Abstract:
A method of inspecting an air-gap of an electrical machine, which method including the steps of directing at least one laser beam (L) into the air-gap; detecting the laser beam (L) after reflection; determining the distance (d OK , d F ) travelled by the laser beam (L); and analysing the travelled distance (d OK , d F ) to detect an irregularity (F pole , F oval , F object ) in the air-gap is provided. The invention further describes an air-gap inspection apparatus for detecting an irregularity in the air-gap of an electrical machine, which apparatus including a laser module realised to direct at least one laser beam (L) into the air-gap and to detect the laser beam (L) after reflection; a computation unit realised to determine the distance (d OK , d F ) travelled by the laser beam (L); and an analysis unit realised to analyse the travelled distance (d OK , d F ) to detect an irregularity in the air-gap.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to EP Application No. 14161812, having a filing date of Mar. 26, 2014, the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The following relates to a method of inspecting a generator air-gap, and an air-gap inspection apparatus for detecting an irregularity in the air-gap of a generator. 
       BACKGROUND 
       [0003]    An electrical motor or generator comprises a rotor and a stator, and the rotor is separated from the stator by an air-gap. The air-gap between rotor and stator plays a very important role in the efficient operation of the machine. Generally, especially in the case of a large electrical machine such as a wind turbine generator, the rotor is realised as the field and bears an arrangement of magnets, while the stator is realised as the armature and bears an arrangement of stator windings. 
         [0004]    In the case of a very large electrical machine such as a direct-drive generator, the outer rotor can have a diameter in the range of several metres. Such a generator has a correspondingly high number of stator teeth and magnet poles. For example, in the case of a 3.0 MW direct-drive generator, the inner stator can comprise several stator segments with a total of more than 300 stator teeth, and the rotor housing may have more than 100 slots, while each slot can accommodate several magnet modules or magnet pole pieces. The depth of such a generator, i.e. the distance between a front end and a rear end, can be between 1.0 m and 3.0 m, and the total generator depth may exceed the length of a rotor slot. For safety reasons, the magnet pole pieces are generally inserted into the rotor slots in an automated or semi-automated workflow using suitable machinery. 
         [0005]    The air-gap of such a large generator may be only a few millimetres wide and should be as uniform as possible in the spaces between opposing stator teeth and magnet poles. A known visual inspection technique can involve shining a light into the air-gap and looking for any anomaly. However, owing to the depth of the generator, a visual inspection of the air-gap is extremely difficult to carry out and is prone to error. An undetected misalignment of just one magnet pole can have a pronounced negative effect on the air-gap. Similarly, an undetected misalignment of a stator segment may result in an uneven air-gap over a section of the generator. This imbalance can cause uneven torque (torque ripple). In a worst case, an irregularity in the air-gap can lead to a catastrophic collision between rotor and stator when the generator is put into operation. Such a collision is associated with very high costs, particularly in the case of an offshore wind turbine. Therefore, it is of paramount importance to ensure that the air-gap of such a generator is free of any irregularities or anomalies before the generator is installed or put into operation. 
       SUMMARY 
       [0006]    As aspect relates to providing a reliable and straightforward way of detecting an irregularity in the air-gap of an electrical machine. 
         [0007]    According to embodiments of the invention, the method of inspecting an air-gap of an electrical machine comprises the steps of directing at least one laser beam into the air-gap; detecting the laser beam after reflection; determining the distance travelled by the laser beam; and analysing the travelled distance to detect an irregularity in the air-gap. 
         [0008]    The laser beam can be generated by a device such as a laser rangefinder, or a comparable device that operates on the same or similar principle, namely that of directing a laser beam at an object and measuring the time taken for the reflected beam to be detected at the device. The total distance travelled by the laser beam (the sum of the outward and return paths) can then be computed. An advantage of the method according to embodiments of the invention is that any irregularity in the air-gap, even only a very minor irregularity, can be quickly and easily determined with a very high degree of accuracy. This is a considerable advantage, particularly in the case of a very large electrical machine such as a direct-drive generator with a diameter in the range of several metres and a correspondingly high number of stator teeth and magnet poles, as described in the introduction, since a visual inspection of such a narrow but extensive air-gap is very difficult to carry out and is also prone to error. The method according to embodiments of the invention provides a way of reliably and accurately detecting any irregularity or anomaly, even in the case of very small deviations or discrepancies in the millimetre range. 
         [0009]    According to embodiments of the invention, the air-gap inspection apparatus for detecting an irregularity in the air-gap of a generator comprises a laser module realised to direct at least one laser beam into the air-gap and to detect the laser beam after reflection; a computation unit realised to determine the distance travelled by the laser beam; and an analysis unit realised to analyse the travelled distance to detect an irregularity in the air-gap. 
