Abstract:
A rotor analyzer for an induction motor or generator checks and quantifies the integrity of a rotor that is not currently installed within its stator. The analyzer includes an electromagnetic coil that exposes the bars of a rotor to a pulsating magnetic field to induce a current through the bars. At the same time, the rotor is slowly rotated to sequentially expose each bar. A magnetic field created by the induced current in the bars induces an analog signal within a search coil. The analog signal is converted to digital and inputted to a microprocessor system. The system interprets the input data and manipulates it to provide a clear, understandable indication of the rotor&#39;s condition, such as the relative impendence of each bar. The system also determines how many bars are within a rotor having an unknown number of bars.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The subject invention generally pertains to induction motors and generators, and more specifically to nondestructive testing of their rotors.  
           [0003]    2. Description of Related Art  
           [0004]    Induction motors typically include a rotor that rotates in response to a rotating magnetic flux generated by alternating current in a stator associated with the rotor. A rotational speed differential (known as “slip”) between the rotor and the rotating flux induces a current through a rotor cage. A rotor cage typically consists of a single aluminum casting having several conductive bars that run axially through the rotor and are joined at each end by two conductive end rings. Current induced in the bars creates a magnetic flux that opposes that of the stator, thus providing the rotor with rotational torque.  
           [0005]    Sometimes it is desirable to inspect the integrity of a rotor before a new motor is assembled or before considering its use in a rebuilt motor. It is especially valuable to know the impedance of each rotor bar to identify rotor faults such as a cracked bar, separation between a bar and an end ring, or porosity of a bar and/or end ring. However, inspecting and identifying such faults is difficult to do, as cast aluminum cages are often cast directly into a laminated steel core of the rotor.  
           [0006]    To provide a nondestructive test for rotors, an apparatus and method disclosed in U.S. Pat. No. 3,875,511 exposes a rotating rotor to what appears to be a constant magnetic field provided by an electromagnetic sending coil. The rotor bars crossing the magnetic lines of flux induce a current through the bars. A receiving coil detects the induced current to provide an analog signal that can be displayed on an oscilloscope.  
           [0007]    An analog display, however, can be difficult to interpret and quantify. For example, in some cases the spacing between adjacent bars is so close that the spikes or peaks of an analog signal may tend to run together, thus making it difficult to distinguish one spike or bar from another. Similar negative results may occur when the bars are slightly recessed below the outer periphery of the laminated core. In such cases, portions of the core overlaying a bar may adversely shield the bar from a sending or receiving coil, and thus reduce the amplitude of the sensed signal. Also, when the bars are hidden below the outer surface of the core, a simple analog display may not provide a clear indication of how many bars are actually in the rotor. Manufacturers of new rotors will, of course, know how many bars are in their own rotors; however, for those that rebuild motors manufactured by others, the number of bars may be unknown.  
           [0008]    With an analog display, electrical noise or a stray spike could be misinterpreted as another bar. Moreover, with an analog display, it can be difficult to establish the repeatability of the readings. Repeatability or comparison of one set of readings to a later one can be valuable not only to establish the credibility of a particular set of readings, but also to determine whether a rotor is deteriorating over an extended period of use.  
         SUMMARY OF THE INVENTION  
         [0009]    To quantify the integrity of a rotor of an induction motor or generator, it is an object of the invention to nondestructively create a digital signature that indicates the impedance of each bar of the rotor.  
           [0010]    Another object of the invention is to repeatedly check the impedance of each bar of a rotor to establish a credible record of the rotor&#39;s integrity.  
           [0011]    Another object is to create and store a digital record that indicates the integrity of a rotor, and later reference that record to determine the extent that the rotor may have deteriorated over an extended period of operation.  
           [0012]    Another object is to determine the number of bars in a rotor by sequentially sensing the impedance of each bar for more than a full revolution of the rotor to create a repeating pattern that indicates that every bar has been checked at least once.  
           [0013]    A further object of the invention is to create digital raw data that indicates the impedance of a rotor&#39;s bars, and to manipulate the data by way of a microprocessor to create an enhanced visual indication of the impedance of each rotor bar.  
