Abstract:
A method for measuring a stator core of an electrical machine includes the steps of winding at least one excitation coil around the stator core, applying a discontinuous voltage to the excitation coil(s) to magnetically excite the stator core, and measuring a quantity of the magnetically excited stator core. Further, a corresponding measuring device is disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims foreign priority benefit under 35 U.S.C. §119 to commonly-owned EP Patent Application No. 15172883.9 filed 19 Jun. 2015, which is hereby incorporated by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    Embodiments of the invention relate to a method for measuring a laminated stator core of electrical machines, in particular of large generators in maintenance. Embodiments of the invention relate in particular to improving a device and a method in which the laminated stator core is subjected to an externally applied field current. Commonly, the rotor of the electric machine is removed during the measurements. 
       BACKGROUND 
       [0003]    Large generators and motors are routinely examined for laminate shorts and stability when stationary. Various methods are available for this purpose. 
         [0004]    One of the methods to determine laminate shorts comprises the magnetization of the entire laminated body by means of an auxiliary coil at the mains frequency, and the measurement of stray fields on the inner surface of the stator bore. The magnetization is carried out to relatively low values of the magnetic induction, typically to about 10% of the normal operating induction. This method of measurement is also known by the name “low-induction laminate short measurement” or by the name “ELCID” (electromagnetic core imperfection detector). 
         [0005]    By way of example, U.S. Pat. No. 4,996,486 describes one method of this type. The prior art is therefore for the laminated stator core to be magnetized by means of an auxiliary coil and a sinusoidal auxiliary voltage applied thereto at the mains frequency, to about one tenth of the operating induction. This auxiliary voltage is normally derived directly from the mains voltage. An electrical recording coil is then moved away from the surface of the stator bore, with the recording coil being located close to the surface of the laminated core. 
         [0006]    The currents which flow as a result of the interlaminar short circuits in the laminated core now induce voltages with a characteristic phase angle and amplitude magnitude in the recording coil. The characteristic phase angles and amplitudes make it possible to distinguish between points where there are laminate-short currents and points where there are no laminate-short currents. It is therefore possible to locate laminate shorts, and to assess the magnitude of the short-circuit currents, by means of this stray-field recording coil. 
         [0007]    The invention provides alternative solutions to determine laminate shorts and the stability of a stator core. 
       SUMMARY 
       [0008]    Embodiments of the present invention relate to a measuring device and a measuring method. For example, in one embodiment a method for measuring a stator core of an electrical machine includes winding at least one excitation coil around the stator core, applying a discontinuous voltage to the excitation coil(s) to magnetically excite the stator core, and measuring a quantity of the magnetically excited stator core. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention will be explained in more detail in the following text with reference to exemplary embodiments and in conjunction with the drawings, in which: 
           [0010]      FIG. 1  shows a schematic circuit diagram of an example of an excitation device with one power supply, a resistor and a capacitor operated in parallel, to supply energy to an excitation coil via a circuit comprising IGFETS with controllers, and a filter; 
           [0011]      FIG. 2  shows a schematic circuit diagram of an example of an excitation device similar to  FIG. 1  with two power supplies connected via diodes; 
           [0012]      FIG. 3  shows a signal diagram of one example of excitation of a stator core provided by the excitation device to a stator core as a discontinuous voltage with rectangular shape, with the time plotted at the horizontal axis and the voltage plotted at the vertical axis; 
           [0013]      FIG. 4  shows a signal diagram of another example of excitation of a stator core provided by the excitation device to a stator core as a discontinuous voltage with burst shapes, with the time plotted at the horizontal axis and the voltage plotted at the vertical axis; 
           [0014]      FIG. 5  shows a signal diagram of another example of excitation of a stator core provided by the excitation device to a stator core as a discontinuous voltage with sinusoid shape, with the time plotted at the horizontal axis and the voltage plotted at the vertical axis; 
           [0015]      FIG. 6  shows a schematic top view of a stator core to be measured with a schematic excitation coil wound around the stator coil to provide an excitation voltage to the stator core for magnetic excitation of the stator core; 
           [0016]      FIG. 7  shows a schematic perspective view of a measuring device with an exciter device connected via a power transformer to excitation windings wound around a stator core and a detection device designed as a camera to optically detect a temperature difference in the vicinity of the stator core; and 
           [0017]      FIG. 8  shows a schematic perspective view of a measuring device with an excitation device connected via a converter to excitation windings wound around a stator core and a detection device designed as two microphones to acoustically detect vibrations in the vicinity of the stator core. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  shows a schematic circuit diagram of an example of an excitation device  12  as part of a measuring device  1  according to an example of the invention. At the left of  FIG. 1  a power supply  25  is provided which provides the electric power necessary to charge an excitation coil  40 . The power supply  25  can be fed by the public grid. The charging voltage is between 50V and 1000V. The charging voltage is adjusted at the power supply  25  to reach typically 50%-100% of the nominal interlaminar voltage. The maximum permanent power of the power supply  25  is typically 50 kW. The power supply  25  is electrically connected via a resistor  27  and a capacitor  30  to the circuit between the power supply  25  and the excitation coil  40  as shown. The capacitor  30  with a high capacity is charged by the power supply  25  and de-charged via a filter  60  to charge the excitation coil  40  in a specific way. A typical value of the capacitor  30  is 1 mF, a typical value for the excitation coil  40  is 1 mH. The filter  60  is commonly an inductivity which is saturated during current rise. During charge of the capacitor  30  the power supply  25  operates as a current source limiting the current output, during de-charge of the capacitor  30  the power supply  25  operates as a voltage source. In the example according to  FIG. 1  the capacitor  30  is connected in parallel with a circuit containing four IGFETs  55 , where each two of the IGFETs  55  are steered by a controller  50 ,  50 ′. The voltage for the excitation coil  40  is tapped from the circuit containing the IGFETs  55  and the assigned controller  50 ,  50 ′. The voltage at the excitation coil  40  can properly be controlled by these means. 
