Electric motor

An electric motor including a stator with a plurality of coils wound around a ring-shaped iron core, and a rotor rotatably disposed inside the stator is provided. The rotor includes a rotor shaft and a yoke. The yoke is provided near its periphery with a plurality of plate-like permanent magnets arranged at equal pitches. Heat generated from the rotor is released through a plurality of air holes formed in the yoke between the rotor shaft and the permanent magnets.

FIELD OF THE INVENTION

The present invention relates to an improvement in an electric motor and, more particularly, to an inner-rotor brushless motor.

BACKGROUND OF THE INVENTION

Such a brushless motor is generally comprised of a stator with a plurality of coils wound around an iron core, a rotor rotatably disposed inside the stator, and a housing containing the stator and the rotor. The housing is provided with a plurality of air holes so that heat generated from the rotor and the stator is released through the air holes.

A brushless motor having a cylindrical housing closed at its opposite ends with covers formed with air holes, and a rotor provided with a fan, is proposed in, e.g., Japanese Patent Laid-Open Publication No. HEI-1-110035. This brushless motor will be described in detail with reference toFIG. 15hereof.

A brushless motor300shown inFIG. 15includes a stator303formed by winding coils302around an iron core301, a rotor307provided inside the stator303and consisting of a yoke305mounted on a rotor shaft304and a permanent magnet306mounted on the periphery of the yoke305, and a cylindrical housing308containing the stator303and the rotor307together.

Opposite ends of the housing308are closed by covers309,309. One cover309is provided with a plurality of air inlet openings310, and the other cover309is provided with a plurality of air outlet openings311.

A fan312is mounted on the rotor shaft304near the air inlet openings310. The fan312is rotated by rotation of the rotor shaft304, drawing air into the housing308through the air inlet openings310.

Air drawn into the housing308cools the heated stator303, and then is discharged from the air outlet openings311.

In the above brushless motor300, however, a central part of the rotor307is not directly cooled, causing the problem that heat persists in the central part of the rotor307.

Also, in the above brushless motor300, the cooling fan312is attached on the rotor shaft304, thus increasing the moment of inertia of the rotor307. Therefore, it is required to increase the rigidity of the housing308and the covers309,309for supporting the rotor307, disadvantageously leading to a weight increase of the brushless motor300.

There is thus a desire for an art which allows heat in a central part of a rotor to be released, and also allows an electric motor to be reduced in weight.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided an electric motor which comprises a stator with a plurality of coils wound around a ring-shaped iron core; and a rotor rotatably disposed inside the stator; the rotor comprising a rotor shaft; a yoke integrally mounted on the rotor shaft; a plurality of magnet holes formed at equal pitches near a periphery, of the yoke; and a plurality of plate-like permanent magnets disposed in the magnet holes, respectively; wherein the yoke has a plurality of air holes for accelerating heat release of the rotor, between the rotor shaft and the permanent magnets.

Thus, in the electric motor of the present invention, the air holes for accelerating heat release of the rotor are formed in the yoke, between the rotor shaft and the permanent magnets, allowing heat in a central part of the rotor to be directly released. Also, the formation of the air holes leads to a reduced weight of the rotor. As a result, the moment of inertia of the rotor is reduced, and a member supporting the rotor can be reduced in rigidity, resulting in reduction in weight of the electric motor.

According to an embodiment of the present invention, with the center of the rotor shaft as C, one of the permanent magnets as M1, a point of intersection of a perpendicular line drawn from the center C to the permanent magnet M1and the periphery of the yoke as P1, a permanent magnet adjacent to the permanent magnet M1as M2, a point of intersection of a perpendicular line drawn from the center C to the permanent magnet M2and the periphery of the yoke as P2, and a point of intersection where a radial line passing through the center C and a midpoint between the adjacent permanent magnets M1, M2intersects with the periphery of the yoke as Q, when a circle of a radius r passing through the intersection point P1and/or the intersection point P2is drawn around the intersection point Q, the air holes are formed outside of the circle.

