Direct current brushless motor for a fan

[0002] 1. Field of the Invention

[0003] The invention relates to a direct current brushless motor for a fan, especially to a motor in which the position of a sensor is altered a certain distance to obtain a better activation effect.

[0004] 2. Description of Related Art

[0005] With reference to FIG. 5, a conventional direct current brushless motor for a fan has a housing (31), a stator assembly (30) and a rotor assembly. The stator assembly (30) is contained in and fixed with respect to the housing (31). The rotor assembly is contained in and able to rotate with respect to the housing (31). The rotor assembly includes a metal cover (312) under the housing (31), four magnetic sectors evenly arranged under and along the metal cover (312), and a shaft (311). Each magnetic. sector is made up of a permanent magnet (313). The stator assembly (30) consists of four stator poles (301) evenly arranged on a round seat, a sleeve (33), a circuit board (32), and a sensor (321) electrically connected to the circuit board (32) and fixed between two stators poles (301). The round seat covers the sleeve (33). The sleeve (33) is connected to the shaft (311) by a bearing (34). The sensor (321) is used to sense the magnetic strength of the permanent magnets (313) and send signals to the circuit to activate the motor and keep the motor running. Before the motor is activated, the sensor (321) always stops with a sensing window (322) stopping right above where two neighboring permanent magnets (313) meet.

[0006] However, this kind of motor has a drawback. The magnetic strength where two neighboring permanent magnets (313) meet is the weakest. Therefore, it is always difficult, or needs a large activation current, to activate the motor.

[0007] The main object of the invention is to provide a direct current brushless motor in which the sensing window of the sensor is altered a certain distance to obtain a better activation effect.

[0008] Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

[0009] FIG. 1 is a cross sectional top view of the direct current brushless motor in accordance with the present invention;

[0010] FIG. 2 is a graph of the magnetic strength along the permanent magnets in the direct C brushless motor in FIG. 1;

[0011] FIG. 3 is a cross sectional top view of another embodiment of the direct current brushless motor in accordance with the present invention;

[0012] FIG. 4 is a graph of the magnetic strength along the permanent magnets in the direct current brushless motor in FIG. 4; and,

[0013] FIG. 5 is a cross sectional view of a conventional direct current brushless motor.

[0014] With reference to FIG. 1, a direct current brushless motor for a fan has a housing (11), a stator assembly (10) and a rotor assembly. The stator assembly (10) is contained in and fixed with respect to the housing (11). The rotor assembly is contained in the housing (11) and able to rotate with respect to the housing (11). The rotor assembly includes a metal cover (112) under the housing (11), four magnetic sectors evenly arranged under and along the metal cover (112), and a shaft (111). Each magnetic sector is made up of a permanent magnet (113). The stator assembly (10) consists of four stator poles (101) evenly arranged on a round seat, a sleeve (13), a circuit board (12), and a sensor (121) electrically connected to the circuit board (12) and fixed between two stator poles (101). The round seat covers the sleeve (13). The sleeve (13) connects to the shaft (111) by a bearing (14). The sensor (121) is used to sense the magnetic strength of the permanent magnets (113) and then send signals to the circuit to activate the motor and keep the motor running.

[0015] The sensing window (122) of the sensor (121) is altered a distance (λ) from a position (102) midway in the gap between the stator poles (101) and directly above where two permanent magnets (113) meet when the direct current brushless motor stops as shown in FIG. 1.

[0016] With reference to FIG. 2, the abscissa (15) is the extended trace the sensing window (122) passes, and the ordinate is the magnetic strength sensed. The curve (16) is the amplitude of the magnetic strength of the permanent magnets (113) sensed by the sensing window (122) during its trace. A complete cycle of the sensing window (122) is represented by the curve (16) as it passes through a distance L. The zero points on the curve (16) in the cycle L are at points above where two neighboring permanent magnets (113) meet. The dashed line (17) is the sensibility of the sensor (BOP: B: flux density; OP: operate point; and BRP: B: flux density; RP: release point). A distance traveled by the sensing window (122) from a preferred point where the curve (16) crosses the dashed line (17) to the nearest zero point is a preferred distance (λ′). When the distance (λ) shown in FIG. 1 is equal to the preferred distance (λ′), the sensor (121) and the circuit board (12) will need the lowest activation current to sense the magnetic strength, and the activation effect of the motor can be much improved.

[0017] With reference to FIG. 3, another embodiment of the motor in accordance with the present invention has a housing (21), a stator assembly (20) and a rotor assembly. The stator assembly (20) is contained in and fixed with respect to the housing (21). The rotor assembly is contained in and able to rotate with respect to the housing (21). The rotor assembly includes a metal cover (212) under the housing (21), eight magnetic sectors evenly arranged under and along the metal cover (212), and a shaft. Each magnetic sector is made up of a permanent magnet (213). The stator assembly (20) consists of eight stator poles (201) evenly arranged around sleeve (13), a circuit board (22), and a sensor (221) electrically connecting with the circuit board (22) and fixed between two stator poles (201).

[0018] The sensing window (222) of the sensor (221) is altered a distance (λ) from a position (202) midway in the gap. between adjacent stator poles (201) and directly above where two permanent magnets (213) meet when the direct current brushless motor stops as shown in FIG. 3.

[0019] With reference to FIG. 4, the abscissa (24) is the extended trace the sensing window (222) passes, and the ordinate is magnetic strength. The curve (23) is the amplitude of the magnetic strength of the permanent magnets (213) sensed by the sensing window (222) during its trace. A complete cycle of the sensing window (222) is represented by the curve (23) as it passes through a distance L. The zero points of the curve (23) in the cycle L are at points above where two neighboring permanent magnets (213) meet. The dashed line (25) is the sensibility of the sensor (BOP and BRP). A distance traveled by the sensing window (222) from a preferred point where the curve (23) crosses the dashed line (25) to the nearest zero point is the preferred distance (λ′). When the distance (λ) shown in FIG. 3 is equal to the preferred distance (λ′), the sensor (221) and the circuit board (22) will need the lowest activation current to sense the magnetic strength, and the activation effect of the motor can be much improved.

[0020] Is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.