Abstract:
A rotor assembly for a brushless motor includes a core circumferentially affixed about a longitudinal surface of a shaft. A rotor magnet covers the entire outer surface of the core to seal the core within the magnet and to prevent exposure of the core to ambient conditions. The rotor magnet has a plurality of portions of alternating magnetic polarity and is formed of a plastic mixed with neodymium-iron-boron particles.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates generally to electric motors, and more particularly relates to rotors in brushless electric motors for use in automotive vehicles. 
     2. Background Information 
     A typical brushless motor includes a stator with teeth and coil windings, such as low resistance copper wires, wound on the teeth. During the operation of the motor, a current is passed through the windings to generate an electromagnetic field that interacts with permanent magnets attached to a core of a rotor positioned within the stator. The rotor is in turn coupled to a shaft mounted on a set of bearings so that the electric current passing through the windings is converted to mechanical rotation of the shaft as a result of the interaction between the permanent magnets of the rotor and the electromagnetic field generated by the windings. The shaft commonly provides a physical transfer of the mechanical energy to some other mechanism that may be coupled to the shaft. 
     In many types of motors, the core of the rotor is laminated steel material and is exposed to ambient conditions. As such, fluid is able to seep through the laminates into the core and thus corrode the core, thereby compromising the structural integrity of the core. Furthermore, since the magnets are usually secured to the core with a retainer mechanism, a post balancing operation may be required to balance the rotor after it has been assembled. 
     From the above, it is seen that there exists a need for a rotor that has reduced susceptibility to corrosion and that eliminates post balancing requirements. 
     BRIEF SUMMARY 
     In overcoming the above mentioned and other drawbacks, the present invention provides a rotor for an electromagnetic motor with a plastic bonded magnet. The plastic bonded magnet is injection molded to a core, which in turn is affixed to a shaft. The percentage of magnetic material bonded to the plastic in the rotor can be tailored to the flux requirements of the motor. 
     In one embodiment, a rotor assembly for a brushless motor includes a core circumferentially affixed about a longitudinal surface of a shaft and a rotor magnet injection molded about the core. The magnet covers the entire outer surface of the core. By covering the entire outer surface of the core, the magnet seals the core within the magnet to prevent exposure of the core to ambient conditions, particularly corrosive fluids. As is typical for rotor construction, the rotor magnet has a plurality of portions of alternating magnetic polarity. In the present construction, these portions are formed of a plastic mixed with neodymium-iron-boron. 
     The foregoing discussion has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, incorporated in and forming a part of the specification, illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the views. In the drawings: 
     FIG. 1 is a perspective view of a rotor and stator of a brushless motor; 
     FIG. 2 is perspective view of the rotor of FIG. 1 in accordance with the invention; 
     FIG. 3A is a side view of the rotor of FIG. 2; 
     FIG. 3B is an end view of the rotor of FIG. 2; 
     FIG. 4A is a cross-sectional view of the rotor taken along the line  4 A— 4 A of FIG. 3A; and 
     FIG. 4B is a cross-sectional view of the rotor taken along the line  4 B— 4 B of FIG. 3B; 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates the main components of a brushless motor  10 , a rotor  12  positioned in a stator  13 . The rotor  12  is supported within the stator  13  to allow the rotor  12  to rotate relative to the stator  13 . 
     Referring also to FIGS. 2 through 4B, there is shown a particular embodiment of the rotor  12  removed from the stator  13 . As seen therein, the rotor  12  includes a shaft  14 , a core  16 , and a rotor magnet  18 . The shaft  14  is supported by a set of bearings positioned on both sides of the magnet  18 . The core  16  provides a suitable structural support to the magnet  18  and a flux path to the opposing poles of the magnet  18 . Moreover, the core  16 , best seen in FIGS. 4A and 4B, is completely encapsulated by the magnet  18 . This encapsulation results in the core  16  being completely sealed to prevent exposure of the core  16  to ambient conditions, particularly corrosive fluids. Hence, the motor  10  can be used, for example, in a fuel pump even though the rotor  10  may be exposed to fuels, such as alcohols, gasoline, diesel fuel, and kerosene, without exposing the core  16  to such fuels. 
     As shown in FIG. 4A, the rotor magnet  18  is a four-pole magnet divided into four portions  18   a ,  18   b ,  18   c , and  18   d  positioned circumferentially about the core  16  and is thus a four-pole magnet. These portions  18   a ,  18   b ,  18   c , and  18   d  are magnetized radially such that the polarity of circumferentially adjacent portions alternate. Thus, the pole portions  18   a  and  18   c  are provided with their S pole radially inward and their N pole radially outward, while the adjacent pole portions  18   b  and  18   d  are provided with their S pole radially outward and their N pole radially inward. 
