Patent Publication Number: US-8542800-B2

Title: Asynchronous motor with features creating magnetic field disturbance

Description:
BACKGROUND OF THE INVENTION 
     Embodiments of the invention relate generally to electric motors and, more particularly, to an asynchronous motor including a component for introducing disturbances into the magnetic field of the motor by altering a reluctance of the motor. 
     The usage of electrical machines in various industries has continued to become more prevalent in numerous industrial, commercial, and transportation industries over time. Due to the prevalence of these motors in industry, it is paramount that the electric motors be operated reliably and efficiently. Motor design parameters and performance parameters are often required by motor management systems to optimize the control and operations of electric motors. Similarly, motor status monitoring enables the electric motors to operate reliably. Many motor status monitoring techniques also look for certain motor design parameters and performance parameters. 
     One such motor performance parameter that is helpful in optimizing the control and operations of electric motors is rotor or motor speed. However, a typical induction motor design does not have the ability to measure rotor speed without some form of physical detection sensor. In many applications, sensor location, alignment, size, and environmental conditions make the sensor option extremely difficult to integrate into the design while still maintaining a high level of reliability and robustness. 
     For example, in an x-ray tube environment, implementation of a physical detection sensor is very challenging because of the increased air gap between the sensor (which would be operating in dielectric oil) and the target material (in vacuum). Additionally, positioning of the x-ray tube casing, which is typically formed of a non-ferrous material such as stainless steel, in the air gap between the stator and the rotor attenuates the magnetic field more than air or vacuum. Also, the sensor target material temperature gradient is critical if the target is a permanent magnet (e.g., magnets formed of Samarium Cobalt, for example, are only rated to 350° C. max). Finally, the size restriction of the sensor itself is a challenge, as it is situated between an x-ray tube&#39;s casing and insert housing. 
     While some systems and techniques for sensorless measurement of rotor speed have been provided in the past, such techniques are typically limited in their implementation. For example, a rotor may be designed to be asymmetrical or have saliencies therein that result in a change in impedance as seen at the stator windings, thereby providing for estimation of the rotor speed based on motor current spectrum analysis based on this change in impedance. However, such signals have a poor signal-to-noise ratio (SNR), which limits the ability to effectively measure such signals. Furthermore, as set forth above, the generation of such signals relies on defects designed into the motor, which is highly undesirable with respect to motor performance (e.g., efficiency, torque capability, etc.). 
     It would therefore be desirable to design an asynchronous motor that provides for detection of rotor speed that is not dependent on measurements acquired via a physical detection sensor, so as to enable the improved motor management and motor status monitoring of asynchronous motors. It would further be desirable for such an asynchronous motor to provide signals having a high SNR and for such signals to be generated without varying an impedance of the motor via the introduction of defects thereto. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention provides embodiments of an asynchronous motor that includes a component for introducing disturbances into the magnetic field of the motor by altering a reluctance of the motor. The component is a separate component from the stator and the rotor and is positioned within the rotating magnetic field generated by the stator. 
     In accordance with one aspect of the invention, an asynchronous motor includes a stator having a plurality of windings that is configured to generate a rotating magnetic field when a current is provided to the plurality of windings. The asynchronous motor also includes a rotor positioned within the stator configured to rotate relative thereto responsive to the rotating magnetic field and a component separate from the stator and the rotor that is positioned within the rotating magnetic field, with the component being configured to alter a magnetic reluctance of the rotor so as create a disturbance in the rotating magnetic field. 
     In accordance with another aspect of the invention, an asynchronous motor including a stator having a plurality of windings and being configured to generate a rotating magnetic field when a current is provided to the plurality of windings. The asynchronous motor also includes a rotor positioned within the stator having a rotor core and a plurality of rotor bar conductors, with the rotor configured to rotate relative to the stator responsive to the rotating magnetic field. The asynchronous motor further includes a component positioned adjacent to the rotor and configured to alter a reluctance of the asynchronous motor so as to generate a disturbance in the rotating magnetic field, with the disturbance in the rotating magnetic field generated by the component introducing a current signal into a stator phase current spectrum of the stator. 
     In accordance with yet another aspect of the invention, an x-ray tube includes a housing enclosing a vacuum chamber, a cathode positioned within the vacuum chamber and configured to emit electrons, and an anode positioned within the vacuum chamber to receive the electrons emitted from the cathode and configured to generate a beam of x-rays from the electrons. The x-ray tube also includes an induction motor configured to rotate the anode, with the induction motor further including a stator having a plurality of windings to generate a rotating magnetic field when a current is provided to the plurality of windings, a rotor positioned within the stator and configured to rotate relative thereto responsive to the rotating magnetic field so as to cause the anode to rotate, and a component positioned on one end of the rotor and being configured to alter a reluctance of the rotor, thereby creating a disturbance in the rotating magnetic field. 
     Various other features and advantages will be made apparent from the following detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate preferred embodiments presently contemplated for carrying out the invention. 
       In the drawings: 
