The present invention relates to a wind turbine comprising a generator with a generator rotor and a stator, with a gap between the stator and the generator rotor and with magnets and electro-conductive windings at opposite sides of the gap. The invention also relates to a method of monitoring the length of the gap, particularly during operation of the wind turbine .

Generally, a wind turbine generator includes a rotor having two or three blades, although turbines with one blade or four or more blades also exist. The rotor is typically mounted to a shaft within a nacelle on top of a tower. The blades, attached to a rotatable hub, transform mechanical wind energy into a mechanical rotational torque that drives the generator rotor of a generator. The generator rotor can be coupled to the rotor through a gearbox or the coupling can be gearless, as in so-called direct-drive wind turbines. The generator converts the rotational mechanical energy to electrical energy, which is fed into a utility grid.

Generators typically comprise a generator rotor and a stator which are separated by a cylindrical air gap. During operation, a magnetic field, generated by a plurality of magnets, passes through a portion of the air gap. The magnets can, e.g., be permanent magnets, wound magnets mounted on the rotor, and/or currents induced in the rotor iron. The effective transmission of the magnetic field through the air gap is at least partly dependent on the length of the air gap. In this context, the length of the air gap is the radial distance between the rotor surface and the stator surface. In operation, loads on the rotor are introduced by wind force via the blades. Such loads can deflect the generator rotor in such a way that the air gap length is changed or becomes nonuniform. If the radial air gap length becomes too small, the rotor and stator components can hit resulting in severe damage to the generator.

EP 1 870 566 Al discloses a wind turbine with a generator comprising a stator and a rotor. The air gap between the stator and the rotor is measured by a number of sensors and adjusted by blade pitch control. During operation of the wind turbine, strong magnetic flux occurs in the air gap. This hinders accurate and reliable measurement of the air gap length. Moreover, since the sensors are located near the air gap, the sensors cannot be easily replaced.

The object of the present invention is to provide a wind turbine having an air gap measurement system that can accurately measure the length of the generator air gap during operation, notwithstanding the strong magnetic flux.

The object of the invention is achieved with a wind turbine comprising a generator with a rotor and a stator, with a gap between the stator and a first surface of the rotor and with magnets and electro-conductive windings at opposite sides of the gap, wherein the wind turbine comprises one or more distance measurement means for measuring the distance (L' ) between a second surface of the rotor and a stationary part of the generator free from magnets and windings and at a radial distance from the gap. This way, the length of the gap can be accurately measured at a location where interference by the magnetic field of the magnets is effectively reduced.

Preferably, the distance measurement means are positioned outside the scope of the magnetic field generated by the magnets to measure the distance between the second rotary surface of the rotor and the stationary part of the generator without effective interference of a magnetic field generated by the magnets over the gap. At the location of the distance measurement means the strength of the magnetic field is too little to reduce measurement accuracy to an unacceptable level.

In one embodiment, the generator rotor comprises an outer surface facing a first surface of the stator and an inner surface facing a second surface of the stator to form a second gap outside the scope of the magnetic field generated during operation. This way, the stator encases the rotor. In such a case, the inner surface of the rotor can comprise a flange extending in the direction of the second stator surface. This way, a second gap can be formed at safe distance from the magnetic field generated in the first air gap.

In an alternative embodiment, the stator comprises an outer surface facing a first surface of the generator rotor and an inner surface facing a second surface of the generator rotor to form a second gap outside the scope of the magnetic field generated during operation. In such an embodiment, the stator is enclosed by the generator rotor.

To obtain even more reliable gap length measurements over the full circumference of the air gap, the length of the air gap can for example be measured at two or three or more locations, which may be circumferentially equidistant. If two distance measurement means are used, they can be arranged to measure the distance in two orthogonal dimensions. The second air gap can have a length of the same order as the first air gap between the stator and the generator rotor, or it can be smaller or wider, if so desired.

The distance measurement means can for example be a non- contact sensor, such as for example an inductive proximity sensor, a capacitive proximity sensor or an optical distance sensor. Combinations of these types of sensors can also be used.

