This application claims the benefit of Chinese patent application serial no. 201310043646.3, filed on Feb. 4, 2013. The entire content of the aforementioned patent application is hereby incorporated by reference for all purposes.
Many existing actuators typically comprise a brushless permanent magnet motor using a Hall Effect sensor for detecting the positions of the permanent magnet. The desire to decrease material costs has spurred the development in the industry towards brushless permanent magnet motors without a Hall Effect sensor. In order to better detect the position of the actuator, it is generally desirable for the difference in inductance between the motor stator windings (the ratio between the maximum inductance and minimum inductance of the stator windings), or saliency, to be large. At the same time, it is desirable for the cogging torque of the motor to be small because high cogging torque would create undesirable vibration and noise during motor operation. Satisfying both of these requirements is difficult for many current brushless permanent magnet motors.
For example, an existing nine pole ten slot brushless permanent magnet electric motor may be able to satisfy the cogging torque requirement. However, due to the number of slots being close to the number of poles, the desired saliency cannot be achieved. Similarly, brushless permanent magnet electric motors able to achieve a desirable saliency, such as a four pole six slot brushless permanent magnet electric motor, typically exhibit high cogging torque.
Accordingly, there exists a need for a brushless permanent magnet motor able to have both high saliency and low cogging torque.
Some embodiments are directed at a brushless permanent magnet electric motor configured to have high saliency and low cogging torque. The brushless permanent magnet electric motor comprises a stator having a plurality of stator teeth defining a plurality of winding slots, and a rotor having a permanent magnet defining a plurality of magnetic poles configured to rotate relative to the stator. In some embodiments, each of the stator teeth contains a pole head with a plurality of grooves. The grooves are configured to be substantially parallel to the axial direction of the motor, and function to increase the number of magnetic stator poles of the motor, thereby lowering cogging torque without lowering saliency. In some embodiments, the permanent magnet is substantially annular, and contains a plurality of notches on one side positioned between adjacent pairs of magnetic poles. The notches are configured to increase the coefficient of pole arc or pole embrace of the rotor, in order to decrease the cogging torque of the motor.
The drawings illustrate the design and utility of embodiments, in which similar elements are referred to by common reference numerals. These drawings are not necessarily drawn to scale. In order to better appreciate how the above-recited and other advantages and objects are obtained, a more particular description of the embodiments will be rendered which are illustrated in the accompanying drawings. These drawings depict only exemplary embodiments and are not therefore to be considered limiting of the scope of the claims.
Various features are described hereinafter with reference to the figures. It shall be noted that the figures are not drawn to scale, and that the elements of similar structures or functions are represented by like reference numerals throughout the figures. It shall also be noted that the figures are only intended to facilitate the description of the features for illustration and explanation purposes, unless otherwise specifically recited in one or more specific embodiments or claimed in one or more specific claims. The drawings figures and various embodiments described herein are not intended as an exhaustive illustration or description of various other embodiments or as a limitation on the scope of the claims or the scope of some other embodiments that are apparent to one of ordinary skills in the art in view of the embodiments described in the Application. In addition, an illustrated embodiment need not have all the aspects or advantages shown.
An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and may be practiced in any other embodiments, even if not so illustrated, or if not explicitly described. Also, reference throughout this specification to “some embodiments” or “other embodiments” means that a particular feature, structure, material, process, or characteristic described in connection with the embodiments is included in at least one embodiment. Thus, the appearances of the phrase “in some embodiments”, “in one or more embodiments”, or “in other embodiments” in various places throughout this specification are not necessarily referring to the same embodiment or embodiments.
Some embodiments are directed at a brushless permanent magnet electric motor configured to have high saliency and lower cogging torque. The brushless permanent magnet electric motor comprises a stator having a plurality of stator teeth defining a plurality of winding slots, and a rotor having a permanent magnet defining a plurality of magnetic poles configured to rotate relative to the stator. In some embodiments, each of the stator teeth contains a pole head with a plurality of grooves. The grooves are configured to be substantially parallel to the axial direction of the motor, and function to increase the number of magnetic stator poles of the motor, thereby lowering cogging torque without lowering saliency. In some embodiments, the permanent magnet is substantially annular, and contains a plurality of notches on one side positioned between the magnetic poles. The notches are configured to increase the pole arc coefficient of the rotor, in order to decrease cogging torque.
In some embodiments, stator 30 comprises a core 32, which may be formed from a magnetic material (e.g., iron).
In some embodiments, stator windings 38 comprise a plurality of winding coils wrapped around the plurality of stator teeth 36. For example, each stator tooth 36 may have one winding coil wrapped around it, wherein each of the coils are wound around each stator tooth 36 a number of turns, and the sides of the coil are accommodated in the winding slots on either side of stator tooth 36. During motor operation, electric current flowing through the winding coils of stator windings 38 create magnetic fields that define a plurality of stator magnetic poles.
