Abstract:
Axial bearing wear in a motor is determined by differencing signals from special coils in the two ends of the motor stator. The signals on the two coils vary in opposite directions with axial displacement of the motor rotor. A dead zone circuit suppresses output when the axial displacement of the rotor is within the normal range of play for the rotor of the particular motor. When the axial displacement exceeds the normal range, the output actuates an indicator in relation to the magnitude of the displacement in excess of the normal range. Sequential LED indicators indicate both magnitude and direction of excess displacement. An analog meter is also disclosed which indicates only the magnitude of the excess displacement.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an axial bearing wear detector device for detecting axial bearing wear in canned motors.  
           [0002]    In general, canned motors are employed mainly for driving pumps and used for chemical plants. Therefore high reliability is required of the canned motors.  
           [0003]    In canned motor pumps, the canned motor and the pump constitute monoblock construction to prevent the leakage of the fluid, thus making it impossible to monitor the inner conditions visually. In most cases, the rotor of the canned motor for rotatably driving the impeller of the pump is journaled by a slide bearing which is lubricated with a pump fluid. To operate the canned motor efficiently, it is necessary to monitor the condition of slide bearing wear from outside.  
           [0004]    Accordingly, axial bearing wear detector devices were suggested, for example, in Japanese Patent Publication No. 21924 of 1982, and Japanese Patent Laid-Open Publication Nos. 80103 of 1998 and 148819 of 1999. The devices suggested therein are adapted to have axial detection coils provided on the both axial end portions of the stator of a canned motor. Voltages induced in these axial detection coils are compared to detect the axial position of the rotor to be rotatably journaled by slide bearings, thereby allowing the amount of bearing wear to be estimated based on the axial position of the rotor.  
           [0005]    Conventional canned motor axial bearing wear detector devices were adapted to estimate the amount of bearing wear based on the axial position of the rotor. However, the rotor journaled by slide bearings has axial play and thus may be displaced in the axial direction depending on the condition of the load even without axial wear occurring on the slide bearings. This made it impossible to clearly determine whether or not the rotor was displaced because wear had occurred on the slide bearings.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0006]    In order to solve the problems, the object of the present invention is to provide a canned motor axial bearing wear detector device which clearly indicates the axial wear of the bearings taking the normal axial play of the rotor into account. The canned motor axial bearing wear detector device can also indicate clearly in which axial direction and how excessively the wear has occurred.  
           [0007]    The canned motor axial bearing wear detector device according to the invention comprises axial detection coils on a canned motor having a stator and a rotor at both axial ends of the stator. A differential amplifier circuit amplifies differentially a rectified and smoothed voltage induced in the axial detection coils outputs a positive or negative voltage signal in response to a direction and position of axial displacement of the rotor relative to the stator. A dead zone circuit and a reference signal generator circuit output a reference signal. An axial wear indicator indicates a degree of bearing wear in an axial direction of the canned motor in response to the signal output from the comparator circuits of the dead zone circuit. Here, the dead zone circuit includes a polarity inverting circuit for inverting positive and negative voltage signals output from the differential amplifier circuit. A first comparator circuit outputs a signal in response to an amplitude of a voltage signal when the voltage signal output from the polarity inverting circuit is greater than the reference signal output from the reference signal generator circuit. A second comparator circuit outputs a signal in response to an amplitude of a voltage signal when the voltage signal output from the differential amplifier circuit is greater than the reference signal output from the reference signal generator circuit.  
           [0008]    In this axial bearing wear detector device, axial displacement of the rotor causes a change in voltage induced in the axial detection coils on both axial ends of the stator. The differential amplifier circuit amplifies differentially a rectified and smoothed voltage induced in the axial detection coils and outputs a positive or negative voltage signal in response to the direction and position of axial displacement of the rotor relative to the stator. The dead zone circuit allows the polarity inverting circuit to invert positive and negative voltage signals from the differential amplifier circuit. The first comparator circuit outputs a signal in response to the amplitude of the voltage signal when the voltage signal output from the polarity inverting circuit is greater than the reference signal output from the reference signal generator circuit. The second comparator circuit outputs a signal in response to the amplitude of the voltage signal when the voltage signal output from the differential amplifier circuit is greater than the reference signal output from the reference signal generator circuit. That is, the amplitude of the reference signal from the reference signal generator circuit is set to a value that is approximately equal to the signal range resulting from normal axial play. When the signal is in the axial-play range, neither of the comparator circuits outputs signals representing axial wear. In this event, only the center position indicator is energized. As axial wear occurs, the signal from the output terminal exceeds the allowed value of axial play. The axial wear indicator then indicates the amount and direction of axial bearing wear of the canned motor in response to the signal output from the comparator circuits of the dead zone circuit. Thus, the dead zone circuit allows the wear indicators to remain unlit for the axial displacement of the rotor corresponding to the play of the rotor and to be lit only for axial displacement of the rotor caused by bearing wear. Therefore, it is possible to determine whether the axial displacement of the rotor is within the range of play or whether the axial displacement is due to bearing wear and clearly indicates axial wear of the bearings.  