The wind turbine can also include an air gap adjustment assembly configured to modulate the air gap length responsive to a signal from the distance measurement means. The air gap adjustment assembly can for instance include a pitch drive for adjusting the pitch angle of one or more of the blades. This way, the load on the blades can be reduced or increased and a more uniform load distribution can be obtained, resulting in a more uniform air gap length. The blades can for example be pitched individually or simultaneously.

Such an air gap control system facilitates an efficient and effective mechanical load transfer scheme. It also enhances efficiency and reliability of the generator and can effectively reduce costs for maintenance, repair, and outages of the wind turbine.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1: shows a cross section of a wind turbine according to the present invention; Figure 2: shows in cross section the upper half of the generator of the wind turbine in Figure 1. Figure 1 is a cross section of an exemplary wind turbine 1 according to the present invention. The wind turbine 1 comprises a rotor 2 with a rotatable hub 3. Attached to the hub 3 are three rotor blades 4. The wind turbine 1 comprises a nacelle 5 and a generator section 6. The generator section 6 comprises a stator 8 connected to the nacelle 5, and a generator rotor 9 connected to the hub 4 via a bearing ring 16. The nacelle 5 is mounted on top of a tower 10, supported by a ground surface such as a platform or sea bottom.

The nacelle 5 is rotatable by means of a yaw drive. During operation, the nacelle 5 is rotated to turn the rotor 2 towards the wind direction, indicated by arrow W in Figure 1...

The generator section 6 is shown in more detail in Figure 2. The generator section 6 comprises a nacelle side 7 bolted to the nacelle 5, and a hub side 12, bolted to the hub 3. A central hollow conical carrier 11 runs centrally through the generator section 6. The conical carrier 11 has a base 13 of a large diameter at the nacelle side 7 and a section 14 of a smaller diameter at short distance form the hub side 12. On the section 14 a cylindrical extension ring 15 is mounted forming the stationary race of a bearing surrounded by a cylindrical ring 16 forming a rotary bearing race of the bearing. A cylindrical flange 17 is mounted on one side of the cylindrical ring 16 to connect ring 16 to the hub 3. On the other side of ring 16, the ring 16 is bolted to an inwardly extending radial flange 18 of a cylindrical generator rotor 9. The cylindrical rotor 9 also comprises a second inwardly extending radial flange 20 at a distance from the first radial flange 18. The second radial flange 20 has a free outer end 21 facing a rib 22 on the outer wall of the conical carrier 11. The outer surface 23 of the cylindrical generator rotor 9 is provided with a series of parallel permanent magnets over its full circumference. A cylindrical stator 25 is provided with an inner surface 24 with windings, which faces the outer surface 23 of the rotor 9. Between the rotor outer surface 23 and the stator inner surface 24 is an air gap 26 with a radial length L. The cylindrical stator 25 is attached to a cylindrical exterior wall 27 capped at one end by a first radially extending ring shaped end wall 28 having its inner peripheral edge 29 connected to the base 13 of the conical carrier 11. At its other end the cylindrical exterior wall 27 is closed by a radially extending end wall 30, having an inner peripheral edge 31 with an air seal 32 to seal the interior of the generator section 6 against the ambient atmosphere.

Under the action of wind, the rotor blades 4 rotate the hub 3 and the generator rotor 9 to transfer kinetic energy from the wind into mechanical energy. Rotation of the permanent magnets on rotor 9 induces electrical voltage in the windings of stator 25. The obtained electric energy is subsequently fed into a utility grid.

The rib 22 comprises a top surface forming a stationary generator surface 35 facing an inner rotor surface 36 at the outer end 21 of flange 20. Between the inner rotor surface 36 facing a second stationary generator surface 35 is a second air gap 33. The length I/ of the second gap 33 corresponds directly to the length L of the first gap 26 between the outer surface of cylindrical rotor 9 and the inner surface of the stator 25. When the length L of the first gap 26 becomes smaller, the length L* at the nearest point of the second gap 33 becomes larger, and the length L ^Λ at the most distant point of the second gap 33 becomes smaller. To monitor the length L of the first gap 26 without substantial interference by the magnetic field caused by the permanent magnets of the rotor 9, three sensors 34 are used to measure the distance between the outer end 21 of flange 20 and the rib 22 of the carrier 11 on three equidistantly arranged positions.

