Patent Application
20140285042
This application claims the benefit of Chinese patent application Ser. No. 201310098562.X, filed on Mar. 25, 2013. The entire content of the aforementioned patent application is hereby incorporated by reference for all purposes.
Due to safety concerns, current guidelines and regulations for alternating current (AC) electric motors require that an AC electric motor operating at a voltage exceeding a certain value must have an insulating sleeve between the motor output shaft and the rotor core of the motor in order to prevent electrical leakage through the output shaft, wherein the minimum thickness of the sleeve is determined based upon the magnitude of the voltage.
On the other hand, direct current (DC) electric motors are not subjected to similar safety guidelines and regulations. As a result, in order to reduce manufacturing and material costs, DC electric motors typically do not contain an insulating sleeve between the motor output shaft and rotor core.
However, Applicants have found that during the operation of many DC electric motors, frequent changes in the polarity of the motor winding coils will create a high-frequency signal. This high-frequency signal is easily coupled to the motor output shaft, allowing the signal to radiate outside the motor via the output shaft, and creating undesirable electromagnetic interference (EMI).
Accordingly, there exists a need for a DC electric motor with improved EMI characteristics.
Some embodiments are directed at a direct current (DC) electric motor, comprising a rotor rotably attached to a stator. The stator comprises an outer shell, a plurality of magnets, and a plurality of brushes. The rotor comprises an output shaft, a rotor core, and a commutator fixed to the output shaft. A plurality of winding coils are wound around the rotor core and electrically connected to the commutator, which is configured to be in sliding contact with the plurality of brushes. An insulating sleeve is disposed between the output shaft and rotor core to reduce the EMI caused by fluctuating current in the winding coils during motor operation. In some embodiments, a first capacitor is formed between the output shaft and rotor core, and a second capacitor is formed between the rotor core and outer shell. In some embodiments, the second capacitor is configured to have a capacitance equal to or greater than that of the first capacitor. In some embodiments, a ratio of the capacitance of the second capacitor and capacitance of the first capacitor is configured to be between 0.1 and 50.
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 direct current (DC) electric motor, comprising a rotor rotably attached to a stator. The stator comprises an outer shell, a plurality of magnets, and a plurality of brushes. The rotor comprises an output shaft, a rotor core, and a commutator fixed to the output shaft. A plurality of winding coils are wound around the rotor core and electrically connected to the commutator, which is configured to be in sliding contact with the plurality of brushes. An insulating sleeve is disposed between the output shaft and rotor core to reduce the EMI caused by fluctuating current in the winding coils during motor operation. In some embodiments, a first capacitor is formed between the output shaft and rotor core, and a second capacitor is formed between the rotor core and outer shell. In some embodiments, the second capacitor is configured to have a capacitance equal to or greater than that of the first capacitor. In some embodiments, a ratio of the capacitance of the second capacitor and capacitance of the first capacitor is configured to be between 0.1 and 50.
In some embodiments, stator 20 comprises an outer shell 22 and a plurality of magnets 24 fixed to an inner wall of outer shell 22. Magnets 24 may comprise one or more permanent magnets. However, it is understood that magnets 24 may comprise any type of component capable of generating a magnetic field. In some embodiments, outer shell 22 is substantially cup-shaped or cylindrical in form, and is made of metal. An end cap 26 may be attached to one end of outer shell 22. A plurality of electric brushes 28 may be disposed on end cap 26. In some embodiments, electric brushes 28 are slidably disposed on end cap 26 configured to contact a commutator 34 on rotor 30. For example, one or more springs (not shown) may be used to urge brushes 28 toward commutator 30 to maintain contact between electric brushes 28 and commutator 34 even as electric brushes 28 may be worn down through wear and tear. In some embodiments, electric brushes 28 may be attached to an inner surface of outer shell 22.
Rotor 30 comprises an output shaft 31, an insulating sleeve 32 fixed to output shaft 31, and a rotor core 33 fixed to insulating sleeve 32, such that insulating sleeve 32 is sandwiched between output shaft 31 and rotor core 33. Thus, output shaft 31, insulating sleeve 32, and rotor core 33 are all fixed to each other and are able to rotate together. Output shaft 31 may be made of metal, and be rotably attached to outer shell 22 and/or end cap 26 of stator 20.
