120 volt axial flux dual rotor

[fanman]

hey guys

i want to build a 120 volt machine for battery charging, and i am brand new to this board, and i do think this site is the best.

i have a 120 volt system now and i have a three phase generater running know that i built that puts out 25 amps at 120 volt not bad considering i made it. now i want to make this hugh design axial flux design gen, and as yet i havent seen any high voltage models to compare with, are there any out there? i want to produce around

5kw at least i have done alot of homework here and im ready to build i was just wondering if i could have your help anybody.

first off im gonna use at least 24" discs, my magnets are gonna be 2x2x1" thick,discs will be 1/2" thick wire will be #14 there will be 24 poles 16 coils for the three phase configuration

my first guestion is instead of using 16 flat coils wouldnt i get more voltage from a double layer winding versus the flat? all three phases would be under a magnet face at all times.there would be 24 poles 72 total coils 24 coils per phase the voltage in each coil would be lower yes there would be more coils to wind . that doesnt matter i just want the most voltage i can get or would i have just as much voltage from one larger coil? remember i want this machine to be 120 volt ac

what do you all think?

[Flux]

Just one or two initial thoughts.

24 pole 16 coil is no use for 3 phase, 18 coils would make sense.

I seriously doubt that you will do any better with more than 1 layer of winding and if you do use multi-layers with overlapped coils watch the insulation with this machine. Be prepared for over 500v if you loose the battery, use plenty of inter phase insulation.

You dont want 120v ac to charge a 120v dc battery. Remember the rectifier voltage will be peak so you should aim at about 85v ac at cut in.

I haven't done any sums but to produce 5kW plus you had better set your cut in for fairly high winds, use a boost or buck converter or consider star delta.

If your total load is not batteries and you are happy to have more modest battery charging and use the excess as heat on windy days then the design will be simpler.

Don't get carried away about volts alone, you need to think carefully about winding resistance with so much power about. I seriously doubt that you can dissipate over 2kW in the stator so you will have to keep efficiency right up.

Flux

[Flux]

I have had a bit more of a look at this.

Efficiency is not going to be your problem, matching the load is.

With those magnets you need discs of about 30". If you squeeze it down to 24" then you would make better use of 20 poles.

If you design for a low wind cut in the thing will go completely into stall with a 5M prop. If you optimise for higher wind cut in you can get extremely good efficiency around 20 MPH.

I have no idea what size prop you intend to use, you have better ideas of what you can manage but you will easily get the 5kW with 5M if you match it right but you are going to have to do something cunning to work over any significant speed range.

My choice would be to match direct at about 16 mph and use a boost converter for the lower winds, but if you want to do it without converters you will probably have to compromise with a cut in around 10mph and use large heaters in the dc line to keep the prop speed up. This will reduce your battery charging efficiency in high winds and will be ok if you can use the heat, if not it may be a nuisance.

Perfect candidate for Nando's mppt tracker if you can live with electronics.

Just a few thoughts for you for now.

Flux

[fanman]

hey flux

fanman here im new at this computer stuff so bare with me im real sorry on the 120 volt axial flux my intentions were tohave at least a 20' prop for low wind speed power . i really dont want to use any converters i would like to keep it as simple as possible. my tower is 158' tall and it is strong enough for a big set of blades .ive made templates of the magnetic discs at 24" and i think i have plenty of room for coils and space so that the magnets dont flux to each other about

3. 34" between them i thought that a double layer winding would give me more poles hence more voltage i do realize the insulating needs to be well between phases
   
85. volts is right i only put 120 volts as a desired voltage.
    
    

 so with 24 mags, 18 coils. that means 6 coils per phase and each coil would have to produce 15 volts each at a cut in speed of say 80 rpm? is this a little high for a 20' prop? that sounds easy enough . what do you think?

                            thanks

                                fanman

[Flux]

With magnets that close together you are wasting a lot of winding space.

In this case it doesn't matter as you have a more powerful alternator than you need.

If you want to save some money I don't think you loose anything by using 20 magnets per disc and making better use of the space.

I wouldn't take the cut in below 80 rpm, it will make things much more difficult.

I think you can do it with 24 magnets with 80 turns per coil, the wire will be incredibly thick and you will have to use several strands of #14 or similar in hand.

You will need to keep the coils over an inch thick for those magnets to make good use of the winding area, up to 1 1/8" thick would be ok.

It will be badly stalled by 15 mph and you will have to add line heaters to keep the speed up, but this does mean that the dissipated heat is not in the stator.

On windy days you may be able to cut the line resistor out and let the thing trundle quietly and peacefully in stall and still do what you want.

It's a big venture, I am sure you can do it, I can offer help but it is way beyond my experience so you must check things for yourself and make your own final decisions.

Flux

[coldspot]

"you will have to add line heaters to keep the speed up".

Flux-

Could you explane this or throw a link up maybe?

Thanks

[Flux]

If you want to match a prop over a speed range without tap change or voltage converters you have to have resistance in the line.

If you make a big powerful generator with low resistance it will run at virtually constant speed and you want the speed to rise with the wind to keep your prop from stalling.  

For small machines the standard procedure is to choose just enough magnets to make an alternator with the desired resistance, this is the cheap option and is fine as long as you can dissipate the heat produced in the resistance of the stator.

With bigger machines it becomes more difficult to get rid of this heat and in the case under discussion he has chosen very large magnets. the internal resistance will be too low to avoid stall. If is better but more expensive to have the resistive loss outside the winding as it doesn't heat the stator. It is ultimately a trade off between magnet cost and what power you can safely loose in the stator. As you  say " it depends on how much you can spend on magnets".

This is why the mppt schemes with voltage converters can extract more power, they match by letting the volts rise and you can reclaim this power lost in heat.

Flux