That is, since the air holes are formed in the yoke in locations outside of the circle, magnetic flux produced between the permanent magnets M1, M2is not interrupted by the air holes. As a result, magnetic flux density can be prevented from being reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, an exemplary snow removing machine equipped with electric motors according to the present invention will be described with reference toFIGS. 1 to 7.

A snow removing machine10shown inFIG. 1includes a transmission case50doubling as a machine body, vertically swingably mounted to a crawler frame40having right and left travel units30R,30L (seeFIG. 3).

Right and left electric motors20R,20L are mounted to the right and left sides of the transmission case50. An engine60is mounted on top of the transmission case50. A snow removing unit70is mounted to the front of the transmission case50. Right and left operating handles80R,80L extend from upper portions of the transmission case50in a rearward and upward direction. An operating panel91is provided between the right and left operating handles80R,80L.

The snow removing machine10is a self-propelled, walk-behind working machine, which is led by an operator walking behind the operating panel91.

The right and left operating handles80R,80L have grips81R,81L at their respective distal ends to be held in the hands of an operator.

The right and left electric motors20R,20L are travel driving sources, which transmit power to the right and left travel units30R,30L for driving via right and left travel transmission mechanisms120R,120L.

The right travel unit30R is a crawler with a crawler belt33R entrained around a front drive wheel31R and a rear idle wheel32R. The drive wheel31R is rotated normally and reversely by the right electric motor20R.

The left travel unit30L is a crawler with a crawler belt33L put around a front drive wheel31L and a rear idle wheel32L. The drive wheel31L is rotated normally and reversely by the left electric motor20L.

The engine60is a vertical engine with a crankshaft61extending downward, constituting an operation driving source to transmit power to the snow removing unit70for driving via an operation transmission mechanism130and a transmission shaft76housed in the transmission case50.

The engine60has a cylinder62oriented rearward, and a front surface63aof an engine body63housing the crankshaft61is adjacent to a rear surface75aof the snow removing unit70(i.e., a rear surface75aof a blower housing75).

The snow removing unit70consists of a front auger71, a rear blower72, an upper chute73, an auger housing74enclosing the auger71, and the blower housing75enclosing the blower72. The auger housing74and the rear blower housing75are integrally formed.

The auger71acts to collect snow accumulating on the ground to the center. The blower72receiving the snow throws the snow to a desired location around the snow removing machine10via the chute73.

The snow removing machine10also includes a swing drive mechanism83for vertically swinging the transmission case50. The swing drive mechanism83is constituted by a telescopic mechanism which vertically swings the transmission case50to adjust the height of the snow removing unit70. The swing drive mechanism83is constituted by an actuator with a rod being able to project from and retract into a cylinder, such as an electric actuator, a hydraulic actuator, or a pneumatic actuator.

InFIG. 1, reference numeral92denotes a scraper;93, a lamp;94, an air cleaner;95, a carburetor; and96, an engine exhaust muffler.

FIG. 2shows the snow removing machine10with the engine60and the surrounding parts enclosed by a cover110. The cover110consists of a lower cover portion111enclosing a lower half of the engine60and an upper cover portion112enclosing an upper half of the engine60.

The engine60is disposed on the transmission case50. The cylinder62of the engine60is oriented rearward, thereby to form a space Sp below the cylinder62. The engine exhaust muffler96is disposed in the space Sp, generally surrounded by the transmission case50located in front of the engine exhaust muffler96, the crawler frame40, the right and left crawler belts33R,33L, and the engine60. Therefore, an operator is prevented from directly touching the relatively hot engine exhaust muffler96.

As shown inFIG. 3, the engine60is disposed in the substantially central part of the snow removing machine10. The center EL of the crankshaft (engine output shaft)61is located on the vehicle longitudinal center line CL. As shown inFIG. 1, the transmission case50and the operation transmission mechanism130housed in the transmission case50are disposed directly below the engine60. The snow removing unit70is disposed in front of the transmission case50and the operation transmission mechanism130. The right and left travel units30R,30L are disposed on the right and left sides of the transmission case50and the operation transmission mechanism130.

