Abstract:
The invention relates to an economical, non-wearing, electrical permanent magnet drive for actively controlling the rotor position in three degree of freedom. The stator windings produce superimposed fields with different pole numbers in the pole pitches by unsymmetrical magnetomotive force distributions.

Description:
BACKGROUND OF THE INVENTION 
   The magnetic bearing technology includes fields of application of machine and apparatus construction with extremely high requirements for the rotary speed range, the working life, the cleanliness and the sealed nature of the drive system—i.e. essentially fields of application which cannot be realized or can only be realized with difficulty using conventional bearing techniques. Various embodiments, such as for example high speed milling and grinding spindles, turbo compressors, vacuum pumps or pumps for high purity chemical or medical products, are already equipped with magnetic bearings. 
   The machine cross-sections shown in the following figures are simply by way of example and partly greatly simplified and serve exclusively for the more precise explanation of the principle of operation. 
   A conventional magnetically journalled machine ( FIG. 1 ) requires, in addition to a machine unit ( 1 ), two radial magnetic bearings ( 2 ), ( 3 ), an axial magnetic bearing ( 4 ), two mechanical touch down bearings ( 5 ), ( 6 ), as well as a total of ten power converter stages ( 7 ), ( 8 ), ( 9 ), ( 10 ) for the control of the motor phases and magnetic bearing phases. 
   In the literature there are proposals ( FIG. 2 ) for the integration of machines and radial magnetic bearings into one magnetic stator unit. In one stator there are two separate winding systems ( 11 ), ( 12 ) for torque winding and levitation force winding which are inserted into slots in multiple layers. The relationship p 1 =p 2 ±2 basically applies in bearingless motors for a largely decoupled levitation force formation and torque formation between the winding pole numbers, with p 1  or p 2  also representing the pole number of the rotor. Both winding systems of the motor in  FIG. 2  are three-phase. The coils are chord-wound and distributed over several slots, whereby an approximately sinusoidal flux linkage is achieved. The two windings are composed as follows:
     four-pole machine winding ( 11 ) (outer): phase  1  ( 13 ), phase  2  ( 14 ), phase  3  ( 15 )   two-pole levitation winding ( 12 ) (inner): phase  1  ( 16 ), phase  2  ( 17 ), phase  3  ( 18 ).   

   In order to achieve a cost-favorable overall system the possibility exists of reducing the number of winding systems and thus to simplify the control electronics in addition to the mechanical layout. 
   As an example of a motor with a reduced number of windings, a motor with common torque winding systems and levitation force winding systems consisting of four concentrated coils should be explained which will be termed a bearingless single-phase motor in the following. 
   In  FIG. 3  the rotor and stator of a four-pole motor is shown in an external rotor embodiment. In this arrangement the rotor ( 35 ) is preferably constructed in ring-shaped or bell-shaped design. With the aid of the four concentrated coils ( 31 ,  32 ,  33 ,  34 ) generation of a two-pole and four-pole circulation distribution is possible, so that a levitation force in the x and y directions and torque can be produced independently of one another. The determination of the individual phase currents takes place paying attention to the desired value setting for the rotor position and speed of rotation, rotor angle or torque after evaluation of the sensor signals for rotor position (x, y) and rotor angle of rotation (φ). 
   In the preceding section in connection with the prior art the bearingless single-phase motor and the multiphase rotary field motors were described. 
   Both embodiments have considerable technical and economical disadvantages: 
   The bearingless single-phase motor ( FIG. 3 ) is only suited for applications with low requirements with respect to the starting torque. These include, for example, drives for pumps, blowers, fans or ventilators. In the simplest constructional form the bearingless single-phase motor requires only four individual coils. The starting weakness of the single-phase drive is brought about by the design. Whereas rotary field windings are used for the building up of the radial levitation forces, the motor part only has one single-phase alternating field winding. There are thus critical angular positions of the rotor in which the starting torque is zero independently of the selected current amplitude. Provision must therefore be made design-wise for the rotor to come to rest only in positions which differ from the critical angular positions. The moment of inertia of the drive thus enables the critical points to be overcome in particular in the starting phase but also in the steady-state operation. For many other drive tasks the starting torque of this type of motor is too small. 
