Patent Publication Number: US-8977511-B2

Title: Method for classifying electrical sheet

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2009/059777, filed Jul. 29, 2009 and claims the benefit thereof. All of the applications are incorporated by reference herein in their entirety. 
     TECHNICAL FIELD 
     The invention relates to a method for classifying electric sheet steel, which is used for producing an electrical machine and is in the form of a strip-wound coil. 
     PRIOR ART 
     The noise emission that occurs during operation of an electrical unit, e.g. a power transformer, is perceived to be disruptive particularly if the unit is installed in the vicinity of a living area. When producing a low-noise power transformer the aim is therefore to use a magnetically soft material which has optimally low magnetostriction. In practice, however, classifying magnetically soft materials proves to be difficult. For the purpose of classifying, a sample is conventionally taken from a strip-wound coil (from which the sheet metal parts for the transformer core are punched in a subsequent processing step) and is analyzed in a measuring device with regard to magnetostriction behavior and other magnetic properties. In this case firstly the problem occurs that known measuring devices do not provide consistent measuring results. Secondly, the measuring result is dependent on the site of sampling: it may be that a sample taken at the start of a strip-wound coil and a sample taken at the end of the strip-wound coil each exhibit very different magnetostriction behavior. A satisfactory prediction of the noise behavior of the electrical machine is virtually impossible with known measuring and classifying methods. 
     EMBODIMENTS OF THE INVENTION 
     It is an object of the present invention to disclose a method for classifying electric sheet steel with which the noise behavior and optionally also prediction of the magnetic losses of an electrical machine may be improved. 
     This object is achieved by a method with the features of the claims. Advantageous embodiments are defined in the dependent claims. 
     For classifying the magnetically soft starting product, in a basic idea the invention does not start from investigating a sample taken from the material supplied but the strip-wound coil as a whole, as is supplied to the production plant. For this purpose the sheet coil, also called a coil, is provided with an excitation winding. A magnetic flux that changes over time is produced in the wound flat strip by means of a feeding device. This magnetic flux that changes over time results in a change in the shape of the strip-wound coil owing to magnetostriction. Vibrations occur in the preferred direction of the magnetic material in accordance with magnetostriction. The changes in the shape of the strip-wound coil that run transversely to the preferred direction are measured by means of a measuring device, as are optionally the magnetic losses, and are supplied to an evaluation device. Noise levels (sound power levels) and loss ratios are calculated in the evaluation device and these values are assigned to categories. The anticipated operating behavior of the electrical machine, i.e. with regard to the anticipated noise behavior and optionally also with regard to the anticipated magnetic losses, can be better predicted in this way. As a result of the inventive method those strip-wound coils which seem to be particularly suitable or less suitable for low-noise operation of an electrical machine can be filtered out from those supplied before production starts. Since the coil is evaluated in its entirety the result of classifying is no longer dependent on the site of sampling. Prognosis is much more accurate with the inventive method. The inventive method is particularly significant financially when producing large machines, such as low-noise power transformers. The evaluation device, which classifies or categorizes the electric sheet steel, can automatically make the classification on the basis of predefined features (noise level, losses). The evaluation unit is e.g. an appropriately modified personal computer (PC) which can be assembled comparatively easily from hardware and software components that are known per se. In production the allocation of the electric sheet steel to noise categories requires only little expenditure. The classification method is largely unsusceptible to interference. 
     The change in shape of the strip-wound coil may be detected particularly easily metrologically in the axial direction of the sheet coil by arranging a sensor device on one or optionally both end face(s) of the strip-wound coil which detect the vibrations in the axial direction of the strip-wound coil. 
     If a plurality of detectors is arranged on each end face the change in length can easily be averaged. 
     Conventionally available acceleration sensors may advantageously be used for detecting the change in shape of the coils. A speed signal may easily be detected from the signal of an acceleration sensors and this can then be broken down by Fourier analysis in a further step into individual frequency components. The vibrational spectrum can then be converted comparatively easily into a spectrum proportional to the noise and allocated in a further step to a noise category. 
     It is expedient if when classifying the electric sheet steel the subsequent operating conditions of the electrical machine have already been taken into consideration, i.e. the excitation winding is fed with 50 Hz (Europe) or 60 Hz (USA) and variable amplitude. 
     It has been found that it may be advantageous if the magnetic flux produced by means of the excitation winding in the strip-wound coil is predefined stepwise in a technically relevant interval between 0.5 T and 2 T. In practice steps of about 0.1 T have proven to be advantageous. The noise behavior to be anticipated can consequently be determined as a function of the magnetic modulation. 
     Good classifying of the noise behavior of the electric sheet steel may be achieved in particular if, when evaluating the measurement signals of the detectors, only speed amplitudes of integral multiples of the predefined frequency (100 Hz, 150 Hz, 200 Hz, . . . or 120 Hz, 180 Hz, 240 Hz, . . . ) are used. 
     It may also be advantageous if the spectral speed fractions are standardized to a certain width of the strip-wound coil, by way of example to a width of 1 m. Classifying independently of the size of the coils supplied is possible as a result. 
     A particularly precise prediction may be made if the vibrations of the strip-wound coil are detected by a plurality of sensors on one or both end face(s) and these measurement signals are averaged. 
     It is advantageous if the measured values are converted into noise levels (sound power levels) and stored in a database, so a certain change in shape of the sheet material that occurs during operation can be allocated to certain values of the magnetic flux density in each case. 
     Different measuring principles may be used as sensors for detecting the deformation; by way of example piezo sensors, laser interferometers and ohmic, capacitive or inductive measuring transducers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To explain the invention further reference will be made in the section of the description below to the drawings, in which further advantageous embodiments, details and developments of the invention can be found, and in which: 
         FIG. 1  shows in a schematic diagram a strip-wound coil with an excitation winding and an inventive measuring and evaluation device for classifying the strip-wound coil with regard to noise emission and losses; 
         FIG. 2  shows a detailed diagram of  FIG. 1 , in which the arrangement of an acceleration sensor device on an end face of the strip-wound coil is shown in section; 
         FIG. 3  shows a detailed diagram of  FIG. 1 , in which the arrangement of an optical sensor device on an end face of the strip-wound coil is shown in section. 
     
