JPS6232787Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a safety device for a motor-driven power tool that controls the motor rotation speed using a switching element such as a thyristor or triax.

Conventionally, power tools, for example, as shown in Fig. 1,
In the disk grinder 7, a grindstone 3 is rotatably attached to the tip of the motor housing 1 via a gear box 2, and a main switch 5 and a circuit block 6 are built into the handle 4 at the rear end of the motor housing 1. A switching element 12 incorporated in the circuit block 6 is controlled by a motor 8 rotation speed detection signal from a magnet ring 11 attached to the armature shaft 9 via a cylindrical body 10 and a magnetic sensitive element (not shown). However, in this case, the capacity of the switching element 12 is large and therefore a considerable space for cooling must be taken, and the external shape of the circuit block 6 including the switching element 12 is, for example, the outer diameter of the motor housing 1. Compared to 100 mm, the longitudinal dimension of the disc grinder 7, L in FIG. 1, reaches 50 to 60 mm, which causes the conventional disc grinder 7 to have the following drawbacks. That is, (1) The power tool as a whole becomes very long and large, making it extremely difficult to work, especially in narrow spaces.

(2) In order to cope with the increase in heat generation during overload, large-capacity heat dissipation fins 13 and large housings 14 that cover them are used, which increases the weight of the tool, making it difficult to use, and requiring the operator to use more energy than necessary. It's giving me fatigue.

(3) The production cost of the power tool as a whole increases significantly because it requires a very expensive large-capacity large heat dissipation fin 13 and a large housing 14 using an expensive mold to house them. .

(4) During overload situations where a large amount of heat is generated, it is necessary to air-cool the motor with a particularly large amount of air, but as a means to prevent local heating, the cooling air inlet has many folds. Since heat dissipation fins with high air friction resistance are specially installed, the air volume decreases and motor burnout often occurs during overload.

In addition, there is a safety device for a power tool that controls the rotation speed of the motor 8 by the switching element 12 in this way, i.e., if the rotation speed of the motor 8 becomes abnormally high, for example, the grindstone 3 of the disc grinder 7 will be damaged and scattered. As a safety device in cases where there is a risk of abnormal rotation, a device is generally used that mechanically or electrically interrupts the gate circuit of the switching element 12 when abnormal rotation is detected. Although it is possible to prevent abnormal rotation of the motor 8 due to a defect, for example, the switching element 1 may be damaged due to heat generation or shock caused by harsh use of the power tool.
It is not possible to prevent abnormal rotation of the motor in the event that the grinder 2 itself becomes uncontrollable or conductivity is broken. For example, in the case of the disk grinder 7 shown in FIG. Exceeding the circumferential speed may cause the grinding wheel 3 to crack and scatter around, causing serious injury to the worker, which is extremely dangerous. , switching element 1
In addition to the case where the rotation detecting circuit 2 breaks down, the same dangerous situation as described above can occur if the abnormal rotation detection circuit becomes defective.

An object of the present invention is to eliminate the above-mentioned drawbacks of the conventional tools by providing a power tool with a safety device that has a simple structure and can reliably prevent abnormal rotation of the motor.

Next, the configuration of an embodiment of the present invention is shown in FIGS. 2 to 7.
This will be explained using figures.

In a disc grinder 20, a grindstone 17 is rotatably attached to the tip of a motor housing 15 via a gear box 16, and a main switch SW1 and a resin-molded control circuit block 19 are built into a handle 18 at a rear end of the motor housing 15. , a switching element connected in series with the motor M1, in this case an insulated type triax TRI1, is attached to the core 21 of the field F1 forming the motor M1 via a mounting bracket 22 as shown in FIG. Alternatively, as shown in Figure 4, transparent insulating vinyl tubes 23 and 2 may be attached directly to the connecting terminals of the triax TRI1.
4 is attached via adhesive.

A rotary ring 27 made of plastic is attached to one end of the armature shaft 25 of the motor M1 via a serration 26, and is opposed to a magnetically sensitive element Q1 embedded in the control circuit block 19 to adjust the rotational speed of the motor M1. Magnet 28 that detects
In addition, the push plate 29 is capable of reciprocating in the radial direction of the rotating ring 27 and the pin 3 is embedded in the push plate 29.
The motor M
When the rotation speed of the motor 1 reaches abnormal rotation, for example, 7000 rpm or more, a centrifugal force greater than the biasing force of the spring 31 acts on the push plate 29, and the push plate 29 is rotated as shown in FIGS. 5a and 6a. Figure 5b from the stop position in the rotating ring 27
Then, the rotary ring 27 is displaced to the outward protruding position shown in FIG. 33 to separate the movable contact 34 from the fixed contact 35 and lock the safety switch 32 in this OFF state via the set spring 36, or the actuator lever 38 of the safety switch 37 shown in FIG. 3
The movable contact 40 is released from the fixed contact 42 through its elastic restoring force by disengaging the movable contact 40 from the claw portion 41 of the lever 38 by rotating it against the counterclockwise biasing force shown in FIG. At the same time, the safety switch 37 is held in this off state.