In the exemplary embodiment, wind turbine 1 is a gearless direct-drive wind turbine. Alternatively, the wind turbine can be a gearbox-driven wind turbine generator.

The pitch angle of a blade 4 is the angle that determines the blades orientation with respect to the direction of the wind. The pitch angle is controlled by a pitch drive in response to measured parameters, such as rotor speed, wind velocity and wind direction. Optionally, the pitch of each blade 4 can be controlled individually. For each blade 4 the pitch drive system comprises a pitch drive motor 40 and a transmission (not shown) enabling the motor 40 to rotate the blade 4 along its longitudinal axis via a bearing (not shown) .

When wind force rotates the rotor with the blades 4, the blades 4 are subjected to centrifugal forces and various bending moments and stresses. The amount of force imparted to the blades 4 by wind and the rotational speed of the wind turbine rotor 2 can be modulated by adjusting the pitch angle of the blades 4.

A plurality of distance measurement sensors 34 are positioned in the second air gap 33 to measure the length of the gap 33. The sensors 34 can for example be capacitive proximity probes. The sensors 34 can be operatively connected to power and signal transmission cables.

While the second gap 33 remains substantially constant, the capacitance of the sensor 34 is also substantially constant and the sensor 34 transmits a substantially constant measuring signal. If the length of air gap 33 changes, the capacitance of the sensor 34 changes and the measuring signal transmitted from sensor 34 is altered.

The sensor 34 is configured to measure the radial length of the air gap 33 between the rib 22 and the outer end 21 of flange 20. The sensor 34 can communicate with a data processing unit via a cable connection and / or wireless, e.g., via a network of transmitters and receivers operating in the radio frequency (RF) band may be used to define input channel. Optionally, Bluetooth® technology can be used. A junction box can be configured to receive a plurality of sensor cables.

The data processing unit can include one or more processors and a memory, an input channel, an output channel and may include a computer, or similar type of programmable circuit. The output channel can for example be a cable, or a wireless network of transmitters and receivers operating in a predetermined portion of a radio frequency (RF) band.

The data processing unit processes the distance measurement signals from the sensors 34. The memory of the data processing unit stores and transfers information and instructions to be executed by the processor.

The control system further includes a feedback channel to the data processing unit. The feedback channel transmits information relating to pitch angles to the data processing unit.

In operation, bending loads and stresses within the blades 4 are induced by wind force. These loads are transferred from the blades 4 via the hub 3 and bearing ring 16 to generator rotor 9. In some instances, loads transferred into the generator rotor 9 deflect the generator rotor 9 such that dimensions of the air gap 26 change and predetermined tolerances are approached. The length of the second air gap 33 changes correspondingly. The sensors monitor the length of the air gap 33 and transmit the measuring signal to the data processing unit. The measuring signal is typically a voltage or an electrical current signal converted to a dimension measurement by at least one resident conversion algorithm within the data processing unit.

The data processing unit uses a resident comparator algorithm to compare this dimension measurement to a predetermined value. If any deviations are observed, the processors generate an adjustment signal which is converted to an output signal. The output signal is transmitted to the blade pitch drive 40. The pitch drive repositions the blades 4 to modulate the pitch angle which in turn modulates the amount of force imparted to the blades 4 by wind.

The pitch drive transmits a feedback signal to the data processing unit. In response, the data processing unit modulates magnitude and duration of the output signal transmitted to the pitch drive. As the blades pitch angle is changed, the blade loads change, which subsequently changes the loads transferred to the generator rotor 9. The length of the air gap 33 is measured throughout the process of blade pitch adjustment and the measuring signals transmitted to the data processing unit facilitate modulation of the magnitude and duration of the output signal transmitted to the pitch drive. When the length of air gap 33 is set to a predetermined value, the pitch drive maintains the pitch angle of the blades 4.