The right and left electric motors20R,20L as electric motors of the present invention are disposed between the right and left drive wheels31R,31L located in the front parts of the right and left travel units30R,30L.

The chute73is disposed at a left front portion of the snow removing machine10, and the lamp93and a battery97are at a right front portion. The air cleaner94is disposed at a right rear portion of the snow removing machine10. A fuel tank98is located in the rear of the snow removing machine10between the right and left operating handles80R,80L. Right and left motor drive controllers99,99are mounted to the right and left sides of the fuel tank98.

As shown inFIG. 1, the left operating handle80L is provided near the grip81L with a travel preparation lever82for making the right and left electric motors20R,20L drivable. The right operating handle80R is provided near the grip81R with an adjusting lever84for operating the swing drive mechanism83.

As shown inFIG. 4, the transmission case50is disposed between the right and left travel units30R,30L.

Referring toFIG. 4, the crawler frame40is a substantially U-shaped member in a plan view, including a pair of right and left side members41,41extending longitudinally of the snow removing machine10, a crossmember42extended between the rear ends of the right and left side members41,41, and a bracket43mounted to a middle portion of the crossmember42.

The right and left side members41,41rotatably support right and left drive wheel axles34R,34L at their front portions, and support an idle wheel axle35at their rear portions. The right and left drive wheel axles34R,34L are rotary shafts to which the right and left drive wheels31R,31L are fixed. The right and left idle wheels32R,32L are rotatably mounted on opposite ends of the single idle wheel axle35.

The crossmember42is configured to be able to move forward and backward for adjustment by right and left adjusting bolts36,36extending rearward from the idle wheel axle35. The adjusting bolts36,36are adjusted forward or backward, thereby moving the idle wheel axle35forward or backward to adjust the tension of the right and left crawler belts33R,33L.

The left electric motor (electric motor)20L includes an annular outer stator (stator)21at the stationary side, an inner rotor (rotor)22enclosed by the outer stator21, and a motor shaft (rotor shaft)23integrally fitted in the inner rotor22. An electromagnetic brake24is mounted on the motor shaft23. The electromagnetic brake24is a brake corresponding to a parking brake of common vehicles.

The left travel transmission mechanism120L is a three-speed reduction mechanism, including a first pinion121mounted on the motor shaft23, a first gear122in mesh with the first pinion121, rotatably mounted on the drive wheel axle34L, a second pinion123integrally formed with the first gear122, a second gear124in mesh with the second pinion123, rotatably mounted on the motor shaft23, a third pinion125integrally formed with the second gear124, and a third gear126in mesh with the third pinion125, mounted on the drive wheel axle34L.

The gears121to126are spur gears. The right electric motor20R has the same configuration as that of the left electric motor20L. The right travel transmission mechanism120R has the same configuration as that of the left travel transmission mechanism120L. Reference numerals37,37denote bearings for the right and left drive wheel axles34R,34L.

The operation transmission mechanism130housed in the transmission case50is a single reduction mechanism including an input shaft131extending nearly vertically, an output shaft132extending nearly horizontally, a driving small bevel gear133mounted on the input shaft131, and a driven large bevel gear134mounted on the output shaft132to mesh with the small bevel gear133.

The output shaft132extends forward from the transmission case50and is connected to the snow removing unit70via the transmission shaft76. That is, the transmission shaft76is connected to the front end of the output shaft132. The transmission shaft76is connected to auger shafts78,78via a worm gear reduction mechanism77, and is also connected to the blower72.

Now, the configuration of the right and left operating handles80R,80L and the mounting structure of the swing drive mechanism83will be described.

The operating handles80R,80L include right and left handle bases85R,85L extending rearward from the right and left sides of the transmission case50, and right and left handle bars86R,86L attached adjustably in mounting angle to the rear ends of the handle bases85R,85L and further extending rearward. A crossmember87is extended between the rear ends of the right and left handle bases85R,85L. A bracket88is mounted to a middle portion of the crossmember87.

