Abstract:
In an electric motor, a conductive ring surrounds, and shields, each pole of stationary magnet. When a changing magnetic flux, produced by an armature, penetrates the ring, by Lenz&#39;s Law, the changing flux causes the ring to produce a counter-flux, which adds to the changing flux. Consequently, the total flux within the ring tends to remain constant. Maintaining this constant flux tends to reduce noise and vibration which the changing rotor flux otherwise causes.

Description:
The invention concerns damping of changes in magnetic flux which occur in electric motors, thereby damping noise and vibration which the flux changes induce. 
     BACKGROUND OF THE INVENTION 
     FIGS. 1-5 provide simplified illustration of some events which occur in electric motors, and give some possible explanations of vibration and noise. 
     FIG. 1A illustrates permanent magnets  3 , having poles north N and south S, as contained within a permanent magnet electric motor (motor is not shown). FIG. 1B illustrates an armature  6 , which includes a single-turn coil  9  and a commutator  12 . In operation, brushes  15  contact the commutator  12 . FIG. 1C illustrates the components of FIGS. 1A and 1B when assembled. 
     FIG. 2A illustrates magnetic field lines  18  produced by the magnets  3  of FIG.  1 A. FIG. 2B illustrates current  21  induced by voltage V+ applied to the brushes  15 , and also the magnetic flux lines  24  which accompany the current  21 . FIG. 2C is a cross-sectional view of FIGS. 2A and 2B, with some of the flux lines  24  removed, and with the brushes  15  shown in contact with the commutator  12 . 
     FIGS. 3A through 3F show the components of FIG. 2C in assembled form, and show how the magnetic flux  24 , produced by the armature  6 , rotates as the armature  6  rotates. In FIG. 3A, the flux  24  is directed to the left, and does not cross the south pole S. (In actual practice, some leakage flux may cross the south pole, but FIG. 3A is a simplification, used to illustrate major principles.) 
     In FIG. 3B, the armature  6  has rotated clockwise, and the armature&#39;s flux  24  occupies the position shown. In FIG. 3C, the armature flux  24  penetrates the south pole S. 
     In FIG. 3D, the armature flux  24  has disappeared, because the commutator  12  is no longer in contact with the brushes  15 . In FIG. 3E, the armature flux  24  has re-appeared, because the commutator re-contacts the brushes  15 . However, the flux  24  has reversed in direction, as indicated by a comparison of FIG. 3E with FIG.  3 C. FIG. 3F indicates the position of the armature flux  24  a time later than in FIG. 3E, wherein the flux does not penetrate the north pole N. 
     The sequence of FIG. 3 provides a simple explanation of one cause of vibration. The sequence of FIGS. 3B through 3F show the following events: 
     
       
         
               
               
             
           
               
                   
               
               
                 Figure 
                 Event 
               
               
                   
               
             
             
               
                 3B 
                 No penetration of south pole. 
               
               
                 3C 
                 Penetration. 
               
               
                 3D 
                 No penetration. 
               
               
                 3E 
                 Penetration, but reversed in 
               
               
                   
                 direction. 
               
               
                 3F 
                 No penetration. 
               
               
                   
               
             
          
         
       
     
