Patent Publication Number: US-2012025653-A1

Title: Aggregate magnetization skew in a permanent magnet assembly

Description:
BACKGROUND 
     The present invention relates to electric motors. More particularly, the invention relates to reducing cogging torque in electric motors. Cogging torque is caused by the magnetic attraction between permanent magnet edges and lamination poles in a motor. Cogging torque is the amount of torque required to move the rotor out of those positions. The net effect of this magnetic attraction is that the rotor of an electric motor does not turn freely. The condition is undesirable because it lowers efficiency and produces torque ripple during motor operation. 
     One method of reducing cogging torque is to design a motor such that magnet edges and lamination poles are not parallel and, therefore, cannot align. This arrangement is known as magnetization “skew.” An ideal skew angle for magnetization is one rotor pole pitch. That is, the skew starts at the front of the motor, at the tip of one pole, and ends at the rear of the motor, at the tip of the next pole. This configuration allows for the smoothest possible transition across attracting features. 
     The disadvantage of magnetization skew in electric motors is that the effective field of the magnet is reduced. Increasing the skew angle comes at the expense of reducing that portion of the magnet which performs useful work. The material of the magnet within a skew area does not fully contribute to the performance of the motor. More specifically, if a coil completes commutation before exiting the transition zone, the coil is exposed to magnets of both polarities. In this condition, opposing forces are generated which cancel each other out. The net effect is that magnetic material is effectively wasted. Thus, where magnetization skew is used to minimize cogging torque, there exists an inherent tension between minimizing cogging torque and maximizing magnet utilization. 
     SUMMARY 
     In one embodiment, the invention provides a permanent magnet assembly for use in an electro-dynamic machine. The permanent magnet assembly has a plurality of ferromagnetic ring members arranged about a longitudinal axis in a co-axial stack. Each ring member has an axial orientation. A plurality of arcuate magnetic poles are arranged around a circumference of the ring member. Pole boundaries between magnetic poles are skewed at an angle Φ that is non-parallel to the longitudinal axis. 
     In another embodiment, the invention provides a method of manufacturing a permanent-magnet portion of an electro-dynamic machine. The method includes manufacturing a plurality of ferromagnetic ring members. Each ring member has a central axis, a radius and an axial height. Each ferromagnetic ring member is magnetized to have a plurality of arcuate magnetic poles arranged around a circumference of the ring member. The arcuate magnetic poles within each ring member contact one another at axially-skewed boundaries. The ferromagnetic rings are arranged in a co-axial stack. 
     In yet another embodiment, the invention provides an electric motor. The electric motor includes a shaft rotatable about an axis, a rotor coupled to the shaft for rotation about the axis, and a stator disposed concentrically about the rotor to provide a magnetic field. The stator includes a first ferromagnetic ring having at least a first magnetic pole and a second magnetic pole adjacent the first magnetic pole. A boundary between the first magnetic pole and the second magnetic pole defines a first skew angle. A second ferromagnetic ring has at least a third magnetic pole and a fourth magnetic pole adjacent the third magnetic pole. A boundary between the third magnetic pole and the fourth magnetic pole defines a second skew angle. The second ferromagnetic ring is stacked axially upon the first ferromagnetic ring. 
     In still yet another embodiment, the invention provides a permanent magnet assembly for use in an electro-dynamic machine. The permanent magnet assembly has a plurality of ferrimagnetic ring members arranged about a longitudinal axis in a co-axial stack. Each ring member has an axial orientation. A plurality of arcuate magnetic poles are arranged around a circumference of the ring member. Pole boundaries between adjacent magnetic poles are skewed at an angle Φ, where Φ is non-parallel to the longitudinal axis. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an electric motor according to one aspect of the invention. 
         FIG. 2  is a top view of a stator of the electric motor of  FIG. 1 . 
         FIG. 3  is a perspective view of a ring magnet with an aggregate magnetization skew according to one aspect of the invention. 
         FIG. 4  is a cross-sectional view of the stator of  FIG. 2  along line  4 - 4 . 
         FIG. 5  is a perspective view of a method of constructing a permanent-magnet assembly according to one aspect of the invention. 
         FIG. 6  is a perspective view of a method of constructing a permanent-magnet assembly according to another aspect of the invention. 
         FIG. 7  is a perspective view comparing design magnetization patterns of the permanent magnet assemblies of  FIGS. 5 and 6  with their actual magnetization patterns. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
       FIG. 1  is a perspective view of an electro-dynamic machine, such as a motor  10 . The motor  10  has a rotor  14  and a stator  18 . A radial air gap  22  separates the stator  18  from the rotor  14 . The rotor  14  is coupled to a shaft  26  for rotation about an axis  30 . A magnetic field of the stator  18  interacts with a magnetic field of the rotor  14  to produce a useful torque about the axis  30 . 