         FIG. 1  is an illustration of an AC induction motor according to an embodiment of the invention. 
         FIG. 2  is an illustration of an AC induction motor according to an embodiment of the invention. 
         FIGS. 3 and 4  are illustrations of an AC induction motor according to embodiments of the invention. 
         FIG. 5  is a schematic view of a motor assembly incorporating the AC induction motor of any of  FIGS. 1-4  according to an embodiment of the invention. 
         FIG. 6  illustrates graphs of a stator phase current spectrum in the time domain and the frequency domain. 
         FIG. 7  is a cross-sectional view of an x-ray tube incorporating the AC induction motor of any of  FIGS. 1-4  according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention are directed to an asynchronous motor that includes a component positioned adjacent to the rotor and stator of the motor. The positioning of the component within the magnetic field generated by the supply of current to the stator changes the rotor reluctance and magnetomotive force (MMF) permeance, which generates a disruption in the magnetic field, thereby causing a measurable change in the stator phase current spectrum. Frequencies measured within the stator phase current spectrum, including the disturbances caused by the component, can then be analyzed to measure the rotational speed of the rotor. 
     Referring to  FIG. 1 , an AC induction motor  10  (i.e., asynchronous motor) is illustrated according to an embodiment of the invention. Asynchronous motor  10  includes a stator  12  and a rotor assembly  14  (i.e., “rotor”). Stator  12  further includes a stator core  16  and windings  18  wound on the stator core  16 . The stator core  16  has a core main body  20  formed, for example, by stacking a large number of annular-shaped thin plates (not shown) made of electromagnetic steel and insulators (not shown) provided on axial end surfaces of the core main body. The stator core  16  is provided with a plurality of teeth  22  at a predetermined pitch along a circumferential direction thereof. According to an exemplary embodiment, windings  18  are wound on the respective teeth  22 , with slots  24  formed between adjacent teeth  22  along the circumferential direction. As further shown in  FIG. 1 , rotor assembly  14  includes a rotor core  26  with end rings  27  and a number of rotor bars  28  coupled to the rotor core  26 . According to the embodiment of  FIG. 1 , a rotor shaft  30  is mechanically coupled to the rotor core  26 . However, according to additional embodiments of the invention, such as when motor  10  is incorporated into an x-ray tube, it is recognized that motor  10  and rotor assembly  14  may be provided without a rotor shaft  30  attached thereto. 
     In operation, an excitation current is provided to stator  12  such that current flows through stator windings  18 . The flow of current through windings  18  creates a rotating magnetic field in an air gap (not shown) between the stator  12  and rotor  14  that induces current flow through rotor bars  28 . These currents interact with the rotating magnetic field created by the stator  12  and, in effect, cause a rotational motion on the rotor  14 . According to embodiments of the invention, asynchronous motor  10  may be in the form of 3-phase motor, however, it is recognized that motor  10  could also be in the form of a single phase motor or another multi-phase motor. 
     Also included in asynchronous motor  10  is a component or mass  32  separate from the stator  12  and rotor assembly  14 . According to one embodiment of the invention, component  32  is attached to rotor shaft  30  on one end of the rotor  14 , such as via bolting welding, or brazing for example. Alternatively, and according to another embodiment of the invention, component  32  is attached directly to rotor core  26  via bolting welding, or brazing for example, as is shown in  FIG. 2   
     Referring again to  FIG. 1 , according to one embodiment of the embodiment, component  32  is constructed as a gear-type component having a slotted design that includes a body  34  and a plurality of projections  36  extending out from the body  34 , with the projections  36  being spaced apart such that a plurality of slots  38  are formed between the projections  36 . Alternatively, it is recognized that component  32  can also be constructed in various other forms, including other uniform or non-uniform constructions and with or without slots or teeth, such as a cylindrical shape as shown in  FIG. 3  or an elliptical shape as shown in  FIG. 4 , for example. Thus, the embodiment of component  32  shown in  FIG. 1  is not meant to be limiting. 
     According to embodiments of the invention, component  32  is configured to generate a disruption in the magnetic field generated by stator  12  (i.e., in the magnetic flux between the stator  12  and rotor assembly  14 ), thereby providing for accurate measurement of the rotation frequency or “rotor speed” of the rotor  14 , as explained in detail below. To generate such a disruption, component  32  is formed as a ferromagnetic or paramagnetic component that alters a magnetic reluctance and magnetomotive (MMF) permeance of rotor assembly  14 . That is, the altering of the magnetic reluctance of rotor  14  caused by component  32  affects an equivalent circuit of the motor  10 , such that the MMF permeance that is produced during motor operation causes an identifiable disruption in the stator current. According to an exemplary embodiment, component  32  is formed from a ferromagnetic material (i.e., a ferrous material, such as electromagnetic steel), and thus here below component  32  is generally described as a “ferrous component” in accordance with a preferred embodiment of the invention. 
     For purposes of measuring the rotor speed, it is recognized that the change in reluctance in asynchronous motor  10  caused by ferrous component  32  generates a disturbance in the magnetic field (i.e., in the magnetic flux in the air gap between the stator  12  and rotor  14 ), thereby causing a subtle but measurable change in the stator phase current spectrum that can be measured. That is, the disturbance of the magnetic field creates harmonics in the stator phase current spectrum at certain identified frequencies. The stator current spectral components introduced by ferrous component  32 , are at frequencies: 
                       f   comp     =       f   s     ⁡     [     1   ±       (     1   -   s     )     ⁢     k   p         ]         ,           [     Eqn   .           ⁢   1     ]               
where k=1, 2, 3, . . . , fs is the supply frequency, s is the per unit slip and p is the number of pair poles.
 