           [0009]    The canned motor axial bearing wear detector device according to a further embodiment of the invention includes a wear indicator that comprises a first axial wear indicator for indicating a degree of bearing wear in an axial direction of the canned motor in response to the signal output from one of the comparator circuits of the dead zone circuit. The axial indicator also comprises a second axial wear indicator for indicating a degree of bearing wear in the axial direction of the canned motor in response to the signal output from the other one of the comparator circuits of the dead zone circuit.  
           [0010]    As mentioned above, the first and second wear indicators are employed as the axial wear indicator. This makes it possible to determine the amount and the direction of axial displacement of the rotor caused by bearing wear and to clearly indicate in which direction and how excessively the wear has occurred.  
           [0011]    The canned motor axial bearing wear detector device includes the first and second axial wear indicator with an LED sequential bar graph. The number of LEDs on the LED bar graph to be lit is varied in response to the signal output from each of the comparator circuits of the dead zone circuit.  
           [0012]    The number of illuminated LEDs is varied in response to the amplitude of the signal output from each of the comparator circuits of the dead zone circuit. This clearly indicates not only the magnitude of the wear, but also the direction of the wear.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a circuit diagram illustrating a dead zone circuit and an axial wear indicator of an axial bearing wear detector device for use in canned motors according to an embodiment of the present invention.  
         [0014]    [0014]FIG. 2 is an explanatory view illustrating the relationship among the axial position of the rotor, the output voltage of the axial wear indicator, and the lighting of the axial wear indicator of the axial bearing wear detector device according to the embodiment of the present invention.  
         [0015]    [0015]FIG. 3 is a partially cut away front view illustrating a radial gap type canned motor pump incorporating the axial bearing wear detector device, for use in canned motors, according to the embodiment of the present invention.  
         [0016]    [0016]FIG. 4 is a perspective partial view illustrating the axial detection coils, mounted on the end portion of a tooth portion of the stator, according to the embodiment of the present invention.  
         [0017]    [0017]FIG. 5 is a schematic diagram of the canned motor pump.  
         [0018]    [0018]FIG. 6 is a circuit diagram illustrating the axial wear detector portion and the differential amplifier circuit according to the embodiment of the present invention.  
         [0019]    [0019]FIG. 7 is a graphical representation of a voltage signal to be outputted from the differential amplifier circuit in response to the axial position of the rotor according to the embodiment of the present invention.  
         [0020]    [0020]FIG. 8 is a circuit diagram illustrating a dead zone circuit and an axial wear indicator of an axial bearing wear detector device for use in canned motors according to another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Referring to FIG. 3, a canned motor pump, shown generally denoted at  11 , includes a pump  12  and a radial gap canned motor  13 , which are integrated with each other in a fluid-tight manner.  
         [0022]    A stator  17  in the canned motor  13  includes stator iron core  14  in a stator frame  18 . A stator winding  16  is wound in a stator groove  15  of the stator iron core  14 . A stator can  19  is tightly held in the inner peripheral surface of the stator  17 . Both end rims of the stator can  19  are welded to the stator frame  18  in a fluid-tight manner. The stator can  19  is made in a thin cylindrical shape of a non-magnetic substance such as stainless steel.  
         [0023]    A rotor shaft  24  is fixed in a rotor  23 . A rotor iron core includes a rotor groove  21  containing a rotor conductor  22 . A rotor can  25  covers the outer peripheral surface of the rotor  23 . The rotor can  25  is made in a thin cylindrical shape of a non-magnetic substance such as stainless steel. The rotor can  25  of the rotor  23  is spaced inward from the stator can  19  by a can gap  26  therebetween.  
