Patent Application
20090236920
Brushless motors often fail to rotate smoothly. Motor structures and phase commutations result in periodic disturbances in a motor, also called torque ripples. The torque ripples may degrade the performance of the motor because of vibrations and noise. Torque ripples also affect a speed of the motor.
Torque ripples may be caused, in part, by cogging torque. Cogging torque is a result of the interaction between slots in a stator and permanent magnets on a rotor.
An interaction between the motor current and the back-EMF of the motor current can also cause a torque ripple. Torque ripples may be measured as a harmonic resonance of varying orders. A motor system that minimizes torque ripples is desired.
The above described and other features are exemplified by the following Figures and Description which includes an exemplary embodiment of a motor comprising, a rotor having a shaped pole operative to minimize cogging torque and electro motive Force (EMF) harmonics, a stator having teeth members, the teeth members including, end portions, wherein the end portions partially define slot openings having a first dimension, and a notch opening defined by the end portions having a second dimension.
An alternate exemplary embodiment including a motor comprising, a rotor having a shaped pole partially defined by a middle portion dimension and an end portion dimension, a stator having teeth members, the teeth members including, end portions, wherein the end portions partially define slot openings having a first dimension, and a notch opening defined by the end portions having a second dimension.
Another alternate exemplary embodiment including a motor comprising, a rotor having a shaped pole partially defined by a middle portion dimension and an end portion dimension, and a stator having teeth members, the teeth members including end portions, wherein the end portions partially define slot openings having a first dimension, wherein the end portion dimension is 0.3-0.9 times the middle portion dimension, a normalized slot depth dimension is 0.01-0.06 from a end surface of the teeth members to a beveled portion of the teeth members, and a slot angle is 30 to 50 degrees defined by the beveled portion and a slot surface of the teeth members.
Referring now to the Figures wherein like elements are numbered alike:
Torque ripple in brushless motors may be caused in part, by cogging torque and back-electromotive Force (EMF) harmonics. Shaping the magnetic poles of a rotor to optimize the amount of cogging torque and back-EMF harmonics reduces torque ripple. Stators also include teeth that are separated with slots. The teeth may include notches in end portions of the teeth. Incorporating notches having widths different than widths of the slots and optimizing the design to reduce cogging torque and back-EMF harmonics may reduce the torque ripple in a motor. By combining shaped poles and notches having widths different than widths of the slots the overall torque ripple in a motor is reduced.
The motor 100 has six poles 103 and nine slots 109. The poles 103 have been optimized using a Finite Element (FE) Analysis software to reduce the cogging torque and back-EMF harmonics in the motor.
Referring to
Though pole shaping may reduce torque ripples, the use of notches in stator teeth further reduces the effects of cogging torque and back-EMF harmonics.
The stator 400 includes a number of other stator teeth similar to the stator teeth 401 and 402. Each of the stator teeth in the stator 400 has a notch 405 and a gap 407. The dimension “d” may be increased or decreased, though it has a minimum dimension that is limited by the size of the stator windings. The dimension “a” of the notch may also be increased or decreased. The dimensions “a” and “d” may be optimized through experimentation and simulation to determine dimensions that reduce the cogging torque and the back-EMF harmonics of the motor.
The following table describes exemplary embodiments of motors having improved cogging harmonics. The table below includes three types of motors, similar to the motor illustrated in
The number of notches indicates the number of notches in each type of motor similar to the notches 405 of
Simulations have shown that embodiments optimized with stator teeth having one notch generally reduce torque ripples, back-EMF harmonics, and cogging torque when the notch dimension “a” is greater than the slot dimension “b,” while embodiments with stator teeth having two notches are optimized when the notch dimension “c” is less than the slot dimension “d.” However, some motors having different numbers of poles and slots may not have the same properties resulting in different relationships between the notch dimensions and slot dimensions for optimization.
The benefits of shaped poles and notches having different dimensions than slot dimensions are enhanced when combined in a motor using both shaped poles and optimized dimensions of the slots and notches. Simulated results of a motor having 9 slots and six poles show a further reduction in cogging torque and back-EMF harmonics when the two optimized configurations are combined.
While the invention has been described with reference to exemplary embodiments, it will be understood by those of ordinary skill in the pertinent art that various changes may be made and equivalents may be substituted for the elements thereof without departing from the scope of the present disclosure. In addition, numerous modifications may be made to adapt the teachings of the disclosure to a particular object or situation without departing from the essential scope thereof. Therefore, it is intended that the Claims not be limited to the particular embodiments disclosed as the currently preferred best modes contemplated for carrying out the teachings herein, but that the Claims shall cover all embodiments falling within the true scope and spirit of the disclosure.
