Patent Publication Number: US-10784757-B2

Title: Synchronous machine with common motor/generator exciter stage

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of co-pending U.S. application Ser. No. 14/498,186, entitled “Synchronous Machine With Common Motor/Generator Exciter Stage” and filed on Sep. 26, 2014, which has since issued as U.S. Pat. No. 10, 305,356. The aforementioned related patent application is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     A synchronous machine is an electric machine which can be operated as either a synchronous motor (synchronous motor mode) or a synchronous generator (synchronous generator mode). Conventionally, a synchronous machine has two separate and independent exciter field windings. Also, conventionally, two separate and independent control units have been used, one control unit for the exciter field winding for the synchronous motor mode and another control unit for the exciter field winding for the synchronous generator mode. The use of two exciter field windings and two control units make the synchronous machine and the system in which it is being used more complicated, heavier, and less reliable. The dual excitation components of a conventional synchronous machine may represent 20 to 30% of the total volume and weight of the synchronous machine. Some conventional systems use only a single, reconfigurable field winding, but still use two separate and independent control units, which then use switches or contactors to connect the appropriate control unit to the field winding. Dual field windings, dual control units, and/or switches and/or contactors add cost, weight, volume, and complexity to the system, and adversely affect the overall reliability of the system. U.S. Pat. No. 5,770,909 to Rosen et al., hereby incorporated in its entirety herein by reference, discloses a synchronous motor-generator system which uses a rotary transformer. 
     Conventional synchronous machines also use a low frequency excitation current and large field windings are used to avoid energy losses. These large field windings substantially increase the amount and weight of the expensive copper used in the windings. Further, with the conventional low frequency excitation current, the back electromotive force generated in the field windings is significantly affected by the rotor speed, and this can cause stability problems during the startup process. 
     SUMMARY OF THE DISCLOSURE 
     A synchronous machine is disclosed which is operable as either a synchronous motor or a synchronous generator. The synchronous machine has a frame, a shaft, a main section, and an exciter section. The main section has a stator (a stationary winding, which may be an armature winding) which is mounted on the frame, and a rotor (a rotating winding, which may be a field winding) which is mounted on the shaft, the stator and the rotor being magnetically coupled to each other. The exciter section has a transformer and a rectifier. The transformer has a primary winding secured to the frame and a secondary winding secured to the shaft. The primary and secondary windings are spaced apart from, and magnetically coupled to, each other. The rectifier is electrically connected to the secondary winding, is mechanically connected to the rotor, and rectifies an output of the secondary winding to provide a rectified output to the rotor. The primary winding and the secondary winding of the transformer are each in the shape of a disk. 
     A control unit provides a control signal to the primary winding to control the operation of the synchronous machine. 
     In one embodiment, the primary winding has an interior radius and the disk defines a plane which is perpendicular to the shaft, and the secondary winding has an exterior radius, which is smaller than the interior radius, so the secondary winding is positioned within the primary winding. 
     In another embodiment, the primary winding is mounted to the frame at an end of the shaft, the disk of the primary winding defining a first plane which is perpendicular to the shaft, and the secondary winding is secured to the shaft near an end of the shaft, the disk of the secondary winding defining a second plane which is perpendicular to the shaft, the second plane being parallel to and spaced apart from the first plane, the shaft does not penetrate the first plane, and the shaft has a channel in which electrical conductors are placed to connect the rectifier with at least one of the secondary winding or the rotor. 
     A method of manufacturing a synchronous machine operable as either a synchronous motor or a synchronous generator is also disclosed. The method includes providing a frame, mounting a stator on the frame, providing a shaft which extends from at least one end of the frame, mounting a rotor on the shaft, mounting a primary winding of a transformer on the frame, mounting a secondary winding of the transformer on the shaft, spaced apart from, but magnetically connected to, the primary winding, securing a rectifier to the shaft, and electrically connecting an input of the rectifier to the secondary winding and an output of the rectifier to the rotor. Either the secondary winding is mounted within the primary winding, such that they are in the same plane, or the secondary winding is mounted facing the primary winding, so that they are in different planes. A channel is provided in the shaft so that electrical conductors may be run from the rectifier to the secondary winding and/or the rotor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an exemplary synchronous machine. 