   The bearingless multiphase motor ( FIG. 2 ) corresponding to the prior art does not have the disadvantage of low torque angular positions. In contrast to the single-phase variant it not only has its own rotary field winding in the bearing part but also in the motor part. However the high coil numbers associated with the two rotary field windings are disadvantageous here. Typical coil numbers of such bearingless motors range as a rule between 36(distributed three-phase windings) and 12 (simple two-phase windings). 
   Bearingless motors, with small numbers of coils and without the restrictions of the field of application which result from the single-phase technology, would be desirable from the preceding considerations and technically and economically extremely interesting for the drive market. 
   SUMMARY OF THE INVENTION 
   The present invention solves the problems encountered in the past with a substantially simplified construction of the magnetically journalled machine and a simplified electrical control which is of particular advantage. 
   A further decisive advantage lies, as a result of the asymmetrical winding construction, in the pronounced chording of the winding and the associated high damping of harmonics in the field layout and circulation layout. This characteristic enlarges the stability range of the position and rotational speed controller, the accuracy of setting and also the quietness of running of the magnetically journalled drive system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will be explained in the following with reference to the drawings. There are shown in schematic representation: 
       FIG. 1  shows a conventional, magnetically journalled system; 
       FIG. 2  shows a bearingless multiphase motor relating to the prior art; 
       FIG. 3  shows a bearingless single-phase motor belonging to the prior art; 
       FIG. 4  shows a bearingless multiphase motor with an asymmetric stator section and concentrated windings; 
       FIG. 5  shows the angle-dependent variation of the phase currents for a constant levitation force in the x direction; 
       FIG. 6  shows an asymmetrical magnetomotive force (MMF) along the periphery of the stator; 
       FIG. 7  shows an angle-dependent variation of the phase currents for a constant torque; 
       FIG. 8  shows an asymmetrical field shape at the air gap periphery with the permanent magnetic field faded out; 
       FIG. 9  shows the flux density variation of the levitation force winding (four-pole) of a motor with symmetrical stator core (rotor: two pole); 
       FIG. 10  shows the flux density variation of the torque winding (two pole) of a motor with a symmetrical stator core (rotor: two pole); 
       FIG. 11  shows a bearingless motor with distributed windings (individual coils); 
       FIG. 12  shows a bearingless motor with distributed windings (coil groups); 
       FIG. 13  shows a bearingless motor with separate winding sets for the generation of levitation force and torque; 
       FIG. 14  shows a bearingless motor windings around the stator yoke; and 
       FIG. 15  schematically illustrates the present invention incorporate in a rotary pump. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A possible embodiment of the invention will be described by way of example in the following. 
     FIG. 4  shows a motor with five concentrated individual coils ( 41 ,  42 ,  43 ,  44 ,  45 ). With this stator arrangement both the two-pole and also a four-pole rotary field, i.e. a two-pole and four-pole MMF, can be achieved at the same time by a corresponding supply of current to the coils with superimposed current components. Thus, in cooperation with the two-poled MMF, torque can be achieved on a two-pole rotor and radial levitation forces can be achieved in cooperation with the four-pole MMF. 
   The odd number of coils or limbs five, which is not whole-numbered divisible by the two-pole numbers four and two that are used, leads to an asymmetrical stator core and to asymmetrical MMF or field distributions at the periphery of the stator or of the air gap. Accordingly, in dependence on the angular position of the rotor, on the demand for levitation force and on the torque requirement, the coil currents are to be determined such that the desired operating point is achieved. 
     FIG. 5  shows for this purpose, for a constant levitation force in the x-direction independently of the rotor angle φ of a two-pole permanent magnet rotor with sinusoidal flux density distribution, the associated levitation force coil current components, with I 1  designating the current through the coil  41 , I 2  the current through the coil  42 , I 3  the current through the coil  43 , I 4  the current through the coil  44  and I 5  the current through the coil  45 . In  FIG. 6  the flux density plot of the winding field in the air gap  61  (with the permanent magnet field of the rotor faded out) is shown for the initial angular position (φ=0) schematically in comparison to an ideal sinusoidal four-pole flux density plot ( 62 ). The shape of a four-pole asymmetrical field can be recognized from the flux density diagram. The asymmetry arises as a consequence of the non-integer ratio of the number of limbs five to the winding pole number four realized via the phase currents. 