           [0014]    A still further object is to induce an electrical current in a rotor by varying the current in an electromagnetic sending coil.  
           [0015]    Another object is to sense the current through an electromagnetic sending coil to acquire an indication of a rotor bar&#39;s impedance.  
           [0016]    Yet another object is to sense the current or voltage of electromagnetic receiving coil to acquire an indication of a rotor bar&#39;s impedance.  
           [0017]    Another object is to distinguish one fault from another, wherein one fault is one or more bars having impedance that exceeds a predetermined limit, and another fault is a pattern of gradually varying impedance or a number of bars of especially high impedance being unequally distributed around the rotor.  
           [0018]    These and other objects of the invention are provided by rotor analyzer that exposes the bars of a rotor to a varying magnetic field to induce a current through the bars. A digital signal is created that varies as a function of the induced current. A microprocessor manipulates the digital signal to provide an enhanced visual indication of the impedance of each rotor bar. 
       
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a perspective cut-away view of a rotor being tested by an exemplary embodiment of a rotor analyzer with some portions of the analyzer being schematically illustrated.  
         [0020]    [0020]FIG. 2 illustrates a plurality of bar signatures, repeating patterns, and an example of an enhanced visual indication of the impedance of each bar of a tested rotor.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Referring to FIG. 1, a rotor  10  is shown in the process of being nondestructively tested by a rotor analyzer  12 . In this example, rotor  10  includes an aluminum cage  14  integrally cast within a rotor core  16  made of a stack of laminated steel sheets. A rotor shaft  18  is keyed, welded, and/or otherwise fixed to laminated core  16 . Rotor cage  14  includes several electrically conductive bars  20  that extend between two opposing end rings  22 .  
         [0022]    To check the resistance, impedance, porosity, or other characteristic that reflects the integrity of bars  20  and their connection with rings  22 , analyzer  12  exposes rotor  10  to a varying magnetic field  24 . This can be accomplished in number of ways, however, in a preferred embodiment a power supply, such as a variac  26  applies an alternating voltage (e.g., 20 to 40 volts, 60 Hz) across an electromagnetic excitation coil  28  to create field  24 . As an alternative, it may also be possible to create an effective varying magnetic field from a pulsating DC voltage or from a moving magnet (oscillating or rotating). Excitation coil  28 , in this example, includes windings  30  of about 300 turns of film-insulated, 19-gage wire wrapped around a preferably U-shaped, laminated steel core  32 .  
         [0023]    Excitation coil  28  is placed near one end of rotor core  16 , while a search coil  34  is positioned near an opposite end. Search coil  34  includes windings  36  having about 400 turns of 20-gage wire and is otherwise similar in construction to coil  28 . The actual positioning and mounting of coils  28  and  34  can be provided by any of a variety of conventional brackets or support structures. Regardless of the chosen mounting structure, the ends of the U-shaped core of coils  30  and  36  are preferably spaced just a few thousandths of inch (e.g., 0.010 inches) away from the surface of rotor  10 . This spacing can be an air gap or can be taken up by some sort of spacer of a non-ferromagnetic material, such as a plastic bearing pad. Although the actual spacing is not critical, preferably the spacing remains substantially constant throughout the testing process of analyzer  12 .  
         [0024]    In operation, varying magnetic field  24  of excitation coil  28  induces a varying electrical current  38  through a first bar  20 ′. The other bars  20  and end rings  22  complete the electrical circuit for current  38 . Current  38  in bar  20 ′ creates a secondary magnetic field  40  that induces an electrical signal  42  in windings  36  of search coil  34 . At the same time, a drive unit  44  (e.g., a set of powered rollers) slowly rotates rotor  10  at about one or two revolutions per minutes relative to coils  28  and  34 . The relative rotation could alternatively be achieved by having coils  28  and  34  revolve while rotor  10  is held stationary. Either way allows current  38  to be generally sequentially induced in each bar  20 . Although, with closely spaced bars  20  and a relatively wide excitation coil  28 , there may be some overlap, whereby some of current  38  is actually induced in a bar adjacent to bar  20 ′. Thus, the inducing of current  38  through each of bars  20  is not necessarily done in sequence or simultaneously.  
