Abstract:
An electric brake circuit for holding an alternating current motor rotor stationary uses a direct current brake acting on an AC motor rotor. The rotor has at least one groove formed therein positioned for alignment with at least one AC motor pole when switching from an alternating current to DC. The AC induction motor holds the rotor stationary against a heavier load by using the grooved rotor with the grooves formed in the rotor aligning with the AC motor&#39;s poles.

Description:
This patent application is a continuation-in-part application of my pending U.S. patent application Ser. No. 11/901,994, filed Sep. 21, 2007 for a Retractable Light Fixture. 
    
    
     BACKGROUND OF THE INVENTION 
     Induction motors are among the simplest and most rugged electric motors and are made up of a wound stator and a rotor assembly. The rotor assembly resembles a squirrel cage from one end so that the motor is commonly called a squirrel cage motor. A shaded pole motor is an AC single phase induction motor which has a squirrel cage rotor. Single phase motors require some means for producing a rotating magnetic field for starting the motor. The shaded pole motor, a part of the face of each field pole carries a copper ring called the shading coil which induces a rotating magnetic field. 
     In many applications, it is desirable to bring an AC motor to a rapid and controlled stop. One technique for accomplishing this in an AC motor is to apply a direct current across the windings of the motor. The application of direct current to the windings generates an electromagnetic force within the motor to stop the rotor with a rapid braking action. In braking systems using a synchronous motor, a starting capacitor in the AC motor may be charged during operation of the motor and then electrically coupled to the windings of the AC motor to produce a braking force on the motor when the motor is switched off. However, sometimes braking affects afforded by the starting capacitor does not stop the motor within the desired time due to the limited charge the starting capacitor can store to increase the braking affects. Some prior art systems utilize a braking capacitor having a greater storage capacity. During the motor&#39;s operation, the braking capacitor is charged. When the motor is stopped, the braking capacitor is applied across the windings of the motor to bring the motor to a rapid halt. 
     The present invention is directed towards an alternating current induction motor which switches from an AC load to applying a DC current to the motor for purposes of holding or locking the rotor in place against torque being placed on the motor shaft and on the rotor and to accomplish a greater holding ability of the rotor with a smaller amount of DC current. 
     Prior art electric braking systems for alternating current motors can be seen in the Hastings U.S. Pat. No. 5,705,903 for an electric brake circuit for bringing an alternating current motor to a rapid halt which uses a braking capacitor charged to a preselected voltage. In the Oltendorf, U.S. Pat. No. 3,475,669, a variable dynamic direct current brake circuit for an AC motor is provided while in the Gross U.S. Pat. No. 3,872,363 an electric motor braking system is utilized. In the Gross U.S. Pat. No. 3,798,523, a single phase induction motor brake uses a capacitor and a current limiting impedance in a DC braking circuit. The Guttmann, U.S. Pat. No. 4,195,255, is for an electric brake for AC motors which has a control rectifying means for applying direct current to the motor. The Ramirez, Jr. et al., U.S. Pat. No. 6,906,493, provides for an electric brake for a motor by applying a direct current voltage from a capacitor across terminals of an alternating current motor. The Gritter et al., U.S. Pat. No. 4,990,844, is a DC braking of an inverter driven AC motor. In the Nagel, U.S. Pat. No. 4,185,770, an automatic flue damper control system is normally energized to hold the damper closed as a mechanical biased mechanism to move the damper to an open position in the absence of motor energization. A braking arrangement applies a direct current to the motor for dynamic braking as the damper closely approaches its open position. 
     In contrast to this prior art, the present invention is directed towards a single phase induction motor in which direct current is applied to the terminals of an alternating current motor for purposes of holding the rotor in a stationary position against a biasing torque placed on the rotor. The present rotor holding circuit is accomplished using a reduced amount of electric power. 
     SUMMARY OF THE INVENTION 
     An electric brake circuit for holding an alternating current motor rotor stationary. An AC power source is connected to an alternating current motor having a stator having at least one pole and a rotor with the drive shaft attached. The rotor has at least one groove formed therein positioned for alignment with at least one AC motor pole when direct current is applied to the AC motor. An AC to DC converter is coupled to the AC source for converting an alternating current to a direct current. The AC to DC converter is coupled to the AC motor. A brake actuator is coupled between the alternating current power and the AC motor for applying an alternating current to the AC motor in one position and to the AC to DC converter in a second position for braking and holding the alternating current motor rotor with each rotor groove aligning with one stator pole to provide an AC motor DC holding brake having a greater holding force for holding the rotor in a stationary position. The AC motor is an induction motor and may be a shaded pole motor having a number of poles while the rotor has grooves matching and aligning with each of the poles. The AC motor electric brake circuit may have an actuator which is a timed delay relay, automatically actuating the electric brake circuit when power is cut to the AC motor. The AC motor shaft is connected to a gear box having an output shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features, and advantages of the present invention will be apparent from the written description and the drawings in which: 
         FIG. 1  is a block diagram of the circuit in accordance with the present invention; 
         FIG. 2  is a perspective view of a shaded pole motor incorporating the improved rotor of the present invention; and 
         FIG. 3  is a perspective view of the rotor of the shaded pole motor of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1  of the drawings, a circuit in accordance with the present invention has an alternating current power source  11  having a power line  12  connecting to one terminal  13  of an alternating current shaded pole motor  14 . The motor  14  has an output shaft  15 . A second line  16  from the AC power source  11  is connected to an actuator  17 . The actuator  17  may be an actual switch or a microswitch or a time delay relay as desired and has the switch element  18  which directs the current flow from line  16  of the alternating current lines into a line  20  which connects to a terminal  21  of the induction motor  14 . The power line  12  is also connected to one terminal  21  of the AC to DC converter. The power supply  22  has a power line  23  connected between actuator  17  and the AC to DC power supply  22 . 
