Abstract:
A method and system for directly measuring the compression of a ripple spring ( 23 ) in a dynamoelectric machine through a corresponding wedge ( 27 ). A compression-assessment tool ( 3 ) is provided that includes a carriage ( 32 ) for supporting a proximity sensor ( 34 ). The carriage ( 32 ) enables the proximity sensor ( 34 ) to be passed over the length of the ripple spring ( 23 ), which produces an output signal that is representative of the compression of the ripple spring ( 23 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the assessment of the condition of dynamoelectric machines, and more particularly to methods and systems for measuring the compression of ripple springs in dynamoelectric machines. Although the following discussion focuses on electric generators, methods and systems consistent with the present invention are applicable to other dynamoelectric machines, including electric motors. 
     BACKGROUND 
     Electric generators include a rotor and a stator. Rotors are generally constructed from a steal forging and include a number of slots that run the length of the rotor. Rotors are electrically wound by placing conductors referred to as rotor windings into the slots of the rotor. 
     Stators are generally constructed from a number of stacked, metal laminations. Stators also include slots, which run the length of the stator. Stators are electrically wound by placing conductors known as stator coils into the slots of the stator. 
     Conventional stator coils are often held in place in stator slots using a wedge and ripple spring configuration. In this configuration, a stator coil is placed into a slot, and a wedge is driven into groove near the top of the slot. A ripple spring is positioned above the stator coil and below the wedge. This ripple spring provides compressive force to keep the stator coils held firmly in the slot. 
     Over time, stator wedges may become loose. If a stator wedge becomes loose, it can permit a stator coil to vibrate, which can cause catastrophic failure in an electric generator. In order to avoid stator-coil vibration and catastrophic failure of a generator, it is desirable to periodically inspect the tightness of ripple springs. However, such inspections present a challenge because ripple springs are difficult to access within a generator and because they are concealed by the corresponding stator wedge. 
     There are a number of conventional approaches to inspecting the compression of ripple springs. One approach involves manually tapping the stator wedges. Another approach involves measuring the depth of the surface of ripple springs through pre-formed test holes in the wedge. A third approach involves physically displacing the wedge and measuring the resulting wedge movement. 
     There are significant challenges associated with the conventional approaches to testing ripple-spring tightness. The first approach, manually tapping stator wedges, is extremely subjective. The results vary greatly between different inspectors. Manually tapping stator wedges is also only possible after a generator&#39;s rotor has been removed from the generator. 
     The second approach, using a depth gauge to take measurements through pre-formed test holes, is time consuming. This approach is also only possible when a generator has pre-formed test holes in its stator wedges. Many generators do not have such pre-formed test holes. 
     The third approach, physically displacing the stator wedge, involves impacting a stator wedge and then measuring the displacement of the stator wedge with a sensor such as an optical or capacitive sensor to give an indirect indication of the compression of the ripple spring beneath the stator wedge. This method is not ideal because it involves only an indirect indication of ripple-spring compression. This approach also requires a relatively complex algorithm for converting the displacement of the stator wedge into an indication of ripple-spring compression. U.S. Pat. No. 5,295,388 to Fischer et al, which is incorporated by reference herein in its entirety, discloses a method and system that utilizes this approach. 
     Despite advances in the area of ripple-spring compression assessment, improved methods and systems are still needed to enable fast, accurate, and direct measurement of ripple-spring compression in generators that do not necessarily have pre-formed test holes. 
     SUMMARY OF THE INVENTION 
     Methods and systems consistent with the present invention enable the direct measurement of the compression of ripple springs in dynamoelectric machines, such as electric generators. A compression-assessment tool that includes a non-contact proximity sensor is passed over the length of the stator wedge. The proximity sensor provides a mapping of the compression of the ripple spring by taking direct measurements of the proximity of the ripple spring through the wedge material that lies over the ripple spring. In a preferred embodiment, methods and systems consistent with the present invention may be utilized to assess ripple-spring tightness on a generator while the generator&#39;s rotor is in situ. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained further by way of example with reference to the following drawings: 
         FIG. 1  illustrates a conventional generator stator suitable for use with methods and systems consistent with the present invention; 
         FIG. 2  illustrates an exemplary embodiment of a compression-assessment tool consistent with the present invention; 
         FIG. 3  is a flowchart illustrating steps in an exemplary embodiment of a method consistent with the present invention; 
         FIG. 4  illustrates exemplary output signals of the compression-assessment tool of  FIG. 2 ; and 
         FIG. 5  illustrates a ripple-spring-compression-assessment system consistent with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example of a stator  10  to which methods and systems of the present invention may be applied. The stator  10  includes stator teeth  15 ; which are formed from multiple, stacked laminations  17 . The stator  10  also includes stator slots  19  in which stator coils  13  may be stacked. The stator coils  13  are retained in the slots  19  by shims  21 , ripple springs  23 , and stator wedges  25  having beveled edges  27  for engaging correspondingly shaped grooves  29  in the sidewalls of the stator teeth  15 . The ripple springs  23  are compressed between the stator wedges  25  and shims  21  to generate a force that firmly holds the stator coils  13  in place. Over time, the ripple springs  23  may lose their resiliency so that the stator wedges  25  become loose. This can permit the stator coils  13  to vibrate, which can result in damage to the stator  10  and eventual failure of the electrical generator. 