         FIG. 2  is a diagram illustrating one exemplary embodiment of the synchronous machine. 
         FIG. 3  is a diagram illustrating another exemplary embodiment of the synchronous machine. 
         FIG. 4  is a diagram illustrating another exemplary embodiment of the synchronous machine. 
         FIG. 5  is a diagram illustrating a rectifier embedded in or within a container. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram of an exemplary synchronous machine  100 . The synchronous machine  100  has a frame  110 , a shaft  115 , a main section  120 , and an exciter section  125 . The main section  120  has a stator  130  (a stationary winding, which may be an armature winding) which is mounted on the frame, and a rotor  135  (a rotating winding, which may be a field winding) which is mounted on the shaft  115 . Part of or all of the frame  110  may be part of, or may be distinct from, a casing which encloses the synchronous machine  100 . 
     The exciter section  125  has a transformer  140  and a rectifier  145 . The transformer  140  has a primary winding  140 A mounted on the frame  110  and a secondary winding  140 B mounted on the shaft  115 . The secondary winding  140 B is spaced apart from, and is magnetically coupled to, the primary winding  140 A. The rectifier  145  is electrically connected by a plurality of electrical conductors  137  to the secondary winding  140 B, is electrically connected by a plurality of electrical conductors  142  to the rotor  135 , and rectifies an output of the secondary winding  1408  to provide a rectified output to the rotor  135 . For convenience and brevity of expression, “electrical conductors”, and a “plurality of electrical conductors”, are sometimes referred to herein simply as “conductors”. The rectifier  145  is secured to the shaft  115 , either by being mounted on the shaft  115  or by another desired and appropriate technique, such as including the rectifier  115  with the secondary winding of the transformer  140 . If desired, the output of the secondary winding  140 B and/or the rectifier  145  may be filtered or smoothed before being applied to the rotor  135 . 
     One may also consider the synchronous machine  100  as having a stationary section  150  and a rotating section  155 , the stationary section  150  comprising the frame  110 , the primary winding  140 A, and the stator  130 , and the rotating section  155  comprising the shaft  115 , the rotor  135 , the secondary winding  140 B, and the rectifier  145 . 
     The electrical lines  165  connected to the stator  130  serve as input lines to provide an electrical input voltage and power to the synchronous machine  100  when operation is in the synchronous motor mode, and serve as output lines to provide an electrical output voltage and power from the synchronous machine  100  when operation is in the synchronous generator mode. 
     A control unit  170  monitors one or more parameters of the electrical lines  165  and provides an output control signal over conductors  180  to the primary winding  140 A. The control unit  170  may monitor parameters such, as but not limited to, the voltage, current, frequency, and/or phase on the electrical lines  165 . The parameters which are monitored may depend in part on whether the machine  100  is being operated as a motor or as a generator. These input parameters may be filtered, if desired, to reduce noise before they are provided to the control unit  170 . 
     The control signal is an alternating waveform voltage (AC voltage) such as, but not limited to, a pulse width modulated (PWM) AC signal. The control signal preferably has a rectangular waveform, such as provided by a pulse width modulation switching system, but may be a sinusoidal waveform, or another desired waveform. The control unit  170  controls at least one of a pulse width, a voltage (which may be a pulse voltage), or a frequency (which may be a pulse frequency) of the control signal. The control signal may be a plurality of pulses or a plurality of cycles of an AC signal, a single pulse or a cycle of an AC signal, a part of a cycle of an AC signal, or a combination thereof. For example, depending upon the monitored input parameters, the control signal may be two pulses or two cycles of an AC signal, may be 6½ pulses or 6½ cycles of an AC signal, or may be less than a full cycle of an AC signal. Pulses may be in sets, with variable lengths, with different numbers in different sets, and/or variable spacing between sets. The control signal may be filtered, if desired, before being provided to the primary winding  140 A. 
