Balancing is a procedure for adjusting the center of gravity of the blade and hub assembly to the exact center of its rotation. There are several good methods for balancing the blades. This one consists of two stages:
- Check that each blade has the same moment of weight, and
- mount the three blades symmetrically on the turbine.
It is not sufficient to simply check that each blade weighs the same since the centers of gravity might be in different places. The final test is to compare the “moment of weight” of one blade with another—the turning effect produced by the blade’s weight. The moment of weight depends on how far this weight is from the center of the shaft. Multiply the weight by the distance.
Make a balancing jig that allows a blade to pivot on an axis passing through its center of rotation. The blade tip must be the same distance from this axis as it would be from the center of the wind turbine. You can then measure the moment of weight by weighing the tip of the blade or by comparing two blades teetering on the same pivot axis. Take care to measure the weight of each blade at the same distance from the pivot so that you obtain an accurate comparison. The actual weight does not matter, but it is important that they all weigh the same when measured at that point and, therefore, have the same moment of weight. It is equally accurate to compare two blades by mounting them on the same pivot axis but projecting them in opposite directions—if they balance, then their moments are effectively the same.
If the blade is solid, you will need to add weights to the root. Much heavier weights will be needed here because they work at a much smaller radius from the axis. Often, blade-fixing bolts are used to attach these balancing weights. Longer bolts may be needed if the weights are thick.To adjust a blade’s weight, add some mass somewhere. In the case of hollow blades, you can inject some filler paste inside the tip. Drill one or two holes into the interior, very close to the tip. Then find out how much body filler is needed by trial and error, placing the paste on a scrap of paper or plastic on the blade tip. When you have discovered the desired weight, mix the paste with hardener and force it all through the holes, smoothing and sanding the surface.
When the blades are identical, mount them symmetrically on the rotor hub and check the distances between the three tips. Each pair of tips must be the same distance apart (±0.5%). Try to pull the blades in the direction of their rotation as you tighten the bolts, so that they will not move thereafter when they are cranking out power.
Verify that the tips of the rotating blades track each other precisely in the same plane, neither upwind nor behind the others, and have the same angle or “pitch.”