Abstract:
A rotating electric machine is provided which includes multiple independent control windings for compensating for rotor imbalances and for levitating/centering the rotor. The multiple independent control windings are placed at different axial locations along the rotor to oppose forces created by imbalances at different axial locations along the rotor. The multiple control windings can also be used to levitate/center the rotor with a relatively small magnetic field per unit area since the rotor and/or the main power winding provides the bias field.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to rotating electrical machines having auxiliary windings in the stator of the machines for providing multiplanar rotor balancing, mechanical stiffening and rotor levitation/centering independent of machine orientation or axis of rotation. 
     2. Related Art 
     Electrical machines, such as motors and generators, may use auxiliary or control windings for rotor levitation/centering and rotor balancing. A typical rotor can have a slight amount of mass imbalance. As the rotor rotates, the mass imbalance leads to an unwanted vibrational force appearing at the motors bearings. Such vibrational forces reduce the lifetime of bearings and degrade the performance of the machine. The electromagnetics associated with the machine can also cause undesired forces which appear at the motor casing due to deflection of the stator core. Prior attempts have been made to cancel the forces created by a mass imbalance and/or electromagnetics by forming a magnetic field with auxiliary control windings in a manner which creates a force on the rotor to cancel the undesired forces. 
     In conventional electrical machines which incorporate auxiliary windings, the control windings are provided along the entire active length of the machine which provides a uniform force distribution. A single sensor measures the effect of undesired forces at a single point and feeds the information to a controller. The controller then drives the control windings with a controlled voltage or current determined from the measurements made by the sensor. With this winding arrangement, optimizing the performance at one axial location along the rotor based on sensor measurements may make the performance at other locations worse. 
     Some prior art machines also include radial magnetic bearings. Magnetic bearings cause the rotor to levitate/center and eliminate the need for mechanical contacts. Conventional magnetic bearings are placed only on the spindle, which has a relatively smaller diameter. Such placement necessitates the use of a high magnetic field per unit area or use of axially longer bearing surface (since the magnetic field may be limited by material characteristics) in order to levitate/center the rotor. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided an electrical machine with multiple independent control windings placed at different axial locations along the stator independent of rotor type. The control windings optimize the performance of the machine by canceling undesired forces produced by 1) mass imbalances of the rotor, 2) variations in rotor position or 3) electromagnetics, and, in one embodiment, also act as magnetic bearings to provide a bearingless, globally balanced machine. 
     In accordance with a first embodiment of the invention, a rotating electrical machine is provided which comprises: a stator; a rotor adapted for rotation relative to the stator and having a longitudinal axis; a first set of control windings mounted on the stator for producing a magnetic field to oppose an undesired force at a first location along the longitudinal axis of the rotor, and a second set of control windings mounted on the stator for producing a magnetic field to oppose an undesired force at a second location (which may overlap the first location) along the longitudinal axis of the rotor spaced from the first location. 
     The first set of control windings preferably has a different distribution than the second set of control windings. It is also preferred that the first set of control windings has an axial length different from that of the axial length of the second set of control windings. 
     In a preferred embodiment, the first and the second sets of control windings produce magnetic fields for levitating, balancing or centering the rotor. 
     In another preferred embodiment, the machine further includes a third set of control windings mounted on the stator for producing a magnetic field to oppose other undesired forces at a third location along the longitudinal axis of the rotor spaced from the first location and the second location. 
     In a first preferred implementation of the first preferred embodiment, the machine also includes: a first set of lead wires connected to the first set of control windings at an edge of the stator; a second set of lead wires connected to the second set of control windings at an edge of the stator; and a third set of lead wires connected to the third set of control windings and placed between or alongside the first set of control windings and arranged to exit at an edge of the stator. 
     In a second preferred implementation of the first preferred embodiment, the machine includes: a first set of lead wires connected to the first set of control windings at an edge of the stators a second set of lead wires connected to the second set of control windings at an edge of the stator; and a third set of lead wires connected to the third set of control windings and arranged to exit radially through a hole in the stator. 
