In this chapter an idea is explored of placing two rotors on a single pillar. In our experiments adding another set of blades to a horizontal-axis turbine, such as that in Figure 5-1, did not improve power output. The extra weight of the second set of blades overcame any added rotation speed. However, adding another generator on the same pillar can provide more power.
Counter-Rotating Blades
Figure 9-1 shows the dual-rotor, dual-generator design. The blades are arranged to counter-rotate to avoid problems in balancing the pillar when the blades are spinning. Counter-rotation means that when viewed from one side of the turbine, one set of blades is going counterclockwise and the other set is going clockwise. This counter-rotation is appealing to watch—videos can be found on hightechlego.com. The same pedestal was used for the other turbines built in earlier chapters. Blades have been used of the same design used for the horizontal-axis turbine in Chapter 5, except a set has been made for clockwise rotation. The counterclockwise blade set is in blue tape, while the clockwise set is in clear tape. Figure 9-2, and instructions following the figure, provides the design for the dual-generator configuration. Mindstorms’ dual-bevel gears are used here for the gearbox for a change of pace from the Technic gears used in earlier chapters.
Figure 9-1
The dual-rotor turbine features two generators on one pedestal
Figure 9-2
The dual-generator design has two #71427c01 generators facing opposite directions
Electrical Connection
Each of the generators needs a connection by a #5306 Wire, and each of these two connectors could go to separate storage batteries. But the connection can also be made in a parallel circuit by snapping the #5306 connectors on top of each other, as shown in Figure 9-3. This combined connector can then be fed to a single rechargeable battery, as was done in Chapter 8 with connection to the #89668 Rechargeable Battery. The power coming out of the dual-rotor turbine is almost twice that of a conventional single-rotor design. It’s not a doubling of power, as might be expected, because there is some loss in the dual-rotor design for two reasons: (1) the front rotor steals some of the wind strength from the rotor behind it, and (2) the parallel electrical connection involves some power from one generator being sent to the second generator and vice versa. The second point of the generators’ electrically interacting with each other can be seen by turning off the test fan and manually turning one rotor—the other rotor will rotate.
Figure 9-3
Cables from the two generators can be connected in parallel by stacking #5306 wire connectors
Summary
This chapter attached two rotors, along with two generators, to a single pillar. The two rotors were made to rotate in opposite directions to balance the forces applied to the pillar when the rotors are spinning. With two generators producing electricity, the electrical power is almost double that of a conventional single-rotor turbine. The power from the two generators can be combined in parallel to charge a battery. In the next chapter, another advanced turbine design will be explored by using pneumatic pressure generation rather than electrical generation.
