Electric motor control circuit

Abstract

An electric motor control circuit that prevents lowering of a motor's rotation speed when a load is applied in the case where a rotation speed is adjusted within a range that doesn't exceed a predetermined upper limit of rotation speed. When a base current of a transistor Q1 starts to flow in response to operation of a trigger switch 3, transistor Q1 turns on and a setting signal output from a switch S2 which sets the upper limit rotation of a motor M is bypassed via a bypass way 22 by transistor Q1. Thus, a signal level of the setting signal inputted to an operational amplifier 7 is changed, and output timing of a pulse signal output from a phase control circuit 5 to a gate pulse output circuit 9 is changed and a conducting angle of a triac Q2 is controlled. Because the trigger switch 3 does not adjust an output level of operational amplifier 7, the rotation speed of motor M does not decrease when the load is applied.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of this invention will now be described in detail with reference to the accompanying drawings.

FIG. 1 is an electric circuit diagram showing an electric motor control circuit according to a first embodiment of the invention. An electric motor control circuit of an interchange commutator motor (hereafter referred as a motor), used for an electromotive tool will be explained as a representative.

The electric motor control circuit 1 has a main switch S1. A triac Q2 is a semiconductor control element which controls an interchange voltage supplied from an interchange power supply 20. An integrated circuit IC 21 provides control of a motor M including: rotation speed, soft start and automatic re-trigger. A tachometer-generator TG detects the rotation speed of motor M and outputs a signal having a frequency representing the rotation speed. A dial switch S2 provides a user with the ability to set an upper limit of the motor's rotation speed. Dial switch S2 is part of a so-called "upper limit rotation speed setting means." A trigger switch S3 allows the user to adjust the rotation speed within an allowable range, bounded by the upper limit. Trigger switch S3 is part of a so-called "rotation speed adjustment means".

The triac Q2 is connected in parallel with a series circuit including a movement stability capacitor C10 and a resistor R10. In this embodiment, IC 21 can be a U209B3- (FP), U211B2, or U211B3 of the German AEGTELEFUNKEN Co. or an equivalent thereto.

The IC 21 has a power supply stabilization circuit 14 which supplies power to IC 21; a reference voltage generation circuit 2; a voltage detection circuit 3 which detects the voltage applied to motor M; an electric current detection circuit 4 which detects a current flowing through motor M; a gate pulse output circuit 9 which outputs a gate pulse signal to triac Q2; a phase control circuit 5, serving as an output timing control means which controls an output timing of the gate pulse signal output from the gate pulse output circuit 9 to control a phase of the voltage applied to motor M; a frequency/voltage converter 6 which inputs an signal output from tachometer-generator TG and converts the signal into a signal having a voltage representing the frequency of the signal output from tachometer-generator TG; an operational amplifier 7; a soft start circuit 8; an automatic re-trigger circuit 10 which corrects any mis-trigger; an electric current limit circuit 11; a power supply watch circuit 12 which watches a movement condition of the power supply stabilization circuit 14; and a tachometer-generator watch circuit 13 which watches a movement condition of the tachometer-generator TG.

The electric current detection circuit 4 prevents an output of the gate pulse signal to the gate of the triac Q2 from the gate pulse output circuit 9 until electric current passes through zeros of the interchange, because the load of triac Q2 is inductive and the electric current is delayed with respect to voltage. An output E3 of frequency/voltage converter 6 is supplied to soft start circuit 8 and controls the electric charge of the capacitor C5 connected to a terminal P13. Soft start circuit 8 prevents rapidly starting rotation when trigger switch S3 is pulled. The electric current limit circuit 11 detects the voltage across both ends of resistor R11 when load electric currents increased drastically, and pulls electric current from the terminal P12 to decrease a voltage U12 so as to decrease the voltage applied to motor M rapidly, after a delay time determined by the values of a resistor R13 and a capacitor C12.