     In another preferred embodiment, the machine further includes a means for providing axial spacings of the first set of control winding end turns and the second set of control winding end turns. In a first preferred implementation of this embodiment, the means for providing axial spacing comprises beams mounted on at least one side of a stator lamination and having regions between the beams for mounting the first set of control winding end turns and the second set of control winding end turns. In a second preferred implementation of this embodiment, the means for providing axial spacing comprises toothless laminations having recessed regions for mounting said first set of control winding end turns and said second set of control winding end turns therein. In a third preferred implementation of this embodiment, the means for providing axial spacing comprises stepped laminations having recessed regions for mounting said first set of control winding end turns and said second set of control winding end turns therein. 
     In accordance with a second preferred embodiment of the invention, a rotating electrical machine is provided which comprises: a stator; a rotor adapted for rotation relative to the stator; a first set of control windings attached to the stator for producing a first magnetic field to apply a force to the rotor at a first axial location of the rotor; a second set of control windings attached to the stator and overlapping and extending beyond the first set of control windings for producing a second magnetic field for opposing undesired forces at a second axial location of the rotor and for, in combination with the first magnetic field, opposing undesired forces at the first axial location of the rotor. 
     The machine preferably includes a third set of control windings attached to the stator and overlapping and extending beyond the first and the second sets of control windings for producing a third magnetic field for opposing undesired forces at a third axial location of the rotor and for, in combination with the first and the second magnetic field, opposing undesired forces at the first axial location of the rotor and for, in combination with the second magnetic field, opposing undesired forces at the second axial location of the rotor. 
     Other features and advantages of the invention will be set forth in, or apparent from, the following detailed description of the preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a stator assembly having multiple control windings in accordance with a first preferred embodiment of the invention; 
     FIG. 2A is a schematic representation of an electrical machine having a stator constructed in accordance with a first implementation of the first preferred embodiment of the invention; 
     FIG. 2B is a schematic representation of an electrical machine having a stator constructed in accordance with a second implementation of the first preferred embodiment of the invention; 
     FIG. 2C is a schematic representation of an electrical machine having a stator constructed in accordance with a third implementation of the first preferred embodiment of the invention; 
     FIG. 3 is a cross-sectional view of an electrical machine in accordance with the first preferred embodiment of the invention 
     FIG. 4A is a schematic representation of a manner of connecting lead wires to the control windings according to a first implementation of the first preferred embodiment of the invention; 
     FIG. 4B is a schematic representation of a manner of connecting lead wires to the control windings according to a second implementation of the first preferred embodiment of the invention; and 
     FIG. 5 is a schematic representation of a stator having multiple control windings in accordance with a second preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown the stator assembly of an electrical machine having multiple control windings  20  and  30  located at different axial zones along the stator  10  in accordance with a first preferred embodiment of the invention. As illustrated, control windings  20  are located in a first axial zone and control windings  30  are located in a second, spaced axial zone. The distribution of the windings associated with each axial zone are optimized for the degree of rotor balancing and levitation/centering required at the given axial zones. The variables of the distribution include: the number of poles, number of turns, number of phases, mechanical displacement, and full or partial peripheral occupancy in each axial zone. Each winding can be controlled at any frequency and at a magnitude and phase angle independent of the other windings. 
     Multiple control windings allow the rotor to be more precisely balanced, when compared to a single set of control windings. Control windings  20  can be used to cancel any mass imbalance and other undesired effects found in the first axial zone. Similarly, control windings  30  can be driven independently of control windings  20  to cancel any mass imbalance and other undesired effects found in the second axial zone. The use of multiple control windings allows the rotor to be balanced in one axial zone independent of another axial zone. 
     In addition to rotor balancing, the control windings  20  and  30  can function as magnetic bearings to perform rotor levitation/centering or mechanical stiffening in their respective axial zones. Conventional magnetic bearings apply the magnetic field at the spindle, which has a relatively small diameter. Applying the magnetic field over the larger diameter of the main rotor body makes for a more robust system. Also, magnetic bearings must supply both the bias and control fields while the auxiliary windings only need to supply the control field, since the bias field is already provided by the rotor and/or the main power winding. The use of multiple windings as magnetic bearings also allows the desired forces to be optimized for each axial zone, which results in improved rotor dynamic performance. The elimination of conventional magnetic bearings also improves the dynamic performance of the rotor by shortening the overall shaft or bearing span. 