The swing drive mechanism83is connected between the bracket43provided at the crossmember42of the crawler frame40and the bracket88provided at the crossmember87of the right and left handle bases85R,85L.

The right and left drive wheel axles34R,34L supported by the crawler frame40are also rotatably supported by the transmission case50. Therefore, the transmission case50is vertically swingable about the drive wheel axles34R,34L. That is, the transmission case50swings vertically (in a front and back direction of theFIG. 4sheet) via the swing drive mechanism83and the right and left handle bases85R,85L.

As describe above, the engine60(seeFIG. 1) and the snow removing unit70are mounted to the transmission case50. Therefore, the engine60and the snow removing unit70are vertically swung with the transmission case50, thereby to adjust the height of the snow removing unit70.

FIG. 5shows a cross section of the transmission case50and the operation transmission mechanism130of the snow removing machine shown inFIG. 1.

The transmission case50is provided at its top with a plurality of mounting bosses51. A bottom portion64of the engine60is bolted to the mounting bosses51so as to mount the engine60on top of the transmission case50. The transmission case50is provided at its front with a mounting flange52. The blower housing75is bolted to the mounting flange52so as to mount the snow removing unit70to the front of the transmission case50.

The input shaft131of the operation transmission mechanism130is disposed in such a manner as to extend upward, concentrically with the crankshaft61. The upper end of the input shaft131is connected to the lower end of the crankshaft61via a clutch65.

The transmission case50has an input shaft housing53in a tubular shape housing the input shaft131, and an output shaft housing54in a tubular shape formed integrally with the input shaft housing53, for housing the output shaft132. The input shaft housing53is closed by a lid55. Reference numerals141,142,143,144and145denote bearings rotatably supporting the input shaft131and the output shaft132.

FIG. 6shows in section the transmission case50, the right and left electric motors20R,20L, and the right and left travel transmission mechanisms120R,120L.FIG. 7shows the left electric motor20L and the left travel transmission mechanism120L. For ease of understanding, the right and left travel transmission mechanisms120R,120L are shown in development.

The electric motors20R,20L and the travel transmission mechanisms120R,120L are housed in the transmission case50.

As shown inFIGS. 6 and 7, at the right and left sides of the transmission case are provided the right and left electric motors20R,20L, the right and left travel transmission mechanisms120R,120L, and the right and left drive wheel axles34R,34L. The transmission case50doubles as part of motor cases (housings)153,153.

The transmission case50includes the output shaft housing54provided along the longitudinal center line CL, motor housings56,56integrally formed at the right and left sides of the output shaft housing54, right and left motor case halves151,151closing openings of the opened right and left sides of the motor housings56,56, and right and left transmission covers152,152fastened by bolts to the right and left sides of the motor case halves151,151.

In this manner, the right and left motor housings56,56and the right and left motor case halves151,151constitute the right and left motor cases153,153. Thus, the right and left motor housings56,56of the transmission case50double as part of the motor cases153,153.

Also, the right and left motor case halves151,151and the right and left transmission covers152,152constitute right and left travel transmission mechanism cases154,154. Thus, the right and left motor case halves151,151double as part of the travel transmission mechanism cases154,154.

The electric motors20R,20L and the electromagnetic brakes24,24are housed in the right and left motor cases153,153. The right and left travel transmission mechanisms120R,120L are housed in the right and left travel transmission mechanism cases154,154, respectively.

As shown inFIG. 7, the left electric motor20L is an inner-rotor-type DC brushless motor of an assembly of the motor shaft23extending horizontally in a transverse direction (right and left direction in the figure), the outer stator21having a plurality of armatures (coils)25arranged in a stator circumferential direction, and the inner rotor22having a plurality of permanent magnets26arranged in a rotor circumferential direction. The right electric motor20R has the same configuration.

