Abstract:
A circuit and a method for controlling excitation current to the field winding of a generator or motor. An uncontrolled current source supplies sufficient excitation current to maintain the generator output voltage at a level slightly below the seated voltage when no load is present. A controlled current source compensates for generator loading to supply additional excitation current sufficient to raise the output voltage to approximately the rated voltage.

Description:
This is a continuation, of application Ser. No. 08/193,519, filed Feb. 8, 1994, now abandoned which is the continuation of application Ser. No. 07/843,415 filed Feb. 27, 1992 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for controlling excitation current to the field winding of a generator or motor, and more particularly to a circuit which uses an uncontrolled current source and a controlled current source to trim excitation current to a field winding. 
     2. Background of the Invention 
     In the drawings referenced herein, like numerals indicate like features. 
     Motors and generators can be classified as brush-type or brushless. In a brush-type machine, electrically conductive brushes connected to slip rings provide excitation current to a rotating field winding.. In brushless machines, excitation current is provided to an excitation field winding. Rotating inductors convert the magnetic flux created by the excitation field winding into current supplied to the rotating field winding, The present invention will be described in terms of a brush-type system. It will be obvious to one skilled in the art to apply the teachings of the present invention to a brushless motor or generator. 
     A typical brush-type system 10 is shown in FIG. 1. System 10 comprises a rotating field winding 12 comprising winding ends 24 and 26, a stationary (main) winding 14 and an automatic voltage regulator (AVR) 18. Magnetic flux created by rotating field winding 12 is converted into an AC voltage supplied to a load 16 by main winding 14. AVR 18 controls the voltage supplied to load 16 by increasing or decreasing the magnetic flux generated by winding 12 as a function of the voltage sensed across winding 14. In a typical system, the voltage supplied to load 16 is controlled by sensing the voltage across winding 14 and supplying an excitation current as a function of the voltage sensed to winding 12 through the slip rings (not shown). 
     A typical AVR circuit is shown generally in FIG. 2. In FIG. 2, two silicon-controlled rectifiers (SCR) 32 and 34 and two diodes 36 and 38 form a two-pulse half-controlled bridge converter capable of converting an AC voltage into a DC voltage used to control excitation current to rotating field winding 12. SCRs 32 and 34 are controlled by excitation current control 30. 
     SCR&#39;s are a well known and used method of controlling field current for a voltage regulator in an AC generator or a DC generator. Excitation current control 30 controls the excitation current provided to winding 12 by increasing or decreasing the turn-on time of SCRs 32 and 34. This increases or decreases the average DC current provided to winding 12 which, in turn, increases or decreases the magnetic flux generated by winding 12. An AVR constructed as in FIG. 2 is useful in the control of AC generators in widely varying conditions and under widely varying loads. 
     A second type of voltage regulation is shown as system 40 in FIG. 3. In FIG. 3, a center tapped winding 42 replaces winding 14 Of FIG. 1. System 40 comprises a rotating field winding 12, a center-tapped stationary (main) winding 42 and an automatic voltage regulator (AVR) 44. Magnetic flux created by rotating field winding 12 is converted into an AC voltage supplied to a load 16 by main winding 42. 
     AVR 44 controls the voltage supplied to load 16 by increasing or decreasing the magnetic flux generated by winding 12 as a function of the voltage sensed across winding 42. In atypical system, the voltage supplied to load 16 is controlled by sensing the voltages between winding ends 20 and 22 and center tap 46. An excitation current is provided to winding 12 as a function of the voltages sensed. 
     A typical AVR circuit for system 40 is shown generally in FIG. 4. In FIG. 4, two silicon-controlled rectifiers (SCR) 50 and 52 form a two-pulse midpoint converter capable of converting an AC voltage into a DC voltage used to control excitation current to rotating field winding 12. SCRs 50 and 52 are controlled by excitation current control 54. Excitation current control 54 controls the excitation current provided to winding 12 by increasing or decreasing the turn-on time of SCRs 50 and 52. This increases or decreases the average DC current provided to winding-12 which, in turn, increases or decreases the magnetic flux generated by winding 12. Like the AVR shown in FIG. 2, an AVR constructed as in FIG. 4 is useful in the control of AC generators in widely varying conditions and under widely varying loads. 
     AVRs 18 and 44 provide feedback control over the magnetic flux generated by winding 12, in a generator or motor, but at a cost. An AVR design based on active components such as SCRs is generally more costly than a purely passive design. For one thing, an SCR is more expensive than a diode. In addition, each SCR requires the addition of the support circuitry needed to turn the SCR on and off as a function of the excitation current required. Passive diodes cannot, however, be used in the place of the SCRs shown in FIGS. 2 and 4 without relinquishing control over the excitation current. 