    
    
     EXECUTION OF THE INVENTION 
       FIG. 1  shows in a three-dimensional sketch a hollow-cylindrical strip-wound coil  1 . When producing an electrical machine, such as a power transformer, such a strip-wound coil  1  constitutes the starting product from which the individual sheet metal parts of the magnetically soft transformer core are punched in a subsequent processing step. Such a strip-wound coil conventionally has a diameter, for instance in large machine construction, of up to 900 mm, a width of 70 cm to 1 m and a mass of about 1 to 5 tons. 
     As already mentioned in the introduction, practice shows that the noise behavior of electrical machines, which are produced from such a starting material, varies from strip-wound coil to strip-wound coil more or less to the same degree. However, when producing a low-noise electrical machine the aim is to use a magnetically soft material with optimum advantageous magnetostriction properties. 
     According to the invention a strip-wound coil or coil  1  (the material is conventionally supplied in this form in the case of production of an electrical machine) is classified or categorized as a whole. The coil  1  is provided with an excitation winding  2  for this purpose. As may be seen from  FIG. 1 , this excitation winding  2  extends transversely to the circumferential direction  11  in the direction of the axis  13  and winds spirally around the outer circumferential surface  9 , the end faces  7  and  8 , and the inner circumferential surface  10  of the hollow-cylindrical strip-wound coil  1 . The excitation winding  2  is connected to a feeding device  5 . The feeding device  5  provides the excitation winding  2  with a sinusoidal input terminal voltage that can be adjusted with regard to frequency and amplitude. 
     A sinusoidal voltage preferably with e.g. 50 Hz or 60 Hz is now applied to the excitation winding  2  for classifying the strip-wound coil with regard to noise emission. The alternating current flowing in the excitation winding  2  causes a magnetic alternating flux to form in the strip-wound coil  1  which is oriented in the circumferential direction  11 . 
     The current fed into the excitation winding is detected by means of a current sensor  23 . The measured value  24  provided by the current sensor  23  is passed to an evaluation unit  4 . 
     To detect the magnetic flux in the sheet coil  1  caused by the excitation winding  2 , a measuring loop  6  is arranged around the strip-wound coil  1 . A voltage is induced in the measuring loop  6  in the event of a change in flux in the sheet coil  1 , and this is proportional to the magnetic flux density B. The induced voltage is passed to the evaluation device  4  as a measured value  22 . 
     The magnetic alternating flux that spreads in the circumferential direction  11  of the strip-wound coil  1  produces a change in the shape of the strip-wound coil  1  due to the magnetostriction of the magnetically soft material. A change in the length of the winding as well as a transverse contraction occurs, i.e. a change in the axial length of the strip-wound coil  1 . 
     A sensor device  3  is arranged on the end face  7  to detect this deformation. The sensors  3  provide measurement signals  21 , which are also supplied to the evaluation device  4 . 
     For classifying the strip-wound coil with regard to noise emission the input terminal voltage of the excitation winding  2  is now changed stepwise until a certain flux density value is attained in the strip-wound coil  1 . The measurement signals provided by the sensors  3  are evaluated in the evaluation device  4  as follows for this adjusted flux density: 
     The measurement signals of the sensors  3  are broken down into frequency components in the evaluation device  4  by means of a Fourier analysis. Only the speed amplitudes of the integral multiples of 50 Hz or 60 Hz are considered in this case. 
     The spectral speed amplitudes are standardized to a length unit of the strip-wound coil  1 , by way of example to one meter sheet width of the strip-wound coil  1 . A mean value is formed for each frequency component: 
     