         [0010]    An advantage of the air-gap inspection apparatus according to embodiments of the invention is that it can be realised at relatively low cost. Another advantage is that it can carry out a very thorough and precise analysis of the air-gap, so that even very small anomalies or irregularities can be quickly and easily identified. 
         [0011]    Particularly advantageous embodiments and features of the invention are given by the dependent claims, as revealed in the following description. Features of different claim categories may be combined as appropriate to give further embodiments not described herein. 
         [0012]    In the following, but without restricting embodiments of the invention in any way, it may be assumed that the electrical machine is a generator such as a direct-drive synchronous generator of the type used in a wind turbine. 
         [0013]    In the following, it may be assumed that the air-gap inspection is performed in a post-assembly procedure, i.e. after the generator has been assembled. In a generator such as a direct-drive synchronous generator with outer rotor, the manufacturing steps generally involve assembling the inner stator by mounting stator segments about a cylindrical core, and then inserting the stator into an empty rotor housing. The rotor housing generally comprises an arrangement of parallel slots into which pre-magnetized magnet modules or magnet pole pieces are then inserted, or it may comprise any other suitable magnet holding means. The method of air-gap inspection according to embodiments the invention is performed after the rotor housing has been filled with all the necessary number of magnet pole pieces. 
         [0014]    In the following, again without restricting embodiments of the invention in any way, it may be assumed that a laser beam is directed from one end of the generator in the direction of the opposite end. For example, the assembled generator may have an open end (referred to as the drive end) and a closed end (referred to as the non-drive end), whereby the open end might correspond to the front end which will later be connected to the hub of the wind turbine, while the closed end corresponds to a rear end facing into a canopy or nacelle of the wind turbine. This closed end can be closed by a flange that interfaces with a brake disk, or by any other suitable element. The rotor housing itself can extend beyond the length of a rotor slot in order to accommodate any winding overhang. The laser beam can be reflected off the surface of the closed end, back in the direction of the open front end. 
         [0015]    A laser beam can be directed into the air-gap according to any suitable scanning pattern, for example a laser beam may be directed diagonally into the air-gap. Preferably, the laser beam is directed into the air-gap in a direction parallel to the axis of rotation of the generator. In this case, the distance travelled by the laser beam can comprise twice the depth of the generator, e.g. twice the depth of the rotor housing, in addition to any offset distance between the laser module and the air-gap and in addition to any offset between the air-gap and the closed end. For any such path taken by the laser beam, a measured distance that exceeds twice the depth of the rotor housing will indicate that the air-gap is free of any irregularity along that path. Preferably, the air-gap inspection apparatus according to embodiments of the invention is realised to detect an irregularity in the air-gap of a generator with a depth of at least 1.0 m, and which has a correspondingly large diameter in the range of 4.0 m to 7.0 m. 
         [0016]    The laser beam might be operated manually, for example by holding a laser rangefinder and directing it into the air-gap. However, unless the operation is performed very exactly, some irregularities may be missed. Therefore, in a preferred embodiment of the invention, the laser beam is controlled to scan area region or volume defined by an outer contour of the stator and an opposing outer contour of the rotor. This approach is based on the insight that, for a particular generator design, a “correct” air-gap will have a certain shape. This shape will be essentially annular, based on the shape of an open-ended cylinder, but with one surface governed by the surface contours of the magnet pole pieces and the rotor housing, and the other surface governed by the surface contours of the stator with its stator teeth and windings. In the following, this annular volume is referred to as the “scan area” or the “area to be scanned”. In a preferred embodiment of the invention, the scan area is preferably slightly larger than the actual air-gap so that an anomaly such as rotor ovalization can also be detected. In the method according to embodiments of the invention, the laser beam can be directed into the scan area towards the rear cover where it is reflected so that it travels back along its return path, and the travelled distance is measured as the laser module moves relative to the scanned area. 
         [0017]    The distance travelled by the laser beam can be measured intermittently or at any appropriate interval. Preferably, the laser beam is controlled to follow a predetermined grid pattern in the scanned area, so that the distance travelled by the laser beam is measured at points corresponding to points in such a virtual grid. This approach allows a more precise analysis of any discrepancy, for example to determine the cause of a “too-short” laser path. Here, a “too-short” laser path is determined if the distance travelled by the laser beam on its outward and return journeys through the air-gap is less that twice the length of the air-gap. A group or cluster of such too-short paths can be analysed to determine the cause, for example by considering the shape that would be projected by these too-short paths onto a virtual plane. 