         [0025]    An amp meter or a voltmeter  46  (e.g. a Hewlett Packard model xyz) effectively includes an analog to digital converter  48  and samples the analog voltage (or current) of signal  42  at a predetermined sampling rate. The sampling rate is preferably several times greater than the product of the rotational speed of rotor  10  times the number of bars  20 . And the product of the rotational speed of rotor  10  times the number of bars  20  is less than the cyclical frequency of varying magnetic field  24 . This allows voltmeter  46  to operate at a predetermined sampling rate that creates several digital signals  50  or values for each bar  20  as rotor  10  rotates, thus providing a plurality of bar signatures (e.g.,  52   a,    52   b, . . .  and  52   n  of FIG. 2) for rotor  10 . Together, the plurality of bar signatures  52   a - n  comprises a digital signature  54  of rotor  10 . Digital signals  50  for bars  20  that create the plurality of bar signatures  52   a - n  and digital signature  54  of rotor  10  can be considered as raw data and, if desired, may be displayed in a table, chart or graphical format on paper or on a monitor screen  56 , as shown in the upper half of FIG. 2.  
         [0026]    Since the raw data can be difficult to interpret, a microprocessor system  58  (e.g. a computer with the appropriate I/O, microprocessor chip, memory, software, and various other related components) receives the raw data or digital signal  50  at an input  60  (e.g., a serial port) and manipulates the raw data to provide an enhanced visual indication of the rotor condition, such as the impedance or other predetermined characteristic of bars  20 . An output  62  conveys the microprocessor-manipulated data to a printer or monitor  56 , which displays the information as a bar graph  64 , as shown in the lower half of FIG. 2. Bar graph  64  is just one of many examples of an enhanced visual indication. Other examples would include, but not be limited to, various other graphical formats; tables; charts; or accept/reject signals, such as lights or text.  
         [0027]    To create an enhanced visual indication, such as bar graph  64 , microprocessor system  58  first determines the number of bars  20  that are in rotor  10 . If the number of bars is already known, the information can simply be manually inputted to system  58 . Otherwise, system  58  analyzes the raw data it receives at input  60  to identify a repeating pattern of bar signatures. To create at least a partially repeating pattern, rotor  10  is rotated more than one revolution. Although between one and two revolutions is possible, rotor  10  is preferably rotated three or four times.  
         [0028]    In some embodiments, programmed software of system  58  starts by assuming that rotor  10  has some particular number of bars, say forty. The average pitch or distance  66  between conspicuously clear signal peaks times forty then identifies what may be a full-cycle or complete rotor signature. System  58  then compares that rotor signature to what it considers as the next full-cycle of readings. A close correlation of the two presumably complete cycles indicates that the rotor actually has forty bars. The process is repeated for various other reasonable numbers of bars, such as thirty-nine, thirty-eight, forty-one, forty-two, and so on. The closest correlation helps determine the actual number of bars of an unfamiliar rotor. Of course, system  58  is preferably provided with some additional logic to eliminate unreasonable numbers of bars. For example, it would be very unlikely or unreasonable to suspect that a rotor would have just one or two bars. If rotor  10  is rotated less than two revolutions, system  58  looks for a correlation between the first few bar signatures of the first revolution and the first few bar signatures of the presumed next revolution.  
