This application claims priority to Japanese patent application serial number 2008-333526, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to electric power tools including a housing body part for housing a blushless DC motor as a driving source, a grip part formed to protrude from a lateral part of the housing body part, a switch part provided in a position allowing a user to engage the switch part by his or her finger while the user holds the grip part, a battery that supplies a power to the blushless DC motor, a three-phase bridge circuit for controlling the blushless DC motor, a switching device constituting a part of the three-phase bridge circuit part, and a capacitor connected in parallel with the three-phase bridge circuit.
2. Description of the Related Art
In many electric power tools such as a driver or a drill for tightening screws or drilling holes, a voltage applied to a motor can be changed in order to control the rotational speed of the motor depending on a pull position of a switch that includes a trigger operation part.
In general, a switching device is controlled by a pulse-width modulation (PWM) to vary the voltage supplied to a motor. When the switching device is driven by this pulse-width modulation, a surge voltage occurs. Further, when the voltage applied to the motor is controlled by the pulse-width modulation, the more the current flows to the switching device, the more the surge voltage increases. Especially, when an on-off control of large current is made by the switching device in order to control the rotational speed of a high-powered motor that may be installed in an electric power tool, a surge voltage that will occur becomes large. In order to absorb this surge voltage, a capacitor is connected in parallel with a three-phase bridge circuit part that includes the switching device.
A switching device whose withstand voltage is high is relatively large in size, and one whose withstand voltage is low is relatively small in size. The usage of a small-sized switching device with low withstand voltage advantageously allows for compact storage of these switching devices, but maximum current is restricted to suppress the surge voltage and a large capacitor with high capacity is required. That is, compactification of the switching device causes the capacitor to become large in size.
In an electric power tool as described in Japanese Laid-Open Patent Publication No. 2003-305667, a capacitor 108 is located in the lower section of a grip part 103 as shown in
In the known electric power tool as described above, a long distance from the electrical circuit board 106, on which the three-phase bridge circuit part is mounted, to the capacitor 108 results increase in length of a lead wire that connects between the electrical circuit board 106 and the capacitor 108. A lead wire can be equivalently represented by a resistance component (R) and an inductance component (L). If the length of the lead wire increases, the resistance component (R) and the inductance component (L) become large in proportion to the length. Especially, if the inductance component (L) becomes large, a surge voltage (e=Ldi/dt), which will be produced when the switching device is operated, will become large.
For this reason, if the lead wire is long and the current applied to the switching device during the pulse-width modulation control is large, this may adversely affect when determining a withstand voltage of the switching device and a capacity of the capacitor connected in parallel with the three-phase bridge circuit part
Further, in the case that a switching device is used with relatively small in size and with low withstand voltage, a current applied to the switching device must be controlled and performance cannot be fully elicited from a high-powered motor that may be installed in an electric power tool. Further, this may adversely affect to increase a battery voltage.
Therefore, there is a need in the art for a power tool that can lower a withstand voltage of a switching device as much as possible and enables a capacity of a capacitor to be decreased as small as possible by suppressing a surge voltage caused by the operation of the switching device.
One aspect according to the present invention includes an electric power tool, in which a capacitor is positioned closer to a motor control circuit than a switch electrically connected to the motor control circuit.
Each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings to provide improved electric power tools. Representative examples of the present invention, which examples utilize many of these additional features and teachings both separately and in conjunction with one another, will now be described in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Moreover, various features of the representative examples and the dependent claims may be combined in ways that are not specifically enumerated in order to provide additional useful embodiments of the present teachings.
In one embodiment, an electric power tool includes a housing body part for housing a brushless DC motor as a driving source, a grip part formed to protrude from a lateral part of the housing body part, a switch part provided in a position allowing a user to engage the switch part by his or her finger while the user holds the grip part, a battery that supplies a power to the brushless DC motor, a three-phase bridge circuit for controlling the blushless DC motor, a switching device constituting a part of the three-phase bridge circuit, and a capacitor connected in parallel with the three-phase bridge circuit. The three-phase bridge circuit is disposed proximate to the brushless DC motor in the housing body part, and the capacitor is disposed at a position proximate to a switch part or located closer to the three-phase bridge circuit than to the switch part. The electric power tool further includes an electric conductor that electrically connects the capacitor and the three-phase bridge circuit, and the three-phase bridge circuit is configured to connect the capacitor and the three-phase bridge circuit at the shortest distance.
With this arrangement, the capacitor is disposed at a position proximate to the switch part or located closer to the three-phase bridge circuit than to the switch part. That is, the capacitor is located in the vicinity of the three-phase bridge circuit. Further, an electric conductor such as a lead wire is configured such that the capacitor and the three-phase circuit are connected at the shortest distance. Thus, the length of the electric conductor such as a lead wire that connects the capacitor and the three-phase bridge circuit can be made as short as possible. Consequently, the inductance component (L) of the electric conductor becomes small and a surge voltage, which is caused by the inductance component (L) when an operation of the switching device having the three-phase bridge circuit part is made, becomes small (e=Ldi/dt). Therefore, the withstand voltage of the switching device becomes small and the capacity of the capacitor becomes relatively small.