     FIG. 7  shows in a manner matched to this, for a constant torque, likewise in dependence on the rotor angle φ, the associated torque coil current components, and here I 1  again designates the current through the coil  41 , I 2  the current through the coil  42 , I 3  the current through the coil  43 , I 4  the current through the coil  44  and I 5  the current through the coil  45 . The current components shown in the two  FIGS. 5 and 7  are superimposed in the five motor coils so that both the desired torque and also the desired carrying force can be achieved. 
   The total MMF of the motor over the stator periphery arises from the electrical superposition of the currents and the geometrical distribution of the coils. In  FIG. 8  there is again shown the flux density plot of the winding field in the air gap ( 81 ) (likewise with the permanent magnetic field of the rotor faded out) for the initial angular position of the rotor (φ±0) in comparison to an ideal sinusoidal two-pole flux density plot ( 82 ). The shape of a two-pole asymmetrical field can be recognized from the flux density diagram. The asymmetry arises as a consequence of the non-integer ratio of the limb number five to the winding pole number two realized via the phase currents. 
   In the same manner bearingless rotary field motors can also be designed with for example six or seven concentrated individual coils. A three-coil solution leads to a bearingless single-phase motor with reduced mechanical cost and complexity. 
   In order to show the difference in the flux density shape to a symmetrically designed motor, a four-pole MMF and flux density plot ( 91 ) which arises with a motor with eight concentrated individual coils is shown in  FIG. 9  in comparison to an ideal sinusoidal four-pole MMF and flux density plot ( 92 ). For this motor configuration there are shown, in associated manner in  FIG. 10 , a two-pole MMF and flux density plot ( 101 ), again in comparison to a sinusoidal circulation and flux density plot ( 102 ). 
   Instead of concentrated individual windings, distributed and optionally chorded windings can be implemented into the stator. Here the tooth number or slot number of the stator is so selected for the above-named reasons that it does not amount to any whole-numbered multiple of the two winding pole numbers which are to be realized. 
   An example to explain the principle construction is shown in FIG.  11 . We see here five coils ( 111 ,  112 ,  113 ,  114  and  115 ) laid in slots with a greater coil width than in FIG.  4 . The coils each surround two teeth and are thus no longer termed concentrated coils. A further variant is shown in  FIG. 12  with an additional coil distribution, with two coils arranged in adjacent slots forming a coil group electrically connected together in series or parallel. 
   Whereas the previously treated winding variants represent integrated variants which can simultaneously build up levitation forces and torques,  FIG. 13  shows an embodiment with separate winding sets for the corresponding functions. The three-phase two-pole winding system  131   a - 131   b ,  132   a - 132   b  and  133   a - 133   b  serves with a two-pole permanent magnet excitation for the generation of torque. With the aid of the three-phase winding system  134   a - 134   b ,  135   a - 135   b  and  136   a - 136   b  a four-pole flux density distribution can be produced which can be used to generate levitation forces. The features of the invention can also be recognized in this variant. 
   A very simple and cost-favorable construction can be achieved by a mechanical arrangement such as is shown in FIG.  14 . Here the coils ( 141 ,  142 ,  143 ,  144  and  145 ) surround the stator yoke ( 146 ) instead of the stator limb. If the stator yoke is assembled from segments, simple-shaped coils can be inserted. The individual segments can be installed and positioned via a segment carrier, such as for example a plastic part matched to the stator contour, via corresponding fastening means. 
   The variant shown in  FIG. 14  offers the advantage that no parts (windings) projecting out of the stator surface are located in the region of the stator teeth, close to the air gap. Accordingly, parts of pumps, blowers, fans, ventilators or others can be attached to the two surfaces of the stator depending on the application. 
   In  FIG. 15  an arrangement of this kind is shown. Here a part of the pump housing is directly located on the surface of the stator teeth.