         [0019]      FIG. 2  shows a schematic circuit diagram of an example of an excitation device  12  similar to  FIG. 1  as part of the measuring device  1 . Here, the main part of the circuit at the right is identical to the circuit of  FIG. 1 . The capacitor  30  however is fed by two power supplies  25 ,  25 ′ instead of one. The two power supplies  25 ,  25 ′ are connected via diodes  27 ,  27 ′ in parallel to the capacitor  30 . The power output is enhanced compared to the example of  FIG. 1  as the second power supply  25 ′ adds additional power to the capacitor  30 . The maximum pulse frequency is in this example according to  FIG. 2  doubled compared to the example of  FIG. 1 . An arrangement adding further power supplies  25 ,  25 ′ to the excitation device  12  is conceivable. 
         [0020]      FIG. 3  shows a signal diagram of one example of excitation of a stator core  4 . The time is plotted at the horizontal axis and the voltage V is plotted at the vertical axis. The signal shown is the voltage generated by the excitation devices  12  to excite the excitation coil  40  as described above. The power supply  25 ,  25 ′ charges the capacitor  30  which is de-charged in a controlled manner by the controllers  50 ,  50 ′ switching the IGFETs  55  of the excitation device  12 . The shown excitation voltage is applied to the excitation coil  40 . As can be seen in  FIG. 3  the voltage signal is discontinuous, the times t a voltage is applied are different to the times t the voltage is zero. Here, during the time t ON  an excitation voltage is applied consisting of two opposite rectangular pulses, one positive pulse followed by one negative pulse. A typical pulse time is 2 ms-5 ms with 50-100 pulses per second. During the time t OFF  the controllers  50 ,  50 ′ switch the IGFETs to apply no voltage to the excitation device  12 . The time t ON  is unequal to the time t OFF . The control of the excitation device  12  in the way described ensures a low real power and a low reactive power from the feeding grid while assuring high voltages to excite the stator core  4 . In the signal shown it equals t ON &lt;t OFF . 
         [0021]      FIG. 4  shows a signal diagram of another example of excitation of a stator core  4  provided by the excitation device  12  as described above. The time t is plotted at the horizontal axis and the voltage V is plotted at the vertical axis. In this example the voltage signal is discontinuous again. The signal shape of the voltage applied to the excitation coil  40  is the shape of bursts as shown in  FIG. 4 . First, a steep nearly vertical edge is applied to a maximum power peak which immediately after reaching the peak decays in a steep curve to zero voltage. The time t with a voltage unequal to zero characterized by the voltage burst is referred to as t ON . A typical pulse time is 2 ms-5 ms with 50-100 pulses per second. During the time t OFF  the controllers  50 ,  50 ′ switch the IGFETs to apply no voltage to the excitation device  12 . The time t ON  is unequal to the time t OFF . In the signal shown it equals t ON &lt;t OFF . 
         [0022]      FIG. 5  shows a signal diagram of another example of excitation of a stator core  4  provided by the excitation device  12  with the time t plotted at the horizontal axis and the voltage V plotted at the vertical axis. In this example the stator core  4  is again excited by a discontinuous voltage, here with a sinusoid shape. The time t with a voltage unequal to zero characterized by the sinusoid voltage is referred to as t ON . A typical pulse time is 2 ms-5 ms with 50-100 pulses per second. During the time t OFF  the controllers  50 ,  50 ′ switch the IGFETs to apply no voltage to the excitation device  12 . The sinusoid voltage is applied with three cycles at the time t ON  after which end of the last cycle the excitation voltage is set to zero. In the signal shown it equals t ON &lt;t OFF . 