Therefore, a withstand voltage of the switching device can be lowered and the capacity of a capacitor can be made relatively small as much as possible by suppressing a surge voltage caused by the operation of the switching device.
The three-phase bridge circuit may be positioned on a rear side of the brushless DC motor and the capacitor is mounted on an electrical circuit board of the switch part or to terminals thereof.
Since the three-phase bridge circuit part is positioned on a rear side of the brushless DC motor in this way, the switching device can be effectively cooled down by a motor cooling fan.
Further, since the capacitor is mounted on the electrical circuit board of the switch part or to the terminals thereof and is located outside the housing body part, the housing body part can be made compact.
The three-phase bridge may be positioned on a rear side of the brushless DC motor and the capacitor may be positioned at the boundary region between the housing body part and the grip part.
Thus, since the capacitor is positioned at the boundary region between the housing body part and the grip part, the lead wire etc. can be made short in comparison with the case where the capacitor is mounted on the electrical circuit board of the switch part.
The three-phase bridge circuit may be located in the rear of the brushless DC motor and the capacitor is mounted on the electrical circuit board on which the three-phase bridge circuit part is mounted.
Thus, the length of the electric conductor between the capacitor and the three-phase bridge circuit part can be minimized.
An electric power tool according to an embodiment of the present invention will be described below with reference to
As shown in
Further, the lower part 15p of the grip part 15 can be opened in a front-back direction with respect to the handgrip part 15h, and a connecting part 16 for a battery pack is provided on the lower side of the lower part 15p for connecting the battery pack (not shown).
Within the housing body part 12, a brushless motor 20, a planetary gear train 24, a spindle 25, a striking power generation mechanism 26, and an anvil 27 are housed coaxially in this order from the rear side of the housing body part 12. A rotative power of the brushless DC motor 20 is transmitted to the spindle 25 via the planetary gear train 24, a rotative power of the spindle 25 is converted into a rotary striking power by the striking power generation mechanism 26, and then the rotary striking power is transmitted to the anvil 27. The anvil 27 is supported by a bearing 12j provided at the front end of the housing body part 12, so that the anvil 27 can rotate about an axis but cannot move in an axial direction. At the front end of the anvil 27, a chuck part 27t is provided, so that a driver bit or a socket bit, and the like (not shown) can be attached to the chuck part 27t.
As shown in
Further, in a rear end part of the stator 23, three magnetic sensors 32 for detecting the positions of magnetic poles of the rotor 22, a three-phase bridge circuit part 45 of a motor drive circuit 40, etc., are mounted on an electrical circuit board 30. Thus, current can be supplied to the drive coils 23c in order by the motor drive circuit 40 to rotate the rotor 22 while the positions of the magnetic poles of the rotor 22 are detected by the magnetic sensors 32.
As shown in
The three-phase bridge circuit part 45 is connected to the power wires 42c in parallel with the capacitor 43c, and three output lines 41 from the three-phase bridge circuit part 45 (hereinafter referred to as power lines 41) are connected to the corresponding windings of the drive coils 23c. For example, field effect transistors (FET) may be used as switching devices 44 of the three-phase bridge part 45.
The control circuit part 46 is constituted by an electronic parts such as a microcomputer, an IC or the like that can operate the switching devices 44 based on an actuating signal from the trigger switch part 18. The control circuit part 46 receives signals from the above-described three magnetic sensors 32, and based on these signals (see an outline arrow in
Further, signals from various switches, such as switches 46a and 46b for switching the tightening speed of fasteners driven by the tool are input to the control circuit part 46.
As shown in
The capacitor 43c is connected by the power wires 42c (at lead wire parts R in
According to the electronic power tool 10 of the present embodiment, the capacitor 43c is located in the vicinity of the switch 18. That is, the capacitor 43c is located relatively close to the three-phase bridge circuit part 45. Further, the electric conductors such as the lead wire parts R are configured to connect between the capacitor 43c and the three-phase bridge circuit part 45 at the shortest distance. Thus, the length of the lead wire parts R (electric conductors) that connect between the capacitor 43c and the three-phase bridge circuit part 45 can be made as short as possible. Consequently, the inductance component (L) of the lead wire parts R (electric conductors) becomes smaller, and a surge voltage that may be caused by the inductance component (L) when the switching device 44 is operated can be lowered. Therefore, the withstand voltage of the switching device 44 can be lowered, and further the capacitance of the capacitor 43c can be relatively small.
Further, since the three-phase bridge circuit part 45 is located on the rear side of the brushless DC motor 20, the switching device 44 can be effectively cooled down by the motor cooling fan 22f. In addition, since the capacitor 43c is mounted on the electrical circuit board 18c of the switch part 18 and is located outside the housing body part 12, the housing body part 12 can be made compact.
The present invention may not be limited by the above-described embodiment and various changes may be made without departing from the scope of the invention. For example, the above embodiment shows that the capacitor 43c is mounted on the electrical circuit board 18c of the switch part 18, but as shown in
Further, as shown in
In the above embodiment, the rotary impact tool (an impact driver) as an example of the electric power tool is driven by the brushless DC motor 20. However, the present invention can be applied to any other power tool, such as an electric screwdriver and an electric drill, having a brushless DC motor.
Number | Date | Country | Kind |
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2008-333526 | Dec 2008 | JP | national |