     While only two sets of windings are shown for illustration purposes, it is understood that several more sets of windings can be provided in a similar manner. The axial spacing between adjacent zones is dictated by the size of the windings. Several different constructions can be used to maintain accurate spacing between the axial zones while providing enough room for the end turns of the control windings. In a first implementation, shown schematically in FIG. 2A, “I”, shaped, rectangular or other shaped beams indicated at  12 , are used in the stator and are placed on one or both sides of a lamination  15  as to support the rest of the stator core through compression during final assembly. The control windings  20  are located between the stator laminations  15  and across an air gap  91  from the rotor which is indicated at  92 . In a second implementation, shown schematically in FIG. 2B, “toothless” laminations  14  are used for the stator material. Toothless laminations  14  aid the stator yoke magnetic circuit and provide mechanical support to the stator. In a third implementation, shown schematically in FIG. 2C, stepped or shortened tooth laminations  16  are used for the stator material to minimize adverse electromagnetic effects caused by the removal of magnetic material and to maximize mechanical support. The end turns of the control windings  20  are preferably held in place so that they do not drop into the air gap  91 . In one preferred embodiment, the end turns are radially wedged under axial wedges (not shown) and tied, if necessary, to the axial wedges. 
     Referring to FIG. 3, there is shown a cross-sectional view of an electrical machine in accordance with the first preferred embodiment of the invention. Conventional main power windings  22  are mounted on the stator  10 . As indicated above, an air gap  91  separates the rotor  92  from the control windings  20  and  30 . The control windings  20  and  30  are preferably mounted on stator  10  at a radial location between the air gap  91  and the main power windings  22  to minimize the leakage reactance and power requirements of the control windings  20  and  30 . Alternatively, the main power winding may be located between the air gap and the control windings, if other design considerations dictate this layout. 
     A first preferred implementation for connecting lead wires to a winding arrangement comprising three separate axially spaced control windings  40 ,  50  and  60  is shown in FIG.  4 A. Windings  40  and  60  which are formed at the edge of the stator  10  have respective lead wires  40   a  and  60   a  attached or connected thereto at the edge of the stator. Control windings  50  are formed at an axial zone located in the middle of the stator  10  and associated lead wires  50   a  are inserted through the frame (not shown) and the stator  10  and are connected to the control windings  50 . FIG. 4B illustrates a second preferred implementation for connecting lead wires to control windings  40 ,  50  and  60 . In this embodiment, the lead wires  40   a  and  60   a  connected to control windings  40  and  60  are connected in the same manner as shown in FIG. 4A but the lead wires  50   a  connected to control windings  50  are twisted and placed between or alongside the control windings  60  and arranged so as to exit at the edge of the stator. Unlike the implementation shown in FIG. 4A, there is no need to place a hole in the frame (not shown) and the stator  10 . 
     FIG. 5 illustrates a second preferred embodiment of the invention. In this embodiment, separate control windings  70 ,  80  and  90  are provided which overlap each other and span different axial lengths. This arrangement of the three sets of control windings  70 ,  80  and  90  creates three separate axial zones. The first axial zone is located where control windings  70 ,  80  and  90  overlap, the second axial zone is located where only control windings  80  and  90  overlap and the third axial zone is located where control windings  90  do not overlap the other control windings. Each of the control windings  70 ,  80  and  90  are separately controlled to produce independent magnetic fields. The magnetic fields produced by the control windings can aid or oppose the magnetic fields produced by the other control windings. The magnetic fields produced by the control windings can be used for rotor balancing and/or rotor levitation/centering in a manner similar to that of the first embodiment of the invention. 
     Although the invention has been described in detail with respect to preferred embodiments thereof, it will be apparent to those skilled in the art that variations and modifications can be effected in these embodiments without departing from the spirit and scope of the invention.