Terminals P1, P17, P6, P14 of IC 21 are connected to resistors R2, R3, R4, R12, respectively. Terminal P9 is connected to a capacitor C9. A terminal P15 is connected in series to a parallel circuit including resistor R13 and capacitor C12. Terminal P12 is connected to a parallel circuit. The parallel circuit includes a series circuit of capacitor C7 and a resistor R7 in parallel with capacitor C4. A terminal P8 is connected to a parallel circuit including resistor RS and a capacitor C8.

Dial switch S2 is a variable resistor which changes the resistance of resistor R5 by turning a dial-shaped member (not illustrated). An output of the dial switch is connected to one input terminal of the operational amplifier 7 via a terminal P11. A bypass 22 is connected between terminal P11 and dial switch S2. A capacitor C3 is connected to bypass way 22. An emitter of transistor Q1 is connected to bypass way 22, and a base of the transistor Q1 is coupled to trigger switch S3. An anode of a diode D1 is connected between trigger switch S3 and the base of transistor Q1. A cathode of diode D1 is connected to a time constant circuit 23. Time constant circuit 23 includes, in series with diode D1, a parallel circuit of resistor R14 and capacitor C13. A resistor R15 couples the base of Q1 with the wiper of switch S3. Switch S3 has a trigger (not shown) which is installed in an electromotive tool, to allow a user to change the resistance of variable resistor R17 based on trigger pull. A voltage is applied across resistor R17 that is the same as that which is applied across resistor R5 of dial switch S2.

The operation of electric motor control circuit 1 will now be explained. When a person using the electromotive tool sets up the requested upper limit of rotation speed by operating dial switch S2 and turns on the main switch S1, the interchange electric current supplied from interchange power supply 20 is half-wave rectified by a capacitor C1, a resistor R1 and a diode D. A direct current voltage is supplied to the power supply stabilization circuit 14 by the half-wave rectified electric current. A reference voltage is supplied to terminal P16 of the reference voltage generation circuit 2. The voltage detection circuit 3 inputs a sawtooth signal into to IC 21. This sawtooth signal has an inclination determined by a time constant of resistor R4 and capacitor C2 and is synchronized with the interchange power supply 20.

When the trigger is hardly being pulled, because the base voltage of the transistor Q1 is lower than the setting signal output to the operational amplifier 7 from the dial switch S2, the setting signal is bypassed into bypass way 22. And the bypassed electric current flows through the emitter of transistor Q1. Therefore, the signal level of the setting signal is lowered.

The rotation speed of the motor M is detected by tachometer-generator TG. The detection signal, corresponding to rotation speed, is input to frequency/voltage converter 6 from the tachometer-generator TG through terminal P8. The input detection signal is converted into a signal having a voltage corresponding to the frequency of the signal from tachometer-generator TG by frequency/voltage converter 6. The voltage U10 of this converted signal is provided to operational amplifier 7. A voltage U11, set by dial switch S2, is provided to operational amplifier 7. Operational amplifier 7 compares voltage U10 and voltage U11, and outputs a voltage U12 indicative of the comparative result. Voltage U12 is compared with the voltage of the above mentioned sawtooth signal by phase control circuit 5, in the agreement point of both voltages, gate pulse signal is output to the triac Q2 from the gate pulse output circuit 9. The triac Q2 turns on to rotate motor M.

Then, when the trigger is further pulled, the base voltage of the transistor Q1 increases gradually and the electric current bypassed by bypass way 22 decreases. The voltages U11 of switch S2 increases. This causes a change in the voltage output voltage U12 of the operational amplifier 7, the agreement point of the voltage and the above sawtooth signal is changed, and a gate pulse signal is output to the triac Q2 from the gate pulse output circuit 9 in the changed agreement point, the conducting angle of triac Q2 changes, and the rotation speed of motor M rises.

In other words, the rotation speed of the motor M is fed back to IC 21, motor M rotates at the rotation speed adjusted by trigger switch S3, i.e., by the rotation speed corresponding the stroke of which the trigger is pulled.

Then, when the pulled stroke of the trigger switch S3 has reached a certain length, transistor Q1 turns off, and the setting signal is not bypassed by transistor Q1. Therefore, the rotation speed of the motor M is decided by the signal set by dial switch S2.