     It is recognized that the amplitude of the stator current spectral components introduced by ferrous component  32 , f comp , is determined in part by the size and mass of the ferrous component. That is, the amount by which ferrous component  32  alters the magnetic reluctance of rotor assembly  14  is based on the size/mass of component  32 , such that the amplitude of the stator current spectral components introduced by ferrous component  32  are also determined in part by the size and mass of the ferrous component. As an example, it is envisioned that ferrous component  32  can have a mass as low as 3% of the mass of rotor assembly  14  or a greater mass, such as 10% of the mass of rotor assembly  14 . For a ferrous component  32  having a mass that is 10% of the mass of rotor assembly  14 , an amplitude of the stator current spectral components introduced into the stator phase current spectrum by ferrous component  32  will be increased by 100 times. 
     According to embodiments of the invention, the change in the stator phase current spectrum can be measured, for example, by a processor connected to asynchronous motor  10 . A motor assembly  40  is illustrated in  FIG. 5  where such a processor  42  is implemented in a motor drive  44  used to drive asynchronous motor  10 . As shown in  FIG. 5 , motor drive  44  may be configured, for example, as an adjustable or variable speed drive designed to receive a three-phase AC power input power input  46   a - 46   c ; however, it is recognized that single or other multi-phase arrangements are also envisioned. According to one embodiment of motor assembly  40 , a drive control unit  48  is integrated within motor drive  44  and functions as part of the internal logic of the drive  44 . Motor drive  44  also includes a drive power block unit  50 , which may, for example, contain a rectification unit, a filtering inductor, a DC bus capacitor or battery, and a pulse width modulation (PWM) inverter (DC to controlled AC). Drive  44  receives the three-phase AC input  46   a - 46   c , which is fed to drive power block unit  50 . The drive power block unit  50  converts the AC power input to a DC power, inverts and conditions the DC power to a controlled AC power for transmission to asynchronous motor  10 . Motor assembly  40  also includes a drive user interface  52  or drive control panel, configured to enable users to input motor parameters and drive operating parameters and other parameters necessary for the drive operation. 
     As set forth above, processor  42  is provided with motor drive  44  and is configured to measure the stator phase current from the asynchronous motor  10 . According to one embodiment, processor  42  is integrated within drive  44  and functions as part of the internal logic of drive  44 . Alternatively, processor  42  may be embodied in an external module distinct from drive  44 , and receive data therefrom (e.g., current and/or voltage signals). In operation, processor  42  functions to receive/measure current signals from the stator  12  ( FIG. 1 ) for purposes of analyzing the stator phase current spectrum. The processor  42  translates the measured stator phase current spectrum from the time domain to the frequency domain by use of a fast Fourier transform (FFT). 
     Upon application of an FFT, frequencies measured within the stator phase current spectrum, including the disturbances caused by the ferrous component, are then analyzed to measure the rotational speed of the rotor. That is, the processor  42  correlates the observed frequencies from the transformed stator phase current spectrum to the actual speed of the rotor. According to an exemplary embodiment of the invention, processor  42  performs a calculation for determining rotor speed in asynchronous motor  10  according to:
 