         [0018]    The accuracy of such an analysis can depend to some extend on the number of such scan points taken over a certain area, for example the number of scan points per square centimetre of the scan area. In a preferred embodiment of the invention, therefore, a resolution of the grid pattern is determined to enable detection of a misaligned magnet pole and/or to detect an ovalization of the generator rotor and/or to detect a foreign object in the scanned area. The distance travelled by the laser beam is then preferably determined at predetermined scan points of the scanned area according to the grid resolution. For example, the scan area can be virtually divided into a grid of 1.0 mm squares. In this case, the resolution comprises 1.0 mm. Preferably, the resolution or the distance between adjacent scan points comprises at most 1.0 mm, more preferably at most 0.5 mm. A suitably fine resolution can permit the detection of a very small foreign object, for example a small piece of metal such as a burr attracted to a permanent magnet. A fine resolution can also allow the detection of a relatively low degree of ovalization of the rotor housing. Of course, the resolution can be variable, so that a fine resolution is used when scanning a particularly critical region such as the narrowest air-gap sections, as described above, and a coarser resolution can be applied when scanning the wider regions between adjacent rotor slots. 
         [0019]    The narrowest point in the air-gap of a generator is generally between a stator tooth and the upper surface of a magnet pole piece. A magnet pole-piece may be essentially flat, but is not necessarily so. A stator tooth generally extends further into the air-gap than a winding arranged between adjacent stator teeth. Therefore, in a further preferred embodiment of the invention, the grid resolution is chosen to comprise at least one, more preferably at least two, most preferably at least three scan points in a most narrow portion of the air-gap. In this way, the air-gap can be analysed very precisely, even if the stator teeth and the magnets are not aligned during the laser inspection procedure. 
         [0020]    As indicated above, the laser inspection could be performed manually, for example by holding a laser rangefinder and guiding it in a controlled manner to scan the air-gap as thoroughly as possible. However, in a preferred embodiment of the invention, the air-gap inspection apparatus preferably comprises a mechanical guiding means or actuator realised to guide the laser module over the predetermined scan area. For example, the guiding means can comprise an annular track, and the laser module can be moved along this annular track that corresponds to the annular shape of the generator, while directing the laser beam into the air-gap. Alternatively or in addition, the laser device can be stationary so that it is not displaced relative to the generator, and a guiding means or actuator can be used to tilt the laser device so that the laser beam is swept over a certain angular portion of the air-gap. In each case, since the laser beam travels at the speed of light, measurements can effectively be made instantaneously, so that the laser module can be moved at any suitable rate while taking the measurements. 
         [0021]    A single laser module, realised to generate a single laser beam, can be used to inspect the air-gap. Of course, in an alternative preferred embodiment of the invention, the laser module can be realised to direct a plurality of laser beams into the air-gap. For example, the laser module can comprise two (or three, four, etc.) laser rangefinders arranged essentially equidistantly about the guiding means. These can be synchronously controlled so that each laser rangefinder covers one half (or one third, one quarter, etc.) of the scan area. The portions of the scan area covered by the individual laser devices can be separate, or they may overlap to some extent. 
         [0022]    As indicated above, the air-gap of a particular generator is associated with a particular shape determined by the number of stator teeth, stator windings and magnet poles. This “expected” or “ideal” area can be determined in advance using the known dimensions of the generator and the stator teeth, windings and magnet poles, and may also take any acceptable tolerances into account, such as rotor eccentricity, magnet thickness tolerances, etc. An allowable or correct air-gap may therefore be regarded as having slightly “fuzzy” contours owing to the acceptable tolerances. The actual air-gap area or shape is computed by collecting laser beam path distances as described above, preferably to a favourably fine resolution. The analysis unit of the air-gap inspection apparatus according to embodiments of the invention is preferably realised to detect discrepancies between the computed air-gap shape and the expected or ideal air-gap shape. Information or data describing the expected air-gap shape can be stored in a memory module that is accessible to the analysis unit. The analysis unit is preferably realised to be able to map the computed air-gap shape to the expected or allowed air-gap shape in order to be able to determine any discrepancies. To this end, the analysis module preferably also receives information defining the distance of the point of origin of the laser beam relative to the generator&#39;s axis of rotation. 