     Note that in other embodiments, the rotor magnet  18  can be magnetized axially so that the change in polarity of the portions occurs along the length of the portions. Moreover, the rotor magnet  18  can be provided as a two-pole magnet or it can be provided with more than four-poles, for example, 10 or more poles. 
     The stator  13  includes a set of teeth  20  about which a coil such as a wire  22  is wound. The wire  22  is made of a conductive material such as copper. As shown in FIG. 1, the stator  13  is a six-slot stator. That is, there are six slots that separate the individual teeth  20 . 
     During the operation of the motor  10 , current flows through the wire  22  producing an electromagnetic field that interacts with the pole portions  18   a ,  18   b ,  18   c , and  18   d  of the rotor magnet  18 . This interaction causes the rotor  12  to rotate relative to the stator  13 . 
     In a typical application, the motor  10  may produce about 75 watts, and the rotor  12  may rotate at about 8,500 rpm. However, the motor  10  can have an output between about 50 and 150 watts, while the rotor  12  can rotate between about 5,000 to 40,000 rpm, depending on the specific application of the motor  10 . 
     The rotational output of the rotor  12  can be harnessed to drive a variety of devices. To achieve this, on end  14   a  of the shaft  14  is coupled to drive a mechanism, such as pump used, for example, in a fuel pump of a vehicle. 
     Preferably, the shaft  14  is formed of stainless steel, such as SS 440, and has a diameter between about 3 to 10 mm. The shaft can be formed of other suitable materials including other steels. Furthermore, the shaft  12  can be made of a magnetic or non-magnetic material. 
     The core  16  is preferably constructed of a powered metal, such as powdered iron, cold rolled steel, a plastic metallized core, or any other suitable material. For example, the core  16  can be made of a polymer, such as polyphenylene sulfide (PPS), and a powered metal, such as a magnetic soft iron powder, that are mixed together with known processing aids. In some implementations, the composition of the core  16  is about 50% to 65% iron powder by volume, with the balance being the polymer and processing aids. The core  16  can be formed about and affixed to the shaft  14  in an injection molding process. 
     The outer diameter of the core  16  will particularly depend on the application of the motor  10 . For the 75 watt motor mentioned above, the ore  16  may have an outer diameter of about 9 mm. 
     The rotor magnet  18  is formed of a magnetic metal powder mixed with a suitable plastic and processing aids. In one implementation, the magnetic metal powder is neodemium-iron-boron particles and the plastic is PPS. The composition of the magnet is tailored to the flux requirements of the motor  10 . For example, with a flux requirement of the 75 watt motor discussed above, the magnet may be composed of about 50% to 65% neodemium-iron-boron by volume bonded with PPS with processing aids. The outer diameter of the rotor magnet is determined by the particular application. For instance, in the above mentioned 75 watt embodiment, the diameter of the rotor magnet is about 16 mm. 
     The rotor magnet  18  is formed onto the core  16  using an injection molding process such that the magnet  18  extends over the ends  16   a  and  16   b  of the core  16 , thereby sealing the core  16  within the magnet  18 . Forming the magnet  18  in this manner eliminates any pathway for permeation of a fluid from outside the magnet  18  into the core  16 , and in particular between the core  16  and the shaft  14 . As such, the core  16  is not exposed to potentially corrosive environments, and the structural integrity of the core  16  and integrity of the bond between the core  16  and the shaft  12  is preserved. 
     Accordingly, separate individual magnets do not have to be attached to the core  16 . Rather, the magnet  18  is formed as a single piece onto the core  16  and subsequently magnetized. Thus, the rotor  12  does not need any additional fabrication steps to retain the magnet  18  to the core  16  beyond the injection molding process. That is, no external or additional retainer is required to affix the magnet  18  to the core  16 . Without such a retainer, the rotor  12  does not require a post balancing process after the rotor has been assembled. By eliminating conventional retention mechanisms and the post balancing process, capital expenses for such fabrication steps are eliminated, thus lowering the overall manufacturing costs of the rotor  12 . 
     Furthermore, as mentioned above, the magnetic properties of the magnet  18  can be modified to tailor the rotor  12  to fulfill the flux requirements of a particular motor  10 . Also, the volume ratio of metal to plastic of the magnet  18  and/or the composition of the magnet  18  can be easily selected or change to produce a desired flux for a particular application. 
     As compared to conventional rotors, the rotor  12  has a higher performance per magnet volume such that it requires less mass to produce a given amount of power. Because of the light weight and resulting low inertia of the rotor  12 , the operation of the rotor  12  produces less vibration and noise, which therefore may extend the life of the bearings supporting the rotor  12 . 
     It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.