         [0024]    The rotor shaft  24  is journaled at bearings (slide bearings)  28   a ,  28   b  mounted in bearing boxes  27   a ,  27   b  via sleeves  29   a ,  29   b . Thrust collars  30   a ,  30   b  limit the axial displacement of rotor shaft  24 .  
         [0025]    The stator iron core  14  includes a pair of radial detection coils C 1 , C 2  angularly spaced apart about the axis of the stator iron core  14  by a spatial angle of 180 degrees relative to the center of the shaft of the stator iron core  14 . The radial detection coils C 1 , C 2  are wound onto the whole tooth portion of the stator iron core  14 .  
         [0026]    A terminal box  33  protrudes from the canned motor  13 . The terminal box  33  is in communication with the interior of the stator frame  18 . The upper portion of the terminal box  33  includes a sealed container  35  having a transparent peephole (not shown) formed of glass with explosion-proof configuration. Part of the axial bearing wear detector device is accommodated inside the sealed container  35 , which is included in an operation monitor device for the canned motor  13 .  
         [0027]    A casing  37  of the pump  12  is mounted in a fluid-tight manner to the end of the stator frame  18  of the canned motor  13 . An impeller  38  is affixed to the rotor shaft  24  inside the casing  37 . The impeller  38  inside the pump  12  is rotatably driven by the rotor  23  supported on the bearings  28   a ,  28   b  via the sleeves  29   a ,  29   b . Axial displacement of the impeller  38  is limited by the thrust collars  30   a ,  30   b  and the bearings  28   a ,  28   b.    
         [0028]    Referring now to FIG. 4, a front axial detection coil Cf, and a rear axial detection coil Cr (only front axial detection coil Cf is shown in FIG. 4) are mounted on end portions of a tooth portion of the stator iron core  14 . A notched groove  41   b  near an end portion  41   a  of a tooth portion  40  of the stator iron core  14  form a small core portion  41  about which front axial detection coil Cf is wound. One axial detection coil Cf is wound in the stator groove  15  around the core portion  41 . Although not shown in FIG. 4, the other axial detection coil Cr is wound on the opposite end of the tooth portion  40  in the same manner.  
         [0029]    Referring now to FIG. 5, the front and rear axial detection coils Cf and Cr are shown on the axial end portions above the stator iron core  14  of the canned motor  13 . The axial detection coils Cf, Cr detect axial wear of the bearings  28   a ,  28   b  through detection of the axial position of the rotor  23 . The radial detection coil C 1 , on a lower tooth portion, detects radial wear of the bearings  28   a ,  28   b . The radial detection coil C 2  (not shown in FIG. 5) is wound on a tooth portion opposite the position of radial detection coil C 1 . The radial detection coils C 1  and C 2  are connected in series, as will be detailed hereinafter.  
         [0030]    In the following description, it is to be understood that the pump  12  side of the canned motor  13  is referred to as the front side (front portion side), whereas the opposite side to the pump  12  is referred to as the rear side (rear portion side).  
         [0031]    The following explains how axial wear of the bearings  28   a ,  28   b  is detected.  
         [0032]    Axial displacement of the rotor  23  toward the front side is limited by contact between the bearing  28   a  and the thrust collar  30   a  at the front side where the impeller  38  is located. Rearward movement of the rotor  23  is limited by contact between the bearing  28   b  and the thrust collar  30   b.    
         [0033]    Axial play of the rotor  23  or the range over which the rotor  23  can move freely in the axial direction thereof, in the absence of axial wear of the bearings  28   a ,  28   b , depends on the size and structure of the pump  12 . However, axial play is generally within a range from approximately 0.5 to 3 mm with the allowable axial wear limit of the front side bearing  28   a  and the rear side bearing  28   b  being approximately 1 mm.  
         [0034]    Under normal operation, the rotor  23  sits in its axial direction at a position where the front side bearing  28   a  and the thrust collar  30   a  rotate in contact with each other, or where the rear side bearing  28   b  and the thrust collar  30   b  rotate in contact with each other. However, when the amount of axial wear of bearings  28   a ,  28   b  exceeds approximately 1 mm, the front or rear surface of the impeller  38  of the pump  12  can contact the casing  37  or the bearing box  27   a.    
         [0035]    Thus, considering the foregoing, the axial displacement of the rotor  23  must be monitored within the range of approximately ±2.5 mm to detect excessive axial wear of the bearings  28   a ,  28   b.    
