Abstract:
An overvoltage protective device for DC motor is disclosed. The device includes a voltage-dividing circuit and a control unit, and is electrically connected to a DC motor in parallel. The voltage-dividing circuit has one end thereof electrically connected to an input voltage of the DC motor, and the other end thereof grounded. The control unit is electrically connected to a part of the voltage-dividing circuit for accessing a voltage level thereof and thereby driving the DC motor.

Description:
BACKGROUND OF THE INVENTION 
   (a) Field of the Invention 
   The invention relates to an over-voltage protective device, and more particularly, to an over-voltage protective device for direct-current motor. 
   (b) Description of the Related Art 
   Direct-current (DC) motors are one of the most commonly applied motors in industrial use. Generally, the amount of current per unit time is raised to enhance rotational speed of DC motor for achieving expected operation efficiency. However, since DC motors are considered as one kind of inductive loads, the input voltages of DC motors are likely to be excessive as the amount of current per unit time is raised, and DC motor coils are prone to be damaged. 
   SUMMARY OF THE INVENTION 
   To overcome the problems of damaging the coils by excessive current of prior DC motors, the invention provides an overvoltage protective device electrically connected to a DC motor to avoid excessive operating voltages of the DC motor. 
   The object the invention is to provide an overvoltage protective device of DC motor, wherein the overvoltage protective device accesses a part of an input voltage of a DC motor for determining whether the part of the input voltage exceeds a default value. If yes, the overvoltage protective device outputs an overvoltage interrupt signal to stop operations of the DC motor. 
   An overvoltage protective device of DC motor according to an embodiment of the invention is in parallel connection with a DC motor, and includes a voltage-dividing circuit and a control unit. The voltage-dividing circuit has one end thereof electrically connected to an input voltage of the DC motor, and the other end thereof connected to ground. The micro controller is in parallel connection with a part of the voltage-dividing circuit to access a voltage level of the part and to control start of the DC motor. 
   An overvoltage protective device of DC motor according to another embodiment of the invention includes a first voltage-dividing circuit with one end thereof electrically connected to an input voltage of a DC motor, and the other end thereof connected to ground; a second voltage-dividing circuit with one end thereof electrically connected to a reference voltage, and the other end thereof connected to ground; a control unit for controlling operations of the DC motor; and an operation amplifier having a non-inverted input end thereof electrically connected to the first voltage-dividing circuit, an inverted input end thereof electrically connected to the second voltage-dividing circuit, and an output end thereof electrically connected to the micro controller. When the input voltage of the DC motor is larger than a rated voltage of the DC motor, the operation amplifier outputs an overvoltage interrupt signal to the micro controller, and the micro controller then stops controlling the start of the DC motor. 
   The invention is excellent for that the DC motor is prevented from damages caused by overvoltage, and the DC motor is ensured to operate only when the input voltage does not go beyond a nominal value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic view illustrating circuit arrangement of the overvoltage protective device for DC motor according to one embodiment of the invention. 
       FIG. 2  shows a schematic view illustrating circuit arrangement of the overvoltage protective device for DC motor according to one application of the invention. 
       FIG. 3  shows a schematic view illustrating circuit arrangement of the overvoltage protective device for DC motor according to another embodiment the invention. 
       FIG. 4  shows a schematic view illustrating circuit arrangement of the overvoltage protective device for DC motor according to another application of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , an overvoltage protective device  11  according to a first embodiment of the invention is electrically connected with a DC motor  12  in parallel, and has the same input voltage Vin as the DC motor  12 . In this embodiment, a circuit of the overvoltage protective device  11  includes a voltage-dividing circuit having two resistors  111  and  112 , and a control unit  113  for controlling driving status of the DC motor. With regard to circuit arrangement, the two resistors  111  and  112  are in series connection with one end of the resistor  112  connected to ground, and the control unit  113  and the resistor  112  are in parallel connection. Here, R 1  and R 2  are used to represent resistances of the resistors  111  and  112 , respectively. In operation, the overvoltage protection device  11  acquires two voltage levels from the resistors  111  and  112 , i.e., terminal voltages V 1  and V 2  of the two resistors  111  and  112 , and the control unit  113  accesses the terminal voltage V 2  of the resistor  112  to make a comparison with a predetermined reference voltage in the control unit  113 . When the accessed terminal voltage V 2  is lower than the reference voltage, the input voltage Vin does not exceed a rated voltage of the DC motor  12 , and the control unit  113  continues driving the DC motor  12  for maintaining operations of the DC motor  12 . To the contrary, when the accessed voltage V 2  is larger than the reference voltage, the input voltage Vin exceeds the rated voltage of the DC motor  12 , and the control unit  113  stops driving the DC motor  12  to cease operations of the DC motor  12 , thereby preventing the DC motor  12  from damages caused by overvoltage operations. 
