Abstract:
A controlling device for a DC motor speed variation including a power supply circuit, a pair of relays and an interlock circuit for alleviating shock arising from the initiation and termination of a DC motor operation. The controlling device protects the DC motor from catastrophic failure through a sudden variation of the armature current. The controlling device interlocks the direction of the motor rotation while simultaneously providing over current protection for the DC motor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to controlling device for DC motor speed variation, particularly, to a controlling device for DC motor speed variation which can alleviate shock arising from starting and stopping of a DC motor to protect it from rapturing by sudden variation of the armature current, and also able to interlock direction of motor rotation, at the same time, it is able to provide overcurrent protection for the DC motor. 
     2. Description of the Prior Art 
     FIG. 1 is a control circuit diagram for a conventional controlling device for DC motor speed variation. As shown in FIG. 1, the (+) input terminal of a DC motor  16  is connected to two relays  11  and  13 , whereas its (−) input terminal is connected to relays  12  and  14 . The normally open points of the relays  11  and  12  are both connected to a contactor  2  which is further connected to the (+) terminal of a power supply circuit  1 . On the other hand, a common point of the relays  13  and  14  is connected to another contactor  6  which is further connected to the (−) terminal of the power supply circuit  1 . All four relays  11 ,  12 ,  13 ,  14  are controlled by a common control circuit  15  for operation, Meanwhile, the contact points of the relays  11 ,  12 ,  13 ,  14  are opened when they are de-energized. 
     In the above described conventional circuit, if it is intended to drive the DC motor  16  to rotate in normal direction, the control circuit  15  energizes the relays  11  and  14  to close their contact points thereby a current flows from the (+) terminal of the power supply circuit  1  to the (−) terminal thereof by way of the contactor  2 , and the initially normally open point (now closed) of the relay  11 , (+) terminal and (−) terminal of the DC motor  16 , the relay  14 , and the contactor  6  therefore forming a complete electrical circuit for the current to circulate so as to enable DC motor  16  to rotate in normal direction. If it is intended to stop the DC motor  16 , the control circuit  15  actuates the relays  11  and  14  to restore their initial state by opening their contact points so as to interrupt the power supply to the DC motor  16 . However, the DC motor  16  can not stop instantly owing to its inertia. Besides, if it is intended to drive the DC motor  16  in reversed direction, the control circuit  15  energizes the relays  12  and  13  to close their contact points, at this time the current flows from the (+) terminal of the power supply circuit  1  to the (−) terminal of the circuit  1  thereof, by way of the contactor  2 , the initially normally open point (now closed) of the relay  12 , (−) terminal of the DC motor  16 , the (+) terminal of the DC motor  16 , the relay  13 , and the contactor  6  therefore forming a complete electrical circuit for the current to circulate so as to enable the DC motor  16  to rotate in reversed direction. 
     Owing to the fact that the rotational direction of the DC motor  16  is controlled by only on/off operation of the relays  11 ,  12 ,  13  and  14 , instantaneous on/off operation of them for starting or stopping the DC motor  16  is sure to irritate a heavy sparking at their contact points by an transient current produced in the circuit resulting in shortening the lifetime of the relays  11 ,  12 ,  13  and  14 . Further to this, when the DC motor  16  suddenly ceases rotating, the sparking at the open contact points of the relays caused by an induced back emf of the DC motor  16  will likely ruin the contact points. Besides, there is no provision of overload protection means for this system, nor provision of device for constant supervision of current during operation of the DC motor  16  resulting in lack of electric security. 
     Aiming at the above depicted shortcomings, the present invention it to propose a newly developed controlling device for DC motor speed variation capable of operating the DC motor securely. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide controlling device for DC motor speed variation which can alleviate shock arising from starting and stopping of a DC motor to protect it from rapturing by sudden variation of the armature current. 
     It is another object of the present invention to provide controlling device for DC motor speed variation which can offer interlocking means for motor rotational direction. 
     It is still another object of the present invention that this control device is able to provide overcurrent protection for the DC motor. 