         [0029]    Once the number of bars  20  has been determined, that same number in readings will be taken of the raw data at a pitch that most closely fits a complete rotor signature. And the readings are preferably, but not necessarily, taken at or near each anticipated peak of each bar  20  (i.e., where a peak would normally occur for a good bar). The readings are then displayed as a first set of discrete digital values to create the upper portion of bar graph  64  of FIG. 2. For rotor  10  having twenty-eight bars  20 , a corresponding twenty-eight columns  68   a,    68   b, . . .    68   n  are displayed. In other words, bar signature  52   a  corresponds to column  68   a,  and bar signature  52   n  corresponds to bar signature  68   n.  The higher the column, the lower the impedance of the corresponding bar. If desired, additional readings taken beyond the first revolution of rotor  10  can also be displayed as a second set of discrete digital values to indicate the repeatability of the readings. The additional readings are displayed as columns  68   a ′,  68   b ′, . . .  68   n ′. Here, bar signature  52   a ′ corresponds to column  68   a ′, and bar signature  52   n ′ corresponds to bar signature  68   n ′. The height similarities of adjacent columns, e.g., columns  68   a  and  68   a ′, provide the indication of repeatability. As an alternative, the repeatability of the readings could also be indicated by a number, such as a ratio of the heights of columns  68   a  and  68   a′.    
         [0030]    In a currently preferred embodiment, valleys  90  between each peak  92  of the raw data are identified and averaged to provide another set of discrete digital values  92   a - n  that are generally lower than the first set  68   a - n  taken near the peaks. This lower set of digital values  92   a - n  are shown underneath their corresponding peak values  68   a - n.  Comparing their relative values, e.g.,  68   a / 92   a,  provides a ratio that can be used as accept/reject criteria for each rotor bar  20 . A rotor bar  20  may be acceptable if its peak-to-valley ratio is above a predetermined level. In some embodiments the predetermined level is relative in that the acceptable level is chosen based on how a particular ratio of one bar compares to that of the others. With some defective bars, a distinct valley may not even exist. For example,  92   n  has a value that is virtually the same as  68   n.    
         [0031]    In some embodiments, rotor analyzer  12  identifies various faults of rotor  10  based on the microprocessor-manipulated data and/or the sampled raw data. For example, one fault may be defined as a bar having an impendence that exceeds a predetermined limit. This is graphically depicted by the columns associated with bars  20 ″ being below a minimum conductivity limit  70 . If desired, such a fault can be distinguished from other predefined conditions or faults, such as a group of bars of relatively poor impendence being unequally distributed about rotor  10 .  
         [0032]    In some embodiments, system  58  includes a memory  72  that stores digital signature  54  and/or  64  for later reference. Memory  72  is schematically illustrated to represent the wide variety of forms that it can assume, which include, but are not limited to, a hard drive of a computer; a floppy disc; a CD (compact disk); magnetic tape; and an electronic chip, such as RAM, EPROM, or EEPROM. With memory  72 , a digital signature taken of rotor  10  when first installed within its stator, can be compared to a later signature taken after rotor  10  has been in operation for a while. The comparison of the two signatures could indicate whether the integrity of rotor  10  deteriorates with use. On a short-term basis, while inspecting a rotor, memory  72  can be used in comparing the set of readings taken during the first revolution of rotor  10  to those of a second revolution, thereby providing an indication of the readings, repeatability.  
         [0033]    Although the invention has been described with reference to a currently preferred embodiment, it should be appreciated by those skilled in the art that other variations are well within the scope of the invention. For example, electrical signal  42  is just one example of a signal that varies as a function of induced current  38 . Other examples of a signal that varies with current  38  include, but are not limited to, amperage  74  or voltage  76  as provided by amp meter  78  and voltmeter  80 , respectively. For voltage signal  76 , however, variac  26  or another power supply should be selected so that its output voltage, which it applies across excitation coil  30 , preferably decreases with an increase in current through coil  30 . An appropriate analog to digital converter  82  converts the analog signal  74  or  76  to a digital signal  50 ′, which in turn is conveyed to input  60 ′ or another similar input  60  of microprocessor system  58 . System  58  then manipulates signal  50 ′ in a manner similar to that of signal  50 , but with appropriate changes to account for any differences between signals  50  and  50 ′. By using signal  74  or  76  instead of signal  42 , search coil  34  may be omitted. In consideration of such modifications, as well as others that would be obvious to those skilled in the art, the scope of the invention is to be determined by reference to the claims, which follow.