     The sequence can be characterized as a repeated sequence of two events: flux penetration of the south pole S, followed by removal of penetration. 
     In effect, a magnetic field is repeatedly applied, and then removed, from the south pole S. The application of the magnetic field applies a force to the south pole S. The removal of the magnetic field removes the force. The sequence of 
     . . . force . . . no force . . . force . . . no force 
     is believed to cause vibration of the south pole S. Similar events occur with respect to the north pole N. 
     A second cause of vibration can be explained with reference to FIGS. 4 and 5. In FIG. 4A, an actual armature  6  comprises a rotor  30  containing slots  33 , which hold conductive bars  36  (also called armature windings). Additional conductors, indicated by the dashed lines  39 , form a conductive loop, analogous to loop  9  in FIG.  1 B. 
     FIG. 4B shows the slotted rotor  30  in cross section, and includes the conductive bars  36 . When current passes through the loop comprising bars  36  and dashed lines  39  in FIG. 4A, the flux lines  40  shown in FIG. 5A are generated. Two positions which the slotted rotor occupies during rotation are shown in FIGS. 5B and 5C. 
     A significant feature of these two positions is that the flux lines must traverse different numbers of slots en route to the south pole S. That is, different flux lines follow paths through different materials. Consequently, different flux lines apply different forces to the south pole S. These differences can also cause vibration, as will now be explained. 
     The slots  33  in FIG. 5A act as an air gap, and reduce the strength of the flux lines  40 . (Even though the slots  33  contain the conductive bars  36 , the slots can be viewed, for present purposes, as being filled with air, because the magnetic permeability of the conductive bars is close to that of air, when compared with the permeability of the material of which the rotor  30  is itself constructed. 
     How an air-gap can change a magnetic field can be explained by an analogy. When a hand-held magnet is brought two inches from a steel nail, the nail hardly “feels” the magnet, because of the large, two-inch, air gap. However, when the magnet is brought sufficiently close to the nail, the nail snaps into contact with the magnet. The very small air gap, created when the magnet approached the nail, caused the strength of the flux lines (more precisely, the magnetic flux density) to increase. 
     Similarly, when the rotor  30  is in the position shown in FIG. 5B, the flux lines must pass through three slots, or air gaps, indicated in insert I, en route to the south pole S. In contrast, in FIG. 5C, the number of slots increases from three to four, as indicated in insert I 2 . 
     In effect, the air gap between the armature and the south pole S has increased from FIG. 5B to FIG.  5 C. Consequently, the “pull” which the rotor  30  applies to the south pole S, because of the flux lines  40 , decreases in FIG. C, compared with FIG. 5B, because of the increased air gap, similar to the case of the steel nail. 
     Therefore, as the armature  30  rotates, the number of slots, through which the flux lines must travel en route to the south pole S, changes, thereby changing the magnetic force applied to the south pole S. This changing magnetic force induces vibration. Some components of the vibration lie within the range of human hearing, and are perceived as audible noise. 
     A similar analysis applies to the north pole N. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to reduce noise and vibration in electric motors. 
     A further object of the invention is to reduce noise and vibration caused by a changing magnetic flux applied to internal components of a permanent magnet electric motor. 
     In one form of the invention, a conductive ring surrounds a stationary pole of a magnet in an electric motor. When armature flux through the hole in the ring changes, a current is induced, which generates a magnetic field which compensates for the change in the armature flux, thereby tending to keep the overall flux constant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1C illustrate exploded views of a simplified DC machine. 
     FIGS. 2A-2C illustrate the components of FIG. 1, in greater detail. 
     FIGS. 3A-3F illustrate rotation of magnetic flux line  24 , caused by rotation of coil  9  of FIG. 1B 
     FIG. 4A is an exploded view of a slotted rotor. 
     FIG. 4B is a cross-sectional view of a slotted rotor contained between two magnets. 
     FIGS. 5A-5C illustrate how the air gap effectively changes between a rotor and stator, during rotation of the rotor. 
     FIG. 6 illustrates one form of the invention. 
     FIGS. 7A-7E illustrate rotation of magnetic flux lines  59 , with respect to ring  50  of FIG.  6 . 
     FIG. 8 illustrates another form of the invention. 
     FIG. 9 illustrates a perspective view of part of the apparatus of FIG.  8 . 
     FIG. 10 is a plot of search coil voltage versus time, when the motor of FIG. 8 was run at no load, with rings  50  and  53  effectively absent. 
     FIG. 11 is a plot of search coil voltage versus time, when the motor of FIG. 8 was run at no load, with rings  50  and  53  in FIG. 6 present. 
