Brushless motor having a permanent magnet rotor

This patent application claims priority to U.S. Provisional Patent Application Ser. No. 62/301,502, filed Jul. 31, 2014 which is incorporated herein by reference in its entirety.

The present disclosure relates to brushless motors and more particularly to rotors of the brushless motors.

For brushless PM motors, the cost of the permanent magnet depends on the shape of the magnet, and is also a function of the amount of finishing operation needed on the magnets. Surface mounted permanent magnets (SPM) with arc shape are desired to obtain lower cogging torque, lower BEMF harmonics and hence less torque ripple in the motor. Arc magnets (ring type or segment type which could be straight segment or skewed segment) need significant grinding to maintain tighter tolerances if the application requires low ripple performances. These magnets have lower material yield and thus the unit cost is higher too.

Interior permanent magnet (IPM) rotor designs with rectangular shaped magnets may thus be desirable to offset scrap cost and increase material yield. However, due to the shape of the magnet, the motor performance in regards to cogging, harmonics, ripple and thus noise are not ideal. Further, the magnets being located inside the rotor core and the traditional methods of cogging and harmonic cancellation through step skew, make the magnets and rotors in particular, difficult to manufacture. Hence, active cancellation of cogging torque and torque ripple is necessary. Moreover, active cancellation of cogging torque requires some voltage budget for cancellation which may negatively impacts the size of the motor.

Accordingly, improved rotor design is desirable to reduce manufacturing cost and improve torque efficiency and magnet retention.

In one embodiment of the present disclosure, a brushless motor includes a stator disposed along an axis and a rotor disposed radially inward from the stator. The rotor includes a core, a plurality of first magnets and a plurality of second magnets configured to have opposite poles relative to the plurality of first magnets. The plurality of first and second magnets may each have a substantially rectangular cross-section and are in contact with a substantially flat surface carried by the core that faces radially outward.

In another embodiment of the present disclosure, a brushless motor includes a stator disposed along an axis, and a rotor disposed radially inward from the stator. The rotor includes a core, a plurality of first magnets and a plurality of second magnets configured to have opposite poles relative to the plurality of first magnets. Each one of the plurality of first and second magnets includes first and second segments having the same pole, and disposed adjacent to and spaced circumferentially apart from one-another. Each one of a plurality of T-shaped retainers of the rotor are disposed circumferentially between respective first and second segments.

These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates a rotor cross-section in accordance with a brushless motor of the present disclosure;

FIG. 2 illustrates a partial enlarged view of the rotor taken from circle 2 of FIG. 1 ;

FIG. 3 illustrates a rotor cross-section in accordance with a second embodiment of a brushless motor;

FIG. 4 illustrates a rotor cross-section in accordance with a third embodiment of a brushless motor; and

FIG. 5 illustrates a partial enlarged view of the rotor taken from circle 5 of FIG. 4 .

Referring now to the Figures, where the invention will be described with reference to specific embodiments, without limiting same, FIGS. 1-5 describe various embodiments of a rotor design with permanent magnets, that may be rectangular, mounted to a core.

The present disclosure seeks to compromise between active cancellation with some form of passive cancellation in the motor design and making a motor cost effective from a manufacturing and a material utilization perspective. Various techniques for what may be an interior permanent magnet (IPM) motor exist to minimize the cogging torque and harmonics but at the expense of added complexity and output torque. Embodiments of the present disclosure include a simpler design technique to achieve these goals while keeping what may be a rectangular shaped magnet as is used in IPM brushless motors. Embodiments of the present disclosure illustrated in FIGS. 1-5 disclose a unique design associated with magnet retention in the rotor that uses low cost rectangular magnets, avoids complexity in the design to reduce cogging torque and also improves the torque output of the motor.

More specifically and referring to FIGS. 1 and 2 , a brushless motor 20 includes a rotor 22 disposed along a rotational axis 24 and a stator 26 concentrically located to, and disposed radially outward from, the rotor 22 . The rotor 22 includes a core 28 general made of a steel-based material, a plurality of first magnets 30 (e.g., three illustrated), a plurality of second magnets 32 (e.g., three illustrated), and a plurality of ribs 34 that may extend axially. The first and second magnets 30 , 32 are orientated such that they have opposite poles to one-another, and are alternatively distributed circumferentially about the core 28 . The magnets 30 , 32 may have a substantially rectangular cross section and are generally adhered, or otherwise engaged, to substantially flat surfaces 36 carried by the core 28 . Each first magnet 30 is spaced circumferentially from the adjacent second magnet 32 , with a respective rib of the plurality of ribs 34 located there-between.

