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 Q 1 starts to flow in response to operation of a trigger switch 3, transistor Q 1 turns on and a setting signal output from a switch S 2 which sets the upper limit rotation of a motor M is bypassed via a bypass way 22 by transistor Q 1. 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 Q 2 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.

What is claimed is:  
     
       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.