ω= f   s   −f   sc   [Eqn. 2],
 
where ω is the rotor speed, f s  is the applied stator frequency, and f sc  is the stator current spectrum peak.
 
     Identification of the “applied stator frequency” and “stator current spectrum peak” from the stator phase current spectrum in the frequency domain, for purposes of determining values thereof in Eqn. 2, is illustrated in  FIG. 6 . That is, the applied stator frequency  54  and stator current spectrum peak frequency  56  are identifiable in the stator phase current spectrum  58  for use in calculating the rotor speed. The effect of ferrous component  32  on the amplitude of the measured stator current spectrum peak frequency  56  (i.e., increasing the amplitude) can be seen in  FIG. 6 , as compared to an amplitude of a measured stator current spectrum peak frequency  59  in a motor  10  not having a ferrous component  32  included therein. While Eqn. 2 is specifically set forth for use in calculating the rotor speed from the disruptions in the stator phase current spectrum caused by the ferrous component  32 , it is recognized that other equations or algorithms could also be implemented in analysis of the stator phase current spectrum for purposes of determining the rotor speed, and thus embodiments of the invention are not meant to be limited merely to the current analysis technique described herein. 
     In analyzing the stator phase current spectrum, it is recognized that it is desirable to increase the signal-to-noise ratio (SNR) of received current signals to increase the robustness of the signal processing and measurement. Therefore, it is desirable to employ methods for increasing the amplitude of stator current spectrum components at desired frequencies. According to an embodiment of the invention, such increasing of stator current spectrum components can be achieved by employing load variation and eccentricity variation techniques. Varying of the load and/or eccentricity generates a disturbance of the air-gap magnetic flux that consequently creates harmonics in the currents as vibrations into the motor. Variations in load and/or eccentricity can thus be purposely introduced to increase the stator current spectrum signal so as to provide for increased robustness of determining rotor speed. 
     With respect to introducing a load variation, the stator current spectral components are at frequencies: 
                       f   load     =       f   s     ⁡     [     1   ±       (     1   -   s     )     ⁢     k   p         ]         ,           [     Eqn   .           ⁢   3     ]               
where k=1, 2, 3, . . . , f s  is the supply frequency, s is the per unit slip and p is the number of pair poles.
 