     In the case of stand-alone generators, AVRs based on active components such as SCRs face an additional problem. Since the AVR needs power to turn on its SCRs, no excitation current is provided until the voltage generated is sufficient to power the SCRs. To Counter this, stand-alone generators must either be provided with an independent energy source to power the active components when the generator is first turned on or the AVR must be designed to remain quiescent for the time necessary for the residual voltage to reach the level needed to power the active components. In the latter case, the generator operates without control while the generated voltage ramps up. This may require additional circuitry for field flashing. 
     It is desirable to minimize the number of active components in an AVR both to minimize cost and design complexity. What is needed is a method of using passive diodes to replace one or more of the SCRs in a automatic voltage regulator while maintaining the control necessary to trim the voltage supplied to a widely varying load. The present invention meets that need. 
     SUMMARY OF THE INVENTION 
     The present invention provides a circuit and a method for controlling excitation current to the field winding of a generator or motor. An uncontrolled voltage converter converts a voltage in order to supply one level of excitation current. A controlled voltage converter converts a voltage in order to supply an additional level of excitation current under control of voltage sensing apparatus. 
     According to another aspect of the present invention, an excitation system is disclosed which can be used to provide an excitation current useful in generating a controlled magnetic flux. The excitation system comprises a voltage reference means for establishing a reference voltage between a magnetic flux sensor such as a main or auxiliary winding and a rotating field winding. The excitation system further comprises a first voltage conversion means for converting voltage from the magnetic flux sensor into a first level of excitation current and second voltage conversion means for converting voltage from the sensor to provide a controlled level of additional excitation current. 
     According to yet another aspect of the present invention, an uncontrolled current source supplies sufficient excitation current to maintain the generator output voltage at a level slightly below the rated voltage when no load is present. A controlled current source compensates for generator loading to supply additional excitation current sufficient to raise the output voltage to approximately the rated voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical block diagram representation of a typical bridge-controlled generator. 
     FIG. 2 is an electrical block diagram representation of a typical two pulse half-controlled bridge converter such as would be used in a generator like that shown in FIG. 1. 
     FIG. 3 is an electrical block diagram representation of a midpoint-controlled generator. 
     FIG. 4 is an electrical block diagram representation of a typical two pulse midpoint converter such as would be used in a generator like that shown in FIG. 3. 
     FIG. 5 is an electrical block diagram representation of a tap-controlled generator having an automatic voltage regulator according to the present invention. 
     FIG. 6 is an electrical block diagram representation of one embodiment of the automatic voltage regulator of FIG. 5. 
     FIG. 7 is an electrical block diagram representation of one embodiment of an automatic voltage regulator useful in bridge-controlled generators according to the present invention, 
     FIG. 8 is an electrical block diagram representation of a bridge-controlled generator which uses an auxiliary winding to sense magnetic flux generated by a rotating field winding. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following Detailed Description of the Preferred Embodiments, reference is made to the accompanying Drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may also be possible and may be utilized and structural changes may be made without departing from the scope of the present invention. 
     A generator 60 is shown generally in FIG.. 5. In FIG. 5, two windings 62 and 64 replace winding 42 of FIG. 3. System 60 comprises a rotating field winding 12, a first stationary winding 62, a second stationary winding 64 and an automatic voltage regulator (AVR) 66. Magnetic flux created by rotating field winding 12 is converted into an AC voltage supplied to a load 16 by windings 62 and 64. 
     AVR 66 controls the voltage supplied to load 16 by increasing or decreasing the magnetic flux generated by winding 12 as a function of the voltage sensed across windings 62 and 64. In one embodiment, the voltage supplied to load 16 is controlled by sensing the voltages between winding ends 20 and 22 and juncture 68 of windings 62 and 64 and providing an excitation current to winding 12 as a function of the voltages sensed. 
     A typical AVR circuit for system 60 is shown generally in FIG. 6. In FIG. 6, a silicon-controlled rectifier (SCR) 70 and a passive diode 72 form a two-pulse converter capable of converting an AC voltage into a DC voltage used to control excitation current to rotating field winding 12. AVR 66 can be powered either with the voltage sensed at 20 and 22 or from an auxiliary winding used in conjunction with stationary windings 62 and 64. 