     FIG. 12 illustrates a plot of accelerometer output versus frequency. 
     FIG. 13 is a plot of motor performance, with rings present, and with rings absent. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 6 illustrates an electric motor comprising one form of the invention. For ease of illustration, no armature coils are shown. In this embodiment, the electric motor comprises two stationary conductive loops  50  and  53 . The loop  50  interacts primarily with the flux penetrating the south pole S, and the other loop  53  interacts primarily with the flux penetrating the south pole S. 
     FIG. 7 provides a simplified explanation of he operation of loop  50 . In FIG. 7A, loop  9 , shown also in FIG. 1B, produces magnetic flux lines  59 . In the sequence of FIGS. 7B through 7E, the loop  9  is shown rotating about motor axis  61 . The flux lines  59  rotate also, as indicated. 
     During the rotation, the flux, which the ring  50  in FIG. 7A surrounds, changes, as indicated by FIGS. 7B through 7E. This change induces a current  65  in FIG.  7 A. By Lenz&#39;s Law, this current produces its own flux (not shown) which compensates for the changing flux, thereby tending to keep the overall flux passing through the ring  50  constant. 
     More specifically, a voltage is induced in the ring, which is proportional to the first time-derivative of the normal (i.e., perpendicular) component of the flux passing through the ring. This voltage induces the current  65  in FIG.  7 A. One normal component N is shown in FIG.  7 C. Normalcy, or perpendicularity, is defined with reference to the plane of the ring  50 . 
     Therefore, the ring  50  in FIG. 6 shields the south pole S from the changes in flux discussed in the Background of the Invention. 
     FIG. 8 illustrates apparatus used in a test undertaken by the Inventors. The upper part of the Figure is a diagram of electrical continuity. Corresponding parts, similarly labeled, are shown in FIG.  9 . The combination of the rods R in FIGS. 8 and 9, together with end plates E, form the conductive ring of FIG.  6 . 
     Specifically, in FIG. 8, rods R 1 , R 2 , and the end plates E (not shown in FIG. 8, but visible in FIG. 9) form a ring analogous to ring  50  in FIG.  6 . Also, in FIG. 8, rods R 3 , R 4 , and the end plate E (not shown in FIG. 8, but visible in FIG. 9) form a ring analogous to ring  53  in FIG.  6 . The two coils, analogous to coils  50  and  53  in FIG. 6, are held at a common DC potential, by virtue of the connection through end plate E, indicated as a thin hoop in FIG.  8 . 
     In the test, an accelerometer  70 , shown at the bottom of FIG. 8, was attached to a casing T to which magnet pole S was attached. A search coil  78 , was used to detect induced voltage in the ring comprised of R 1 , R 2  and the two end plates E, shown in FIG.  9 . The search coil  78  infers flux changes in the magnetic field passing between rods R 1  and R 2  in FIG.  8 . 
     FIG. 9 illustrates an exploded perspective view of part of the apparatus of FIG.  8 . 
     FIG. 10 is a plot of search coil voltage versus time, when the motor of FIG. 8 was run at no load, with no dampers present (the coils  50  and  53  were open-circuited, or, from another viewpoint, each ring was split open). FIG. 11 is a plot of search coil voltage versus time, when the motor of FIG. 8 was run at no load, with rings  50  and  53  in FIG. 6 present, as indicated in FIG. 8 (the rings  50  and  53  were not split, but present in ring-form). 
     The difference in the two plots indicates that the rings, or dampers, reduced the search coil voltage, thereby supporting the inference that flux changes through the rings  50  and  53  were reduced by the rings. 
     FIG. 12 illustrates a plot of accelerometer output versus frequency. The solid line indicates the damped case, and is, in general, smaller in amplitude at most frequencies than the dashed line, which indicates the undamped case. FIG. 12 supports the inference that the damping rings  50  and  53  in FIG. 6 reduce vibration of the motor. 
     FIG. 13 indicates that the presence of the dampers does not significantly affect motor performance. 
     It should be observed that the magnets N and S in FIG. 6 need not be permanent magnets, but can take the form of electromagnets. 
     It should be appreciated that the rings  50  and  53  are electrically independent of the motor, with the exception of the current  65 , shown in FIG. 7A, which is induced. That is, neither stator nor rotor current passes through the rings  50  and  53 . 
     Notice that one effect of ring  50  in FIG. 6 can be characterized as reducing interaction between (a) the time changing flux  59  in FIG.  7  and (b) the magnet pole S in FIG. 6, by virtue of reducing the magnitude of changes in the flux which reach the pole S. 
     As FIG. 9 indicates, the components used to construct the rings can also be used as part of the motor&#39;s structural housing. For example, rods R may provide support for end rings E. 
     In FIG. 9, the end rings E are electrically part of the rings of the type shown in FIG.  6 . However, in FIG. 9, the end rings E are not part of the case structure, which includes tube T, although they could be so constructed. 
     Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.