Hugh Piggott's blog

I mostly like everything that people do with small wind turbines, especially hands-on and do-it-yourself, so I got excited to discover Missouri Wind and Solar, and I watched this video that I found on the home page with interest.

Unfortunately I was a bit disappointed as there was almost nothing the Jeff said that could begin to agree with.  Here are just some of the main points of disagreement.  If anyone wants to chime in and explain what Jeff was thinking of I would be grateful but to be honest I am afraid I just don’t get it 🙁

• AC versus DC transmission in cables.  At 4.40 Jeff states that “3-phase AC output, you can simply use an extension cord and run it 200 feet with literally no loss whatsoever.”  In fact the 3-phase AC cable supplying a rectifier has to carry the same current as the DC cables that follow after, but the difference is that they are only working part time.  So you need 3 wires the same size as the two DC ones would be to achieve the same efficiency.  I wonder if Jeff has tried putting 50 amps of AC down his extension cord, or maybe not.  I mostly use AC transmission and my rule of thumb is that for the same voltage you need the same wire size for AC as you do for DC (to have the same power losses).
• Solid core wiring.  At 6.50 Jeff says to never use solid core wiring for DC current.  I think he has some garbled idea about skin effect however this only affects AC current, and at low frequencies it would only be significant at huge wire sizes having solid cores 3/4″ or more thick.  solid core wire will work fine for AC or DC.  Often stranded wire is easier to work with but weight for weight they have the same resistance.  the coils in the alternator are wound with solid core wires.  I use both types of wire on AC and on DC.
• Seven blades will start at lower windspeeds.  AT 8.50 Jeff contends that 3-bladed wind turbines cannot start in low winds whereas machines with multiple blades can start at lower windspeeds.  In fact the start-up depends also on the friction in the seals, the losses in the stator core, and above all upon the diameter of the blades.  Blade diameter matters more than blade number, and this is not something Jeff mentions at all.  You can use more blades to get more torque (at lower rpm) to overcome friction etc (and to sell more blades), but the key to low wind performance is large blade diameter (longer blades to catch more wind).  A multi-blade will turn slowly in low winds but there is no real power to be had from small rotors in low winds, and the rpm is too low as well.
• Odd numbers of blades.  At 10.40 things get really bizarre when Jeff tells us that blade rotors need to have odd number of blades.  I wonder if he ever heard of the old Dutch windmills with 4 blades? 2-bladers have their issues with vibration which you can read about elsewhere but they are faster than 3-bladers.  For many years (1970s and 80s) when I needed maximum rpm speed I only made 2-bladers.  3-bladers run more smoothly and if you don’t have excessive friction or a tiny diameter then 3 is all you need.  Odd or even makes no difference.
• Blade pitch and stall.  At 14.30 Jeff tells us about aluminum blades with coarse pitch.  He claims that “at 15 mph these blades will throw themselves out of the wind and stall”.  In fact the coarse pitch is a good strategy for a low speed, high torque blade rotor.  If you combine the large pitch of these blades with the large blade number that Jeff uses (to achieve the low wind start-up) then you would get the best result for an efficient low speed rotor, suitable for pumping water etc.  The problem is not “stall” in this case but simply coarse pitch which limits rpm.  Rpm in relation to windspeed is determined by “tip speed ratio” and these aluminum blades are designed to work at low tip speed ratio so they need a very slow generator to work well.  Stall however is the opposite problem, when high speed rotors (with a flat pitch angle and fewer blades) have too much load torque holding them back so they cannot achieve their best (high) tip speed in relation to the wind.  Best results are achieved with larger diameter blades (with high tip speed ratio) and a good understanding of how to match the generator load to what the blades can do over a range of windspeeds.  There is not much sign of these lessons being learned, unfortunately.
• Towers at 17.40 we learn that “Most wind turbines fit on an inch and a half pipe”.  This confirms that Jeff’s experience is pretty much limited to the world of vehicle alternator wind turbines with diminutive blade rotors that can’t produce more than a few amps (or he would have already set fire to his extension cords).
• 18.10 “you can simply take a pipe, stick it in the ground, concrete it in up to twenty feet” yikes!  that would get bent over pretty quickly where I live!  You need to put guys on the tower, fairly close up to the tips of the blades so as to support the pipe.  That is if you are generating serious power output.

I won’t get into my comments on the missouri wind and solar controllers here.  I have just touched above on a few of the problems I have with Jeff’s instruction material.  I hate to be negative about this, and I have no wish to start a big argument, but I can’t really leave this stuff out there without challenging it.