FIGS. 2(A), 2(B) show the results of operating electric motor control circuit 1 according to the first embodiment. FIG. 2(A) is a graph showing the relation between a rotation speed N of the motor M and an electric current I flowing through motor M. FIG. 2(B) is a graph showing the relation between the rotation speed N of the motor M and the stroke L of the trigger.

As shown in FIGS. 2(A), 2(B), it is understood that, at each upper limit rotation speed set up by the dial switch S2, the rotation speed of the motor M becomes constant when the stroke of the trigger exceeds fixed values. For example, until the stroke of the trigger becomes L4, the rotation speed N of the motor X rises in proportion to the stroke L, when the stroke L of the trigger exceeds L4, the rotation speed N of motor M becomes constant.

Trigger switch S3 controls the voltage of the setting signal output from switch S2 which is an input to operational amplifier 7, but it does not alone control the output of operational amplifier 7. Thus, a function which maintains the rotation speed of the motor M at the adjusted rotation speed of the motor M by operational amplifier 7 and phase control circuit 5 isn't lost.

Even if a load is applied to motor M, the rotation speed of the motor M does not decrease. Especially, when control circuit 1 of the first embodiment is utilized in an electromotive tool such as a sander or a polisher, because the rotation of the motor doesn't fall even if a pad for grinding is pressed down to a grinding side with a certain power in grinding work, working efficiency can be improved.

In the case where the base voltage of the transistor Q1 increases suddenly when the trigger is pulled, the voltage U11 increases, a big torque suddenly occurs in the motor M and the electromotive tool shakes. The electric motor control circuit 1 however can gradually increase the base voltages by time constant circuit 23 and can slowly raise the rotation speed of motor M when the trigger is pulled.

A electric motor control circuit according to a second embodiment of the present invention will now be explained with reference to FIG. 3 showing an electric circuit diagram thereof.

The electric motor control circuit of the second embodiment can linearly increase the rotation speed of a motor M in proportion to the stroke of the trigger. In this second embodiment, shown in FIG. 3, switch S3 is wired differently than in the first embodiment. In this embodiment, one end of trigger switch S3 is connected to bypass way 22 rather than to the top of switch S2 as in the first embodiment. Switches S2 and S3 are arranged such that by operating trigger switch S3, a level of a setting signal output from dial switch S2 can be adjusted from the minimum value to the maximum value.

FIGS. 5(A), 5(B) show the result of operating this second embodiment of the invention. FIG. 5(A) is a graph showing the relation between a rotation speed N of the motor M and an electric current I flowing through motor M. FIG. 5(B) is a graph showing the relation between the rotation speed N of the motor M and a stroke L of which the trigger is pulled.

As shown in FIG. 5(B), the rotation speed N of the motor M is linearly increased in proportion to the stroke L which the trigger is pulled within the range that does not exceed the upper limit rotation number. In other words, the rotation speed of the motor M can be linearly varied corresponding to the stroke of the trigger. Therefore, the entire trigger stroke can be fully used.

This is especially useful in certain works such as in grinding using a tool such as a sander and a polisher. The finishing condition of the grinding side is rather delicately determined by the rotation speed of the pad for grinding. Therefore, a craftsman wants to change the rotation speed of the motor often. The electric motor control circuit 1 can freely control the rotation speed of the motor M by the operation of the trigger, thereby satisfying this requirement.

An electric motor control circuit according to a third embodiment of this invention will now be explained with referring to FIG. 4.

The electric motor control circuit of the third embodiment includes a switch arrangement that allows it to operate like either the first embodiment (FIG. 1) or the second embodiment (FIG. 3). As shown in FIG. 4, a switch S4 is provided at one end of trigger switch S3. Switch S4 is connected to allow the top of switch S3 to connect with either terminal P19 or terminal P20. Contact point P19 is connected to bypass way 24 which is connected to bypass way 22. terminal P20 is connected to an electric way 25 which applies a voltage from an interchange power supply 20 to dial switch S2.

