Intelligent wind power unit with tandem wind rotors - Wind Power Generation and Wind Turbine Design - page 345

that both torques show the same absolute value irrespective of the relative rota-
tional speed and the output/load, because the rotational torque should be always
dynamically counter-balanced between the inner and the outer armatures.
Figure 19 shows the rotational torques T F and T R at the various relative speeds
N T against the induced electric current I G , where these torques include the mechan-
ical torques in the generator and T R is given by the absolute value. That is, these
are the shaft torques in practical use. The rotational torques of both shafts T F , T R
are directly proportional to the induced electric current I G irrespective of the rota-
tional speeds and/or the external load but the shaft torque T F of the outer armature
is slightly larger than T R of the inner armature due to the mechanical torque of the
larger bearings.
The output P and the induced voltage E against the induced electric current I G
are determined by the relative rotational speed N T (= N F N R ), as shown in Fig. 20.

The output increases with the increase of the induced voltage E at the same I G ,
while E is proportional to the relative rotational speed N T and I G determines the
rotational torque as presented in Fig. 19.

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5 Demonstration of intelligent wind power unit

5.1 Preparation of the tentative tandem wind rotors

5.1.1 Front blade design
Figure 21 shows the drag and the lift coeffi cients C D , C L against the angle of attack
a for MEL012 aerofoil [10]. The angle giving the maximum C L is a = 11°, but the
angle a = 8° was chosen as the design dimension irrespective of the radial position,
so as to avoid the fl ow stall on a large scale at the lower rotational speed. The chord
of the blade was designed taking the traditional blade element theory [11] into

24

N T N F N R T F - T R

20

min -1 min -1 min -1
300 150 -150
900 150 -750
900 750 -150
1500 750 -750

16

12

8

4

0

0 2 4 6 8 10 12 14

I G A

Figure 19: The output and the induced voltage against the induced electric current.