DIY that offers the best output with the least amount of losses while being relatively easy to construct is the axial flux dual rotor design by Hugh Piggott.
It uses air coils surrounded by two rotors of magnets allowing for a large flux to the coils without the large losses associated with iron core/stator designs.

Hear are some links to get you started.  The 1st site has a .pdf you can download with details of an early model: http://www.scoraigwind.com/pmgbooklet/index.htm

http://www.scoraigwind.com/
http://www.fieldlines.com/
http://www.windstuffnow.com/main/

[irethedo]

thanks Capthook-

I will check these out...

[khabe]

It is difficult to advice any generator when given info about rotor is just only:  very efficient rotor design which can spin (without load) in 1.5 MPH wind
Iron core or ironless ... there I have seen also many misunderstandings about.
Efficency could be higher than 90% for both - iron core or iron-less brushless motor/generator.
Ironless design has no cogging but well designed iron-core generator could have near zero cogging as well.
Iron-less generator need to have at least twice bigger magnets as iron-core brother. Art least! In reality all the mass of Stator need to be added for rotor magnets ...
Most of ironless generators made by "masters" are made totally unprofessionally - they are speaking about "coils" as well as design based on the "coils" - forgotten about
basic physics laws about left/right hand rules (moving_magnetic_field_wire_current) wheres "coil" need to meet two magnet poles at the same time - N-left and S-right side of "coil" and conversely.
And only part of "coil" is useful when we are speaking about ironless machine - only wires what crossways to moving direction of magnet poles.
Iron core generator is bit easier to build for first-timer, especially when low rpm and low power. There is several ways for reduce cogging torque near to zero:
Choice number of stator slots versus number of rotor poles - Pole-Slot Combinations
Skewing rotor poles or stator slots.
Unequal tooth width - influence of pole number
Unequal distance between rotor magnets ...
etc ...
And combination of mentioned above
There are a lot of perfect styles and designs worldwide - it will take full year to expand upon even part of more precise ...
The best choice is some older brushless servo motor from ebay - older because cheaper. Usually you can find out unique Pole-Slot Combinations like 39 slots / 12 poles ... + skewd rotor poles or skewed stator slots ... Nominal power and KV (rpm/volt) gives enough info about is it useful or not for your project.
When you like to choice axial flux ironless desigh anyway - then do not use too weak (thin) magnets and too large airgap between. Airgap (between magnets or between magnet and return iron) must to not exceed 1/2 of thickness of magnet when one-side magnets or 1 thickness of magnet when two-side magnet design.  Less airgap = less "turns of coils" for the same Voltage ... well wound you can use larger total cross area of wire ...
You have to think about Eddy losses - do not use thicker wire than 0,5mm. When higher current then stranded wire. Litz wire is the best choice when airgap machine (ironless generator).
Hereby one good PDF file how its look correctly made windings for slotless generator. Please look at only like "coils" must to be placed. Please note - 6 pole machine, but actual magnets are too weak ... and just 6 poles - requests higher rpm ...
http://www.windstuffnow.com/3phasekit.pdf
But you can see how windings need to look like - and that there are no "coils" literally - just wires between magnetic flux!
Regards,
khabe

[irethedo]

Are there any other motors out there used in other applications which are low RPM
and require little force to turn them that can be used for a reasonable output generator?

Perhaps a washing machine or dryer motor or a treadmill or electric scooter type motor???

Not sure if these would work or not or if they are difficult to turn or not...

Any ideas?

thanks