Electric power tool

There is disclosed an electric power tool in which magnetic saturation seldom occurs in a tooth of a stator without increasing the size of a motor. The electric power tool includes a motor having a stator with a plurality of teeth formed circumferentially at equal intervals and axially extending groove-like slots between each tooth, with winding wires being wound around each tooth by use of the slots, and having a tubular rotor coaxially surrounding the stator with a permanent magnet of a plurality of poles on an inner peripheral surface facing distal end surfaces of the teeth of the stator. Further, the ratio of the number of poles of the permanent magnet of the rotor to the number of slots of the stator is set to be 4 to 3.

This application claims priority to Japanese patent application serial number 2011-114484, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric power tool including a so-called outer rotor type motor in which a tubular rotor rotates around a stator.

2. Description of the Related Art

A prior art relating to an electric power tool including an outer rotor type motor as described above is disclosed in Japanese Patent Application Laid-Open No. 2006-150571.

The electric power tool disclosed in Japanese Patent Application Laid-Open No. 2006-150571 is an electric sander including an outer rotor type motor. By using an outer rotor type motor, the diameter of the rotor can be set large, and a large rotational torque can be obtained due to a high centrifugal force.

Generally, in an outer rotor type motor that is used in an electric power tool, a rotor102has four poles of a permanent magnet103, and a stator105has six slots106, as shown inFIG. 7.

In case where there are four poles of the permanent magnet103, N-poles and S-poles are alternately provided on the inner peripheral surface of the rotor102at an interval of 90 degrees. Further, in case where there are 6 slot-stators105, teeth107around which winding wires (not shown) are wound are formed at an interval of 60 degrees.

In the outer rotor type motor, the rotor102is arranged on the outer side of the stator105, and accordingly, the width in the circumferential direction of each magnetic pole of the permanent magnet103(length of arc) is relatively large, as compared with an inner rotor type motor (a motor in which the rotor is arranged on an inner side of the stator). As a result, in the outer rotor type motor, the amount of magnetic flux passing a magnetic pole of the permanent magnet103is larger than that in the inner rotor type motor. Thus, in the outer rotor type motor, the amount of magnetic flux passing through the teeth107of the stator104becomes large in the magnetic circuit including the rotor102and the stator105. Further, a high power magnet of large magnetic flux density has been used in recent years in order to improve a motor performance.

Thus, during the rotation of the rotor102, when a magnetic flux from the magnetic poles of the permanent magnet103(refer to the narrow lines inFIG. 7) passes through a tooth107of the stator (refer to A and B inFIG. 7), magnetic saturation is occurred, and in some cases, further passage of the magnetic flux becomes impossible.

Accordingly, even if a high performance magnet is used, its performance cannot be fully achieved. Magnetic saturation can be suppressed by enlarging the size of the teeth107of the stator105, but this approach is not desirable since it causes the size of the motor to increase.

SUMMARY OF THE INVENTION

Thus, there is a need in the art to fully utilize the performance of the magnet by preventing the magnetic saturation from occurring in the tooth of the stator without increasing the size of the motor.

One construction for an electric power tool can include a motor having a stator with a plurality of teeth formed circumferentially at equal intervals and with axially extending groove-like slots between each tooth, with winding wires being wound around each tooth by use of the slots, and also having a tubular rotor coaxially surrounding the stator and having a plurality of poles of a permanent magnet on an inner peripheral surface facing distal end surfaces of the teeth of the stator, wherein the ratio of the number of poles of the permanent magnet of the rotor to the number of slots of the stator is set to be 4 to 3.

According to this construction, the ratio of the number poles of the permanent magnet of the rotor to the number of slots of the stator is set to 4 to 3. For example, the number of poles of the permanent magnet can be set to be eight, and the number of slots of the stator to six (6-slot motor). In a 6-slot motor, the number of poles of the permanent magnet can be reduced from the conventional number of four to eight, and accordingly the width of each magnetic pole of the permanent magnet can be reduced by half in the circumferential direction (length of arc) as compared with that in the prior art, while remaining the size of each part of the motor unchanged. As a result, the amount of magnetic flux passing through a magnetic pole of the permanent magnet can be reduced approximately by half as compared with that in the prior art.

Thus, during the rotation of the rotor around the stator, even when the magnetic flux from the magnetic poles (e.g., N-poles) concentrates in one tooth of the stator, magnetic saturation seldom occurs since the amount of magnetic flux with regard to one magnetic pole is reduced by half. Accordingly, even if a high performance magnet is used, its performance can be fully achieved. That is, a high performance of the magnet can be achieved with the size of the motor unchanged.

According to another construction, an electric power tool is characterized in that the permanent magnet of the rotor is made of a rare-earth magnet.