         [0036]    The axial detection coils Cf, Cr, on both end portions of the stator iron core  14  detect axial displacement of the rotor  23  by a difference in voltage induced in the axial detection coils Cf, Cr.  
         [0037]    Referring now to FIG. 6, an axial wear detector portion  50  includes the axial detection coils Cf, Cr, described above, on the opposed ends of the stator iron core  14 . Output terminals  51 ,  52  of the front side axial detection coil Cf are connected to one input of a differential amplifier circuit  59  via an amplifier circuit  55  and a rectifier smoothing circuit  57 . Output terminals  52 ,  53  of the rear side axial detection coil Cr are connected to the other input of the differential amplifier circuit  59  via the amplifier circuit  56  and the rectifier smoothing circuit  58 . The output of the differential amplifier circuit  59  is connected to an output terminal  60 .  
         [0038]    Referring now to FIG. 7, voltage signal outputted from the differential amplifier circuit  59  in response to the axial position of the rotor  23  is shown. In the figure, the horizontal axis represents the axial position of the rotor  23  journaled by the bearings  28   a ,  28   b , relative to a center position, and the vertical axis represents the output voltage from the output terminal  60  of the differential amplifier circuit  59 .  
         [0039]    The relationship between the position of the rotor  23  and the output of the voltage signal is independent of the axial play of the rotor  23  and the axial wear of the bearings  28   a ,  28   b  but is dependent on the position of the rotor  23  relative to the stator  17 .  
         [0040]    A change in axial position of the rotor  23  relative to the stator  17  causes a change in the distribution of magnetic flux between the stator  17  and the rotor  23 . The change in the distribution of magnetic flux appears as a change in amplitude of the voltage signals induced in the axial detection coils Cf, Cr on opposite ends of the stator  17 . Thus, the difference between the outputs of the axial detection coils Cf, Cr provides a voltage signal responsive to the axial position of the rotor  23  with the common change being eliminated.  
         [0041]    In the example shown in FIG. 7, the output terminal  60  provides a voltage signal of 0V when the rotor  23  is located at the center of the axial play thereof. A positive (+) voltage signal is outputted when the rotor  23  is located closer to the front side from the center of the play and a negative (−) voltage signal is outputted when the rotor  23  is located closer to the rear side from the center.  
         [0042]    Referring now to FIG. 1, an axial wear indicator  61  energizes a sequential LED (light emitting diode) bar graph having a center position indicator  62 , a front side wear indicator  63 , and a rear side wear indicator  64 . The center position indicator  62  is located at the center of the LED bar graph. The front side wear indicator  63  is located to the left of the center position indicator  62  where it functions as a first axial wear indicator. The rear side wear indicator  64  is located to the right of the center position indicator  62  where it functions as a second axial wear indicator.  
         [0043]    The center position indicator  62  has a green LED G 0  that is always lit. The front and rear side wear indicators  63 ,  64  each have a LED bar graph with a green LED G 1 , LED G 2 , LED G 3 , and LED G 4 , a yellow LED Y 1  and LED Y 2 , a red LED R 1 , arranged in that order from the center to outside. The LEDs of the LED bar graph are lit sequentially from the center to outside in either direction in response to the frontward or rearward displacement of the rotor  23  caused by bearing wear, thereby indicating the state of axial bearing wear.  
         [0044]    The transmission path for the positive and negative voltage signal from the output terminal  60  is divided into two systems. This is done to allow each of the wear indicators  63 ,  64  to be lit in response to a positive or negative voltage signal outputted from the output terminal  60  of the differential amplifier circuit  59 . One of the systems is connected with a polarity inverting circuit  71 , a first comparator circuit  72 , and a first LED driver circuit  73 . The polarity inverting circuit  71  inverts the positive and negative polarities of the voltage signal from the output terminal  60 . The first comparator circuit  72  outputs a signal in response to the amplitude of the voltage signal when the absolute value of the negative voltage signal outputted from the polarity inverting circuit  71  is greater than a reference signal. The first LED driver circuit  73  drives plurality of LEDs of the front side wear indicator  63  in accordance with the output from the first comparator circuit  72 . The other system is connected with a second comparator circuit  74  and a second LED driver circuit  75 , but without a polarity inverting circuit. The second comparator circuit  74  outputs a signal in accordance with the amplitude of the voltage signal when the absolute value of a negative signal from the output terminal  60  is greater than a reference voltage. The second LED driver circuit  75  drives LEDs of the rear side wear indicator  64  in accordance with the output from the second comparator circuit  74 .  