     With respect to introducing an eccentricity variation, the stator current spectral components are at frequencies: 
     
       
         
           
             
               
                 
                   
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     As can be seen in Eqns. 1, 3, and 4, the stator current spectral components introduced by the ferrous component, the load variation, and the eccentricity are at a same/common frequency. Thus, the stator current spectrum components introduced by the load variation and eccentricity variation serve to increase the amplitude of the stator current spectrum component introduced by the ferrous component, providing for a more robust determination of rotor speed. In calculating the rotor speed according to Eqn. 2, both main signals and/or harmonics can be measured in the stator phase current spectrum. Such signals are measured at an increased or higher frequency in the range of 1 kHz or above, as measurement at such a higher frequency creates harmonic frequency separation. The signal measurements made at the higher frequency are thus analyzed to measure/identify the stator current spectrum peak for purposes of determining rotor speed, as set forth in Eqn. 2. 
     Referring now to  FIG. 7 , implementation of an asynchronous motor  10  (such as shown in  FIG. 1 ) into an x-ray tube  60  is shown according to an embodiment of the invention. While x-ray tube  60  is shown as incorporating cathode, anode, and bearing arrangements and/or structures described in detail below, it is recognized that x-ray tubes incorporating other varied cathode, anode, and bearing arrangements and/or structures are also within the scope of the invention. As such, the exemplary x-ray tube  60  shown in  FIG. 7  incorporating the described cathode, anode, and bearing structures is not meant to limit the scope of the invention. 
     As shown in  FIG. 7 , according to an embodiment of the invention, asynchronous motor  10  is incorporated into an x-ray tube  60  that includes a casing or housing  62  having a radiation emission passage  64  formed therein. The casing  62  encloses a vacuum  66  and houses an anode  68 , a bearing assembly  70 , and a cathode  72 . X-rays  74  are produced when high-speed electrons are suddenly decelerated when directed from the cathode  72  to the anode  68  via a potential difference therebetween of, for example, sixty thousand volts or more in the case of CT applications. The electrons impact a material layer  76  at focal point  78  and x-rays  74  emit therefrom. The point of impact is typically referred to in the industry as the track, which forms a circular region on the surface of the material layer  76 , and is visually evident on the target surface after operation of the x-ray tube  60 . To avoid overheating the anode  68  from the electrons, the anode  68  is rotated at a high rate of speed about a centerline  80  at, for example, 90-250 Hz. 
     The bearing assembly  70  includes a center shaft  82  attached to the rotor  14  of asynchronous motor  10  at first end  84  and attached to the anode  78  at second end  86 . A front inner race  88  and a rear inner race  90  rollingly engage a plurality of front balls  92  and a plurality of rear balls  94 , respectively. Bearing assembly  70  also includes a front outer race  96  and a rear outer race  98  configured to rollingly engage and position, respectively, the plurality of front balls  92  and the plurality of rear balls  94 . 
     As shown in  FIG. 7 , the rotor  14  of asynchronous motor  10  resides inside a rotor can  100  of casing/housing  62  and is attached to center shaft  82 . The stator  12  of asynchronous motor  10  resides outside rotor can  100  in either air or oil for cooling thereof, with the stator  12  being connected to a low or high efficiency motor drive  44  (LEM or HEM drive). In operation, the stator  12  functions to generate a magnetic field between the stator  12  and the rotor  14  by having a high current passed through a plurality of windings (not shown) included therein, as described above with respect to  FIG. 1 . The high current passing through the stator windings generates the magnetic field, thereby transmitting torque from the stator  12  to the rotor  14  according to known principles. 
     As further shown in  FIG. 7 , ferrous component  32  is included in asynchronous motor  10  and is positioned on an end of rotor  14  inside casing  62 . According to one embodiment of the invention, the ferrous component  32  is secured to a rotor core (not shown) of rotor  14 . When current is provided to stator  12  from motor drive  44 , ferrous component  32  causes a disturbance in the rotating magnetic field, thereby inducing a voltage in the stator  14  and causing a subtle change in the stator phase current spectrum that can be measured, for example, by processor  42  of motor drive  44 . The processor  42  analyzes the stator phase current spectrum and applies an FFT thereto to translate the measured stator phase current spectrum from the time domain to the frequency domain. Upon application of an FFT, the processor  42  then correlates the observed frequencies from the transformed stator phase current spectrum to the actual speed of the rotor  14 , such as by way of Eqn. 2 set forth above or by way of other suitable current signature