An Affordable Wind Generator You Can Make Yourself: The Blue Max
(Page 3 of 4)
May/June 1985
By the Mother Earth News editors
Whether you're merely curious about the viability of wind power or determined to apply the potential of this form of alternative energy, our home-built plant represents a cost-effective means to that end . . . and a chance to be one of "them that's doin' "!
RELATED CONTENT
Rock Port, Mo., took advantage of the power generated from the nearby Loess Hills Wind Farm in a bi...
More than 100 schools in 30 states have installed wind generators, and many more are doing the rese...
This 1975 article on wind generators includes notes on firsthand experiences, thoughts on finding u...
Wind power has enormous potential, is the fastest growing form of electricity and creates no pollut...
The American Wind Energy Association is predicting a record-breaking year for wind power....
Fine-tuning: Torque vs. Power
Our windplant's sail-wing design eliminates many of the construction headaches associated with hard-skinned, internally supported blades . . . while still retaining an acceptable portion of their efficiency. This is due, in part, to the fact that we've combined traditional fabric-over-frame construction with simple but effective blade pitch controls to allow the rotor near-optimal performance at both start-up and working velocities.
Because the alternator requires a fairly stiff (15.75-to-1) gear-up to bring it into its prime-efficiency RPM range at reasonable wind speeds, a good deal of initial torque is needed to start the plant from a dead stop. Conversely, once the alternator and the mass of the rotor are set in motion, power (the rate at which work is accomplished) — not torque — becomes the critical factor.
To satisfy this inconsistency, we incorporated a series of mercury switches into the jackshaft output sheave to control energization of the alternator's field windings. These keep the field circuit open (and the alternator unloaded) until the rotor reaches a speed of about 56 RPM, at which point centrifugal force closes the switch bank to activate the alternator.
However, this cut-in governor is only the first step in prompting an efficient transition from start-up to full operation. More effective — especially at greater wind velocities — is aerodynamic control, in our case accomplished by varying the blades' pitch to suit generator load and wind speed. Simply stated, an extreme pitch angle is necessary to develop torque under high-load conditions . . . but the broad profile presented by those cocked blades only gets in the way as wind (and rotor) velocities increase. The solution is to decrease the pitch angle under these conditions and allow the fabric airfoils to pull the blades along their rotary path with minimal drag.
The very nature of a rotating body such as the hub and blade assembly favors automatic pitch control: The blades "want" to seek their plane of rotation as the rotor gains momentum . . . so much so, in fact, that we've included several compensating features in our design to regulate the rate and degree of this in-transit "upshift."
First, you'll note in the blade detail diagram that the blade shaft joins the frame mount off-center, so the blade's trailing section is broader than its leading one. This inequality is adjustable (notice the series of mounting holes) and assures that even mild wind pressures return the blades to the full-pitch position for start-up. It also serves to buffer the potentially damaging effects of strong gusts at speed; these should be enough to re-pitch the blades to their "slow" position, thus spoiling airflow and reducing the rotor velocity.
