First try at dual rotor

[WoodSpinner]

Hi, I've been lurking for a while and thought I might give it a try.  I have a bunch of 3/4" x 1/8" round neos and I'm wondering if its even worth the effort?  All I'd like to do is be able to recharge my one deep cycle 12v battery that I keep in the shop to power 12v motors I also play with, it doesnt need to put out much total power but if it would charge this battery at all I'd be a happy camper.

I have 24 of these mags and was thinking about a 12/9 rotor coil arrangement in 3 phase just for kicks, what do you think?

WoodSpinner

[JeffD]

About 2 years ago I built a small turbine (1.7 ft diameter) using 32 3/4" x 1/8" round neos.  There are photos of the unit in my files.

  I use the little turbine for charging two ten year old 12v car batteries that provide power for my computer (11w) and two 13w compact fluorescent lights .  The turbine generates about 43 watt hours (1.8Wavg x 24hrs) on an average wind day (9 mph) and about 450 watt hours (19Wavg x 24hrs) on windy days (20 mph).

[WoodSpinner]

Thank you for the link to the pix. I'd be happy with that sort of output.  Now I just have to figure out the rest of it.

[WoodSpinner]

So, can you tell me how big the rotor is and how many wraps per coil you used just to give me a starting point for testing?

[ghurd]

To get decent output, need to have the distance from one magnet face to the next the same as the combined thickness of 2.

Meaning the space for the stator, and mechanical clearance on both sides, is only 1/4" total.

Might consider getting 24 more and doubling them up.

More 'kick value' when it works.

G-

[WoodSpinner]

I take it that mean the 1/8" magnets on dual rotors would only allow a space for the stator of 1/4"?  and would that double if I stack them two to a pole?  

So many factors to consider, so many unknowns in this, guess thats the attraction for me.

[ghurd]

Yes, it would double.  Almost, not quite.

Half inch is a bit of space to work with.  1/4" is not.  At least for me.

It may not be in the spirit of using what you have, but given the cost of the copper, resin, etc, spending a few $ more for more magnets would be a smart move economically.

And the total cost per watt would be a lot less.  Small increase in cost with a large increase in output.

G-

[ghurd]

Also, could stack those on one disk, use 3/4x1/4 on the other.  Might be cheaper.

Not sure why ebay sells small magnets in pointless quantities for us, like 10!

G-

[JeffD]

The rotors have a diameter of 4" and are 1/8" thick.

For the number of turns for the coils its going to depend on what you want for output, your wind conditions, and your blades.  It becomes quite the balancing act trying to match the alternator output with the blade output.

  I made my blades before building the alternator and had a rough idea what the blades could output in terms of power vs wind speed vs blade rpm.  The blades I made have a calculated Tip Speed Ratio (TSR) of 4.5 at which maximum power extraction occurs.  Knowing the TSR at which max power extraction occurs for each wind speed allows you to determine the cutin speed for the alternator.  

    I wanted a very low cut in of about 4.5 mph but also wanted high output in high winds.  This goal can not be attained using just coils and rectifiers hooked up to a battery.  With just coils and rectifiers you have to decide if you want low, middle, or high wind response.  If you use some form of load matching mechanism then you can have it all, sort of.

I used a boost converter for the low end of the power curve to match the load better so that the turbine can extract power from low and high winds without burning out. Each coil has 80 turns of 22awg wire which gives a wind speed cut in (using main rectifiers) of about 13 mph which is about 990 RPM.  I could have used 20awg wire with 75 turns per coil since there was more room available in the stator but I had lots of 22 awg wire on hand.  20awg would make a slightly more efficient alternator but at a higher cost which I wasn't willing to do at the time.  I made use of what I had on hand.

The boost converter allows the turbine to start generating power at about 400 rpm (4.5 mph) and can go up to 1650 rpm (30 mph) before furling starts.  At 30 mph the alt is  putting 46 watts into the battery. At about 33 mph (when it starts to furl) the alt is putting about 55 watts into the battery.  The data logger has recorded 3 second peaks of 63 watts (4.7A @ 13.5 volts) into the battery when high wind gusts occur and the turbine is in the processes of furling.  

With out the boost converter, I tried a number of different coils to see what kind of output I could get.

Using coils with 135 turns 22awg gave a cut in of 600 rpm ( 7mph).  Unfortunately the blades start to stall around 12 mph and by 30 mph are severely stalled (TSR down to 1.9) and only putting 27 watts into the battery.

Coils with a turn count of 110 gave a good middle ground response.  Cut in was 750 rpm (9mph). At 30 mph the rpm was 1220 and the alt was putting 35 watts into the battery which is about 11 watts less than the boost converter configuration.

[JeffD]

Definitely have to shop around.  It was cheaper for me to buy 32 3/4" x 1/8" ($16) than 16 3/4" x 1/4" ($24).  

[ghurd]

Completely OT.

I forgot about your mill.  Kind of eagerly anticipated a post.

I can not reconcile the cut in and stall numbers in my head.

G-

[WoodSpinner]

The only magnets my local store has in stock are these little guys so I'll see how many more I can pick up.  

I guess I'm going at this kinda sideways, starting off with the available magnets and all but one does with what one has available.

So the next question is to figure out the diameter of the rotors, from what I've read the "magnet spacing should be about 5* the airgap"? is that correct? and if so...duh, is that the airgap from the coils, or the total airgap between rotors? in my case a bit less than 1/2" I guess so...gosh this gets kinda complicated for me, any help would be appreciated.

[WoodSpinner]

Wow, a ton of good info. Thank you.  since I will have more coils I will probably be able to reduce the number of turns per coil, testing will be done once I build the rotors, I hadnt actually thought a lot about the blades, thought I would start with some bent aluminum and decide if I need larger or smaller blades to keep the thing charging in moderate wind.Of course thats also probably going at it sideways again. and I do eventually want to carve some nice blades matched to the alternators needs.

[WoodSpinner]

looking at jeffd's rotor and assuming a 3/4" magnet it looks like about a half inch for the spacing so if I use 12 poles, thats 9" and .5" spacing thats another 6" that gives me 15" circumfrence at magnet center line, so 15/3.14159 gives about 4 and 3/4" diameter to the magnet centerline, how far out to the edge of the rotor should the magnets be placed? I was thinking that an 8th"would be sufficient? giving me a rotor diameter of 5 and 3/4". does this sound good, maybe round it to 6"?