Abstract:
A control circuit for controlling the excitation current in a compound motor includes a limiting switch connected in series with the shunt field winding of the compound motor. The limiting switch is closed whenever either the speed of the motor is excessive, the armature current is excessive, or there is insufficient back EMF due to a light load.

Description:
This application is a continuation of application Ser. No. 08/087,414, filed on Jul. 8, 1993, which is now abandoned. 
    
    
     SUMMARY OF THE INVENTION 
     The present invention is a control circuit for controlling the excitation current in the shunt field winding of a compound motor by means of a limiting switch, the limiting switch being controlled in response to sensing of an operating state of the motor, and particularly to a control circuit of the above-stated type in which the limiting switch closes when the motor running speed is too high, such that the cumulative excitation type shunt winding of the compound motor is excited while the motor running speed is being reduced. In addition, the action point of the limiting switch can be controlled to change the motor running speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing a preferred compound motor control circuit which controls the shunt field winding excitation by means of a limiting switch. 
     FIG. 2 is a schematic diagram showing a preferred compound motor control circuit which controls the shunt field winding excitation by means of a centrifugal switch. 
     FIG. 3 is a schematic diagram showing a preferred compound motor control circuit which controls the shunt field winding excitation by means of a limiting switch made up of a transistor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A conventional series motor limiting control usually includes a centrifugal switch connected in series between the power supply and the series, as opposed to shunt, motor. The conventional centrifugal switch is arranged to cut off the power supply during an over-speed condition, and to close when the speed is reduced to provide a speed limiting function. In such an arrangement, the cut-off power involves the full power of motor, so that the motor capacity is greatly limited. 
     The present compound motor limiting control circuit, in contrast, involves controlling the shunt field winding, as opposed to the series field winding, by means of the limiting switch, based on the operating state of motor. Specifically, the limiting switch of the present design includes either: (1) a solid state or electro-mechanical switch controlled by a motor speed-sensing device; (2) a motor EMF sensing control; (3) a motor armature current control; or (4) centrifugal type switch sensing control. 
     FIG. 1 is a diagram illustrating the basic principles of the compound motor control circuit of the invention, which includes: 
     a series field winding S101 and an armature A101 mutually connected in series and connected in parallel to the power supply; 
     a limiting switch device SW101 made up of an electro-mechanical or solid state type switch controlled by the motor centrifugal force, or by a central control unit CCU101 with reference to the motor loading current, running speed or armature EMF. The limiting switch may be connected in series with the shunt field winding F101 of the compound motor and then parallel connected to the series field winding S101 and the armature A101 in a long shunt configuration, or directly parallel connected to the armature in a short shunt configuration, the series field winding and shunt field winding both having the same polarity for cumulative excitation; and 
     a flywheel diode CR101 parallel connected to the shunt field winding F101 for absorbing EMF when the limiting switch device SW101 is cut off. 
     The circuit may further include a motor running speed sensing device SD101 for measuring motor speed and transmitting the data to central control unit CCU101 in either analog or digital form in order to cause the limiting device to close or open at a desired speed; or an armature EMF sensing device EMFD101 which controls the limiting switch device in order to power up the shunt field winding F101 when the EMF is rising; or an armature current sensing device ID100 which controls the limiting switch in order to power up the shunt field winding F101 when the armature current is reduced and the speed is rising due to a light load. 
     Central control unit CCU101 is made up of a circuit which is able to set the motor speed value and compare it with the actual running speed, and an output driving circuit for driving the limiting switch device SW101. 
     FIG. 2 shows an embodiment in which the limiting switch is a centrifugal switch. The circuit of this embodiment includes: 
     a series field winding S201 and an armature A201 mutually connected in series and connected in parallel to the power supply; 
     a limiting switch device SW201 made up of an electro-mechanical switch controlled by the centrifugal force of the motor, and which may be series connected with the shunt field winding F201 of the motor and then parallel connected to the armature in a short shunt configuration, with the series field winding and shunt field winding both having the same polarity to provide cumulative excitation; and 
     a flywheel diode connected in parallel to the shunt field winding F301 for absorbing EMF when the limiting switch device SW201 is cut off. 
     The aforesaid limiting switch device SW201 is driven by centrifugal force, but the action point of the centrifugal switch can be controlled by an appropriate adjustment mechanism. 
     FIG. 3 shows an embodiment in which the limiting switch is made up of a power transistor. The control circuit of this embodiment includes: 
     a series field winding S301 and an armature A301 mutually connected in series and connected in parallel to the power supply; 
     a limiting switch device Q301 made up of a solid-state switch controlled by central control unit CCU301, which may be connected in series with shunt field winding F301 of the compound motor and then parallel connected to series field winding S301 and armature A301 in a long shunt configuration, or directly parallel connected to the armature in a short shunt configuration, with the series field winding and shunt field winding both having the same polarity in order to provide cumulative excitation; and 
     a flywheel diode CR301 parallel connected to the shunt field winding F301 for absorbing EMF when the limiting switch device Q301 is cut off. device in order to power up the shunt field winding F301 when the EMF is rising; or an armature current sensing device ID300 which controls the limiting switch in order to power up the shunt field winding F301 when the armature current is reduced and the speed is rising under a light load. 
     Central control unit CCU301 is made up of a circuit which is able to set the motor speed value and compare it with the actual running speed, and an output driving circuit for driving the limiting switch device SW301. 
     In summary, the invention provides a DC compound motor control circuit for limiting the shunt field winding excitation current, as opposed to the series field winding excitation current, by means of a limiting switch. The result is an improved motor design with increased life span and controllability.