Abstract:
A compliant stator motor is disclosed. In one implementation, the motor ( 200 ) includes a rotor ( 210 ), a stator ( 220 ) having main and auxiliary windings, an outer motor case ( 230 ), and a group of isolators ( 240 ) positioned between the stator ( 220 ) and outer motor case ( 230 ) to enhance forces applied to the foundation ( 260 ) due to excitation of the auxiliary windings. In another implementation, the motor ( 300 ) includes a rotor ( 310 ), a stator ( 320 ) including main and auxiliary windings, linear bearings ( 320 ) configured to constrain a motion of the stator ( 320 ) to an axial direction, and isolators ( 330 ) connected to the stator ( 320 ) and configured to enhance axial forces applied to the foundation due to excitation of the auxiliary windings.

Description:
RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 based on U.S. Provisional Application Ser. No. 60/279,398, filed Mar. 28, 2001, the disclosure of which is incorporated herein by reference. 
    
    
     GOVERNMENT CONTRACT 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00014-97-C-0075 awarded by the Office of Naval Research. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electric motors and, more particularly, to systems and methods for providing a compliant connection between the stator of an electric motor and the motor case. 
     2. Description of Related Art 
     Auxiliary windings in electric motors can be made to generate forces and moments in six-degrees-of-freedom. In this way, a motor can be made to act like an actuator offering the potential for canceling forces applied externally to the motor shaft or housing. A problem that arises, however, is that the high stiffness commonly found between the rotor and the stator reduces the magnitude of the resultant forces that can be applied to the foundation through the rotor, especially at low frequency. 
     Accordingly, there is a need in the art for systems and methods that allow the magnitude of the resultant forces applied to the rotor to be enhanced. 
     SUMMARY OF THE INVENTION 
     Systems and methods consistent with the present invention address this and other needs by providing a compliant connection between a motor&#39;s stator and outer motor case. The compliant connection allows the auxiliary windings to transmit higher forces to the foundation at lower frequency than would be possible for conventional rigidly-connected stators. 
     In accordance with the principles of this invention as embodied and broadly described herein, a motor includes a rotor, a stator including main and auxiliary windings, an outer motor case, and a group of isolators positioned between the stator and the outer motor case. 
     In another implementation consistent with the present invention, an electromechanical machine includes a rotor, a stator including main and auxiliary windings, linear bearings configured to constrain a motion of the stator to an axial direction, and a group of isolators connected to the stator and configured to attenuate a force exerted by the stator in the axial direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, explain the invention. In the drawings, 
         FIG. 1  illustrates an exemplary overall configuration of a compliant stator motor according to implementations consistent with the present invention; 
         FIGS. 2A and 2B  illustrate an exemplary detailed view of a compliant stator motor in an implementation consistent with the present invention; and 
         FIG. 3  illustrates an exemplary alternative configuration of a compliant stator motor in an implementation consistent with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of implementations consistent with the present invention refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents. 
     Implementations consistent with the present invention provide compliance between the stator and the motor case to increase the effectiveness of the auxiliary windings in generating forces on the motor foundation. Introducing compliance into the rotor mounting system might lead to unacceptable shaft dynamics. Because the shaft rotates, any imbalance in it could lead to unacceptably large lateral deflections of the rotor. These problems substantially go away if the compliance is introduced into the stator/motor case rather than the rotor. Accordingly, systems and methods consistent with the present invention provide a very stiff bearing assembly with a resiliently mounted stator/motor case assembly. The stiff rotor mounting minimizes the deflections due to any imbalances and efficiently transmits any external loads, such as thrust, from the shaft to the foundation. 
     Exemplary System Configuration 
       FIG. 1  illustrates an exemplary overall configuration of a compliant stator motor  100  according to implementations consistent with the present invention. Motor  100  includes a rotor  110  and a stator/motor case  120  connected to a foundation  130 . The rotor  110  may include any type of rotor, such as those used in permanent magnet electric motors. The stator/motor case  120  includes main windings  121 , auxiliary windings  122 , and, as will be described in detail below, isolation mounts that add compliance between the stator and outer motor case. 
     As illustrated, the auxiliary windings  122  apply forces (F aux ) equally to the rotor  110  and stator  120 . External forces (F excit ) may be applied to the rotor  110 . The stiffness with which the rotor  110  connects to the foundation  130  is high and the stiffness with which the stator/motor case  120  connects to the foundation is low. During those instances where the resonant frequency of the stator/motor case  120  on its mounting stiffness is lower than the frequencies at which forces (F actuator ) on the foundation  130  are to be generated, the stator/motor case  120  mass reacts the auxiliary winding forces and transmits these forces with little attenuation through the rotor  110  to the foundation  130 . 