According to this construction, the permanent magnet of the rotor is made of a rare-earth magnet whose main components are neodymium, iron, and boron.

For this reason, the magnetic flux density of the permanent magnet increases, and the power of the motor can be increased.

According to another construction, the number of poles of the permanent magnet of the rotor and the number of slots of the stator can be set to eight and six, respectively. Further, they can be set to twelve and nine, or sixteen and twelve, respectively.

According to the above, magnetic saturation seldom occurs at the tooth of the stator, and thus the performance of the magnet can be fully achieved, and the performance of the motor can be improved without increasing the size of the motor.

DETAILED DESCRIPTION OF THE INVENTION

In the following, an electric power tool according to an example will be described with reference toFIG. 1toFIG. 6. The electric power tool according to the example is a portable circular saw10provided with an outer rotor type brushless motor.

As shown inFIG. 1, the portable circular saw10is provided with a flat-plate-like base12held in face contact with an upper surface of a work W (indicated by the chain double-dashed line), and a circular saw main body portion20provided on the base12. The circular saw main body portion20includes a brushless motor30serving as the drive source (hereinafter termed motor30), a speed reduction mechanism22for reducing the rotational speed of the motor30, and a disc-like rotary blade26mounted to a spindle25that is an output shaft of the speed reduction mechanism22.

The lower side of the rotary blade26protrudes downwardly from the lower surface of the base12, and the protruding part of the rotary blade26can cut off the work W.

As shown inFIG. 1, the motor30is provided with a stator32fixed within a motor housing31so as to be coaxial to the motor housing31, and a tubular rotor37coaxially arranged within the motor housing31so as to surround the stator32. The motor housing31has an opening (at the left-side end as seen inFIG. 1) covered with a disc-like stator support fitting33, which is fixed to a housing22cof the speed reduction mechanism22. The stator support fitting33is a fitting that supports the stator32within the motor case31, and includes a disc portion33ethat covers the opening of the motor case31, and a tubular portion33fthat axially protrudes at the center of the disc portion33r. Further, the stator32is coaxially connected with the distal end of the cylindrical portion33fof the stator support fixing33.

As shown inFIGS. 1 and 2, the disc portion33eand the tubular portion33fof the stator support fitting33, and the stator32have at their center an axially extending through-hole34, and the output shaft35of the motor30is rotatably inserted into the through-hole34. And, as shown inFIG. 1, both end portions of the output shaft35are rotatably supported by a first bearing36aprovided on the speed reduction mechanism22side and a second bearing36bprovided on the bottom plate side of the motor housing31.

A disc-like bottom plate portion38provided at one axial end (the right-hand end as seen inFIG. 1) of the rotor37is coaxially connected to the output shaft35of the motor30so as to be incapable of relative rotation. As a result, the output shaft35of the motor30can rotate together with the rotor37. Further, between the bottom plate portion38of the rotor37and the second bearing36b, a cooling fan39is connected to the output shaft35of the motor30so as to be incapable of relative rotation. The cooling fan39is configured to rotate with the output shaft35to create an airflow, which can cool the inside of the motor case31. In the bottom plate portion38of the rotor37, there is provided a ventilation hole38hthrough which a cooling air created by the cooling fan39passes.

The rotor37of the motor30includes a rotor main body370arranged around the stator32, and the above-mentioned bottom plate portion38that connects the rotor37to the output shaft35of the motor30. As shown inFIG. 2, the rotor main body370includes an outer cylinder portion372made of a tubular ferromagnetic member (for example, iron), and a permanent magnet374provided on the inner peripheral surface of the outer tubular portion372. And, the permanent magnet374is circumferentially divided into eight equal portions, alternately forming N-poles and S-poles.

As the permanent magnet374, a neodymium magnet is used which is a rare-earth magnet whose main components are neodymium, iron, and boron.

As shown inFIG. 2, the rotor32of the motor30is provided with six teeth321formed at equal circumferential intervals, and with slots323formed as groves extending axially between each tooth321. Further, winding wires325are wound around each tooth321of the stator32by use of the slots323. A three-phase connection by star connection or delta connection is applied with the winding wires325wound around each tooth321.

In this way, the motor30is an 8-pole/6-slot outer rotor type brushless motor.

Next, with reference toFIGS. 3 and 4, the 8-pole/6-slot outer rotor type motor according to the example will be explained compared with the conventional 4-pole/6-slot outer rotor type motor.

FIG. 3shows how the magnetic flux of the permanent magnet374(indicated by the narrow lines) passes through the magnetic circuit formed by the rotor37and the stator32in the 8-pole/6-slot outer rotor type motor30(rotation angle: 9 degrees). Further,FIG. 4shows the relationship between the rotation angle of the rotor37and the amount of magnetic flux when the magnetic flux of the permanent magnet374passes through a specific tooth321of the stator32(Refer to section A ofFIG. 3). All of the six teeth321of the motor30are of the same configuration and the same size, and thus the characteristics as shown inFIG. 4are applied to all of teeth321as well as the specific tooth321.