In the safety switch 37 shown in FIG. 6, the movable contact 40 is pressed against the elastic restoring force of the reset button 43 to make pressure contact with the fixed contact 42, thereby turning the safety switch 37 on. In this on state, the actuator lever 38 is rotated in the counterclockwise direction shown in the figure by the spring 39, thereby moving the movable contact 40 in the on state to the lever 38.
A reset device is attached that engages the claw portion 41 of the safety switch 37 to maintain and return the safety switch 37 to the on state.

Next, FIG. 7 is an electrical circuit diagram of the present embodiment, and shows a main switch SW1, a safety switch 32 or 37, and a full-wave rectifier DS1 connected to the AC power source at the X and Y terminals via a resistor R1. A main switch SW is connected between both ends of the capacitor C1, which smooths the output of
1 and a magnetically sensitive resistor R2, R that is sensitive to a magnet 28 attached to the rotating ring 27 of the motor M1, which is connected to the AC power supply via the triax TRI1.
3, a rotation speed detection circuit 44 consisting of a reference voltage setting resistor R4, R5, and an operational amplifier comparator Q2, and a monomulti stable Q3 whose pulse width is set by a resistor R6 and a capacitor C2. and monomulti stable Q
The output from 3 is smoothed through diode D2, resistor R7, and capacitor C3, and the variable resistor
A smoothing circuit (integrator circuit) 45 that controls the base potential of the transistor Tr1 via VR1, and an output from the smoothing circuit 45 and a Zener diode ZD.
The Zener for trapezoidal waveform generation is connected to the overload current limiting circuit 46 which controls the base potential of the transistor Tr2 by comparing the voltage of the transistor Tr2 with the voltage of the transistor Tr2 by the comparator Q4 of the operational amplifier. Diode ZD2 includes transistor Tr3, capacitor C4, and transistor
The programmable union transistor PUT1 is controlled via the transistors Tr1 and Tr2 and the capacitor C5 via the resistors R9 and R10 connected in parallel with the bias resistor R8 between the base and emitter of Tr3. A phase control reference pulse generation circuit 48 for controlling the light emitting diodes LED1 and LED2 of the photocouplers PC1 and PC2 forming the trigger circuit 47 of the triax TRI1 is connected thereto.

Next, the operation of this embodiment will be explained.

First, in the disk grinder 20 configured as described above, a centrifugal force greater than the biasing force of the spring 31 acts on the push plate 29 when the motor M1 is stopped and normally rotating at a normal speed of, for example, 6000 rpm or less. Therefore, in this stopped and normal rotation state, the push plate 2
9 is a rotating ring 27 shown in FIGS. 5a and 6a.
Inside, the safety switch 32 or 37 is also secured in the switch-on state shown in FIGS. 5a and 6a.

Therefore, when the main switch SW1 is turned on while the safety switch 32 or 37 is on, the potential of the capacitor C5 is zero at that moment, so the transistor Tr3 is turned off and the PUT1 is turned on.
There is no output from PUT1 and the triac TRI1 is not triggered, but as capacitor C5 is charged and its potential becomes higher, the conduction resistance of transistor Tr3 decreases and the charging time of capacitor C4 becomes shorter. The conduction phase angle of the triax TRI1 due to the trigger pulse gradually increases, and as a result, the motor M of the disk grinder 20
Since motor M1 gradually rotates at the same time as the motor M1 is started, the operator is not affected by the reaction caused by the start of the motor M1 and can proceed with the work safely.

When the motor M1 rotates in this manner, an output is generated from the comparator Q2 of the rotation speed detection circuit 44 for each rotation of the armature A1, and an output is generated from the monomulti stable Q3.
As a result, a pulse of a constant width is output every rotation of the
From the capacitor C3 of the smoothing circuit 45, the motor M1
The phase control reference pulse generation circuit 48 generates a voltage corresponding to the rotational speed of the motor M1, and is controlled via the variable resistor VR1 according to the rotational speed of the motor M1 by the conduction resistance of the transistor Tr1 and the bias resistor R8. From PUT1, motor M1 is set based on the specific rotation speed set by variable resistor VR1.
When the rotational speed exceeds a certain rotational speed, the potential of capacitor C3 increases, the conduction resistance of transistor Tr1 decreases, the conduction phase angle of triax TRI1 via PUT1 becomes small, and motor M1
When the number of rotations of motor M1 decreases and the number of rotations of motor M1 falls below a certain number of rotations, the potential of capacitor C3 also decreases, the conduction resistance of transistor Tr1 increases, and the phase angle of conduction of triax TRI1 via PUT1 increases. As a result of the increase in the rotational speed of the motor M1, the rotational speed of the motor M1 of the disc grinder 20 can be stably maintained at a constant rotational speed despite load fluctuations.

