24" Rotor questions

[wgatenson04]

I have a 24" steel disk that I am using for an axial-flux genny.  It has 40 (2" x 3/4" x 1/4") neos on it.  I was planning on doing it 3 phase, with 30 coils.  What gauge wire would you recomend? (I was thinking #18AWG)?  My goal for the genny is to acheive 20KW at less than 400RPM.  I know I may need to double up the mags, or add another set of magnets and coils to the other side of the rotor to acheive the 20KW.  Also, have you guys heard anything bad about using epoxy to adhere the mags to the rotor?  I will have pictures up soon.

[Flux]

You have one disc? Do you intend to use some form of laminated core or are you hoping to get away with the one disc and a stator.

What voltage?, ac or dc?

Small magnets and thin wire, can't comment without a lot more detail but as a first thought you are miles out.

What is going to drive it at 400rpm to make 20kW?

Flux

[old55olds]

Just thinking out loud

20000. Watts
       
12. volts  
    
1. ,666 Amps ( a fair size welder)
   
24. volts  833.3 Amps. (Still fair sized)
    
48. volts  416. Amps ??
    
    

Wire size?? maybe a little larger.

For a 60 Hp Freon cooled I use 4  16gauge in parallel.

On my 1 KW I used 8 turns of 18 gauge 8 in hand ( or parallel)

I would use a little heavier wire.

[devoncloud]

you are going to need alot bigger wire, more than double your magnets, and come up with some blades like noone has ever seen to come up with the kind of output you are talking about.

1.  Are you using a laminate core?
   
2.  If not, yes you need another steel disk and double up on magnets
   
3. If you will be using a laminate core, what kind?  will it be slotted? if so, look at windstuff now's designs and you can pick up some more output by making a bunch of small coils. If not, you will only produce about a third (if  that) of what you are tying to achieve.
   
4. Remember:  The bigger the wire size the less resistance.  Given the amount of power you want, you are going to need some big wire (i dont think it's possible with your design).  
   
5.  Figure out how many turns you will need on your coils for your design and then figure out the biggest size wire you can fit in your stator.
   
6.  what size prop are you looking at here?  Where are you getting your 400 rpm number from?
   
   

Devon

[chux0r]

Hmm..... let's say you have a 30MPH wind.  I don't know if that happens a lot where you are, but I'm trying to come up with 20KW, so let's go with that.  Someone check my math, but you will need a 35ft diameter blade to get 20KW (Assuming 15% efficiency.  Is that about in the ballpark, or can we do better?).  This blade will need a TSR of 3.8 for 400RPM at that wind speed.  Holy cow, I'll want pictures.

[Victor]

20 kw Jacobs wind turbines have a 31 foot diameter (originally 29') and a rated wind speed of 25 mph at, if I remember correctly, 175 rpm.

www.windturbine.net

[jimovonz]

I think your TSR calcs are a little askew. By my calcs a 35ft turbine would have to have a TSR of 16.5 to get 400rpm in a 30mph wind. Would still be a sight to see tho...

[chux0r]

Oopsie, you're right.  I jabbed it into a spreadsheet then read off the wrong cell... hah!  16.6 is the right answer.

[Victor]

Check  out Hugh's nirvana turbine. He has duel rotor 28 magnets each at roughly 2x1x1/2 inches, very close to the same magnet area as 40  2x 3/4  assuming doubling up your magnets and he is only getting a "toasty" 4 kw at 225 rpm

[wgatenson04]

It is to be a 2 blade arround 30' in diameter, only I was using 30% efficency from the blades.  The rated speed is going to be 30mph and a TSR arround 9 to 10.  20KW is the "goal", not expecting to get that though.  Yes I was planning to have slotted laminates.  Blades are going to be hand lay-up fiber-glass/carbon composite hybrid with a expandable polystyrene core.  Pics for all as soon as I get them completed.

[hvirtane]

I don't think that your plan is unrealistic.

But I think that 400 RPM for about 10 m diameter

blades is quite fast. I would plan for about

60 RPM - 200 RPM, because the speed of the tip

of the blades would be quite fast already

at that speed.

I've been working sometimes with a man,

who has got about 15 kW wind turbine

with the blades as described here:

http://www.otherpower.com/cgi-bin/webbbs/webbbs_config.pl?read=15028

The machinery with that turbine

is quite complex

with automatic electric - hydraulic

pitch control mechanism etc.

There is a gearbox, too.

I think that with a direct drive generator

it is probably easier to make such

big machines.

What is your plan for 'the furling' or other

kinds of control systems?

- Hannu

[Flux]

With 30 ft   tsr 10, it is nearer 280 rpm than your previous 400.

At 280 rpm with a slotted laminated core you may possibly be able to get near 10kW but its a big leap from any of mine and I don't know how it will scale. I don't know what losses you can tolerate before it burns out.

For a slotted core I would go for 120 slots with 120 coils, the 30 coil arrangement would not be sensible with a slotted core.

This is also assuming you let the volts rise directly with wind speed, heating load or something. For battery charging only a few kW to load over a reasonable speed range.

Assuming about 300v at full speed I think nearer #12 0r #14 wire But I think you have a lot of tests to do before you settle that.

Fortunately I am far enough away not to worry about a 30ft rotor running at tsr 10 in a 30 mph wind.

Flux

[wgatenson04]

For furling I plan to have a electronicially controlled tail to furl it out of high wind speeds.  For genny/speed control I plan to have a voltage controlled variable resistors.  Also a disk redundant disk breaking system on the main shaft.  For the blades, not as sure yet... considering "free" pitching blades for a lower speed start-up, and a better "genny matching" TSR.

I am looking to get 10k from arround 400 RPM, and with the 20k with the added magnetic disks and copper at a rpm arround 180-220.  Probiblly going to use a 3 blade system for the 20k also.

Also, has anyone used "moving laminates" i.e. another rotor with laminates behind the coils that moves with the magnets?

[Flux]

If you rotate the return magnetic circuit there should be no need to laminate it.

There will be a second order eddy current loss due to ripple pulses from harmonics in the stator current but I doubt if you need worry about even at 10kW, especially  if you do not rectify the output

Voltage controlled resistors should be a good way to match the load.

I would go for 3 blades it will be a lot smoother.  Your revised speed seems much more sensible.

Flux

[DanB]

Agreeing with Flux here... if the back rotor moves with the front rotor... there is no need to have laminates.

But  - if you're going to have 2 rotors there on either side of the stator, it makes a lot of sense to have magnets on both sides.  Half your magnets on both sides will produce more power than all your magnets on one side with a blank disk on the other.