The motor shaft23extending into the left travel transmission mechanism case154has the first pinion121and the second gear124mounted on its distal end. The integrally formed second gear124and third pinion125are rotatably supported by the transmission cover152. The motor shaft23and the drive wheel axle34L are rotatably supported by bearings161,162,163,164,165and166.

Harnesses (conductive wires)171,171for the right and left electric motors20R,20L are partially located inside the transmission case50doubling as part of the motor cases153,153. More specifically, the harnesses171,171connected to the electric motors20R,20L in the motor housings56,56are guided to the outside through harness apertures172formed in the transmission case50as shown inFIG. 5. The harnesses171,171in the transmission case50are not exposed to the outside, thus having increased durability at the unexposed portions.

Now, the configuration of the right and left electric motors20R,20L shown inFIG. 6will be described as an “electric motor20.”

FIG. 8shows a cross section of the electric motor20in the present invention shown inFIG. 7.

Referring toFIG. 8, the electric motor20includes the outer stator21(hereinafter, only referred to as a “stator21”), the inner rotor22rotatably disposed inside the stator21(hereinafter, only referred to as a “rotor22”), and the motor case153housing the stator21and the rotor22(hereinafter, only referred to as a “housing153”).

The stator21includes a ring-shaped iron core14formed by combined teeth13, and a plurality of armatures25(hereinafter, referred to as “coils25”) wound around the teeth13, respectively. Each tooth13has a substantially T shape, and is constituted by a laminated body of a plurality of iron sheets12. The teeth13and the coils25constitute a coil piece15.

FIG. 9shows the rotor22. The rotor22includes the motor shaft23(hereinafter referred to as a “rotor shaft23”), a disc-shaped yoke27integrally mounted on the rotor shaft23, and a plurality of permanent magnets26arranged at equal pitches near the periphery of the yoke27.

The yoke27is a laminated member of a plurality of iron sheets16,17,17. The yoke27has a shaft hole18formed to press fit the rotor shaft23, a plurality of magnet holes19formed to house the permanent magnets26, respectively, and a plurality of air holes28formed to release heat generated from the rotor22.

The yoke27is formed by housing the permanent magnets26in the respective magnet holes19, and then covering the permanent magnets26by the iron sheets17,17located at the opposite ends. The iron sheets16are formed with the shaft hole18, the magnet holes19, and the air holes28. The iron sheets17are formed with the shaft hole18and the air holes28.

As shown inFIGS. 10 and 11, the rotor22is comprised of the rotor shaft23, the disc-shaped yoke27integrally mounted on the rotor shaft23, the magnet holes19formed at equal pitches near the periphery of the yoke27, and the plate-like permanent magnets26disposed in the respective magnet holes19. The yoke27has the air holes28formed in the region between the rotor shaft23and the permanent magnets26to accelerate release of heat generated from the rotor22. The air holes28have a contour or shape that includes an angled portion having an apex or tip28athat points toward a mid-portion of respective inward faces26bof the permanent magnets26.

The yoke27is thus provided with the air holes28for accelerating heat release of the rotor22between the rotor shaft23and the permanent magnets26, allowing heat in a central part of the rotor22to be directly released, and resulting in acceleration of heat release of the rotor22.

In addition, the formation of the air holes28in the yoke27leads to a reduced weight of the rotor22. As a result, the moment of inertia of the rotor22is reduced, and a member supporting the rotor22(e.g., the housing153shown inFIG. 8) can be reduced in rigidity, resulting in reduction in weight of the electric motor20.

The permanent magnets26have plate-like bodies with one side magnetized as the north pole and the other side as the south pole in a plan view. The permanent magnets26are arranged in circumferentially spaced relation from one another about a peripheral portion of the yoke27and are arranged in a polygonal shape in such a manner that the north poles and the south poles of the adjacent permanent magnets26are alternate. The permanent magnets26each have opposed major faces26a,26b(FIG. 11) interconnected by sides26cand are disposed in side-by-side equidstantly-spaced relation so that the inward faces26bof the permanent magnets26face the rotor shaft23.

FIG. 12is a diagram illustrating the formed locations of the air holes28shown inFIG. 11.