     To achieve these and other objects mentioned above, the controlling device for DC motor speed variation essentially comprises a power supply circuit, two relays, and an interlock circuit. Wherein the power supply circuit is for providing a DC power source; each of the two relays includes an exciting coil, a common point, a normally closed point, and a normally open point, before energizing the exciting coil, the common point is communicated with the normally closed point, but is not communicated with the normally open point, after the exciting coil is energized, the common point is communicated with the normally open point, but is not communicated with the normally closed point. The interlock circuit includes two output terminals each of them being connected to the exciting coil of one of the two relays respectively, the exciting coil is energized when the output terminal is energized, but the two output terminals are never energized simultaneously. The two input terminals of the DC motor are respectively connected to the common points of both relays, and the two normally closed points of both relays are connected to one terminal of the power supply circuit, whereas the two normally open points of both relays are connected to the other terminal of the power supply circuit. 
     Further to this, an electronic switch is connected to the power supply circuit, this electronic switch is actuated by a starting control circuit capable of delaying actuating time for the electronic switch. Besides, a current detecting circuit is provided for the power supply circuit for detecting whether the motor is over loaded. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To enable a further understanding of the innovative and technological content of the invention herein, refer to the detailed description of the invention and the accompanying brief description of the drawing appended below in which: 
     FIG. 1 is a control circuit diagram for a conventional controlling device for DC motor speed variation. 
     FIG. 2 is a control circuit diagram for the controlling device for DC motor speed variation according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2, the drawing shows a control circuit diagram for the controlling device for Dc motor speed variation according to the present invention. As shown in FIG. 2, the (+) terminal of a DC motor  17  is connected to a common point  40  of a relay  51 , whereas the (−) terminal of the DC motor  17  is connected to a common point  40  a of another relay  52 . A normally open point  41  of the relay  51  and the normally open point  41   a  of the relay  52  are connected together to a contactor  7  before being connected to the (+) terminal of a power supply circuit  1  via a contactor  7 , on the other hand, a normally closed point  42  of the relay  51  and a normally closed point  42   a  of the relay  52  is connected together to a common contactor  3  which is connected with an electronic switch  4 , and the electronic switch  4  is in series with a current detecting circuit  5 , and then the connection returns to a (−) terminal of the power supply circuit  1 . Meanwhile, each relay  51  or  52  is respectively equipped with an exciting coil  21  or  22 . The normally open point  41  of the relay  51  is communicated with the common point  40  when the exciting coil  21  is energized, whereas the normally closed point  42  of the relay  51  is communicated with the common point  40  when the exciting coil  21  is de-energized. The normally open point  41   a  of the relay  52  is communicated with the common point  40   a  when the exciting coil  22  is energized, whereas the normally closed point  42   a  of the relay  52  is communicated with the common point  40   a  when the exciting coil  22  is de-energized. An interlock circuit  44  has two output terminals  31  and  32  which are respectively connected to the coils  21  and  22  of relays  51  and  52  such that the coils  21  and  22  are energized when the output terminals  31  and  32  are energized. The interlock circuit  44  allows only one output terminal  31  or  32  to be energized at a defined time interval. 
     A starting control circuit  55  includes a voltage detecting terminal  33 , and ON/OFF control terminal  34 , and current feedback terminal  35 . The voltage detecting terminal  33  is connected to the normally closed points  42  and  42   a  of the relays  51  and  52 . The ON/OFF control terminal  34  is for controlling ON/OFF of the electronic switch  4 . The current feedback terminal  35  is connected to the current detecting circuit  5 . 