In some embodiments, each stator tooth 36 comprises a base or a stem 35 extending radially from central portion 34 and a pole head 37 on an end of stem 35 remote from central portion 34 and extending circumferentially to either side of stem 35. Pole heads 37 of stator teeth 36 contain, on the outer surface facing away from central portion 34 and towards rotor 50, one or more slots or open grooves 39 (hereinafter, collectively “grooves”) extending substantially in the axial direction of motor 10. It should be understood that the term “substantially,” such as in “substantially in the axial direction” is used herein to indicate certain features, can refer to either an exact feature or a feature that is slightly offset or otherwise not perfect. For example, grooves 39 may extend perfectly parallel to the axial direction of motor 10, or be slightly offset from being exactly parallel to the axial direction of motor 10.
Each groove 39 may be offset from the center of its associated stator tooth 36 by an angle α (corresponding to the angle between a line connecting the center of stator core 32 and the center of groove 39, and the center line of stator tooth 36). In the illustrated embodiment, angle α is configured to be approximately 10 degrees (°). The radius of each groove 39 may be configured to approximately 0.75 millimeters (mm).
The effect of grooves 39 is to increase the effective number of stator poles of stator 30. For example, stator core 32 in the illustrated embodiment comprises six stator teeth 36, wherein the pole head 37 of each stator tooth 36 contains two grooves 39. Thus, each stator tooth 36 forms the equivalent of three stator poles, such that stator core 32 forms a total of eighteen stator poles. When the number of stator poles is not equal to an integer multiple of the number of rotor poles (increasing the smallest common multiple of the number of stator and rotor poles), the amount of cogging torque experienced by the motor is reduced. The six winding coils of stator windings 38 may be formed into three phases (U, V, W), each phase wound around two stator teeth 36 on opposite sides of stator core 32.
Rotor 50 comprises an output shaft 52 and a permanent magnet 56. In some embodiments, a substantially cylindrical rotor housing 54 made of a magnetic material may be attached to output shaft 52. Permanent magnet 56 may be attached to an inner surface of cylindrical rotor housing 54 and be substantially annular in form. In the illustrated embodiment, permanent magnet 56 is configured to be disposed around stator 30, with an air gap located between permanent magnet 56 and stator 30, such that rotor 50 is able rotate around stator 30.
Notches 58 function to lower the coefficient of pole arc or pole embrace of motor 10. As illustrated in
Through measurement and experimentation by the inventors, it has been found that when using a four pole six slot motor 10, wherein stator teeth 36 each have an pole head 37 with two grooves 39, and notches 58 in annular magnet 56 each define an arc angle β of between 30° and 45°, the maximum cogging torque of motor 10 is greatly reduced, with the maximum value of the detent torque being 2.0 milli-Newton meter (mNm) or less. The saliency, defined as the ratio between the maximum inductance and minimum inductance of the stator windings for each rotation of rotor 50, is greater than 1.3.
For instance, Table 1 illustrates measurements for a four pole six slot motor 10, wherein notches 58 define an arc angle β of 45°, and have a depth or axial length L of 4.5 millimeters (mm) As can been seen for the measured results, the maximum values of the detent torque (and thus also the cogging torque) are 1.8 mNm or less, with the majority of the measured motors having detent torque of 1.6 mNm or less. In addition, the saliency exceeds 1.45 for all measurements.
As illustrated in
Motor 10 may further comprise a circuit board 70 and/or a cover board 80 on an open end of rotor housing 54. Circuit board 70 and cover board 80 may be configured to be axially spaced apart from the stator windings 38, and be positioned such that circuit board 70 is adjacent to an inner surface of cover board 80. Circuit board 70 may be configured to connect to an external power supply through connection terminals 72, and to supply power to stator windings 38 during motor operation.
Stator 30 may further comprise a sleeve 40 located within the central portion 34 of stator core 32. Output shaft 52 of rotor 50 may be configured to pass through one or more bearings 42 rotatably mounted to sleeve 40. In some embodiments, bearings 42 comprise a bushing bearing. Cover board 80 may be attached to sleeve 40, and circuit board 70 fixed to the inner surface of cover board 80.
While the illustrated embodiments depict a four pole six slot electric motor, different motor configurations may be used for other embodiments, such as six pole seven slot motors, six pole nine slot motors, eight pole twelve slot motors, or any other type of brushless permanent magnet motor having a high saliency.
In the foregoing specification, various aspects have been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of various embodiments described herein. For example, the above-described systems or modules are described with reference to particular arrangements of components. Nonetheless, the ordering of or spatial relations among many of the described components may be changed without affecting the scope or operation or effectiveness of various embodiments described herein. In addition, although particular features have been shown and described, it will be understood that they are not intended to limit the scope of the claims or the scope of other embodiments, and it will be clear to those skilled in the art that various changes and modifications may be made without departing from the scope of various embodiments described herein. The specification and drawings are, accordingly, to be regarded in an illustrative or explanatory rather than restrictive sense. The described embodiments are thus intended to cover alternatives, modifications, and equivalents.
Number | Date | Country | Kind |
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201310043646.3 | Feb 2013 | CN | national |