     The illustrated motor  10  is a brushed direct-current (DC) motor, such as for use with power tools, though the invention is applicable to other types of motors and motor uses. It should also be appreciated that the invention is also applicable to generators. 
     The motor  10  generates an oscillating current in the rotor  14 , or armature, with a commutator  34 . The rotor  14  includes one or more coils of wire (not shown) wound around a metallic core  38  on the shaft  26 . An electrical power source is connected to the rotor coil through the commutator  34  and commutator brushes (not shown), causing current to flow in the coils, producing electromagnetism. The commutator  34  causes a current in the coils to be switched as the rotor  14  turns, keeping the magnetic poles of the rotor  14  from ever fully aligning with the magnetic poles of the stator  18 , such that the rotor  14  never stops but rather keeps rotating indefinitely. In the illustrated construction, a fan  42  is coupled to the rotor  14  to provide cooling during operation. 
     Although the embodiments of the invention are described below in the context of a permanent-magnet stator, it should be appreciated that the principles of the invention are equally applicable to permanent-magnet rotor construction. 
       FIG. 2  is a top view of the stator  18  shown in  FIG. 1 . A metallic flux ring  46  forms an outer radial surface  50  of the stator  18  about axis  30 . A permanent magnet assembly  54  is disposed adjacent an inner radial surface  56  of the flux ring  46  about axis  30 . In the illustrated embodiment, the permanent magnet assembly  54  has four magnetic poles (N, S, N, S)  58  arranged circumferentially about axis  30 . Each pole  58  forms an arcuate segment  62  of the permanent magnet assembly  54 . In other constructions, a stator with a different number of poles may be used. For example,  2 ,  6 ,  8  or other numbers of poles may be used. 
       FIG. 3  is a perspective view of a ferromagnetic ring  66 .  FIG. 3  illustrates an exemplary way to maximize an effective magnetization skew (and thereby minimize cogging torque), while simultaneously maximizing magnet utilization for producing useful torque. The ferromagnetic ring  66  is magnetized such that a design skew pattern  70  is formed at the boundary between magnetic poles  58 . The design skew pattern  70 , illustrated in the upper half of  FIG. 3 , is formed as an aggregate of several linear segments,  74 , and can thus be described as an “aggregate” magnetization skew  70 . 
     A skew angle Φ for linear segment  74  can be defined relative to a reference line  78 . The reference line  78  is parallel to the orientation of the axis  30 , though the skew angle Φ could be defined relative to another reference. Each of the linear segments  74  provides a large effective skew angle Φ that minimizes cogging torque. Simultaneously, the design aggregate magnetization skew  70  is confined to a relatively narrow annular segment  82  of the ferromagnetic ring  66 , thereby maximizing the area of the permanent magnet available to produce useful torque. The skew angle Φ will typically be in the range from approximately 15 degrees to approximately 75 degrees; preferably from approximately 30 degrees to approximately  60  degrees, and even more preferably from approximately  40  degrees to approximately 50 degrees. For a commutated DC machine, such as that illustrated in  FIG. 1 , the design may be optimized by confining the design aggregate magnetization skew  70  within an area of the magnet that does not interact with the armature due to commutation. This “inactive” magnet area is a function of commutator and brush geometry for a given motor configuration, and can be determined by well-known methods. 
     A design aggregate magnetization skew  70  such as illustrated in  FIG. 3  is not easily manufacturable within a ring magnet using existing techniques. Shaping magnetic poles in a magnetization fixture to form the design magnetization pattern may be impractical since magnetic field lines follow the path of least resistance. The lower half of  FIG. 3  illustrates the actual magnetization pattern  86  resulting from the design aggregate magnetization skew  70  pattern illustrated in the upper half of  FIG. 3 . As illustrated, magnetization boundaries  90  formed in this manner tend to be thick, fuzzy lines. This is a result of the well-known tendency of magnetic field lines to bypass abrupt corners and to diffuse sharp edges (i.e., to follow the path of least resistance). The design aggregate magnetization skew  70  is diminished as the rounded, blurred magnetization boundaries  90  significantly reduce the capacity of the skew to minimize cogging torque. 