   For instance, when the rated voltage of the DC motor  12  is 12V, the reference voltage in the control unit  113  can be set to 5V depending on requirements, with the resistances R 1  and R 2  of the resistors  111  and  112  being 7Ω and 5Ω, respectively. Thus, when the terminal voltage V 2  accessed by the control unit  113  is larger than 5V, the input voltage V in  of the DC motor  12  exceeds 12V and the control unit  113  then automatically stops driving the DC motor  12  using, for example, embedded programs that automatically stop driving the DC motor  12 . 
   Referring to  FIG. 2 , the overvoltage protective device  11  according to the first embodiment of the invention is in parallel connection with a DC motor  22  having four power switches  221 ,  222 ,  223  and  224 . In addition, a micro control unit (MCU) driver  113   a  is adopted as the control unit  113 . In this example, four output terminals D 1 , D 2 , D 3  and D 4  of the MCU  113  control the four power switches  221 ,  222 ,  223  and  224  of the DC motor  22 , respectively. In addition, the four power switches can be transistors. 
   Referring to  FIG. 3 , an overvoltage protective device  21  according to a second embodiment of the invention is electrically connected to a DC motor  32  in parallel, and has the same input voltage V in  as the DC motor  32 . In this embodiment, the overvoltage protective device  21  includes four resistors  211 ,  212 ,  213  and  214 , and a comparator  215 . The comparator  215  is a type of operation amplifier. With respect to circuit arrangement, the two resistors  211  and  212  are in series connection to form a voltage-dividing circuit, and have resistances of R 1  and R 2 , respectively, while the other two resistors  213  and  214  are in series connection to form another voltage-dividing circuit, and have resistances of R 3  and R 4 , respectively. The comparator  215  operates in linear region to take a divided voltage V 2  of the resistor  212  for a non-inverted input end voltage V p , and take a divided voltage V 4  of the resistor  214  for an inverted input end voltage V n . The inverted input end voltage V n  of the comparator  215  is mainly decided by a reference voltage V ref  and resistances of the resistors  213  and  214 . Therefore, we can select appropriate resistors  213  and  214  and the reference voltage V ref  to determine the inverted input end voltage V n  and to regard it as a default value. In this embodiment, the comparator  215  compares the non-inverted input end voltage V p  and inverted input end voltage V n . When V p  is larger than V n , the input voltage V in  exceeds the rated voltage of the DC motor  32  and the comparator  215  outputs an overvoltage interrupt signal to a control unit  216 , to cease the micro controller  216  from driving the DC motor  32 . 
   For example, when the rated voltage of the DC motor  32  is 12V, we can set the reference voltage V ref  to 12V and select resistors  211 ,  212 ,  213  and  214  having resistances of R 2  and R 3  of 5Ω and R 1  and R 4  of 7Ω. Thus, the inverted input end voltage V n  is 5V. When the non-inverted input end voltage V p  accessed by the comparator  215  is larger than 5V, the input voltage V in  of the DC motor  32  exceeds 12V. Meanwhile, the comparator  215  immediately outputs an overvoltage interrupt signal to the micro controller  216  to stop the micro controller  216  from driving the DC motor  32 , thereby preventing the DC motor  32  from damages caused by overvoltage operations. 
   Referring to  FIG. 4 , an overvoltage protective device  21  for DC motor according to the second embodiment of the invention is in parallel connection with a DC motor  42  having two power switches  421  and  422 , and adopts a drive IC (integrated circuit)  216   a  as the micro controller  216 . In this example, two output terminals E 1  and E 2  of the drive IC  216   a  controls and drives the two power switches  412  and  422  of the DC motor  42 , respectively. While over-voltage, the comparator  215  outputs an overvoltage interrupt signal to the drive IC  216   a  to further shut down the power switches  421  and  422 , thereby ceasing operations of the DC motor  42 . 
   It is to be noted that, the aforementioned overvoltage protective devices of DC motor can be applied not only in common axial-flow fans and centrifugal fans, but also in DC driving devices so as to prevent the driving devices from damages. 
   The embodiments and examples according to the invention are fully illustrated as in the above descriptions. For those who are skilled in this art, it is understood that the embodiments according to the invention are illustrative but not limitative. Without departing from the true spirit and scope of the invention, various modifications and changes of the overvoltage protective device for a direct current fan motor shall be included by the appended claims of the invention.