Brushless Motor

The present invention relates to a brushless motor, particularly relates to a flat brushless motor with a short axial length preferable for a compact electric device.

As shown in FIG. 12, a conventional brushless motor used in the compact tape recorder has a body 101 receiving a stator coil, a rotor including permanent magnets and a sub-rotor. A rotor shaft 102 projects axially outwardly from the body 101, and a pulley 103 is mounted on the rotor shaft 102 at the outside of the body 101 so that an output power of the brushless motor can be transmitted by a belt 104 wound on the pulley 103.

An object of the present invention is to provide a brushless motor which has a small axial length although the brushless motor has output means for transmitting an output power of the brushless motor to the outside of the brushless motor.

According to the present invention, a brushless motor comprises,

rotor means which is rotatable on an axis of the brushless motor and which include permanent magnets arranged circumferentially thereon,

output means connected to the rotor means to rotate with the rotor means, and

stator means which include electro-magnetic coils energized in order to rotate the rotor means, and have a first space which receives the output means therein and a second space which extends from the first space to the outside of the stator means so that an output power of the brushless motor is transmitted from the output means to the outside of the brushless motor through the second space.

Since the stator means have the first space which receives the output means therein and the second space which extends from the first space to the outside of the stator means so that the output power of the brushless motor is transmitted form the output means to the outside of the brushless motor through the second space, a distance in the axial direction of the brushless motor between the stator means and the output means is substantially zero, a distance in the axial direction of the brushless motor between the output means and the rotor means is very small, and a distance in the axial direction of the brushless motor between the output means and a magnetically energized field between the stator means and the rotor means is very small. Therefore, a length of the brushless motor in the axial direction of the brushless motor is very small, and a difference in axial position between a force transmitted from the output means to the outside of the brushless motor through the second space and a force generated on the rotor means by the stator means to drive the output means is very small so that a capacity of a bearing for supporting rotatably the rotor means and the output means in relation to the stator and for keeping accurately a space of the magnetically energized field between the stator means and the rotor means may be small.

FIG. 1 is a cross-sectional view taken along a line 1--1 of FIG. 2 to show an embodiment of a flat brushless motor according to the present invention.

FIG. 2 is a front view showing the embodiment of FIG. 1.

FIG. 3 is a side view showing the embodiment of FIG. 1.

FIG. 4 is a front view showing an embodiment of a stator according to the present invention.

FIG. 5 is a schematic front view showing an arrangement of permanent magnets on a rotor.

FIG. 6 is a front view showing an arrangement of the stator, the rotor and a pulley.

FIG. 7 is a front view showing an arrangement of the rotor and the pulley.

FIG. 8 is a diagram showing a control device for energizing in order electro-magnets of the stator.

FIG. 9 is a schematic view showing a structure for mounting the stator and terminal members of the stator.

FIG. 10 is a cross-sectional view showing another embodiment of the present invention.

FIG. 11 is a cross-sectional view showing another embodiment of the present invention.

FIG. 12 is a side view showing a conventional brushless motor.

FIG. 13 is a front view showing an arrangement of the stator, the rotor and a gear.

As shown in FIGS. 1 to 7, an embodiment of a flat brushless motor according to the present invention includes a stator 1 containing electro-magnet driving coils 1a. An outer periphery of the stator 1 has a terminal holding member lb including terminals 1c, and a first space 1d of the stator 1 is formed at a central portion of the stator 1. As shown in FIG. 4, the driving coils 1a (FIG. 6) constituted by individual coils L1a, L1b, L2a, L2b, L3a and L3b (FIG. 4) whose coil turns may be substantially equal to each other to form respective magnetic flux densities identical to each other are arranged on a circular line separately with angular distances of 40 degrees. Alternatively, a total amount of coil turns of L1a and L1b, a total amount of coil turns of individual coils L2a and L2b, and a total amount of coil turns of individual coils L3a and L3b may be equal to each other so that the respective magnetic flux densities of three-phases magnetic energizing are equal to each other. Three-phase driving combinations of individual coils L1a-L1b, L2a-L2b and L3a-L3b are energized in order so that a magnetically energized field by the driving coils 1a rotates along the circular line. A second space 1e of the stator 1 is formed between individual coils L1a and L3b (FIG. 4). Each of the three-phases driving combinations has the same number of the driving coils 1a, that is, two of the driving coils 1a so that driving forces by the three-phases driving combinations of the driving coils la are substantially equal to each other. Therefore, a driving force by the driving coils la is always constant. In a preferred embodiment, as clearly seen in FIG. 4, the electromagnetic coils adjacent to each other do not overlap each other in the axial direction of the brushless motor.