     As an alternative to magnetizing a single ferromagnetic ring  66 , an aggregate magnetization skew  70  may be formed using an axial stack of ferromagnetic rings, where each ring is individually magnetized with a straight magnetization skew. An assembly of multiple ferromagnetic rings allows the use of standard manufacturing methods to generate complex skew patterns. Assembly of the ring magnets requires little deviation from typical processes. The rings slip over or inside other rotor or stator components and are secured with existing techniques, such as an epoxy-resin potting compound. 
       FIG. 4  is a cross section of the stator  18  shown in  FIG. 2 . In the illustrated construction, the stator  18  includes five ferromagnetic rings  94  arranged in an axial stack  98  to form the permanent-magnet assembly  54 . Each ferromagnetic ring  94  within the stack  98  has substantially the same circumferential orientation such that the poles  58  of each ferromagnetic ring are aligned with the poles  58  of the other rings within the stack  98 . In other words, the S poles of each ring magnet are substantially aligned with the S poles of the other ring magnets within the stack. Similarly, the N poles of each ring magnet are substantially aligned with the N poles of the other magnets within the stack. In the construction of  FIGS. 1 ,  2  and  4 , the flux ring  46  and each of the ferromagnetic rings  94  includes an alignment notch  106 . The alignment notch  106  is provided to assist in aligning the poles  58  of each ferromagnetic ring during assembly. 
     As illustrated in  FIG. 4 , linear boundaries  110  between magnetic poles  58  within each ferromagnetic ring  94  are skewed relative to reference lines  78 , to define the skew angle Φ. In the illustrated construction, the skew angle Φ at each linear boundary  110  within a ferromagnetic ring  94  is the same. Similarly, the skew angles within each subsequent ring of the stack  98  are the same, forming the illustrated “saw-tooth”  118  profile of magnetization. The skew angle Φ will typically be in the range from approximately 15 degrees to approximately 75 degrees; preferably from approximately 30 degrees to approximately 60 degrees, and even more preferably from approximately 40 degrees to approximately 50 degrees. As discussed in greater detail below, other constructions may arrange the ring magnets in alternating axial orientations, to form a “zig-zag” magnetization pattern. In still further constructions, alternative skew patterns may be formed using combinations of rings with different skew angles, or with non-linear pole boundaries. 
       FIG. 5  illustrates one method of constructing a permanent magnet assembly  122  for use in a stator or rotor. A plurality of ferromagnetic rings  126  (rare earth, iron, etc.) are formed using conventional manufacturing techniques. As shown in the upper half of  FIG. 5 , each of the ferromagnetic rings  126  is substantially identical in all dimensions, though in other constructions the rings may have various axial heights or wall thicknesses. 
     Each of the ferromagnetic rings  126  is then magnetized in a magnetization fixture. Because the skew angle Φ at each pole boundary  130  is linear, a conventional magnetization fixture may be used. After magnetization, the individual ferromagnetic rings are assembled into a stack  134  shown in the lower half of  FIG. 5 , to form a complete permanent magnet assembly  122 . The combination of pole boundaries  130  forms the illustrated saw-tooth magnetization pattern  142 . 
       FIG. 6  illustrates an alternative construction of a permanent magnet assembly  146  using stacked ring magnet aggregate magnetization. A “zig-zag” magnetization pattern  150  is created by alternating the direction of skew with every ring magnet. In other words, the individual ferromagnetic rings  126  within the stack  134  are identical to those shown in  FIG. 5 , but the axial orientation  154  of every other ring is reversed to create the illustrated zig-zag pattern  150 . 
       FIG. 7  is a comparison of (a) the design saw-tooth  142  and zig-zag  150  magnetization patterns versus (b) the actual magnetization patterns  158 ,  162 . In both cases, the magnetization patterns  158 ,  162  are sharper and closer in geometry to the design magnetization than that illustrated in  FIG. 3 . Because each of the rings is magnetized individually, the resulting magnetization pattern after assembly is sharper and better defined. 
     While the invention has been described in the context of a stator for a DC motor, the invention has uses in other electrodynamic machinery. For instance, a ring magnet magnetized with an aggregate magnetization skew could be used in the rotor of a DC motor or generator. 
     Furthermore, while the embodiments of  FIGS. 3-7  have each been described as having ferromagnetic rings, it should be appreciated that each of the previous embodiments may alternatively be constructed with ferrimagnetic rings. A ferrimagnetic material is one in which the magnetic moments of atoms on different sublattices are opposed, yet unequal, such that a spontaneous magnetization remains. 
     Thus, the invention provides, among other things, a ring-magnet assembly for use in electro-dynamic machinery. Various features and advantages of the invention are set forth in the following claims.