analysis techniques. In implementing the rotor speed estimation technique of Eqn. 2, the applied stator frequency and the stator current spectrum peak introduced by ferrous component  32  are identified in the stator phase current spectrum. According to one embodiment of the invention, load variation and/or eccentricity variation for motor  10  can be employed to increase the amplitude of the stator current spectrum peak and increase the SNR, so as to provide increased robustness for the rotor speed estimation. The disturbances in the stator phase current spectrum introduced by ferrous component  32  (with or without the added load/eccentricity variation) thus provide for accurate determine of the speed of rotor  14  without the need for any sensors, thereby providing for more efficient control of asynchronous motor  10  by motor drive  44 . 
     According to embodiments of the invention, inclusion of ferrous component  32  in asynchronous motor  10  allows for continuous rotor speed measurement, enabling closed loop drive and rotor control. By doing so, the drive scheme can be optimized such that it can run at higher slip, thereby reducing the input power required. More specifically, the stator can be driven at an applied frequency above the expected run speed, reducing the required drive power. Furthermore, with continuous rotor speed feedback, power can be modulated to maintain the rotor speed within specifications. Additionally, by reducing the power required by the drive and delivered to the stator, losses are reduced, heat generation is minimized, and the motor drive and motor design have reduced performance requirements. This decrease in performance requirements allows for drive components to be rated accordingly, reducing cost and increasing reliability. 
     While  FIG. 7  is illustrative of an asynchronous motor  10  such as shown and described in  FIG. 2  as being incorporated into an x-ray tube environment, it is recognized asynchronous motor  10  can be implemented in a wide variety of applications and settings. In general, however, it is recognized that particular benefits are derived from asynchronous motor  10  when it is applied in an x-ray tube application or other application where the rotor may be in a vacuum, such that the rotor speed can&#39;t be easily measured with a physical sensor. 
     Furthermore, while embodiments of the invention described above are described with respect to measuring rotor speed via the introduction and analysis of a stator current spectral component into the stator phase current spectrum, it is also recognized that embodiments of the invention can also be directed to analysis of the stator voltage spectrum. That is, the altering of the rotor reluctance caused by component  32  can be utilized to determine rotor speed via analysis of the stator voltage spectrum rather than the stator current spectrum. 
     Therefore, according to one embodiment of the invention, an asynchronous motor includes a stator having a plurality of windings that is configured to generate a rotating magnetic field when a current is provided to the plurality of windings. The asynchronous motor also includes a rotor positioned within the stator configured to rotate relative thereto responsive to the rotating magnetic field and a component separate from the stator and the rotor that is positioned within the rotating magnetic field, with the component being configured to alter a magnetic reluctance of the rotor so as create a disturbance in the rotating magnetic field. 
     According to another embodiment of the invention, an asynchronous motor including a stator having a plurality of windings and being configured to generate a rotating magnetic field when a current is provided to the plurality of windings. The asynchronous motor also includes a rotor positioned within the stator having a rotor core and a plurality of rotor bar conductors, with the rotor configured to rotate relative to the stator responsive to the rotating magnetic field. The asynchronous motor further includes a component positioned adjacent to the rotor and configured to alter a reluctance of the asynchronous motor so as to generate a disturbance in the rotating magnetic field, with the disturbance in the rotating magnetic field generated by the component introducing a current signal into a stator phase current spectrum of the stator. 
     According to yet another embodiment of the invention, an x-ray tube includes a housing enclosing a vacuum chamber, a cathode positioned within the vacuum chamber and configured to emit electrons, and an anode positioned within the vacuum chamber to receive the electrons emitted from the cathode and configured to generate a beam of x-rays from the electrons. The x-ray tube also includes an induction motor configured to rotate the anode, with the induction motor further including a stator having a plurality of windings to generate a rotating magnetic field when a current is provided to the plurality of windings, a rotor positioned within the stator and configured to rotate relative thereto responsive to the rotating magnetic field so as to cause the anode to rotate, and a component positioned on one end of the rotor and being configured to alter a reluctance of the rotor, thereby creating a disturbance in the rotating magnetic field. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.