An anemometer of this type is disclosed in FR 2 514 512 A. With this anemometer the rotor on which the wind-catching profile is located is continuously electrically driven.

As a result of the prevailing wind there will be variations in the speed of revolution of the rotor within each r'otir. These c"m be detected by a signal processing unit and, on the basis thereof, wind speed and wind direction can be determined. However, the wind speed detennination displays systematic deviations because the determination is dependent on the air density, which usually is not accurately known.

Reliable determination of wind speed and wind direction is of growing importance in view of the increasing growth in the generation of energy with the aid of wind. Especially in the case of offshore wind turbines, knowledge of the wind speed is of particular importance, both with regard to the operation of the turbines and for determination of the point in time when these are switched off. Reliability is of crucial importance because of the difficult accessibility and the associated high maintenance costs.

Although such a meter functions well in practice, because of the severe conditions to which such an anemometer is subjected there is, in the long term, a not negligible risk that the latter no longer functions correct. For instance, it is possible that the friction in the bearings increases or that during a revolution there is higher friction at certain points than at other points.

This leads to inaccuracies. Consequently, in the state of the art it is necessary regularly to maintain, for example to calibrate, such anemometers in order to obtain a reliable measurement. Because such anemometers are frequently installed in inaccessible locations, for example offshore, this maintenance is associated with appreciable costs.

The aim of the present invention is to avoid these disadvantages and to provide a method by means of which it is possible to check such anemometers in a simple manner.

This aim is achieved with a method as described above, in that, in the event of a lack of such changes in speed, the power taken by the motor is compared with a target value and

the condition of said anemometer is determined from any difference.

According to the invention, it is possible by electrical means not only to determine the wind speed and wind direction as is known from the prior art, but also to determine the condition of an anemometer. This means that it is no longer necessary for the tester to obtain physical access to such an anemometer. Such a determination can also be carried out remotely. If the results of the determination are unsatisfactory, it is still to possible to decide to replace such an anemometer.

However, according to an advantageous embodiment of the invention it is possible to incorporate a correction signal in the evaluation unit, as a result of which the meter is still able to function and does not have to be replaced and the measured result can be corrected to rectify the fault that has been detected. That is to say, the wind speed can be calculated independently of said friction. Consequently, it is possible to operate the anemometer for a very long period. After all, it is possible with the aid of the signal processing unit to correct inaccuracies during operation. Moreover, the meter can transmit a condition signal to the maintenance cngineers.

According to the invention, the determination of the condition of the anemometer can also be carried out in that the rotor is driven at two speeds of revolution. Starting from the first state and the first drive power, the power is switched to a higher or lower second drive power and the state thus obtained is compared with the calculated state. Of course, it is also possible to change to a second state and to compare the power. With this procedure, the condition of the anemometer can be determined from the variation in the torque and the average torque (wind speed).

Furthermore, it has proved possible to determine the air density, or the density of the fluid in which the meter is used, by means of the present invention. To this end, according to the invention not only the measured variation in torque, but also the average torque is determined. It has been found that there is a relationship between variation in torque on the rotor, the density and the average torque. The variation in torque on the rotor bears a linear relationship to the density, a linear relationship to the wind speed and a linear relationship to the speed of revolution. The torque averaged over one revolution likewise bears a linear relationship to the air density, but a relationship to the square of the speed of revolution (of the rotor). From these two relationships it is possible to determine the density of the fluid.

This variation in torque and average torque can yield the air density only if the friction in the bearings is essentially disregarded. Should it be the case that the friction in the bearings

does play a role, measurements must then be carried out at two speeds of revolution, as has been described above. If the density is known, the wind speed can be determined more accurately because the systematic error introduced by the unknown density, as when measuring the variation in torque at one speed of revolution, is no longer present.

It will be understood that by analysing both the average torque and the fluctuation in torque at two speeds of revolution it is possible to determine whether the anemometer is functioning correctly. Rights are also explicitly applied for in respect of a method for operation of an anemometer where the condition thereof is not determined.

It is possible to construct the device as a wireless installation by transmitting the signals obtained to a processing unit, which, for example, is on the ground, optically, by radio or by means of sound. In the case of an electric motor the power supply can be provided by a battery installed close to the electric motor and optionally charged by a solar cell or another source of energy. Such another source of energy can be a small wind turbine. Consequently, long connecting cables are not needed for some applications and the whole can be set up as a unit. In the case of applications high in the air, this means that a lightning strike does not result in extensive damage.

The invention will be described below with reference to the drawing of an illustrative embodiment of the invention. This is shown in the single figure.

The anemometer according to the invention is indicated by 1. This anemometer consists of a rotor 2, which, for example, can be provided with a number of permanent magnets arranged around the periphery. The rotor shaft is indicated by 3 and is mounted on bearings in a manner not shown in more detail. Close to one end, the rotor shaft is provided with a rotor arm 4. One end of arm 4 is connected to a sphere 6, whilst the other end is provided with a balance weight 5.

The stator which operates in conjunction with the rotor is indicated by 7. The stator consists, for example, of a number of coils 8, each connected to a signal processing unit 10 which is also provided with a power source 12. A display is indicated by 11.

The installation described above functions as follows. An accurately determined power supply (alternating voltage) is provided by the power source 12, by means of which the rotor/stator assembly functions as an electric motor and rotates at constant speed of revolution. With the aid of the signal processing unit 10 it is possible to check how much power is required (when there is no wind) to maintain a constant speed of revolution. By predicting the fluctuation in torque at another speed of revolution and comparing this with

measurements it is possible to determine and if necessary to correct the condition of the anemometer. In addition, there can be a memory in which the"nominal"data of the anemometer are stored. These can comprise the calibration data which, furthermore, have been determined from tests in a wind tunnel.

If there is wind, a larger torque will arise during part of a revolution as a result of the wind catching sphere 6 and there will be a smaller torque during another part of the same revolution. The average torque is also affected. This has consequences for the signal fed to the signal processing unit. The signal, which, for example, is sinusoidal, will be deformed.

This deformation gives an indication both of the wind speed and wind direction and of the air density. These variables can be determined by determining the change in torque.

However, it is also possible to take a decision immediately with regard to the wind condition without first determining the torque. Determination of the air density can be important in some cases. Wind turbines are nowadays switched off at a certain speed to prevent overloading. However, this loading is partly dependent on the air density. By determining the air density in accordance with the invention it is possible in practice to keep the wind turbine in operation up to higher wind speeds without the risk of it being damaged.

The condition of the anemometer described above can, for example, be determined as follows. If no fluctuation in torque is detected it can then be concluded that the wind speed is zero. The torque that is needed to move the rotor results partly from the aerodynamic resistance and partly from (lower) friction. The aerodynamic resistance is known and is determined by the geometry of the profile that is under the influence of the fluid and the fluid itself (such as the density). It is possible to conclude from the measured values whether or not the low friction falls within the desired limits.

On the other hand, if the density is determined and large fluctuations, for example of more than 10%, are found, it can be concluded that problems are being experienced with the rotational behaviour of the rotor.

The signal-processing equipment is able to determine whether the rotor is rotating.

With the aid of the invention it is possible to establish whether the rotor is still operating as desired.

It will be understood that, by adopting the approach of constructing the anemometer as an electric motor, which is completely contrary to the prior art, numerous new possibilities open up for determination of the various variables associated with the speed and direction in which the wind blows. These will be immediately obvious to a person skilled in the art after reading the above description and fall within the scope of the appended claims. It must also be understood that the installation according to the invention is suitable for determining the flow behaviour of other types of gases and even liquids.