         [0023]      FIG. 6  shows a schematic top view of a stator core  4  of an electric machine. A rotor inside the stator core  4  is removed which is commonly done in measurement mode. In a schematic way the winding of the excitation coil  40  around the stator core  4  is shown which is the excitation coil  40  described under  FIG. 1  and  FIG. 2 . The excitation coil  40  is divided into four connected parts in this example. As described above an excitation voltage u(t) is applied to the excitation coil  40 . According to the electro-magnetic theory the current flow i(t) in the excitation coil  40  induces a magnetic flux density B in the stator core  4  in the direction indicated by the arrow. The electric exposure of the stator core  4  is reduced with all three exemplary signal curves with discontinuous voltages. The magnetic flux density B in the stator core  4  is detectable by different measures from which two are described below as two different embodiments of the invention. 
         [0024]      FIG. 7  shows a first embodiment of the invention. Shown is a schematic perspective view of a measuring device  1  which comprises an excitation device  12  connected via a power transformer  13  to excitation windings  10 ,  10   a,    10   b  wound around the stator core  4  constituting the excitation coil  40 . The stator core  4  is illustrated in a perspective view with partly cut faces and an axis  2 . The stator core  4  has a weight of 53 t and a length of 5 m for example. At the inside the stator core  4  commonly has notches  7  to house stator bars (not shown). The excitation coil  40  has ten turns wound around the stator core  4  in this example, shown are only two turns. Switching the excitation voltage by the excitation device  12  and thus magnetizing the stator core  4  leads to a rise in temperature at the surface of the stator core  4 . To the end of measuring temperatures and especially temperature differences an optical detection device  14  is provided next to the stator core  4 . The quantity to be measured is the temperature in this embodiment. The optical detection device  14  is in this first embodiment an Infrared (IR) camera. The IR camera is suitable to measure the temperatures at the stator core  4  with a high sensitivity. The optical detection device  14  is moved along the surface of the stator core  4  by an operator and records temperature data. The temperature data is compared to stored data such that deviations between the gathered data and stored data can be determined. The detection device  14  comprises a calculation unit  70  and an electronic memory to this end. When the temperature measured at the stator core  4  with magnetic excitation takes an extraordinary high value it can be deduced that at the corresponding spot an interlamination short exists. In particular, when a large temperature difference between specific measured spots occurs the calculation unit  70  identifies an interlamination short at the spot at which the temperature is higher. Interlamination shorts are electric shorts between the insulations of the separate laminated sheets building the stator core  4  impairing the power efficiency of the electric machine and reducing the operation safety. The temperature rise at these spots are mainly caused by undesirable eddy currents flowing there. The temperature rise at these spots was found to have a linear relation to the averaged power brought into the spot. 
         [0025]      FIG. 8  shows a schematic perspective view of a second embodiment of the invention with a different detection device  14  than the first embodiment. In this embodiment the excitation device  12  is designed the same as in the first embodiment. However, here one typical permanent power is 15 kW. The excitation device  12  is connected to the excitation windings  10  via a converter  13 . The excitation windings  10  forming the excitation coil  40  are similar wound around the stator core  4  as in the first embodiment. Here, the windings  10  are arranged in two turns around the stator core  4 . The detection device  14  is hereby designed as two microphones, a first microphone or detection device  14  arranged at the left and a second microphone or detection device  14 ′ at the right of the stator core  4 . The detection devices  14 ,  14 ′ can also be designed as vibration sensors. After magnetizing the stator core  4  with discontinuous voltages as described above the microphones are suitable to detect the small vibrations acoustically which are caused by the application of power to the stator core  4 . For this purpose the microphones are arranged in close vicinity to the stator core  4  and have a high sensitivity. Generally, this way of detection is based on the fact that the variable magnetic excitation creates vibrations in the whole stator core  4  due to magnetostrictive forces. These vibrations of the stator core  4  are detected by the detection devices  14 ,  14 ′. The detection devices  14 ,  14 ′ are guided along the stator core  4  and measurement results are taken at different spots. The detection devices  14 ,  14 ′ comprise a calculation unit  70  and an electronic memory. In case of extraordinary values of detected vibrations it can be deduced that a quantity of the stator core  4  diverges at a specific spot. In particular this quantity hereby is the stability of the stator core  4 .