nubie with questions

[albert sun]

hi everyone! i, for fun, made a wind mill and now i can really see the potential for wind power. i made 22" blades from 6" PVC that seem to always be moving. since designing begins with finding the potential of the blades can someone assist me in guessing/ figuring this? i did already purchase 25- 1"x 1/4" N52 neodymium mags. are these too big for my blades? i want to make a dual rotor 9 coil/24 pole generator. i want to charge a 12 volt battery so i can power led strip lights around my hot tub for when i have parties. what kind of power are my mags capable of making? what gauge wires should i get for making my test coils? will a quarter pound be enough? when i calculate the ohms required for my voltage/amperage, is that divided by 9? how thick of metal do i need to contain the amount of flux the magnets have?

[Adriaan Kragten]

The steel sheets of an axial flux generator must have at least that thickness that the steel is not saturated. I have written several KD-reports about axial flux generators and in every report there is a calculation of the flux density in the air gap and it is checked if the steel armature sheet is not saturated for the chosen thickness. The reports can be found at the menu KD-reports on my website: www.kdwindturbines.nl. You can choose for instance report KD 571, KD 596 or KD 607.

[electrondady1]

1"dia mags +.5"space between
1.5 x 12 mags = 18"circumference @ center of mags
+.5" radius of mags x 2 = 19" circumference
19/3.14=6.05" dia. backing rotor

you could go to a machine shop and get them to build two 6" disks 1/4" thick
or you could buy 4 x 7.25" skill saw blades.
use two saw blades behind each set of mags for your rotors.

[kitestrings]

A wind power dado head.  I like it

[kitestrings]

what kind of power are my mags capable of making?

My sense is that you probably have more potential alternator here than you have blades to power it.  If you have one of Hugh or Dan's plans/books you could compare, but I'm thinking 4' diameter may be a bit small.  You're probably talking 50-100 watts at best and average quite a bit lower than that (10-30W).

I think you mean to say that you are planning a 12-pole (9-coils, 24 magnets) alternator.  My back of the napkin WAG (with several assumptions) is you might need ~60-70 turns to get near cut-in (12 DC nom plus ~1.4V for diode drop).  I'm guessing maybe 250 rpm.  Others may be able to help more.  The test coil doesn't have to be exact; you can adjust accordingly.

Good luck,  ~ks

[albert sun]

thank you for the replies. i did mean to say; 12 pole/9 coil alternator. would i be better off making a smaller alternator? would an 8 pole be better for me? how can i get a lower cut in speed? i do not have any way to really test the RPM's but i feel like it is in the 100- 200+range in my average winds. what if i made 36" blades? i do live in a windy site, i just do not want to build a big tower and all that. i would be happy with making two or three different turbines if i would get better efficiency with the mags i have than making one BIG one. would 36" blades and a 8 pole/ 6 coil alternator be better for me?

[Adriaan Kragten]

The size of a PM-generator should be chosen such that there is good matching in between the windmill rotor and the generator. Every windmill rotor has an optimum cubic line which gives the load for which the rotor turns at the design tip speed ratio. The generator should be designed such that the Pmech-n (Pmech = mechanical power in W, n = rotational speed in rpm) curve for the choosen load is lying close to the optimum cubic line of the rotor. The optimum cubic line of the rotor can be calculated if the rotor is designed according to the aerodynamic theory. The Pmech-n curve of the generator has to be measured and this requires a sophisticated test rig because one has to measure the torque and the rotational speed. A simple test rig for measuring of small generators which can be connected to the shaft, is decribed in report KD 595. For big generators one needs a test rig as described in report KD 78.

I advice to start with the generator and build and test a prototype for different load conditions. In my report KD 78, measured characteristics are given for a radial flux PM-generator for three different load conditions; constant voltage, constant current and constant resistance. When the characteristics of the generator are known for the wanted load condition, it is rather easy to design a rotor which matches to the measured Pmech-n curve of the generator. Starting with the rotor and designing the generator afterwards is much more difficult. Matching is described in chapter 8 of my report KD 35. All reports can be copied from my website: www.kdwindturbines.nl

[kitestrings]

The size of a PM-generator should be chosen such that there is good matching in between the windmill rotor and the generator.

While we probably have different opinions and advice as to how you might proceed, I agree entirely with Adriaan here.

Broadly, if you are direct-tied to batteries, and have too much prop (rotor area) you start out good in low winds, but run the risk of burning out the alternator in high winds.  On the other end of the spectrum if you don't have enough rotor area, you never really get the blades 'flying' where they are most efficient.  The rotor wants to follow a cube power curve as suggested, but the alternator without electronics (MPPT, or at least change in line resistance) pretty much wants to rise in linear manner relative to speed.  The result is we are forced to try to find a happy middle where the load match is reasonable - but there are compromises at best.

To complicate thing further, we spend an inordinate amount of time on theory, blade profile and coil configuration and the like, only to find out the load is also changing as the batteries are charged, loads are applied, and or other charge sources (PV) are introduced.

how can i get a lower cut in speed?

Stronger magnets is one way.  You've got that...  but, if you look at some of the designs that Hugh, the Dan's and others have worked out, I think you'll see rotors in the 7'-10' range with comparable magnet/coils to your initial proposal.  I would look to those as a guide, and either increase the rotor diameter, or build a less robust alternator.  They've both got some excellent books on the subject.  There are also several blade calculators - Hugh's, Alton Moore's (and, I've only glanced at Adriaan's compilations).