     The control signal is a “high frequency” control signal; that it, it has a frequency which is higher than the input frequency (motor mode), that is, the frequency of the input signal on electrical lines  165 , and higher than the output frequency (generator mode), that is, the frequency of the output signal on electrical lines  165 . More preferably, the frequency of the control signal is at least several times higher than the frequency of the voltage on electrical lines  165 . Even more preferably, the frequency of the control signal is at least  10  times the frequency of the voltage on electrical lines  165  in order to minimize the effects on excitation caused by the rotation speed of the rotor  135 . Higher frequencies may also be used. Lower frequencies may also be used, but the size, weight, and cost of the windings  140 A,  140 B may increase as the frequency is lowered, and coupling between the primary and secondary windings may become affected by the rotational speed of the shaft. In one implementation, the frequency of the control signal provided to the transformer  140  is 10 kHz if the frequency of the voltage on electrical lines  165  is 400 Hz. In addition, the use of such a higher frequency for the control signal allows the transformer  140  to use smaller windings, and less iron, that the exciter armature windings of conventional systems. 
     The control unit  170  may also monitor other parameters or aspects of the operation of the synchronous machine  100  such as, by way of example and not of limitation, the rotation speed, the shaft angular position, the changes therein, etc. For example, a shaft position encoder (not shown) may be connected to the shaft to provide the angular position of the shaft. The control unit  170  may then adjust the control signal on conductors  180  accordingly. For example, if the machine is being operated as a motor and the load is such that the changes in the shaft angular position indicate that the motor may not be able to maintain synchronous operation then the power provided to the primary winding  140 A, and therefore to the rotor  135 , may be increased. As another example, if the machine is being operated as a generator and the output voltage on lines  165  is increasing then the power provided to the primary winding  140 A may be decreased. The control unit  170  may vary the power by adjusting, for example, the pulse width, the pulse repetition rate, the amplitude of the control signal on conductors  180 , and/or the pulse pattern (e.g., how many pulses are provided in a set of pulses, the time between each set of pulses, etc.). 
     This synchronous machine design provides for the use of a single compact high frequency exciter stage  125  for both synchronous motor mode and synchronous generator mode. As mentioned, the primary winding  140 A and the secondary winding  140 B are in a spaced apart relationship; that is, they do not contact each other, and the secondary winding  140 B moves as the shaft  115  rotates whereas the primary winding  140 A, mounted to the frame  110 , does not move. The control unit  170  provides the high frequency control signal (input voltage) to the primary winding  140 A, which induces a high frequency AC output voltage on the secondary winding  140 B. This high frequency AC output voltage is rectified by the rectifier  145  to provide a direct current (DC) to the rotor  135 . The rectifier  145  may be, by way of example and not of limitation, a full-wave rectifier or a bridge rectifier. 
     The high frequency output from the control unit  170  allows for the use of a smaller transformer  140 , thereby reducing the size of the exciter section  145  and also reducing copper and iron losses. The high frequency also enables a wider control bandwidth, which provides for better machine speed stability and better torque control. This single exciter section  145  also provides a simplified machine architecture, reduced weight of copper and/or iron used therein, reduced volume, and reduced number of excitation sources (smaller component count). This single, high frequency exciter section  145  thereby provides better efficiency and higher reliability than the conventional systems mentioned above. 
     As seen from  FIG. 1 , only one rotor  135  and only one control unit  170  are used for both synchronous motor operation and synchronous generator operation. Elimination of the duplicate rotors and control units used in conventional designs reduces the volume, weight, and number of components of the synchronous machine  100 . 
     Further, by using a high frequency AC input voltage to the transformer  140 , the voltage provided to the rotor  135  is more stable than in conventional synchronous machines. A more stable voltage to the rotor  135  improves the stability and control in the process of starting the synchronous machine  100 . 