       FIGS. 2A and 2B  illustrate an exemplary detailed view of a compliant stator motor  200  according to an implementation consistent with the present invention. As illustrated, the compliant stator motor  200  includes a rotor  210 , a stator/motor case  220 , an outer case  230 , and isolation mounts (or isolators)  240 . The rotor  210 , stator/motor case  220 , outer case  230 , and isolators  240  are mounted in a pedestal  250  that is hard mounted to the foundation  260 . 
     The outer case  230  provides a mounting location for the resilient isolators  240  that support the stator/motor case  220  and reacts the torque applied by the stator  220  to the rotor  210 . The outer case  230  rigidly attaches to the pedestal  250 . The impedance of the outer case where the isolators are attached should be much greater than the impedance of the isolators. 
     The inner motor case  220  supports the stator and connects to the outer case  230  by a number of resilient isolators  240 . In an implementation consistent with the present invention, the isolators  240  may be formed of an elastomeric (i.e., rubber-like) material. The size of the isolators  240  depends on the size of the motor being supported. The larger the motor size, the larger the size of the isolators  240 . The isolators  240  may be oriented to be in shear for radial and axial motor case deflections (presumably high compliance) and in compression for rotation about the motor axis (better able to carry the high torque loads). The isolators  240  may be symmetrically located about the axis of the motor, so that when the isolators  240  deflect under the torque load, the axis of the stator/motor case  220  does not move out of alignment with the rotor  210 . Eight isolators  240  are illustrated in  FIG. 2B  for simplicity. In practice, more or fewer isolators  240  may be used. 
     If the stator  220  is allowed to move radially, as well as axially, the isolator  240  stiffness should be chosen appropriately. The isolator  240  cannot be made too compliant in the radial direction because if the isolators  240  do not provide sufficient stiffness, the attractive magnetic forces in the rotor-stator gap could cause the gap to close and the rotor  210  and stator  220  to lock together. On the other hand, it is desirable to make the natural frequency of the stator/motor case  220  on the isolators  240  to be as low as practical (requiring very compliant isolation mounts  240 ) so as to increase the effectiveness of the auxiliary winding forces over as broad a range of frequency as possible. 
     The motion of the stator  220  relative to the rotor  210  in the radial direction may result in torque modulation due to the decrease in the rotor/stator gap over a portion of the circumference of the motor  200 . Because of the nonlinear relationship between motor torque and gap size, displacement of the stator  220  relative to the rotor  210  in the radial direction may result in an increase in torque. Consequently, when the auxiliary winding forces are reacted against the stator/motor case  220 , the stator/motor case  220  may move changing the gap and potentially modulating the motor torque. In such an event, the auxiliary or main winding system may be used to modulate the torque in order to counteract the unwanted modulation. 
     Alternative System Configuration 
       FIG. 3  illustrates an exemplary alternative configuration of a compliant stator motor  300  according to an implementation consistent with the present invention. As illustrated, the compliant stator motor  300  includes a rotor  310 , a stator  320 , and vibration isolation mounts (or isolators)  330 . 
     The rotor  310  may include any type of rotor, such as those used in permanent magnet electric motors. The stator  320  may include main and auxiliary windings (not shown). In this configuration, the stator  320  is mounted in linear bearings in a known manner. The linear bearings constrain the motion of the stator  320  in the radial and tangential (torque) directions, while allowing the stator  320  to move in the axial direction. The linear bearings may be the type commonly used in motors, or alternatively may be special linear bearings designed for more rugged applications. 
     The vibration isolators  330  provide isolation in the axial direction. Similar to the description above with respect to  FIGS. 2A and 2B , the isolators  330  may be formed of an elastomeric (i.e., rubber-like) material. The size of the isolators  330  depends on the size of the motor  300  being supported. The larger the motor size, the larger the size of the isolators  330 . Since the isolators  330  would be required to support a lower load (only axial direction) than in the configuration illustrated in  FIGS. 2A and 2B  where the isolators  240  support axial, radial, and tangential directions, the isolators  330  may be smaller and have a lower load capacity. Four isolators  330  are illustrated in  FIG. 3  for simplicity. In practice, more or fewer isolators  330  may be used. 
     In general, the auxiliary windings produce very robust forces in the radial and tangential directions while producing much weaker forces in the axial direction. Consequently, the axially oriented compliant stator  320  would ensure that the weaker axial auxiliary winding forces would be transmitted with little attenuation to the foundation. The stronger radial and tangential auxiliary winding forces may be attenuated, but because they are so robust, the attenuation might be less detrimental. 
     CONCLUSION 
     Systems and methods, consistent with the present invention, provide compliance between the stator and the motor case of an electric motor to increase the effectiveness of the auxiliary windings in generating forces on the motor foundation. These forces can be used to cancel external forces applied to the motor shaft or housing of the motor. 
     The foregoing description of exemplary embodiments of the present invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. 
     The scope of the invention is defined by the claims and their equivalents.