When switch S4 is connected to terminal P20, the electric motor control circuit 1 looks like that of the first embodiment. The voltage applied across trigger switch S3 is the same as the voltage applied across dial switch S2. Whereas, when switch S4 connects with terminal P19, circuit 1 looks like that of the second embodiment. The voltage applied across trigger switch S3 is controlled by dial switch S2.

As mentioned above, in the electric motor control circuit of the third embodiment, by operating switch S4, the operating characteristics shown in either FIG. 3(B) or FIG. 5(B) can be chosen.

If the electric motor control circuit of the third embodiment is used in an electromotive tool such as a sander and a polisher, the operator has the ability to switch operating modes at will. During a coarse grinding stage switch S4 can be set to terminal P20 and grinding can be carried out at a constant speed of rotation with the trigger fully pulled.

Whereas during a finishing stage requiring delicate adjustment of rotation speed, switch S4 can be made to terminal P19, allowing fine adjustment of rotation speed based on trigger pull.

Using the third embodiment, if it is not desirable to allow mode switching of the tool, switch S4 can be implemented utilizing a dip-switch that is set at the time the tool is manufactured so as to allow for operation in only one of the modes depending upon the intended use of the tool.

In each embodiment, circuit 23 is connected to the base of the transistor Q1. Circuit 23 however may be omitted for the purpose of preventing the decline of the rotation speed when the load is applied. Further, in each above-described embodiment, the electric motor control circuit applied to the electromotive tool is explained only as a representative example. The electric motor control circuit of this invention also can be applied to a machine tool as well, or to other tools.

Although the invention has been disclosed in the context of a certain preferred embodiments, it will be understood that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments of the invention. Thus, it is intended that the scope of the invention should not be limited by the disclosed embodiments, but should be determined by reference to the claims that follow.

Claims

1. A control circuit for controlling an electric motor comprising:

rotational speed detection means for detecting a rotational speed of the electric motor and producing a detected speed signal in response thereto;

pulse signal generating means for providing a pulse signal;

a semiconductor control element having (i) a first input for receiving the pulse signal, (ii) a second input for receiving a speed comparison signal, and (iii) an output for providing a rotational speed adjustment signal to the electric motor based upon the speed comparison signal and the pulse signal, the rotational speed adjustment signal for maintaining the rotational speed of the motor at a predetermined level;

rotational speed selection means for receiving a selected speed value as an input and producing a selected speed signal in response thereto;

upper limit speed setting means for (i) selecting an upper limit to the rotational speed, and (ii) producing a setting signal as an output, the setting signal being representative of the upper limit and responsive to the selected speed signal;

wherein the rotational speed selection means adjusts a signal level of the setting signal output from the upper limit speed setting means; and

comparison means for comparing the detected speed signal with the setting signal and producing the speed comparison signal as an output thereby.

2. An electric motor control circuit according to claim 1, wherein the rotational speed selection means comprises:

a variable resistor; and

a transistor having a base connected to the variable resistor, the transistor controlling the setting signal.

3. An electric motor control circuit according to claim 2, further comprising switching means for adjusting a voltage applied to the variable resistor, the voltage applied to the variable resistor being equivalent to a voltage across a variable resistor of the upper limit speed setting means.

4. An electric motor control circuit according to claim 2, further comprising a delay circuit for delaying an increase in a base voltage of the transistor.

5. An electric motor control circuit according to claim 4, further comprising switching means for adjusting a voltage applied to the variable resistor, the voltage applied to the variable resistor being equivalent to a voltage across a variable resistor of the upper limit speed setting means.

6. A control circuit for controlling an electric motor powered by an alternating current (AC) power source comprising:

a voltage detector coupled to the AC power source, said voltage detector generating a wave signal synchronized with the AC power source;

a speed sensor detecting a rotational speed of the electric motor and generating a voltage signal in response thereto;

a variable voltage source;

an operational amplifier having a first input coupled to said speed sensor, a second input coupled to said variable voltage source, said operational amplifier generating a speed comparison signal, and an output;

a driving circuit having a first input coupled to said voltage detector, a second input coupled to the output of said operational amplifier, and an output, said driving circuit generating a pulse signal at the output based upon said speed comparison signal, said pulse signal comprising a rotational speed adjustment signal for maintaining the rotational speed of the motor at a predetermined level; and

a semiconductor control element having a gate coupled to the output of said driving circuit and a conduction path coupled in series with the electric motor and the AC power source, said gate receiving said pulse signal such that said semiconductor control element maintains the rotational speed of the motor at a predetermined level.