InFIG. 12, the center of the rotor shaft23is indicated at C, one of the permanent magnets26(seeFIG. 11) at M1, a point of intersection of a perpendicular line drawn from the center C to the permanent magnet M1and the periphery of the yoke27at P1, a permanent magnet adjacent to the permanent magnet M1at M2, a point of intersection of a perpendicular line drawn from the center C to the permanent magnet M2and the periphery of the yoke27at P2, and a point of intersection where a radial line passing through the center C and a midpoint between the adjacent permanent magnets M1, M2intersects with the periphery of the yoke27at Q. With this, when a circle H of a radius r passing through the intersection point P1or P2is drawn around the intersection point Q, the air holes28are formed in locations outside of the circle H.

FIGS. 13A and 13Bshow a comparative example and the present embodiment, illustrating the relationship between magnetic flux passing through a yoke and air holes. In this embodiment, the apexes28aof the air hole contours lie along the lines passing through the center C and the points P1, P2and the apexes28apoint toward the mid-portion of the inward faces26bof the permanent magnets M1, M2.

An electric motor200in the comparative example shown inFIG. 13Ais an example in which when a plurality of air holes203are provided in a yoke202of a rotor201, the air holes203are provided radially between adjacent permanent magnets204,204, respectively. Magnetic flux shown in thin lines is interrupted by the air holes203, and magnetic flux density is reduced.

In the electric motor20in this embodiment shown inFIG. 13B, the air holes28are provided outside of the circle H of the radius r passing through the intersection point P1or P2with its center at the intersection point Q, so that magnetic flux produced between the permanent magnets M1, M2does not pass through the air holes28, and magnetic paths are ensured. As a result, magnetic flux shown in thin lines is not interrupted, resulting in prevention of reduction in magnetic flux density even with the provision of the air holes28.

Further, in this embodiment, since the air holes28are formed on perpendicular lines from the center C of the rotor shaft23to the plate-like permanent magnets M1, M2, and so on, circumferentially of the rotor shaft23, unnecessary magnetic flux such as magnetic flux passing through the rotor shaft23can be interrupted.

Furthermore, in this embodiment, in the yoke27of the rotor22, the distance between the shaft hole18and the air holes28is set at a distance which allows the rigidity of the yoke27to be sufficiently maintained.

FIGS. 14A,14B and14C show first, second and third modifications in which the shape of air holes formed in a rotor in this embodiment is modified.

Air holes38in the first modification shown inFIG. 14Aare deformed squares, and the apexes38aof the air hole contours lie along lines passing through the center C and the points P1, P2. The air holes38can thus be formed large, resulting in acceleration of heat release.

Air holes48in the second modification shown inFIG. 14Bhave a pentagonal shape, and the apexes48aof the air hole contours lie along lines passing through the center C and the points P1, P2. The air holes48can thus be formed large, resulting in acceleration of heat release, like the air holes38shown inFIG. 14A.

Air holes in the third modification shown inFIG. 14Care exemplarily shown as pairs of symmetrical air holes58,68formed in a triangular shape, each pair of air holes being associated with one permanent magnet. The apexes58a,68aof the air hole contours of each pair of air holes58,68lie on opposite sides of lines passing through the center C and the points P1, P2. Consequently, rigidity is also provided between each pair of air holes58,68, and the rigidity of the yoke27can be sufficiently maintained.

The electric motor shown in this embodiment is an inner-rotor-type electric motor as shown inFIG. 8, but is not limited thereto, and may be an inner-rotor-type generator or an inner-rotor-type generator/motor.

In the electric motor shown in this embodiment, the magnet holes19are provided in the yoke27as shown inFIG. 11, which is not limiting. Alternatively, magnet holes may be notches or the like.

Also, this embodiment has been described with the example in which the teeth13are arranged in a ring shape to constitute the iron core14as shown inFIG. 8, but the present invention is not limited thereto. It may alternatively be possible to wind a plurality of coils around an integrated ring-shaped iron core.

Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.