The** ET 4063 research project**, funded by the German Federal Ministry of Research and Technology and the Industry [1], has clearly shown that the wind energy can make a contribution to securing our energy supplies. Fig. 1 shows the entire pilot plant used at that time.

# Torque measurement in wind turbines - as relevant today as it was in the past

## Background

### Torque transducers used in the 1980s

In the course of the research project, the **T30FN torque transducer** offering 10 kN•m nominal (rated) torque was used.

The** F** in the type name refers to the **frequency-modulated signal transmission method**. This means contactless measurement signal and energy supply of the rotor without any influence of the coupling factors, for example air gap variations.

The **N** in the type name designates** integrated magnetic rotational speed measurement**. The mechanical power as the generator input quantity can be determined from torque and rotational speed.

**Fig. 2** shows the **torque transducer** installed on top of the mast.

**Fig. 3** clearly shows the relationship between** wind speed and torque** [2]. Torque increases with increasing wind force - while rotational speed remains unchanged. The result: Additional mechanical power is generated. However, this also means that the generator can produce more electrical power.

## Status quo: Wind energy today

Today, the wind energy is one of the major renewable energies and continues to be a **market of the future** with attractive growth rates. Furthermore, energy requirements have rapidly increased and the demand for alternative forms of energy has virtually exploded as a result of nuclear phase-out.

At the beginning of the 1990s, a wind turbine's **average rated output** was **200 kW**. Today, it amounts to **2 MW**. There has been an increase in rated output by factor 10 over just under 15 years. This increase mainly results from **larger rotor diameters**. Doubling the rotor diameter gives a quadrupling of the effective area.

## Mechanical measurement quantities of a wind turbine

**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].

## Gear unit

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** convert****the 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

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*

## Conclusions

This article clearly shows how important torque measurement in wind turbines was years ago and still is today. There is **no power generation without rotation**, hence, there is **no power without angular speed and torque**.

### References

[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] http://www.energiewelten.de/

[5] Christian Scheer, Rainer Schicker: Energie wird knapp. Getriebe und moderne Drehmomentmesstechnik tragen zur Energieerzeugung aus Windenergie bei, Windkraftkonstruktion, http://www.windkraftkonstruktion.vogel.de/automation/articles/327594/