       FIG. 2  is a diagram illustrating one exemplary embodiment of the synchronous machine  100  showing the frame  110 , the shaft  115 , the stator  130 , the rotor  135 , the transformer windings  140 A,  140 B, the rectifier  145 , and the bearings  160 A,  160 B. Also shown are conductors  180  which connect to the primary winding  140 A through a hole, grommet, or other opening  110 A, preferably but not necessarily sealed, in the frame  110 . Also shown are conductors  137  and  142 . For convenience and clarity of illustration, these conductors  137  and  142  are shown as being apart from the shaft  115 . In practice, however, these conductors would preferably be mounted directly to the shaft  115  so as to minimize the centrifugal forces on these conductors. They could also be placed in a groove (not shown) in the shaft. The groove would be as shallow as possible so as to have the minimum effect on the strength and integrity of the shaft  115 . If desired, the conductors  137  and  142  could be placed in a channel in the shaft  115 , such as is shown in  FIG. 3 . 
     In the embodiment of  FIG. 2 , each transformer winding  140 A,  140 B is preferably in the shape of a disk, which may have a width, length, depth, wire size, and number of turns as convenient and appropriate for a particular implementation. Primary winding  140 A may be considered to be an “outer” winding, and secondary winding  140 B may be considered to be an “inner” winding. The primary winding  140 A has an interior radius  140 A 1  with respect to the centerline  115 A of the shaft  115 , and the secondary winding  140 B has an exterior radius  140 B 1  with respect to the centerline  115 A of the shaft  115 . The exterior radius  140 B 1  is less than the interior radius  140 A 1 , so that winding  140 B is fits inside of and is interior to winding  140 A. The spacing between the windings  140 A,  140 B is sufficiently small that the windings  140 A,  140 B are magnetically coupled to each other. Preferably, windings  140 A and  140 B are in substantially the same plane  175 . Windings  140 A and  140 B need not be in exactly the same plane  175 , they may be slightly offset from each other. Windings  140 A and  140 B are considered to be in substantially the same plane, even if offset from each other, if the magnetic coupling between them is sufficient to provide the appropriate power and control to the rotor  135 . The windings  140 A and  140 B are in a container, such as  140 A 2  and  140 B 2 , respectively, to protect the windings and hold the windings in place. The containers are preferably made of ferrite or other material which serves to provide a closed path for the magnetic lines of force from the windings and to increase the magnetic coupling between the windings. The shaft  115  may also serve to concentrate the magnetic flux and increase coupling if the shaft  115  is made of or includes a ferromagnetic material, especially if the containers are not made of a material which increases the coupling. 
     Although the frame  110  is illustrated as being a stepped frame, where one part of the frame has a different radius than another part of the frame, this is not a requirement; the frame may have a different shape, such as having the same radius throughout its entire length, as shown in  FIG. 3 . Also, although the frame  110  is illustrated as being single-ended, that is, end  1108  is open and end  110 C is closed, so that the shaft  115  only extends from end  1108  of the frame, this is not a requirement. The end  110 C may also be an open end so that the shaft  115  may extend from both end  1108  and end  110 C. In addition, although the exciter section  125  is illustrated as being at the closed end  110 C of the frame  110 , it could instead be at the open end  1108  of the frame  110 . 
       FIG. 3  is a diagram illustrating another exemplary embodiment of the synchronous machine  100 . In this embodiment transformer windings  140 A,  140 B are not “inner” and “outer” windings, they are parallel or facing windings but they are not in the same plane. Rather, winding  140 A is in plane  175 A, and winding  140 B is in plane  1758  so that they face each other. They are again preferably in the shape of a disk. In this embodiment the conductors  137  from the secondary winding  140 B to the rectifier  145  are at least partially within a channel or hollow section  1158  in the shaft  115  so that the conductors  137  do not interfere with the bearing  160 B. 