When an overload is applied to the grinding wheel 17 while the motor M1 is rotating, and the motor M1 rotation speed falls below the overload rotation speed preset in response to this overload, the Zener diode is As the ZD1 voltage becomes higher and the transistor Tr2 is turned on, the conduction resistance of the transistor Tr3 increases due to a decrease in its bias resistance, and the charging time of the capacitor C4 of the phase control reference pulse generation circuit 48 becomes longer, so that the triac The conduction phase angle becomes small, and the load current of the motor M1 at the time of overload is suppressed to a low level, and burnout of the motor M1 is prevented.

In this way, the disk grinder 20 is operated with stable operating characteristics such as slow start, stable rotation, and overcurrent prevention. In order to maintain sufficient working capacity even during descent, the power supply voltage
In some cases, the rated voltage of power tools is set to about 75V compared to 100V, but in this case, the work capacity of the power tool increases due to the increase in load current during overload, but switching due to the increase in load current may occur. If an element, for example, the triax TRI1 is destroyed, the triac TRI1 becomes conductive regardless of trigger control, and in particular, if the motor M1 rotates under no load in this conductive state, the rotational speed of the motor M1 will greatly exceed the specified rotational speed. However, due to the increase in centrifugal force caused by this increase in rotational speed, the push plate 29 in the rotating ring 27 attached to the armature shaft 25 resists the biasing force of the spring 31, causing the As shown in Fig. 6b, the rotary ring 27 protrudes outward, and the actuator rod 33 of the safety switch 32 or the actuator lever 38 of the safety switch 37 is pushed and the contacts 34, 35 or 40, 42 are turned off. held and motor M
As a result, the power supply to motor M1 is cut off.
The runaway rotation of the grinding wheel 17 is prevented and the motor M1
At the same time, the inertial rotation of the grinding wheel 17 gradually stops without being affected by the reaction of sudden braking, etc., so that the safety of the worker can be sufficiently ensured when this abnormal rotation occurs, and after the safety switch 32 or 37 is activated. The disk grinder 20 can be made usable again by replacing the safety switch 32 or by pressing the reset button 43 of the safety switch 37.

Note that each transistor Tr in this example
1, Tr2, and Tr3 can be replaced with operational amplifiers, and safety switches 32 and 37 can be replaced with the movable contact 3 using a toggle mechanism, in addition to the non-return type and return type shown in the figure.
4, 40 can be reversely moved, and the brake shoe 31 of the copper ring 40 can be lightly contacted with the brake shoe 31 described in the specification and drawings of Utility Application No. 55-151689, which was previously filed by the applicant of the present application. Melting of the metal fuse due to heating when no braking effect is expected or main switch SW1 via temperature sensor
The supply of power to the motor M1 can also be forcibly cut off by turning off the motor M1.

Next, the effects of the present invention will be explained.

The present invention provides a power tool in which a switching element for a phase control circuit that is connected in series with a motor for driving a power tool and adjusts the rotational speed of the motor is attached to the field core of the motor, and the heat of the switching element is dissipated. A main switch, a motor for driving the power tool, a switching element for the phase control circuit, and a switch part of a safety device that opens the main power circuit to stop and hold the motor when the motor reaches an abnormal rotation speed, for example. push plate 29
This power tool is equipped with a safety device in which safety switches 32 and 37 each operated by a safety switch 32 or 37 are connected in series.

As a result, the present invention not only makes the power tool smaller and lighter than its capacity, improving workability using the power tool, but also prevents burnout of the motor and reduces the field core part. Because the air flow is especially large, the cooling effect of the switching element is high, and it is possible to effectively prevent thermal damage to the switching element itself, and even if the switching element is destroyed and becomes conductive, it will prevent runaway rotation of the motor. This has the effect of reliably preventing this by cutting off the power supply, and by gradually stopping the motor, it is possible to reliably maintain the safety of the operator when the motor runs out of control.

[Brief explanation of the drawing]

Fig. 1 is a cutaway front view of a conventional embodiment, Fig. 2 is a cutaway front view of an embodiment of the present invention, Figs. 3 and 4 are perspective views of the field core 21 portion, Figs. 6b is a detailed view of the safety switch 32 portion, FIGS. 6a and 6b are detailed views of the safety switch 37 portion, and FIG. 7 is an electric circuit diagram thereof. 20...disc grinder, 21...field core, 25...armature shaft, 27...
...Rotating ring, 29...Push plate, 32,
37... Safety switch, 44... Rotation speed detection circuit, 45... Smoothing circuit, 46... Load current limiting circuit, 47... Trigger circuit, 48... Phase control reference pulse generation circuit, M1... Motor, TRI1 ... Triack, Q3 ... Monomulti stable,
Tr3...transistor, C4, C5...capacitor, R8...bias resistor.

Claims (1)

[Scope of utility model registration request] In a power tool configured to connect a power tool drive motor in series and adjust the motor's rotation speed, a phase control circuit switching element is attached to the field core of the motor, and the heat of the switching element is dissipated. , the motor for driving the power tool, the switching element for the phase control circuit, and the switching part of the safety device that opens the main power circuit to stop and hold the motor when the motor reaches an abnormal rotation speed are connected in series. A power tool with a safety device characterized by being connected to.