7. The control circuit of claim 6, said voltage detector generating a sawtooth wave signal synchronized with the AC power source.

8. The control circuit of claim 6, said speed sensor comprising:

a tachometer coupled to the electric motor, said tachometer sensing the rotational speed of the electric motor and generating a frequency signal in response thereto; and

a signal converter coupled to said tachometer and generating the voltage signal in response to the frequency signal.

9. The control circuit of claim 6, said variable voltage source comprising:

an upper limit voltage setting element setting an upper limit voltage level; and

a voltage adjusting element adjusting an output voltage of said variable voltage source within a range up to the upper limit voltage level.

10. The control circuit of claim 6, said driving circuit comprising:

a comparison circuit comparing an output signal of said operational amplifier and the wave signal of said voltage detector and generating a phase signal; and

a pulse circuit generating the pulse signal in response to the phase signal.

11. The control circuit of claim 6, said semiconductor control element driving the electric motor in response to the pulse signal at the gate of said semiconductor control element.

12. A method for controlling an electric motor comprising:

generating a wave signal having a phase shift from an alternating current (AC) power source;

setting a voltage level;

sensing a rotational speed of the electric motor;

generating a speed comparison signal based upon said voltage level and said rotational speed;

generating a voltage signal in response to the voltage level and the speed comparison signal;

generating a pulse signal in response to the voltage signal and the wave signal, said pulse signal comprising a rotational speed adjustment signal for maintaining the rotational speed of the motor at a predetermined level; and

driving the electric motor by transmitting the pulse signal to a gate of a semiconductor switching element coupled in series with the electric motor and the AC power source, said gate receiving said pulse signal such that said semiconductor control element maintains the rotational speed of the electric motor at a predetermined level.

13. The method as claimed in claim 12, the step of generating a wave signal further comprising generating a sawtooth wave signal.

14. The method as claimed in claim 12, the step of setting a voltage level further comprising:

setting an upper limit voltage level to select a maximum speed of the electric motor; and

adjusting the voltage level within a range up to the upper limit voltage level to adjust the rotational speed of the electric motor up to the maximum speed.

15. The method as claimed in claim 12, the step of generating said speed control signal further comprising:

generating a frequency signal having a frequency determined in accordance with the rotational speed of the electric motor; and

converting the frequency signal to a voltage signal as the feedback signal.

16. The method as claimed in claim 12, the step of generating a pulse signal further comprising:

generating a phase signal by comparing the voltage signal and the wave signal; and

generating the pulse signal in response to the phase signal.

17. The method as claimed in claim 12, the step of generating a pulse signal further comprising:

increasing a stroke length of the pulse signal in response to the voltage level set at a higher level; and

decreasing the stroke length of the pulse signal in response to the voltage level set at a lower level.

18. The method as claimed in claim 12, the step of generating a pulse signal further comprising:

increasing a stroke length of the pulse signal in response to a decreasing rotational speed of the electric motor; and

decreasing the stroke length of the pulse signal in response to an increasing rotational speed of the electric motor.

19. The method as claimed in claim 12, the step of driving the electric motor further comprising establishing a conduction path under the gate of the semiconductor switching element in response to the pulse signal.