         [0045]    The reference signal inputted to each to the comparator circuits  72 ,  74  is produced by a reference signal generator circuit  76 . The reference signal generator circuit  76  has a variable resistor VR 1  to allow adjustment of the output voltage of the reference signal to be inputted to the input side via a resistor R 1 . The input side of each of the comparator circuits  72 ,  74  is connected to the output side of the variable resistor VR 1  via resistors R 2 , R 3 , respectively.  
         [0046]    The amplitude of the reference signal outputted from the reference signal generator circuit  76  is adjusted using the variable resistor VR 1 . The reference signal generator circuit  76 , the comparator circuits  72 ,  74 , and the polarity inverting circuit  71  constitute a dead zone circuit  77 . The dead zone circuit  77  allows the wear indicators  63 ,  64  to remain unlit over a range of axial displacement of the rotor  23  corresponding to the play of the rotor  23 . The wear indicators  63 ,  64  thus are lit only for in response to axial displacement of the rotor  23  in excess of the normal axial displacement caused by bearing wear.  
         [0047]    Referring now to FIG. 2, the relationship among the axial position of the rotor  23 , the output voltage of the axial wear indicator  61 , and the lighting of the axial wear indicator  61  is shown. Without axial wear in the bearings  28   a ,  28   b , the dead zone circuit  77  allows the variable resistor VR 1  of the reference signal generator circuit  76  to be adjusted as follows. That is, the green LED G 1  of the front side wear indicator  63  is lit when the thrust collar  30   a  and the bearing  28   a  of the front side are brought into contact with each other. In addition, the green LED G 1  of the rear side wear indicator  64  is lit when the thrust collar  30   b  and the bearing  28   b  of the rear side are brought into contact with each other. As axial wear of the front side bearing  28   a  increases, the LEDs G 1 , G 2 , G 3 , G 4 , Y 1 , Y 2 , and R 1  of the front side wear indicator  63  are successively lit. As axial wear of the rear side bearing  28   b  increases, the LEDs G 1 , G 2 , G 3 , G 4 , Y 1 , Y 2 , and R 1  of the rear side wear indicator  64  are successively lit.  
         [0048]    When the bearing  28   a  wears excessively toward the front side, the output signal of the differential amplifier circuit  59  becomes positive and the input signal to the second comparator circuit  74  becomes positive, causing no output to be generated from the second comparator circuit  74 . Conversely, the input signal to the first comparator circuit  72  is inverted by the polarity inverting circuit  71  to become negative. When the absolute value of the output signal from the polarity inverting circuit  71  becomes greater than the reference signal outputted from the reference signal generator circuit  76 , a signal is outputted from the first comparator circuit  72 . The first LED driver circuit  73  drives the front side wear indicator  63  to illuminate the LEDs G 1 , G 2 , G 3 , G 4 , Y 1 , Y 2 , and R 1 . The number of illuminated LEDs depends on the level of the output signal from the first comparator circuit  72 .  
         [0049]    That is, when the front side thrust collar  30   a  and the front side bearing  28   a  rotate in contact with each other, the green LED G 1  of the front side wear indicator  63  is lit in addition to the green LED G 0  located at the center. When bearing wear is excessive toward the front side, the green LED G 2  of the front side wear indicator  63  is also lit. When the bearing wears more excessively toward the front side, the green LED G 3 , green LED G 4 , yellow LED Y 1 , yellow LED Y 2 , and red LED R 1  are all. In this manner, the state of axial bearing wear of the front side is indicated.  
         [0050]    When the bearing wears excessively toward the rear side, the output signal of the differential amplifier circuit  59  becomes negative and the input signal to the first comparator circuit  72  is inverted by the polarity inverting circuit  71  to become positive, causing no output to be generated from the first comparator circuit  72 . On the other hand, the input signal to the second comparator circuit  74  becomes negative. Thus, when the absolute value of the output signal from the differential amplifier circuit  59  becomes greater than the reference signal outputted from the reference signal generator circuit  76 , a signal is outputted from the second comparator circuit  74 . The second LED driver circuit  75  drives the rear side wear indicator  64  to illuminate the LEDs G 1 , G 2 , G 3 , G 4 , Y 1 , Y 2 , and R 1 . The number of illuminated LEDs depends on the level of the output signal from the second comparator circuit  74 .  