       FIG. 2  illustrates a compression-assessment tool  30  for assessing the tightness of ripple springs that is consistent with an exemplary embodiment of the present invention. The compression-assessment tool  30  is shown resting on a conventional stator wedge  27 . A ripple spring  23  is installed between the stator wedge  27  and the stator coil  13 . For purposes of explanation, the ripple spring  23  is shown as having the shape of a sign wave. However, the differences between the ripple spring&#39;s peaks and crests are actually less pronounced. 
     Methods and systems consistent with the present invention may be utilized with any stator wedge  25  that is made of a substantially non-conductive material. Most conventional stator wedges  25  are made of a non-conductive material, such as fiberglass. Methods and systems consistent with the present invention may be utilized with any ripple spring  23  that is made of either a substantially non-conductive material or a combination of conductive and non-conductive materials. 
     The compression-assessment tool  30  illustrated in  FIG. 2  includes a carriage  32 , which supports a proximity sensor  34 . For purposes of methods and systems of the present invention, the proximity sensor  34  may be an inductive or capacitive sensor or any sensor that permits the compression-assessment tool  30  to measure the proximity of the ripple spring  23  through the material of the stator wedge  25 . If the ripple spring  25  is non-conductive a capacitive-type sensor is preferred. One example of such a sensor is the capacitive sensor with model number PM475, which is manufactured by Lion Precision, 563 Shoreview Park Road, St. Paul, Minn. 55126. In another embodiment, the ripple spring  25  may include a conductive lining or conductive layer and the proximity sensor may be of the inductive type. The compression assessment tool  30  may also include an amplifier for amplifying output signals from the sensor  34 . One example of such an amplifier is the amplifier with model number 99343-04, also available from Lion Precision. 
     Referring again to the compression-assessment tool  30  illustrated in  FIG. 2 , the carriage  32  may include wheels  36  or sliding surfaces to facilitate the movement of the compression-assessment tool  30  over the length of the stator wedge  25 . The carriage  32  may also include one or more cables  38  for transmitting signals to and from the compression-assessment tool  30 . The carriage  32  may also include an adjustment device  40  for adjusting the height and/or position of the proximity sensor  34  relative to the stator wedge  27 . 
       FIG. 3  illustrates exemplary steps for using a compression-assessment tool that is consistent with the present invention. Before assessing the compression of a ripple spring, the compression-assessment tool should be calibrated. The compression-assessment tool may be calibrated, for example, using known information about the size and material of the stator wedge and ripple spring. This may be accomplished, for example, by taking a wedge and ripple spring of the same type as the ones to be tested; putting them into a test fixture; and applying a known force, for example, using a hydraulic press. The compression-assessment tool may then be passed over the text fixture to obtain initial output signals. The compression-assessment tool may then be adjusted to match the output signals to the tightness created by the known force. 
     Alternatively, the compression-assessment tool may be calibrated using a stator with a ripple spring of known tightness value. The tightness value may come, for example, from one of the conventional compression-assessment methods. With this approach, the compression-assessment tool is next swept over the stator wedge of the reference ripple spring with known tightness (step  50 ) to produce an output signal that is representative of the tightness of the reference ripple spring. This data may then be used to map the compression of the reference ripple spring over its length (step  52 ). 
     Once the reference data has been established, the compression-assessment tool may be swept over the test stator wedge (step  54 ) to produce an output signal that is representative of the tightness of the test ripple spring. This data is then used to map the compression of the test ripple spring over its length (step  56 ). The mappings of the reference ripple spring and the test ripple spring may then be compared (step  58 ) to determine the relative compression of the test ripple spring. 
       FIG. 4  illustrates typical output signals for a relaxed and a compressed ripple spring. As the compression-assessment tool is passed axially over a stator wedge, an output signal (represented by Voltage Amplitude (Y) in  FIG. 4 ) is generated by the proximity sensor. This output signal is representative of the tightness or compression of the underlying ripple spring over the Axial Travel of Sensor (X). The output signal may be utilized to create a mapping of the ripple-spring compression over the length of the stator wedge. 
       FIG. 5  illustrates an exemplary embodiment of a compression-assessment system  50  consistent with the present invention. The compression-assessment system  50  includes a low-profile robotic carriage  51  that may be inserted in a narrow gap  60  between the rotor  72  and stator  74  of a dynamoelectric machine  70  such as an electric generator. The robotic carriage  51  may be guided by an operator along the length of a stator slot to inspect the tightness of the wedges in the slot. Electrical signals may be transmitted between the carriage  51  and a data processing system  55  via electrical cables  53  to control positioning of the carriage  51 . Output signals from the proximity sensor on the carriage  51  representing the ripple-spring tightness may also be transmitted between the carriage  51  and data processing system  55  over one or more of the electrical cables  53 . 
     The present invention has been described with reference to the accompanying drawings that illustrate preferred embodiments of the invention. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Thus, the scope of the invention should be determined based upon the appended claims and their legal equivalents, rather than the specific embodiments described above.