     In case it is intended to drive the DC motor  17  in normal direction, by energizing the terminal  31  of the interlock circuit  44  so as to energize the coil  21  of the relay  51  and communicate the common point  40  with the normally open point  41 , at the same time, by de-energizing the terminal  32  of the interlock circuit  44  so as to de-energize the coil  22  of the relay  52  and communicate the common point  40   a  with the normally closed point  42   a . In this state, a current flows from the (+) terminal of the power supply circuit  1  to the (+) terminal of the DC motor  17  by way of the contactor  7 , the normally open point  41  of the relay  51 , and the common point  40 , and flows from the (−) terminal of the DC motor  17  to the contactor  3  by way of the common point  40   a  and the normally closed point  42   a  of the relay  52 . At this moment the potential at the contactor  3  is equal to that at the (+) terminal of the power supply circuit  1 . As the voltage detecting terminal  33  of the starting circuit  55  is connected to the contactor  3  so that the voltage detected at the terminal  33  is equal to that of the (+) terminal of the power supply circuit  1 . At this time the current feedback terminal  35  of the starting circuit  55  is kept at the same potential level as that of the (−) terminal of the power supply circuit  1  as there is no current flowing through the current detecting circuit  5  to cause a voltage drop therefore there is a voltage difference between the voltage detecting terminal  33  and the current feedback terminal  35 . As soon as the starting circuit  55  has detected this signal of voltage difference, the ON/OFF control terminal  34  delays actuation of the electronic switch  4  such that current flows through the electronic switch  4  and the current detecting circuit  5  back to the (−) terminal of the power supply circuit  1  thereby completing a circuit for the DC motor  17  to rotate in normal direction. In this version, the operation of the electronic switch  4  is delayed by the ON/OFF control terminal  34  after completion of relay operation so that an immense momentary starting current of the DC motor is alleviated thus protecting contactors of the relays  51  and  52  from rapturing. 
     On the other hand, in case there happens an overload, the inherent resistance contained in the current detecting circuit  5  produces a large voltage drop thereon sufficient to feedback from the motor  17  or the current feedback terminal  35  of the starting circuit  55  to monitor. If the current exceeds the prescribed monitoring value, the electronic switch  4  is switched off by the ON/OFF control terminal  34  so as to interrupt overcurrent thereby protecting the DC motor  17  and the relays  51 ,  52  from rapturing. 
     In case it is intended to stop driving the DC motor  17 , at first an instruction to switch off the electronic switch  4  is delivered from the ON/OFF control terminal  34 , and then the relays  51  and  52  are restored their initial state by the interlock circuit  44  so as to eliminate possibility of rapturing of relays  51  and  52  due to a high back emf induced by the DC motor  17  from its sudden stopping. 
     If it is intended to reverse rotating direction of the DC motor  17 , by energizing the output terminal  32  of the interlock circuit  44  so as to provide an exciting voltage for energizing the exciting coil  22  of the relay  52  and communicate the common point  40   a  with the normally open point  41   a , at the same time de-energizing the output terminal  31  of the interlock circuit  44  so that the exciting coil  21  of the relay  51  can not be energized, and communicate the common point  40  with the normally closed point  42 . At this moment current flows from the (+) terminal of the power supply circuit  1  to the (−) terminal of the DC motor  17  by way of the contactor  7 , normally open point  41   a  of the relay  52 , and the common point  40   a , and again flows from the (+) terminal of the DC motor  17  to the contactor  3  by way of the common point  40  and the normally closed point  42  of the relay  51 . Meanwhile the contactor  3  and the (+) terminal of the power supply circuit  1  are at equal potential level, the voltage detecting terminal  33  of the starting control circuit  55  detects this voltage to delay actuation of the electronic switch  4  through the ON/OFF control terminal  34  so that the electronic switch  4  is in ON state. At this time current flows back to (−) terminal of the power supply circuit  1  via the electronic switch  4  and the current detecting circuit  5  thereby completing an electrical circuit. For the DC motor  17 , current flows into its (−) terminal and flows out of its (+) terminal thereby providing a reverse rotation circuit. 
     Since current monitoring for the DC motor  17  can be carried out during its operation, an immense momentary current caused by sudden starting or stopping of the DC motor  17  can be effectively prevented thereby protecting the DC motor  17  and the relays  51 ,  52  from rapturing. 
     It is understood from the above description that the controlling device of the present invention has several noteworthy advantages which the conventional controllers do not have, those are: 1. alleviating shock arising from abrupt starting and stopping of a DC motor to protect it from rapturing by sudden variation of the armature current; 2. providing interlocking means for motor rotational direction; 3. providing overcurrent protection for the DC motor. 
     Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.