         [0023]    The steps of the method are preferably carried out in an automatic or automated manner. To this end, the steps of the method are preferably carried out by a computer program product, when the computer program product is loaded into a memory of a programmable device of an air-gap inspection apparatus according to embodiments of the invention. For example, various suitable software algorithms can be used to guide the laser device(s) about the scan area; to time the measurements according to a desired resolution or grid; to analyse the measurements in order to obtain an actual picture of the air-gap; and to compare this information with an expected or allowable air-gap to determine the presence of any anomalies or irregularities in the air-gap. 
     
    
     
       BRIEF DESCRIPTION 
         [0024]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0025]      FIG. 1  shows a simplified schematic showing an inspection setup with an embodiment of the air-gap inspection apparatus; 
           [0026]      FIG. 2  is a simplified rendering of a cross-section through a direct-drive generator; 
           [0027]      FIG. 3  shows a first situation in an air-gap inspection procedure using the method; 
           [0028]      FIG. 4  shows a second situation in the air-gap inspection procedure of  FIG. 3 ; 
           [0029]      FIG. 5  shows a grid pattern overlaid on an air-gap of a generator; and 
           [0030]      FIG. 6  shows potential irregularities in the air-gap of  FIG. 5 , as detected by the apparatus. 
       
    
    
       [0031]    In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale. 
       DETAILED DESCRIPTION 
       [0032]      FIG. 1  is a simplified schematic showing an inspection setup comprising an embodiment of the air-gap inspection apparatus  3  according to the invention and a generator  2  undergoing inspection. For the sake of clarity the diagram only shows a simplified cross-section through the generator  2  and only shows relevant components such as the rotor  22 , magnet poles  220 , the stator  21 , stator windings  211 , and a rear cover  223  of the rotor housing. The length of the air-gap  1  corresponds essentially to the length D of a rotor slot holding a number of magnet poles  220 . The stator  21  and rotor  22  are arranged about the axis of rotation R of the generator. The air-gap inspection apparatus  3  comprises a laser module  30 , a computation unit  31 , an analysis unit  32 , and a processing unit  34  such as a microprocessor, as well as a memory module  35 . The memory module  35  is used to store data describing a number of features of an ideal or expected air-gap shape, as will be explained below. During inspection, the laser module  30  generates a laser beam L, which is directed along the air-gap  1  towards the rear covering  223  of the rotor housing. The path travelled by the laser beam L on its outward and return journeys can therefore be at most twice the distance d OK  between the laser module  30  and the rear cover  223 , whereby the laser beam L can travel in a path parallel to the axis of rotation R, or can be directed at an angle to the axis of rotation R. If there is an irregularity in the air-gap, the path of the laser beam L is interrupted and reflected back to the laser module  30 , so that it travels a shorter distance d F . The laser module  30  can be guided to direct the laser beam L over the entire air-gap  1 , covering as much of the air-gap  1  as practicable. This is achieved by using an appropriate guiding means or actuator (not shown) to control the motion of the laser module  30 . The laser beam L is directed over a pre-defined grid pattern so that measurements can be taken at specific scan points, as will be explained below. The distances d OK , d F  measured are provided by the computation unit  31 , which in this case is realised as part of the laser module  30 . This can be a known type of laser rangefinder, for example. Information from the computation unit  31  is forwarded to an analysis unit  32 , which can use the measured distances to make a ‘picture’ of the actual air-gap shape  1   32 . This is then compared to an expected air-gap shape  1   35  stored in the memory  35 . If the actual and expected air-gap shapes  1   32 ,  1   35  match up, it may be assumed that the air-gap  1  is ‘clean’ and free of any irregularities. However, a mismatch between the air-gap shapes  1   32 ,  1   35  can indicate that the air-gap  1  is not satisfactory. A comparison between the air-gap shapes  1   32 ,  1   35  can be performed by the processor  34 , which may also be able to determine the reason for the mismatch. For example, a comparison between the air-gap shapes  1   32 ,  1   35  may indicate that a specific magnet pole piece is misaligned so that one side of it protrudes too far into the air-gap. An appropriate output signal  4  may inform a technician of the nature of the irregularity. 