     SCR 70 provides a controlled voltage converter which operates under the control of excitation current control 74. Excitation current control 74 controls the excitation current provided to winding 12 by increasing or decreasing the turn-on time of SCR 70. This increases or decreases the average DC current provided to winding 12 which, in turn, increases or decreases the magnetic flux generated by winding 12. Like the AVRs shown in FIGS. 2 and 4, an AVR constructed as in FIG. 6 is useful in the control of AC generators in widely varying conditions and under widely varying loads. 
     As can be seen in FIG. 5, a voltage reference is established between the flux sensing means comprising windings 62 and 64 and the rotating winding. In the embodiment shown, this is done by connecting juncture 68 and winding end 24 to ground. It will be understood by those skilled in the art that other mechanisms, such as a conductor connecting end 24 to juncture 68, could be used to establish the voltage reference. 
     The AVR constructed as in FIG. 6 offers an advantage over the AVRs of FIGS. 2 and 4. Passive diode 72 provides an uncontrolled voltage converter that appears as just a single diode drop in line with half the excitation system. A system constructed according to the present invention will begin feeding excitation energy to winding 12 at anything over the diode drop. This eliminates the design complications inherent in AVRs constructed with active components described above. 
     In one embodiment of the circuits of FIGS. 5 and 6, windings 62 and 64 are sized such that the voltage across 64 is sufficient to maintain the generator at slightly below the nominal voltage when there is no load present. This approach reduces the range of operating parameters required of SCR 70. In this embodiment, SCR 70 is part of a trimming circuit used to provide any additional excitation current necessary to compensate for variations in operating conditions or in load 16. 
     The teachings of the present invention can be applied to the bridge-controlled generator shown in FIG. 1. In one embodiment, AVR 18 in FIG. 1 is replaced by an AVR 80 constructed according to the present invention. AVR 80 is shown generally in FIG. 7. In FIG. 7, SCR 34 shown in FIG. 2 is replaced with a passive diode 82 to form an uncontrolled voltage converter. Excitation current control circuit 30 is replaced by excitation current control circuit 84. AVR 80 can be powered either with the voltage sensed at 20 and 22 or from an auxiliary winding used in conjunction with stationary winding 12. 
     Excitation current control 84 controls the excitation current provided to winding 12 by increasing or decreasing the turn-on time of SCR 32. This increases or decreases the average DC current provided to winding 12 which, in turn, increases or decreases the magnetic flux generated by winding 12. As is well known in the art, the voltage reference is provided by the bridge configuration used for the elements of AVR 80. 
     In one embodiment of a bridge-controlled circuit constructed according to the present invention, generator 10 and AVR 80 are designed such that the excitation current across the passive side of the full-wave converter is sufficient to maintain the generator at slightly below the nominal voltage when there is no load present. This approach reduces the range of operating parameters required of SCR 32. In this embodiment, the half of the full-wave converter which includes SCR 32 is part of a trimming circuit used to provide the additional excitation current necessary to compensate for variations in operating conditions and loads 
     An AVR constructed as in FIG. 7 is useful in the control of AC generators in widely varying conditions and under widely varying loads. It has an advantage over the circuit of FIG. 2 in that passive diode 82 provides just a single diode drop in line with half the excitation system. A system constructed according to FIG. 7 will begin feeding excitation energy to winding 12 at anything over the diode drop. 
     Auxiliary windings can be used in conjunction with the main windings to isolate the circuitry used to sense and control the magnetic flux generated by field winding 12. In one embodiment, AVR 18 of FIG. 1 is removed and windings 62 and 64 of FIG. 5 are mounted as a set of auxiliary windings used in conjunction with stationary winding 14. AVR 66 is connected to windings 62 and 64 and operates in conjunction with windings 62 and 64 to control the excitation current supplied to field winding 12. In this embodiment, both the energy to power AVR 66 and the sensed voltage are obtained from windings 62 and 64. 
     In an alternate auxiliary winding embodiment, a single auxiliary winding is connected to an AVR like AVR 80 in FIG. 7. Such an embodiment is shown illustrated generally in FIG. 8. In FIG. 8, auxiliary winding 92 is placed in the magnetic flux generated by rotating field winding 12. AVR 8C is powered by the voltage induced across auxiliary winding 92. In addition, the voltage induced across winding 92 is rectified by AVR 80 so as to provide excitation current to rotating field winding 12. The operation of AVR 80 is as described previously. 
     Although the present invention has been described with reference to the preferred embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. In particular, although the embodiments described teach the use of an SCR to control the flow of excitation current as a function of the sensed voltage, it should be obvious that other active switching devices (such as transistors) could be used without departing from the scope of the present invention. In addition, the present teachings can be applied advantageously to provide excitation current to the control windings of electric motors.