Each magnet 30 , 32 may have opposite and substantially flat faces 38 , 40 with the face 38 facing radially inward and in contact with surface 36 of the core 28 , and the opposite face 40 facing radially outward toward and in close proximity to the stator 26 . Each rib 34 projects radially outward from the core 28 and includes a distal end 42 that is generally disposed radially inward from the face 40 of the magnets 30 , 32 . The ribs 34 may be non-magnetic, may be made of a polymer, and may be molded directly to the core 28 , as one, non-limiting example. It is further contemplated and understood that the ribs 34 may be of a magnetic material and may further be an extension of the core; however, as a magnetic material there may be some flux leakage between opposite poles.

Using rectangular magnets 30 , 32 on the core surface may benefit the motor design from both a cost and an effectiveness of passive cancellation. FIGS. 1 and 2 illustrate a 6-pole motor design but several different designs are contemplated including 8 or more poles. The brushless motor 20 may generally be a surface permanent magnet (SPM) motor.

Referring to FIG. 3 , a second embodiment of a brushless motor is illustrated wherein like elements to the first embodiment have like identifying numerals except with the addition of a prime symbol suffix. A brushless motor 20 ′ includes a plurality of magnets 30 ′ each having first and second segments 30 A, 30 B of the same polarity, and a plurality of magnets 32 ′ each having first and second segments 32 A, 32 B of the same polarity and opposite the first and second segments 30 A, 30 B polarity. The segments 30 A, 30 B of the first magnet 30 ′ are circumferentially spaced from, and adjacent to, one-another (i.e., no other magnets located circumferentially there-between). Similarly, the segments 32 A, 32 B of the second magnet 32 ′ are circumferentially spaced from, and adjacent to, one-another. Ribs 34 ′ may be located circumferentially between the magnets 30 ′, 32 ′, between the segments 30 A, 30 B and between the segments 32 A, 32 B. The segments 30 A, 30 B, 32 A, 32 B may generally have a rectangular cross section. The motor 20 ′ may be a SPM motor.

Referring to FIG. 4 , a third embodiment of a brushless motor is illustrated wherein like elements to the first embodiment have like identifying numerals except with the addition of a double prime symbol suffix. A brushless motor 20 ″ may include first magnets 30 ″ each having segments 30 A″, 30 B″, and second magnets 32 ″ each having segments 32 A″, 32 B″. The first and second magnets 30 ″, 32 ″ are spaced circumferentially from one-another by a rib 34 ″. The first and second segments 30 A″, 30 B″ may be separated by a retainer 44 that may be T-shaped. The first and second segments 32 A″, 32 B″ may also be separated by a similar retainer 44 .

Each retainer 44 may include a flange portion 46 and a base portion 48 , which is engaged to and spans radially between a core 28 ″ of the rotor 22 ″ and a midpoint of the flange portion 46 . The flange portion 46 spans circumferentially and over (i.e. radially outward from) a portion of substantially flat faces 40 A, 40 B of the respective segments 30 A″, 30 B″ and the respective segments 32 A″, 32 B″. The retainers 44 may be made of a magnetic material (e.g., same material as the core). The retainers 44 may be cast with the core 28 ″ and/or machined. The retainers 44 may further be manufactured separate from the core 28 ″ and attached to the core via a mechanical means, welding, and/or other adherence.

The boundaries of an opening 50 for receipt of any one of the magnet segments 30 A″, 30 B″, 32 A″, 32 B″ may be defined by a flat surface 36 ″ of the core 28 ″, the retainer 44 and the rib 34 ″. During assembly, the magnet segments 30 A″, 30 B″, 32 A″, 32 B″ may be mounted to the core 28 ″ by sliding the segments 30 A″, 30 B″, 32 A″, 32 B″ in a substantially axial direction and through the opening 50 .

Because manufacturing of the motor 20 ″ does not require the more traditional over mold processes that create internal pressure, the bulging and breakage (i.e., typically at more traditional web locations) is avoided. By bridging the segments 30 A″, 30 B″ of magnet 30 ″ and bridging the segments 32 A″, 32 B″ of magnet 32 ″ with the retainers 44 , passive cancellation with a step skewed rotor 22 ″ is easier since the magnets 30 ″, 32 ″ are closer to or at the outer rotor face. Moreover, the bulging and breaking problems with more traditional IPM motors at web locations is eliminated with the T-shaped retainer design. Yet further, the more traditional web is completely eliminated with use of the T-shaped retainer 44 . Because the ribs 34 ″ of the present disclosure generally replace the more traditional bridges made of the core material, there is no flux shorting bridge between opposite poles, rather the T-shaped retainer 44 facilitates concentration of the flux from the magnets belonging to the same pole. Advantageously, magnet retention of the motor 20 ″ is about as good as magnet retention for more traditional IPM motors.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.