         [0033]      FIG. 2  is a simplified rendering of a cross-section through a direct-drive generator  2 . The diagram shows an outer rotor  22  and an inner stator  21 . The stator  21  comprises several sections each with a number of stator windings  210  separated by stator teeth  211 . The rotor  22  comprises a housing with rotor slots  221 , and each rotor slot  221  can accommodate a number of magnet pole pieces  220 . The contours  21 C,  22 C of the stator  21  and rotor  22 , respectively, are shown in greater detail in the enlarged portion of the diagram. The air-gap  1  between rotor  22  and stator  21  is also shown. This is at its narrowest when a stator tooth  211  passes over a magnet pole piece  220 . In a stationary position of the generator  2 , for example during the laser inspection method according to embodiments of the invention, the shape of the air-gap  1  is well-defined, since the geometries of the stator  21  and rotor  22  are known, as are any relevant tolerances of the generator  2 . Therefore, for a particular position of the stator  21  relative to the rotor  22 , the ideal or expected shape  1 A of the air-gap  1  can be determined in a relatively straightforward manner. The enlarged part of the diagram indicates a portion of this expected air-gap shape  1 A indicated by hatching. The completed air-gap shape  1 A will be essentially annular. The annular shape  1 A should be the same for a cross-section taken at any depth in the generator  2 , assuming that the rotor slots  221  run parallel to the axis of rotation R, since the air-gap is a volume with a shape corresponding essentially to an open-ended cylinder. For a generator with skewed rotor slots or staggered magnet pole pieces, the air-gap cross-sectional shape will progressively change with increasing depth through the generator. Again, this geometrical or topological information is known and can be easily taken into consideration when determining an acceptable or expected air-gap shape. 
         [0034]      FIG. 3  shows a first situation in an air-gap inspection procedure using the method according to embodiments of the invention. The diagram shows a simplified cross-section through the generator and only shows relevant components such as the rotor  22 , a row of six magnet pole pieces  220  in a rotor slot, the stator  21 , a stator winding  211 , and a rear cover  223  of the rotor housing. The length of the air-gap  1  corresponds essentially to the length D of a rotor slot or the length of the six pole pieces  220 . Here, a laser beam L originating from a laser module  30  is being directed through the air-gap  1 , in a direction essentially parallel to the axis of rotation of the generator. Ideally, the laser beam L will pass without interruption through the air-gap  1  to the rear cover  223  of the rotor housing, and will return to the laser module  30 , which can then determine the length of the path travelled. In the absence of any irregularities in the air-gap  1 , the laser beam L travels a distance that is twice the distance d OK  between the laser module  30  and the rear cover  223 . This can be computed by the computation unit and stored for later use by the analysis unit, as described above. A guiding means  33  is used to guide the laser module  30  along an annular path as indicated by the arrow. The guiding means  33  can also effect a radial displacement of the laser module  30  (relative to the axis of rotation of the generator) so that the laser beam L can be directed over the desired grid pattern as the length of the laser beam is repeatedly measured. 
         [0035]      FIG. 4  shows a second situation in the air-gap inspection procedure of  FIG. 3 . Here, the laser beam L cannot reach the rear cover  223  for reflection, since a misaligned pole piece  220  (the fourth pole piece from the front opening of the generator) is protruding into the air-gap  1 . Therefore, the length of the path travelled in this case is only twice the distance d F  between the laser module  30  and the misaligned pole piece  220 , indicating the presence of a fault. Here also, this information is computed by the computation unit and stored for later use by the analysis unit, as described above. 
         [0036]      FIG. 5  shows a grid pattern G overlaid on the scan area  1 A of the air-gap  1  of a generator. The diagram indicates an exemplary pattern of scan points P, corresponding to instances at which a laser measurement is taken. For example, the laser can be guided to travel alternately up and down as it traverses the grid pattern from left to right. At the widest part of the air-gap, as shown for example in the left-hand side of this diagram, five measurements may be taken during an upward or downward motion of the laser. Towards the centre of the diagram, the laser is guided to scan the narrowest portion  10  of the air-gap  1 , and up to three measurements are taken during an upward or downward motion of the laser. To improve the accuracy of the inspection the laser can be guided such that the distance between upward and downward paths is less, i.e. the grid pattern G is denser about the narrower portion  10  of the air-gap  1 . 
         [0037]      FIG. 6  shows potential irregularities F pole , F oval , F object  in the air-gap  1  of  FIG. 5 , as detected by the apparatus according to embodiments of the invention. By comparing the computed air-gap shape to the expected or allowable air-gap shape, any discrepancy detected, even allowing for the known tolerances, can be assumed to be an irregularity that must be dealt with before activating the generator. For example, an irregularity F pole  owing to a misaligned pole piece must be dealt with by removing the misaligned pole piece and re-inserting it correctly. An irregularity F object  indicating the presence of an object in the air-gap can be dealt with by drawing the object out of the space between stator and rotor, for example manually or by vacuum extraction. An irregularity F ova , indicated by the broken line showing a displaced rotor contour can arise owing to an ovalization of the rotor  22 . This can be corrected, for example by later securing an anti-ovalization ring to the rotor front face. 
         [0038]    Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
         [0039]    For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module.