Commutator 34 is fixed to output shaft 31 and electrically connected to a plurality of winding loops 35 (one of which is illustrated in
During operation of a typical DC electric motor such as, for example, motor 10, DC current travels through electric brushes 28 and commutator 34 to corresponding winding loops 35, which generate a magnetic field that interacts with the magnetic field of magnets 24, causing rotor 30 to rotate. As rotor 30 rotates, electric brushes 28 are in contact with different commuator bars of commutator 34.
During operation of motor 10, rotor 30 may rotate at speeds of thousands of revolutions per minute (rpm) or even higher. Consequently, the on and off switching of current within winding loops 35 may be very rapid, which generates high-frequency electromagnetic radiation. In many conventional DC electric motors, due to output shaft 31 and rotor core 33 being in direct contact, this high-frequency radiation is coupled to output shaft 31 and outer shell 22 through rotor core 33.
In accordance with an embodiment of the present invention, insulating sleeve 32 is disposed between output shaft 31 and rotor core 33, and rotor core 33 and output shaft 31 are not in direct contact. Rotor core 33, output shaft 31, and insulating sleeve 32 form a capacitor that may effectively prevent the high-frequency signal coupling to output shaft 31 through rotor core 33, reducing the amount of electromagnetic radiation emanating from output shaft 31 and improving the electromagnetic interference (EMI) characteristics of motor 10.
In addition, there is a gap between rotor core 33 and outer shell 22 of stator 20. Outer shell 22, rotor core 33, and the gap there between also form a capacitor. In some embodiments, outer shell 22 is grounded during operation of motor 10, providing for absorption of the high-frequency signal generated by winding coils 35.
The capacitance of a capacitor may be expressed with the following equation:
wherein ε corresponds to a dielectric constant, A corresponds to area of the capacitor plates, and d corresponds to the distance between the capacitor plates.
In general, the greater the total impedance between winding coils 35 and outer shell 22, and between winding coils 35 and output shaft 31, the better the reduction of high-frequency signals coupled to outer shell 22 and output shaft 31 will be. Because the value of R is generally small, the total impedance between rotor core 33 and outer shell 22 can be substantially determined by the impedance of C1 and C2 in parallel connection.
A lower value of C1 results in a lower capacitance and thus a higher impedance between winding coils 35 and output shaft 31. The higher impedance reduces the electromagnetic radiation that is able to be coupled to output shaft 31. The parameters of ε, A, and d may be configured in order to obtain a desired C1 value. For example, it is desirable for the thickness of insulating sleeve 32 (corresponding to d in the above equation) to be as large as possible. However, the thickness of insulating sleeve 32 is also constrained by limits of the size of motor 10. In some embodiments, the thickness of insulating sleeve 32 is configured to be at least 0.001 millimeter (mm). To configure the value of ε, different materials may be used for insulating sleeve 32. For example, in some embodiments, insulating sleeve 32 may be made of Teflon.
According to experimental results, when the ratio of C2 to C1 (i.e., C2/C1) is between 0.1 and 50, and preferably between 0.5 and 10, a good balance can be achieved between suppression or reduction of electromagnetic radiation, motor performance, and manufacturing costs. More preferably, having C2/C1 between 1 and 5 may allow for higher absorption of high-frequency signals coupled to outer shell 22, and further reduce the electromagnetic radiation of output shaft 31. This is because, in general, the smaller C1 is, the less the high-frequency radiation will be coupled to shaft 31; while the larger C2 is, the more the high-frequency radiation will be grounded by outer shell 22.
In some embodiments, as illustrated in
In addition, an inner surface of rotor core 33 facing insulating sleeve 32 may comprise a plurality of recesses 37 configured to accommodate protrusions 38 on an outer surface of insulating sleeve 32. The interface between recesses 37 and protrusions 38 may function to prevent rotor core 33 from rotating relative to insulating sleeve 32. In some embodiments, insulating sleeve 32 and rotor core 33 are connected through an interference fit, while insulating sleeve 32 and output shaft 31 may be connected by knurling.
It is understood that while embodiment illustrated in
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 |
|---|---|---|---|
| 201310098562.X | Mar 2013 | CN | national |
