Abstract:
A capacitor is placed in series with a solenoid to bear a substantial portion of the voltage so that a reduced voltage energizes the solenoid. The solenoid may operate a brake on a motor. Applying voltage to operate the motor, energizes the solenoid which retracts the brake.

Description:
The present application claims priority from U.S. Provisional Application No. 60/158,816, filed Oct. 12, 1999, the full disclosure of which is hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is in the field of drive circuits for solenoids, particularly for use as a brake release in connection with a motor. 
     A prior art reversible motor  10  is shown in FIG.  1 . The motor is a three terminal capacitor-start capacitor-run motor, including two wound coils. One terminal of the motor is connected to a clockwise winding. A second terminal is connected to a counterclockwise winding in the motor. A third terminal is for connection to the AC common input. A capacitor  12 , referred to herein as the motor capacitor, is connected between the first and second windings to impose a phase angle shift on the AC input voltage to the opposite coil. The motor capacitor is selected to nominally provide about a 90° phase shift. An AC input voltage may be applied either to the clockwise winding or the counterclockwise winding to cause the motor to turn in one direction or the other. It has been found desirable in such a motor to provide a brake to more rapidly stop the motor when voltage is removed from the inputs. A typical brake is spring loaded so as to be biased into the stop position. A solenoid  14  is used to retract the brake when power is applied to the motor. The solenoid  14  is connected between the first and second terminals. When power is applied to either the first or second terminals of the motor, the solenoid is energized retracting the brake. The problem with this design is that the voltage across the two terminals and thus across the solenoid, is approximately twice the magnitude of the AC input voltage. For a 115 volt line, a solenoid is required that can handle over 200 volts. For a motor that plugs into a 230 volt line, the solenoid would need to be able to handle over 400 volts. Such solenoids are not commonly found and are unnecessarily expensive. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the invention, a solenoid drive circuit is incorporated into the circuitry accompanying the motor in order to reduce the voltage across the solenoid. A capacitor is provided in series with an AC solenoid so as to divide the AC voltage between two terminals. Thus, an advantageously reduced voltage is provided across the solenoid to energize it for releasing the motor brake. 
     In an alternative embodiment of the invention, the brake release is formed by a DC solenoid. A diode bridge rectifier circuit is connected across the DC solenoid to steer an AC current through the DC solenoid in only one direction. A capacitor is coupled in series with the diode bridge rectifier and solenoid to divide the AC voltage. Thus, the DC voltage produced across the solenoid is reduced in magnitude relative to the AC voltage across the terminals. Thus, a reversible motor with an AC input voltage may be supplied with the brake release advantageously operated by a common low voltage solenoid. 
    
    
     Other objects and advantages of the invention will become apparent during the following description of the presently embodiments of the invention taken in conjunction with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit schematic of a reversible motor of the prior art. 
     FIG. 2 is a circuit schematic of a first embodiment of the present invention. 
     FIG.  3 . is a circuit schematic of a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with the first embodiment of the invention, a drive capacitor  20  is placed in series with an AC solenoid  30  as shown in FIG.  2 . The capacitor  20  and solenoid  30  serve as a voltage divider in that a substantial amount of the voltage between the first terminal A of the motor  10  and the second terminal B of the motor  10  is developed across the drive capacitor. In a preferred embodiment, the voltage across the drive capacitor  20  is at least a quarter of the voltage between the first terminal A and the second terminal B. In a more preferred embodiment, the voltage across the drive capacitor  20  exceeds that across the solenoid  30 . The voltage across the solenoid has been reduced by including the drive capacitor so that a less expensive solenoid could be used in the circuit. When AC voltage is applied across terminals A and B to drive the motor  10 , the brake is pulled away from the motor  10  by the solenoid  30  and the motor is allowed to turn. When the AC voltage is removed, the solenoid is deactivated and the motor is braked. 
     The use of a drive capacitor  20  may be further adapted for use with a DC solenoid  40  as shown in FIG.  3 . Here, a diode bridge rectifier  50  is connected across a DC solenoid to steer a current in only one direction through the solenoid. The drive capacitor  20  is in series with the diode bridge rectifier  50  and the DC solenoid  40 , all coupled between the A and B terminals of the reversible motor. Thus, the AC voltage across terminals A and B is divided between the drive capacitor  20  and the DC solenoid  40 . The divided AC voltage is applied as direct current by the rectifier to the DC solenoid. The DC solenoid is used as the brake release for the motor. When input voltage is applied to one of the motor terminals, the brake is pulled away from the motor by the solenoid and the motor turns. When the input voltage is removed, the solenoid is deactivated and the motor is braked. 
     In accordance with the presently preferred DC solenoid embodiment, for a 115 volt AC input voltage, a 0.33 microfarad capacitor rated for 200 volts is used as the drive capacitor. For a 230 input voltage, a 0.12 microfarad capacitor rated at 250 volts is used. Note that the voltage across terminal A and terminal B of the motor is twice the input voltage. By using the drive capacitor, a 4 watt 110 volt DC solenoid of 3.2K winding resistance may be used for the brake release. The motor capacitor  12  connected at either end to the A and B terminals as used in such present embodiments were 4 microfarad and 1 microfarad for 115 VAC and 230 VAC motors, respectively. Thus, the brake circuit provided only a light load on the motor capacitor. 
     Of course, it should be understood that various changes and modifications to the preferred embodiments described above will be apparent to those skilled in the art. For example, the solenoid drive circuit may be used in other applications where an AC voltage is the input and a solenoid is required. Such changes can be made without departing from the spirit and scope of the invention and without diminishing its attendant advantages. It is, therefore, intended that such changes and modifications be covered by the following claims.