As shown inFIG. 3, by setting the number of poles of the permanent magnet374to eight and by setting the number of slots323of the stator323to six, the width of each magnetic pole of the permanent magnet374in the circumferential direction (length of arc) can be reduced by half as compared with that in the prior art, while remaining the size of each portion of the motor30unchanged. Accordingly, the amount of magnetic flux passing through each magnetic pole of the permanent magnet374can be reduced approximately by half.

Thus, during the rotation of the rotor37around the stator32, even when the magnetic flux from the magnetic poles (e.g., N-poles) of the permanent magnet374concentrates in one tooth321of the stator32, the amount of magnetic flux of each magnetic pole is reduced by half as compared with that in the prior art, thus avoiding occurrence of magnetic saturation.

InFIG. 4, characteristic I (indicated by the solid line) shows a change in the amount of magnetic flux passing through a specific tooth321of the 8-pole/6-slot motor according to the example, and characteristic II (indicated by the dotted line) shows a change in the amount of magnetic flux passing through a specific tooth of the conventional 4-pole/6-slot motor.

As indicated by the characteristic II, in cases where the conventional 4-pole/6-slot motor is used, magnetic saturation occurs when the magnetic flux concentrates in a specific tooth321at a rotation angle of approximately 60 degrees to 90 degrees, and more magnetic flux may not be passed through the tooth31.

In contrast, as indicated by the characteristic I, in cases where the 8-pole/6-slot motor is used, such magnetic saturation does not occur, and thus there is no such limitation to the passage of magnetic flux as mentioned above when the flux passes through a specific tooth321. Accordingly, as compared with the conventional 4-pole/6-slot motor, in cases where the 8-pole/6-slot motor is used, the electromotive force induced in the winding325(induced voltage V (V=n×dΦ/dt)) can increase (refer toFIGS. 5 and 6), where n is the winding number of the winding325, Φ is the magnetic flux, and t is the time.

FIG. 5shows the induced voltage V induced in each winding325(U-phase, V-phase, W-phase) in the 8-pole/6-slot motor30when the rotor37rotates.FIG. 6shows the induced voltage V induced in each winding (U-phase, V-phase, W-phase) in the 4-pole/6-slot motor30when the rotor rotates.

As is apparent fromFIGS. 5 and 6, in cases where 8-pole/6-slot motor is used, the induced voltage V induced in each winding325becomes large as compared with the case where the conventional 4-pole/6-slot motor is used, and thus the output of the motor30becomes large. As a result, a rotational torque can be increased even when the RPM (revolution per minute) of the motor30is not changed.

According to the example, the number of poles of the permanent magnet374of the rotor37is set to eight, and the number of slots of the stator32is set to six. That is, the ratio of the number of poles of the permanent magnet374of the rotor37to the number of slots of the stator32is set to 4 to 3.

In this way, by increasing the number of poles of the permanent magnet374from four to eight, the width of each magnetic pole of the permanent magnet374in the circumference direction (length of arc) can be reduced by half, while remaining the number of slots of the motor30at six and the size of each portion of the motor30unchanged. Thus, the amount of magnetic flux passing through a magnetic pole of the permanent magnet374can be reduced approximately by half.

As a result, during the rotation of the rotor37around the stator32, even when the magnetic flux from the magnetic poles (e.g., N-poles) of the permanent magnet374concentrates in one tooth321of the stator32, the amount of magnetic flux can be reduced by half as compared with that in the prior art, thus avoiding occurrence of magnetic saturation. Accordingly, when a high performance magnet374is used, its performance can be fully achieved. That is, a high performance of the magnet374can be achieved without changing the size of the outer rotor type motor30.

The above construction may not be limited by the above-described example and various changes may be made without departing from the scope of the invention. For example, the above example shows that the number of poles of the permanent magnet374of the rotor37is set to eight, and the number of slots of the stator32is set to six. However, it is also possible to change the number of poles and the number of slots as needed, while the ratio of the number of poles of the permanent magnet of the rotor to the number of slots of the stator being set to 4 to 3. For example, it is also possible to set the number of poles of the permanent magnet374of the rotor37to twelve, and to set the number of slots of the stator32to nine. Further, it is also possible to set the number of poles of the permanent magnet374of the rotor37to sixteen, and to set the number of slots of the stator32to twelve.

As a result, a high performance of the motor30can be achieved without increasing the diameter of the motor of the electric power tool.

Further, the above example shows that the construction is applied to a portable circular saw, but the construction may be applied to another type of electric power tools other than a portable circular saw, for example an electric sander.