20. An electric motor system powered by an alternating current (AC) power source, comprising:

an electric motor;

a voltage detector coupled the AC power source and generating a sawtooth wave signal synchronized with the AC power source;

a speed sensor detecting a rotational speed of the electric motor and generating voltage signal in response thereto;

a voltage source having a first element setting an upper limit of a voltage level and a second element adjusting the voltage level up the upper limit;

a comparison circuit having a first input coupled to said speed sensor, a second input coupled to said voltage source, and an output, said output of said comparison circuit comprising a speed comparison signal;

a driving circuit having a first input coupled to said voltage detector, a second input coupled to the output of said comparison circuit and receiving said speed comparison signal, and an output, said driving circuit generating a pulse signal at the output, said pulse signal comprising a rotational speed adjustment signal for maintaining the rotational speed of the motor at a predetermined level; and

a semiconductor control element having a gate coupled to the output of said driving circuit and a conduction path coupled in series with said electric motor and the AC power source, said gate receiving said pulse signal such that said semiconductor control element maintains the rotational speed of the electric motor at a predetermined level.

21. The electric motor system of claim 20, wherein said semiconductor control element drives the electric motor in response to the pulse signal at the gate of said semiconductor control element.

22. The electric motor system of claim 20, wherein said speed sensor comprises:

a tachometer coupled to said electric motor, said tachometer sensing the rotational speed of said electric motor and generating a frequency signal having a frequency determined in accordance with the rotational speed of said electric motor; and

a signal converter coupled to said tachometer and converting the voltage signal to the frequency signal.

23. The electric motor system of claim 20, wherein said driving circuit comprises:

a phase control circuit comparing an output signal of said comparison circuit and the wave signal of said voltage detector and generating a phase signal; and

a pulse circuit coupled to said phase control circuit, said pulse circuit generating the pulse signal in response to the phase signal.

24. The electric motor system of claim 20, wherein:

said speed sensor and said comparison circuit generate a speed comparison signal; and

said driving circuit and said semiconductor control element maintain a stable rotational speed of said electric motor under various load conditions in response to an output voltage of said voltage source and the feedback signal.

25. An apparatus for controlling an electric motor comprising:

means for generating a wave signal having a phase shift from an alternating current (AC) power source;

means for setting a voltage level;

means for sensing a rotational speed of the electric motor;

means for generating a feedback signal in response to said rotational speed of the electric motor and said voltage level;

means for generating a speed comparison signal in response to the voltage level and the feedback signal;

means for generating a pulse signal in response to the speed comparison signal and the wave signal, said pulse signal comprising a rotational adjustment signal for maintaining the rotational speed of the motor at a predetermined level; and

means for driving the electric motor by transmitting the pulse signal to a gate of a semiconductor switching element coupled in series with the electric motor and the AC power source, said gate receiving said pulse signal such that said semiconductor control element maintains the rotational speed of the electric motor at a predetermined level.

26. The apparatus of claim 25 wherein said means for generating a wave signal further comprises a means for generating a sawtooth wave signal.

27. The apparatus of claim 25 wherein said means for setting voltage level further comprises:

means for setting an upper limit voltage level to select a maximum speed of the electric motor; and means for adjusting the voltage level within a range up to the upper limit voltage level to adjust the rotational speed of the electric motor up to the maximum speed.

28. The apparatus of claim 25 wherein said means for generating a feedback signal further comprises:

means for generating a frequency signal having a frequency determined in accordance with the rotational speed of the electric motor; and

means for converting the frequency signal to a voltage signal as the feedback signal.

29. The apparatus of claim 25 wherein said means for generating a pulse signal further comprises:

means for generating a phase signal by comparing the speed comparison signal and the wave signal; and

means for generating the pulse signal in response to the phase signal.

30. The apparatus of claim 25 wherein said means for generating a pulse signal further comprises:

means for increasing a stroke length of the pulse signal in response to the voltage level set at a higher level; and

means for decreasing the stroke length of the pulse signal in response to the voltage level set at a lower level.

31. The apparatus of claim 25 wherein said means for generating a pulse signal further comprises:

means for increasing a stroke length of the pulse signal in response to a decreasing rotational speed of the electric motor; and

means for decreasing the stroke length of the pulse signal in response to an increasing rotational speed of the electric motor.

32. The apparatus of claim 25 wherein said means for driving the electric motor further including establishing a conduction path under the gate of the semiconductor switching element in response to the pulse signal.