     In an alternative embodiment illustrated in  FIG. 4 , the rectifier  145  may be, if desired, positioned outside of the bearing  160 B, that is, between the bearing  160 B and the end  110 C. In this alternative embodiment the conductors  137  may or may not be in the channel  1156 , but the conductors  142  from the rectifier  145  to the rotor winding  135  would be at least partially within the channel  115 B in the shaft  115  so that the conductors  142  do not interfere with the bearing  160 B. As shown, the container  140 B 2  is secured to an endface  185  of the shaft  115 . 
     Although the rectifier  145  is shown in  FIGS. 2 and 3  as being separate from the secondary winding  140 B, this is not a requirement. For example, the rectifier  145  could be embedded in or within the container  140 B 2 , as illustrated in  FIG. 5 . 
     Also, the channel  115 B design can be used with the embodiment of  FIG. 2  if, for example, it is desired that the exciter section  125  be between the bearing  160 B and the end  110 C. 
     The embodiment of  FIG. 2 , in addition to the advantages and benefits described above, is also advantageous in another respect. If the synchronous machine  100  is used with, for example, a screw drive, then the compressive and tensile forces on the shaft  115  may cause the shaft  115  to shift slightly along its length, that is, toward, or away from, an end  1108  or  110 C, but a shift will have little effect upon the magnetic coupling between the windings  140 A and  140 B. 
     The embodiment of  FIG. 3 , in addition to the advantages and benefits described above, is also advantageous in another respect: reduced centrifugal forces exerted upon the windings  140 A and  140 B. As the windings  140 A and  140 B are closer to the axis  115 A, the centrifugal forces exerted upon them will be less than the forces exerted in the embodiment of  FIG. 2 . This reduction in centrifugal forces may be significant for a synchronous machine  100  which is to be operated at a very high revolution per minute rate, as might be the case for some smaller-size synchronous machines. 
     Thus, the use of a single exciter stage transformer  140 , instead of the use of two separate excitation stage transformers or reconfigurable windings, reduces the weight of copper and iron in the machine, and reduces the number of switches and contactors required when two transformers are used. Further, only one exciter source, control unit  170 , is used, rather than two or more excitation sources. The single control unit  170  controls the synchronous machine  100  for both motor mode and generator mode of operation, simplifies the control design, and reduces the number of components. A high frequency control signal, instead of a low frequency control signal, provides for better control. 
     A method of operating the synchronous machine as either a synchronous motor or a synchronous generator includes (1) applying a first alternating voltage to the primary winding and applying a second alternating voltage to the stator to cause the synchronous machine to operate as a synchronous motor providing an output torque, or (2) applying a first alternating voltage to the primary winding and applying an input torque to the shaft to cause the synchronous machine to operate as a synchronous generator to provide an output voltage. At least one of a voltage, a frequency, or a duty cycle of the first alternating voltage is adjusted to control an output torque when operating the synchronous machine as a synchronous motor or an output voltage when operating the synchronous machine as a synchronous generator. 
     “About”, “approximately”, “substantially”, and similar terms, as may be used herein, are relative terms and indicate that, although two values may not be identical, their difference is such that the apparatus or method still provides the indicated or desired result, or that the operation of a device or method is not adversely affected to the point where it cannot perform its intended purpose. 
     The subject matter described herein is provided by way of illustration for the purposes of teaching, suggesting, and describing, and not limiting or restricting. Combinations and alternatives to the illustrated embodiments are contemplated, described herein, and set forth in the claims. Various modifications and changes may be made to the subject matter described herein without strictly following the embodiments and applications illustrated and described, and without departing from the scope of the following claims. 
     The subject matter described above is provided by way of illustration only and are not to be construed as limiting. Various modifications and changes may be made to the subject matter described herein without following the exemplary embodiments and applications illustrated and described herein. Although the subject matter presented herein has been described in language specific to components, features, and operations, it is to be understood that the appended claims are not necessarily limited to the specific components, features, or operations described herein. Rather, the specific components, features, and operations are disclosed as example forms of implementing the claims.