         [0051]    That is, when the rear side thrust collar  30   b  and the rear side bearing  28   b  rotate in contact with each other, the green LED G 1  of the rear side wear indicator  64  is lit in addition to the green LED G 0  located at the center. When the bearing wears excessively toward the rear side, the green LED G 2  of the rear side wear indicator  64  is also lit. When the bearing wears even more toward the rear side, the green LED G 3 , green LED G 4 , yellow LED Y 1 , yellow LED Y 2 , and red LED R 1  are additionally lit. In this manner, the state of axial bearing wear of the rear side is indicated.  
         [0052]    The dead zone circuit  77  allows the wear indicators  63 ,  64  to remain extinguished while the axial displacement of the rotor  23  corresponds to the normal axial play of the rotor  23 , and to be lit only for the axial displacement of the rotor  23  caused by bearing wear. This makes it possible to determine whether the axial displacement of the rotor  23  is within the range of play or due to bearing wear. This makes it possible to clearly indicate the axial wear of the bearings  28   a ,  28   b  as well as in which axial direction the wear is directed and how excessive the wear is.  
         [0053]    Furthermore, since the wear indicators  63 ,  64 , the number of LEDs in the LED bar graph varied in response to the signal outputted from each of the comparator circuits  72 ,  74  of the dead zone circuit  77 . This clearly indicates how much, and in which axial direction the wear has occurred.  
         [0054]    The wear indicators  63 ,  64  can be formed easily at low cost using the LED bar graph. However, a liquid crystal display device may be used as the bar graph. Alternatively, without limiting the indicator to the bar graph, any indicator which can indicate the degree of bearing wear may be used to clearly indicate how excessively and in which axial direction the wear has occurred.  
         [0055]    In addition, when the voltage induced by the axial detection coils Cf, Cr is comparatively high, the connection sequence of the amplifier circuits  55 ,  56  and the rectifier smoothing circuits  57 ,  58  may be reversed. Alternatively, the amplifier circuits  55 ,  56  may be omitted.  
         [0056]    Referring now to FIG. 8, an embodiment of the invention is shown which uses an analog meter  79  in place of the axial wear indicator  61 . The output signal of the first comparator circuit  72  or the output signal of the second comparator circuit  74  are inputted to a combining circuit  78  comprising two diodes D 1 , D 2 . Then, the output signal of the combining circuit  78  is indicated on an axial wear indicator  79  such as an analog voltage meter. In this case, like the embodiment shown in FIG. 1, the dead zone circuit  77  allows the axial wear indicator  79  to remain deenergized over a range of axial displacement of the rotor  23  corresponding to the play of the rotor  23 . However, it should be noted that the degree of bearing wear due to the axial displacement of the rotor  23  caused by bearing wear can be indicated but it cannot be determined whether the wear has happened on the front side bearing  28   a  or on the rear side bearing  28   b . A simple direction-indicating addition may be sufficient to overcome this.  
         [0057]    According to the canned motor axial bearing wear detector device of the invention, the dead zone circuit allows the wear indicators to remain unlit for the axial displacement of the rotor corresponding to the play of the rotor. Thus, the wear indicators are lit only for axial displacement of the rotor caused by bearing wear. Therefore, it is possible to determine whether the axial displacement of the rotor is within the range of play or whether there is bearing wear and clearly indicate the axial wear of the bearings.  
         [0058]    The canned motor axial bearing wear detector device according to the invention has the following effect in addition to that described above: the first and second wear indicators are employed as the axial wear indicator, thereby making it possible to determine the amount and the direction of axial displacement of the rotor caused by bearing wear and to clearly indicate in the direction of the wear, and how excessive the wear is.  
         [0059]    The canned motor axial bearing wear detector device according to the invention the following effect in addition to that described above: the LED bar graph is employed as the axial wear indicator. The number of LEDs illuminated is varied in response to the signal outputted from each of the comparator circuits of the dead zone circuit. This clearly indicates in which direction and how excessively the wear has occurred.  
         [0060]    Although the problem of bearing wear detection is especially acute in canned motors, the present invention may be applied to other types of motors without departing from the spirit and scope of the invention.  
         [0061]    Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.