A disk-shaped rotor 2 is rotatable on an axis of the brushless motor and has permanent magnets 2a facing to the driving coils 1a of the stator 1 with an axial magnetically energized clearance therebetween. A number of poles of the permanent magnets 2a is twelve and the permanent magnets 2a are arranged on the rotor 2 with angular distances of 30 degrees, as shown in FIG. 5. A pulley 2b is fixed to the rotor 2 and a central axis of the pulley 2b is kept on that of the rotor 2. The pulley 2b is mounted on a shaft 2c so that the pulley 2b is rotatably supported through the shaft 2c by a bearing 3a fixed to a bracket 3. The pulley 2b has a V-shaped belt groove 2d at an outer periphery thereof, and has a cylindrical projection 2e.

The bearing 3a has a radial bearing 3b and a thrust bearing 3c. The bracket 3 has a magnetic shield plate 3d which is made of a magnetically conductive material and extends along an outer periphery of the brushless motor so that an electromagnetic wave generated by the stator 1 is prevented from acting on an electronic device, for example, a tuner circuit or amplifier circuit or a magnetic head. The stator 1 is supported on the bracket 3 through a plurality of support columns 3e by screws 6.

A sub-rotor 4 is fixed to the pulley 2b through a holder member 4b fixed to the projection 2e of the pulley 2b so that the sub-rotor 4 and the rotor 2 are rotatable on the axis of the brushless motor. The sub-rotor 4 has permanent magnets 4a which are the same as the permanent magnets 2a and face the driving coils 1a of the stator 1 with an axial magnetically energized clearance therebetween so that the stator 1 is arranged between the sub-rotor 4 and the rotor 2. N poles of the rotor 2 face S poles of the sub-rotor 4 through the stator 1, and S poles of the rotor 2 face N poles of the sub-rotor 4 through the stator 1. From the above description and FIGS. 1 and 4, it will be immediately apparent that the N poles of the rotor 2 must necessarily face the S poles of the sub-rotor 4 through the second space 1e and similarly that the S poles of the rotor 2 must necessarily face the N poles of the sub-rotor 4 through the second space 1e. The holder member 4b extends radially outwardly from the projection 2e of the pulley 2b so that a diameter of the first space 1d is small although an inner diameter of the sub-rotor 4 is large. The magnetically energized clearance between the permanent magnets 4a of the sub-rotor 4 and the driving coils 1a of the stator 1 is substantially equal to the magnetically energized clearance between the permanent magnets 2a of the rotor 2 and the driving coils 1a of the stator 1 so that a magnetic force in an axial direction of the brushless motor between the sub-rotor 4 and the stator 1 is substantially equal to a magnetic force in the axial direction of the brushless motor between the rotor 2 and the stator 1. Therefore, a force in the axial direction of the brushless motor is not applied to the stator 1 and a movement or deformation of the stator 1 in the axial direction of the brushless motor is prevented. If a magnetomotive force of the permanent magnets 4a is different from that of the permanent magnets 2a, the magnetically energized clearance between the sub-rotor 4 and the stator 1 and the magnetically energized clearance between the rotor 2 and the stator 1 is adjusted to make the magnetic force in the axial direction between the sub-rotor 4 and the stator 1 substantially equal to the magnetic force in the axial direction between the rotor 2 and the stator 1. That is, when the magneto-motive force of the permanent magnets 4 a is smaller than that of the permanent magnets 2a, the magnetically energized clearance between the sub-rotor 4 and the stator 1 is made smaller than the magnetically energized clearance between the rotor 2 and the stator 1 so that a magnetic flux density by the permanent magnets 2a is made substantially equal to a magnetic flux density by the permanent magnets 4a, and when the magnetomotive force of the permanent magnets 4a is larger than that of the permanent magnets 2a, the magnetically energized clearance between the sub-rotor 4 and the stator 1 is larger than the magnetically energized clearance between the rotor 2 and the stator 1 so that a magnetic flux density by the permanent magnets 2a is made substantially equal to a magnetic flux density by the permanent magnets 4a.