       
         
           
             
               
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     V j  . . . mean speed at frequency j 
     M . . . number of sensors 
     j . . . j&#39;th frequency component (e.g. 100 Hz comp.) 
     An averaged sound power level is then calculated for each frequency component.
 
 L   W,j =10· C   j ·log(   V     j ) 2    C   j  . . . interface factor
 
     The interface factor C j  takes account of differences in noise emission of different frequency components j. The interface factors C j  are quantitatively selected such that they represent the connection between a vibrating surface of 1 m 2  and the sound power L w . 
     In a further step the non-rated and A-rated sound power levels are calculated according to formula 
     
       
         
           
             
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     A storage device  20  designed as a database is implemented in the evaluation device  4 , in which measured values of the flux density, the calculated relative total sound power level and spectral components are stored. The strip-wound coils can be graded using these values and the effect of a strip-wound coil on the noise behavior of an electrical machine can be better predicted therefore. 
     The magnetic loss value and the magnetization requirement of the strip-wound coil are simultaneously also determined for each predefined value of the magnetic flux density by means of evaluation device  4 . These values are also stored in the database  20 . It is consequently possible to also predict the magnetization losses and the magnetization power requirement of an electrical machine more accurately. 
       FIG. 2  shows a possible arrangement of a sensor device  3 . An exemplary embodiment is shown in which the sensor  3  is designed as an acceleration sensor  14  and is arranged by means of a connecting piece  16  and by means of a permanent magnet  15  on an end face  7  of the hollow-cylindrical strip-wound coil  1 . The acceleration sensor  14  detects the axial deformation of the strip-wound coil  1 , which is produced by the magnetic alternating flux in the sheet strip  1 . The measuring information is supplied to the evaluation unit  4  as measurement signal  21  (see  FIG. 1 ). The sensor  3  could also be secured in a different way, by way of example by a glued or wax joint. As already mentioned, a plurality of sensors can be secured to the end face  7  in this or a different way. 
       FIG. 3  shows a different embodiment of the sensor device. The sensor device  3  is designed here as an optical sensor device. An optical transmitting device  17  produces a light beam  12  directed onto a reflector  19 . This light beam  12  is reflected in different ways according to the magnetostriction of the magnetically soft material, and this is detected by an optical detector  18 . The measuring information of the detector  18  is again passed to the evaluation unit  4 . 
     As already mentioned, other measuring methods are also conceivable for detecting the deformation of the strip-wound coil  1 , by way of example ohmic, capacitive or inductive sensors. 
     The inventive method allows strip-wound coils (strip-wound coils) to be complied into a category and, more precisely, simultaneously with regard to the noise emission to be anticipated, the magnetic losses and the magnetization power requirement. Different material properties at the start and end of the strip-wound coil cease to apply as a result of the overall evaluation of the strip-wound coil.