As development of rotor spinning machines progresses, the goal is not only to improve the quality of the yarns produced, but above all to increase production capacity. A key factor in increasing production capacity is the rotary speed of the spinning rotor. For this reason, varied kinds of drives and bearings for spinning rotors have been developed, in order to reach rotary speeds of markedly over 100,000 rpm. Reducing the rotor diameter and mass and lowering friction losses enables not only greater rotary speed but also reduced energy consumption when driven.
In this respect, a shaftless spinning rotor, which is embodied as the rotor of an axial field motor, can be considered especially advantageous by providing a combined magnetic and gas bearing which assures relatively low friction losses.
An axial field motor with a combined magnet/gas bearing is disclosed in WO 92/01096, wherein the spinning rotor has a bearing face remote from the rotor opening in opposed facing relation to a bearing face the stator of the motor at a spacing defining an air gap between the two bearing faces which thereby form the combined magnetic/gas bearing. The axial field motor has means associated with both the stator and the rotor for conducting the magnetic flux of magnetic fields for driving and guiding the rotor. The stator is annular in shape and has a segmental winding, disposed symmetrically to the rotational axis of the rotor, for generating the surrounding driving magnetic field. This winding is embodied as a so-called gap winding, i.e., wrapped around the unslotted stator core, so that it extends in the region of the bearing face within the gap between the stator core and the rotor base. This kind of gap winding necessitates a limitation to a certain winding geometry, because the nonmagnetic properties of copper dictate keeping a relatively small width in the gap between the magnetically conductive materials of the stator core and of the rotor base in order to limit the magnetic reluctance. In such a gap winding, only one layer is therefore typically wound, and typically the copper wires also have a flattened cross-section, which limits the number of windings per phase and consequently the attainable magnetic saturation. Moreover, if the stator bearing face is damaged, the current-carrying winding can be directly exposed and damaged. Occupational safety aspects play an additional role.
To circumvent the unavoidable disadvantages of a gap winding, i.e., the large magnetic reluctance in the gap region and the limited magnetic field intensity attainable because of the limited maximum number of windings, the attempt has been made to place the winding, at least in the bearing region, in slots of the stator core. However, this leads to significant localized heating, especially of the parts of the rotor that conduct the magnetic flux. The consequence of this heating is thermal strain resulting from differing coefficients of thermal expansion of the rotor components, and deformation of the bearing face, which is especially critical at the relatively small widths typical across the air gap between the bearing faces, normally in the range of hundredths of a millimeter. Enlarging the gap, required in such cases to avoid damage to the bearing face, leads to a marked increase in air space and hence energy consumption. If drive magnets are used in the rotor of a brushless direct current motor, then over a relatively long period of time the heating which occurs can cause temperature-dictated reversible or nonreversible demagnetization, or detachment of the composite material of the powdered magnetic composition of the magnets. It must be remembered that as a rule the magnets are embedded in carbon fibers, which are incapable of dissipating the heat buildup because of their poor thermal conductivity.
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