A belt 5 is wound on the belt groove 2d of the pulley 2b in the first space 1d and passes through the second space 1e so that an output power of the brushless motor can be taken out to the outside of the brushless motor. A gear 12 (FIG. 13) may be fixed to the rotor 2 instead of the pulley 2b and may engage with another gear (not shown) which is rotatable on an axis at the outside of the brushless motor and is arranged in the second space 1e so that the output power of the brushless motor can be taken out to the outside of the brushless motor through the second space 1e, as shown in FIG. 6, although the pulley 2b or the gear 12 is received in the stator 1. Since the pulley 2b or the gear 12 for transmitting the output power of the brushless motor to the outside of the brushless motor is received in the stator 1, a size of the brushless motor, particularly an axial length of the brushless motor is small, and a moment which is generated by the pulley 2b or the gear 12 for transmitting the output power of the brushless motor to the outside of the brushless motor and by the stator 1 for driving the rotor 2 and/or the sub-rotor 4 and which is born by the bearing 3a is small.

As shown in FIG. 8, the coils L1a and L1b are connected to a terminal 1c1 in series, the coils L2a and L2b are connected to a terminal 1c2 in series, the coils L3a and L3b are connected to a terminal 1c3 in series, and the combinations of the coils L1a-L1b, L2a-L2b and L3a-L3b are connected together to a terminal 1c4 so that the three-phases driving combinations of L1a-L1b, L2a-L2b and L3a-L3b are energized in order by a control circuit 7 to rotate the rotor 2 and/or the sub-rotor 4. As shown in FIG. 9, since the terminals 1c of the stator 1 project outwardly from the terminal holder member 1b, the stator 1 can be easily connected to a printed circuit plate 8.

As shown in FIG. 10, a sub-rotor 11 includes permanent magnets 11a which are the same as the permanent magnets 2a or 4a with the twelve magnetic poles, and may be directly fixed to the projection 2e of the pulley 2b on the rotor 2 without the holder member 4b through a central recess 11b of the sub-rotor 11. A depth of the recess 11b is determined in such a manner that a magnetically energized clearance between the sub-rotor 11 and the stator 1 is substantially equal to the magnetically energized clearance between the rotor 2 and the stator 1. An inner diameter of an opening of the recess 11b is equal to that of the holder member 4b so that the sub-rotor 4 can be easily exchanged for the sub-rotor 11. N poles of the rotor 2 face to S poles of the sub-rotor 11 through the stator 1, and S poles of the rotor 2 face to N poles of the sub-rotor 11 through the stator 1 so that the sub-rotor 4 and the rotor 2 is rotated in accordance with changes of energizing the coils 1a of the stator 1.

As shown in FIG. 11, the sub-rotor 4 with the permanent magnets 4a may be replaced by a flat sub-rotor 12 made of a magnetically conductive material, for example, iron. The sub-rotor 12 is fixed to the projection 2e of the pulley 2b on the rotor 2 without the holder member 4b. An inner diameter of an opening of the sub-rotor 12 is equal to that of the holder member 4b so that the sub-rotor 4 can be easily exchanged for the sub-rotor 12 or each of the other various sub-rotors including a common opening diameter A magnetically energized clearance between the sub-rotor 12 and the stator 1 is substantially equal to the magnetically energized clearance between the permanent magnets 2a of the rotor 2 and the stator 1.

The coils 1a may be made by an etching process or by a plating process, and the sheet shaped coils 1a made on a surface of the stator 1 by an etching process or by a plating process may be securely fixed to the surface of the stator 1 by an electric insulating material covering the sheet-shaped coils 1a.