It is not that long ago - end of the 1970s, early 1980s - that it was not yet certain whether the wind energy would be able to generate electric power in a productive way. To clarify this, measurements were required - and the result is present everywhere today.
The output of a rotating body is obtained from the product of torque and angular speed.
P = Output in N•m/s (1N•m/s = 1 W = 0.00136 metric hp)
M = Torque in N m
ω = Angular speed in s-1
N = Rotational speed in rpm
Transformation and some other steps give the relationship for torque, the quantity to be measured.
The calculated torque must by no means be directly used as the basis for selecting the torque flange, because it does not take into account any additional influencing factors, for example starting performance or vibration. General information about torque measurement is provided in [3].
In wind turbines, there is a "conflict of interests" between the rotor's drive speed, limited, for example, by pitch speed, and the required rotational speed of the generator. With two pole pairs, a rotational speed of 1500 rpm is required for a mains frequency of 50 Hz. [4]. The solution is to use a gear unit. Gear units convert rotational speed and torque and transmit high power. In a modern multi-megawatt wind turbine [5], they are used to convertthe rotor's low rotational speed of approximately 14 rpm into the generator shaft's high rotational speed of approximately 1400-1650 rpm. This conversion involves a reduction of the high rotor torque according to the gear ratio. Fig. 4 shows a type T10FM* torque flange from HBM with 40 kN•m nominal (rated) torque used at the generator input end. Wind turbine gear units weigh many tons and in most cases are compact, combined planetary-spur gear units. Even though wind turbines without a gear unit are being discussed today, the torque generated by the rotor blades will always need to be very high to generate sufficient electrical power. * The T10FM torque flange isn't sold by HBM anymore. Follow-up model is the digital T40FM torque flange.
Torque to be measured often ranges from the kilonewton range (kN•m) up to several Mega-Newtons (MN•m).This is to be illustrated by the following example:
Generator: P=2 MW
Gear unit: 1:100
The generator power of 2 MW and a rotational speed of 1500 rpm give the following formula:
(1) MD=12.74 kN•m / n=1500 rpm
(2) MD=1.3 MN•m / n=15 rpm
Bigger generators with lower rotational speeds are being discussed. However, the torque transducers will then reach their limits as well. Fig. 5 shows the implementation of a 1.5 MN transducer and a design proposal for higher nominal (rated) torques.
Fig. 5: Implementation of a 1.5 MN transducer and design proposal
However, the traceability of a calibration of this huge torque transducer is not guaranteed. The German National Metrology Institute (PTB) in Brunswick, Germany, houses the world's largest torque calibration machine at present. It enables test equipment up to 1.1 MN torque to be calibrated with 0.1% measurement uncertainty [6]. HBM's current torque calibration offer is shown in Fig. 6.
Fig. 6: HBM's torque calibration offer
[1] Herbert Lauer: Die Windkraft meßtechnisch erfaßt, Markt&Technik No. 44 dated October 30, 1981
[2] MESSTECHNISCHE BRIEFE, MTB 17 (1981) Issue 2, Published by Hottinger Baldwin Messtechnik GmbH, 64293 Darmstadt
[3] Rainer Schicker, Georg Wegener: Measuring Torque Correctly, ISBN 3-00-008945-4
Published by Hottinger Baldwin Messtechnik GmbH, Darmstadt
[4] energiewelten.de
[5] Christian Scheer, Rainer Schicker: Energie wird knapp. Getriebe und moderne Drehmomentmesstechnik tragen zur Energieerzeugung aus Windenergie bei, Windkraftkonstruktion