Early on you stated the project was "for fun".  I'd say jump in, and have at it, but use some of what's readily available.

~ks

[Adriaan Kragten]

There are two critical wind speeds which determine when a windmill starts generating energy.

The starting wind speed is the wind speed for which the rotor torque is equal to the sticking torque of the generator. The sticking torque of a direct drive axial flux PM-generator with no iron in the coils is only determined by the friction of the bearings and the seals. The sticking torque of a PM-generator with iron in the coils can be much higher and can strongly fluctuate if the generator isn't designed well. Another point is that you need some extra torque to accelerate the rotor. If the wind speed is only a little higher than the starting wind speed, it takes a long time for the rotor to reach the rotational speed where the generater voltage is equal to the battery voltage.

The cut in wind speed is the wind speed for which the generator starts to generate energy once the rotor is started. The most optimal situation is when the starting wind speed is equal to or lower than the cut in wind speed. A starting wind speed a little higher than the cut in wind speed is also acceptabel because the wind speed is always fluctuationg. But if the starting wind speed is much higher than the cut in wind speed, there may be long periodes with a non rotating rotor even if the wind speed is higher than the cut in wind speed. Low cut in and low starting wind speeds are only possible if the generator bearings and seals have minimal friction and if the windmill rotor has a starting torque coefficient which is high enough.

[Adriaan Kragten]

Assume that the starting wind speed is lower than the cut in wind speed, so there is no hysteresis in the Pel-V curve. For this condition the cut in wind speed is determined by the number of turns per coil. The more turns per coil, the lower the rotational speed for which the open DC generator voltage becomes equal to the battery voltage and the lower the cut in wind speed. However, the disadvantage of increasing of the number of turns per coil for a certain copper volume is that the coil resistance increases. So for a low cut in wind speed you get some power at low wind speed but this goes at the costs of a reduced generator efficiency at high rotational speeds and therefore you will loose a lot of power at high wind speeds.

Another problem of a low generator efficiency at high rotational speeds is that the generated heat has to be transferred to the surrounding air. If there isn't enough cooling, the generator winding can become too hot and will burn. So a low cut in wind speed is only allowed for windmills with a low rated wind speed. The rated wind speed depends on the safety system which is used and the safety system must be adjusted such that turning of the rotor out of the wind starts already at a low wind speed of about 6 m/s other wise the maximum rotor power at very high wind speeds will be too high for the generator and probably also for the electronic components in between the generator and the battery.

[kitestrings]

Particularly for these ironless-core axial's the start-up never seems to be an issue, and in my experience occurs well below the cut-in.  As you say, you basically just need to overcome the friction of the seal(s) and bearings.  Selection of where the cut-in occurs is probably more important in this application.  It's worth mentioning, however, that there is very, very little power in these low-end winds...

There are probably wind conditions where the turbine is spending a lot of time in range of speeds fluctuating near the cut-in, and perhaps some benefit is derived if the thing is spinning sooner, and sustaining inertia in the lulls near, but below, where production begins.  Otherwise, I don't see it as a "critical" wind speed except in a theoretical context.

~ks

[Adriaan Kragten]

Recently I measured a Chinese axial flux generator type TGET165-1.15kW of manufacture Hefei Top Grand. The measurements are described in report KD 595. This generator is standard supplied without a seal on the rotor shaft and the sticking torque is therefore very low. However, if no seal is placed, the generator can't be use for a horizontal axis wind turbine because water will pentrate in the back bearing. The back generator cover was provided with a chamber for a seal and I have placed a seal with dust lip. But only this seal gave a friction torque of 0.13 Nm resulting in a starting wind speed of more than 4 m/s for a rotor with a diameter of 1.65 m and a design tip speed ratio of  5.5. A reason of the high friction torque is that the shaft has a rather large diameter of 25 mm for such a small generator. Sometimes old car wheels are used for self made axial flux generators. These wheels have tapered roller bearings and these bearings can have a large friction torque if adjusted a little too tight. So one has to be alert other wise the windmill will simply not start at low wind speeds.

[albert sun]

i have come to the conclusion that i should just make bigger blades/machine. 30"-36" blades. i feel like at this speed i might feel better using wood that pvc, but i do have 8" pvc to use for blades.

electrondaddy1, can you explain your math a little better for me? i printed a circle divided by 12 and measured a 6" diameter circle. i placed my magnet at the edge of the circle and the space between magnets was a 1/4". is 6" the center of the magnet and is 6" really big enough for a backing rotor? i have two used 10" saw blades plus two 7.5" blades. i placed 4 magnets on one blade and stacked 4 blades high. i still had a paper clip stick through 4 blades, will a 1/4" inch be a thick enough for a backing rotor?

1"dia mags +.5"space between
1.5 x 12 mags = 18"circumference @ center of mags
+.5" radius of mags x 2 = 19" circumference
19/3.14=6.05" dia. backing rotor

i saw a build where someone used pillow block bearings. i was thinking that was a good idea but then again, if i am going to have someone cut disk for me, couldn't i just have them drill out holes and have them press bearings in my rotors? does anyone have good advice on bearings? i know about bearing slop, what are some of the tried and true methods that people have used?