     As seen in  FIG. 1 , when the actuator is connected to the terminal in line  20 , there is no power from line  16  passing to line  23  and to the AC to DC power supply. The power line  12  is always connected to both the AC to DC power supply and to one side of the AC shaded pole motor. The AC to DC power supply has an electric line  24  connected to the output and to the terminal  21  of the motor  14 . The AC to DC power supply  22  also has an output line  25  which connects directly to the terminal  13  of the motor  14 . Referring to the actuator of  FIG. 1 , there are two principle ways to physically switch power from AC to DC. The first uses a limit switch that switches the AC off and DC on when it is activated. The second uses a timed delay relay which switches AC to DC after a predetermined time frame. The timed delay relay has the advantage that there would be no need to install a limit switch remotely or otherwise to active the hold system. This would be desirable in the installation of a timed delay DC hold circuit in an existing fire dampener, for instance. 
     In this circuit, the AC power is applied to the AC motor for operating the motor when the actuator switch is switched to operatively connect lines  16  and line  20 . Both sides of the AC power are directly connected to the motor  14 . When the actuator  17  switches the power from line  16  to line  23 , the AC power is switched to the AC to DC converter. However, the AC to DC power supply is activated so that DC current is applied to lines  24  and  25  and to the terminals  21  and  13  of the shaded pole motor  14  to apply a braking force to the motor  14 . In the present invention, it is desired to hold the shaft  15  from the rotor in position using as little electric power as possible until such time it is desirable to actuate the AC motor. 
     Turning to  FIGS. 2 and 3 , the AC shaded pole motor  14  is illustrated in  FIG. 2  and has a rotor  26  rotatably mounted in a stator  27  and having an output shaft  28  therefrom feeding into a gear box  30  which also has an output shaft  31 . The rotor  26 , as seen in  FIG. 3 , is a squirrel cage motor rotor  26 . Shading coils  32  may be seen forming a part of the face of each pole and carries a copper ring called a shading coil. The shading coil produces a rotating magnetic field for starting the motor. The squirrel cage motor rotor, as seen in  FIGS. 2 and 3 , has had grooves  33  and  34  cut or formed therein across the iron surface of the rotor on each side of the rotor for use in a two-pole shaded motor. A four pole shaded pole motor might use four grooves. This groove into the rotor of the motor creates a stronger magnetic field when the groove and pole of the motor align under DC power. This allows the motor to hold a heavier load against the torque of the motor and allows the use of less power to hold light loads. This advantageously allows for the use of less energy. 
     Turning to  FIGS. 2 and 3 , AC shaded pole motor is illustrated in  FIG. 2  having a rotor  26  rotatably mounted in a stator  27  and having an output shaft  28  therefrom feeding into a gear box  30  which also has an output shaft  31 . The rotor  26 , as seen in  FIG. 3 , is a squirrel cage rotor  28 . Shading coils may be seen forming a part of the face of each pole and carries a copper ring called a shading coil. The shading coil produces a rotating magnetic field for starting the motor. The squirrel cage rotor, as seen in  FIGS. 2 and 3 , has had grooves  33  and  34  cut or formed therein across the surface of the rotor on each side of the rotor for use in a two-pole shaded motor. A four pole shaded pole motor might use four grooves. This groove into the rotor of the motor creates a stronger magnetic field when the groove and pole of the motor align under DC power. This allows the motor to hold a heavier load against the torque of the motor and allows the use of less power to hold light loads. This advantageously allows for the use of less energy. 
     Many applications require a drive and hold-type process, such as in the case of automatic fire dampeners which are held open using electric power until disrupted upon which they close by a spring. In the shaded pole motors of  FIG. 2 , the grooves cut in the rotor serves to create a stronger magnetic field when the groove and pole of the motor are aligned under DC power. 
     It should be clear at this time that an electric brake circuit for holding alternating a current motor rotor stationary has been provided which utilizes a smaller amount of energy and the locking of the rotor in place while provided the rotor with greater holding force. However, the present invention is not to be construed as limited to the forms shown which are to be considered illustrative rather than restrictive.