Sat, 30 Aug 1997 10:42:55 -0700
Tom W. Rentz (econergy@whidbey.net)
 
I promised to provide information on the low rpm DC motor once it had arrived. It was purchased from the Surplus Center, 10151 W. "O" St., PO Box 82209, Lincoln NE, 68509, 1.800.488.3407 Not sure how many they have in stock??
 

>From my previous posting:
They have an interesting 194 rpm 24 vdc, 13 amps, motor, item #10-1519 ($49.95) that weighs in at 40 lbs. Since it is a permanent magnet design it may be suitable for a moderate output direct propeller design system (about 300 watts)
 

The heavy box arrived (Approx. 40 lbs). I opened it immediately and the first thing I checked was the torque required to turn the shaft (at least by hand rotation). It was stiff.. I thought perhaps it had a built-in unanticipated gearbox. But alas, I removed the brush cover and confirmed that the main drive shaft speed matches the rotational speed of the main shaft. The main component of drag is the permanent magnet "notch" that is typical for permanent magnet motors. I am impressed with the heavy duty construction, heavy duty wiring, and the adaptability for weather-tight sealing. It likely was intended for indoor use rather than outdoor, so some modifications should help for outdoor use. A high quality rust preventing enamel or epoxy paint should help a lot. A sealer such as silicon around the removable brush cover and end covers should help keep moisture out. Be sure to reseal this if you later remove the cover to work on the brushes, etc. A good fitting but not too tight "O" ring or rubber washer, which is lubricated at the shaft only and secured with silicone should help prevent moisture from entering the shaft bearing. I would advise painting the drive shaft as well to prevent rust.
 

It looks like it is suitable for home experimenters as it definitely performs as a 194 rpm motor at 24 volts. It will require a turbine with substantial starting torque to get the shaft started, due to the strong "notch" effect, and the 4 brushes, which do create a certain amount of drag, as well. Once it starts, the notch effect is no problem for efficiency, since the magnets repel as well as attract during rotation. This may be a good unit for experimenters since its cost is minimal and it offers much potential for results. It will require a 40-50 Amp. blocking diode to prevent the battery bank from discharging when wind speeds are low.
 

My first estimations are that it will perform best (with higher output) on a 24 volt system with a turbine as described below. It has a continuous duty rating and heavy windings. It may be possible to run it at up to 300 or 350 rpm to get more output since it is such a heavy duty unit, especially if one outfits the smooth outer casing with some cooling fins by removing the paint and using industrial grade epoxy to fasten them in-line with the shaft (then refinish as per previous paragraph). Remember that it has to begin charging at about 200 rpm for a 24 volt system, so the turbine must be capable of  higher speeds. If you use it on a 12 volt system, it will begin to charge at about 100 rpm, but it will have a much lower output. However, the unit is heavy enough that it may surprise us.
 

Perhaps a light weight but strong wood turbine, hand laminated with water proof glue and carved from Sitka Spruce or other clear/strong wood would make a good experimental turbine (2 ea. 2" X 8" clear per blade), it will require a lot of carving and a lot of wood needs to be removed to get an approximate 45 degree pitch from the root of the blades with a gradual taper to 10 or 12 degrees at the tips). I have carved them using a draw knife (used to debark logs). I have successfully made light weight blades from Redwood, but I advise using fiberglass and cloth for extra strength.  The turbine should have 3 or 4 blades, with a strong pitch within about a 2 to 3 foot radius of the shaft, to assist in starting and overcoming the notch effect. This should  gradually taper to about 10 to 12 degrees at the tip -- I would estimate a turbine of about 7.5 feet in diameter for a 3 or 4 blade unit. Blade tips should be thin and no wider than about 3-4 inches to effect good tip speed and adequate rpm. (Suggest purchasing or checking out from the library some of Hugh Piggot's books, David Egglestons if you can really get into technical jargon, or Michael Hackleman's. These all have a great deal of valuable information).  Sealing the wood blades with an epoxy paint is a good idea since moisture can eventually load up in the blades and cause an out of balance condition. The turbine can be statically balanced by hanging it from the center with a rope positioned exactly on center. Weights can be added to the lighter blades until the unit sits level. When doing this, try to determine the center of gravity for the heavier blades by balancing them on a knife blade locked into a vise. This will give you an idea as to where to place the weights on the lighter blades, trying to match the weight distribution of the heavier blades.
 

I make no claims as to its performance, just that for those who like to experiment, this one might be a good one to work with, since low rpm generators are quite difficult to come by unless you are willing to spend big bucks -- even then, they are hard to find. (Please note that today's commercially available wind generators do not need brushes, they use rotating magnets, not a rotating armature. The brushes will need replacing eventually, and the armature will need to be surfaced.) These operate and are maintained a bit like the generators of the automobile pre-alternator era.
 

Tom Rentz,  Sun/Wind Concepts

 

Further notes:
 

You can make successful wind generators with higher speed alternators up to several thousand rpm. However, the gearing up required consumes power, therefore, the turbine must be quite large to compensate. This all increases the price per watt of output. It is necessary for large turbines, but smaller turbines are more cost effective if they can be direct drive. In 1998 we made 2 experimental units with epoxy and fine fiberglass cloth coated styrofoam. The styrofoam was easy to cut, shape and form. These can be shaped in sections of 4 to 6 inches in length. The epoxy was a bit messy to work with but made effective turbine blades. The smoother the finish, the higher the attainable speed. They are very light and required little balancing. We used 3 foot pieces of 1" EMT conduit to mount the blades to the hub, utilizing 1" I.D. U bolts to connect them to the hub. This allows the turbine blades to be adjusted for maximum starting torque and most effective operating speeds. Hubs can be metal or even heavy plywood. The hub I built used two 3/4" X 1 foot diameter discs, laminated with waterproof wood glue and screwed together (creating a 1 1/2" thick hub). A 6" heavy-duty pulley with the correct shaft size served as an adequate mount for the hub and also provided a means to secure the turbine to the motor shaft.  A 1" hole was drilled in each segment of the styrofoam as it was laminated and carved. We used 'water based' pressurized can foam to laminate and glue the sections together and to securely connect each piece to the 1" conduit. (Note that liquid fiberglass will chemically melt styrofoam.. this is why epoxy is a good choice for this blade). A Trace C40 can be used as a battery shunt regulator with a diversion load to keep the battery bank from overcharging.