Patent Publication Number: US-8542085-B2

Title: High frequency rotary transformer for synchronous electrical machines

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to synchronous electrical machines, and more particularly relates to transformers used in connection with wound-rotor synchronous machines and the like. 
     BACKGROUND OF THE INVENTION 
     Modern wound-rotor synchronous machines typically require a stationary rotor field to interact with the stator field and produce torque at the machine shaft. The power to produce this stationary field is supplied from outside the motor in the form of DC current. Since the rotor of the machine rotates, it is necessary to supply power to the rotor through a rotating interface. Typically, this rotating interface is achieved through the use of brushes (stationary side) and slip rings (rotating side). This approach can be unsatisfactory with respect to long term durability (e.g., wear-out of brushes) and reliability (degradation of brush-to-slip-ring electrical contact in adverse environments). 
     Another approach, seen primarily in the power generation industry for large generators, is the use of a low frequency rotating transformer. The primary winding of the transformer is connected to the power grid through a rheostat or an autotransformer in order to adjust the input power. The secondary winding of the transformer rotates together with the rotor of the synchronous generator. A solid state or mechanical rectifier converts the AC power from the transformer secondary into DC power to be supplied to the field winding of the generator. Since such transformers operate at a relatively low grid frequency (e.g., 60 Hz), such a devices tend to be prohibitively large and heavy. 
     Accordingly, there is a need for more compact and efficient transformer designs for use in wound-rotor synchronous machines. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION 
     In accordance one embodiment of the invention, a high frequency rotary transformer for an electrical machine includes a primary transformer component having a primary transformer winding, and a secondary transformer component having a secondary transformer winding. The primary transformer winding is configured to be coupled to a DC power source via a DC-AC converter (inverter). The secondary transformer winding is configured to be coupled (e.g., indirectly, through a rectifier/filter circuit) to a winding of the rotor. Each of the primary and secondary transformer components are mechanically coupled to either the stator or the rotor. The secondary transformer component is configured to rotate with respect to the primary transformer component. The AC current in the primary produces a magnetic flux via the primary transformer winding and the secondary transformer winding. 
     A rotary transformer power supply system in accordance with one embodiment includes an inverter module configured to receive a DC input and a rotor current command; a rotor having a rotor winding provided therein; a rotary transformer, the rotary transformer comprising: a primary transformer component having a primary transformer winding, the primary transformer winding configured to be coupled to the inverter module; and a secondary transformer component having a secondary transformer winding coupled to the winding of the rotor, wherein each of the primary and secondary transformer components are mechanically coupled to either the stator or the rotor; and wherein the secondary transformer component is configured to rotate with respect to the primary transformer component to produce a magnetic flux via the primary transformer winding and the secondary transformer winding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIG. 1  is a conceptual block diagram of a rotary transformer power supply system associated with a synchronous machine in accordance with one embodiment; 
         FIG. 2  is a schematic cross-sectional views of an axial gap rotary transformer in accordance with one embodiment; and 
         FIG. 3  is a schematic cross-sectional views of a radial gap rotary transformer in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, embodiments of the present invention relate to compact, light-weight, high frequency rotary transformers configured to provide power to the field windings of a wound rotor synchronous machine. For simplicity and clarity of illustration, the drawing figures depict the general structure and/or manner of construction of various embodiments. Elements in the drawings figures are not necessarily drawn to scale: the dimensions of some features may be exaggerated relative to other elements to assist understanding of the exemplary embodiments. In the interest of conciseness, conventional techniques, structures, and principles known by those skilled in the art may not be described herein, including, for example, fundamental principles of motors and rotary machines, and basic operational principles of transformers. 
     Referring to the conceptual block diagram shown in  FIG. 1 , a rotary transformer power supply assembly (or simply “assembly”)  100  generally includes an DC-AC converter (inverter)  104  (and associated control processor or “processor”  105 ) electrically coupled to a synchronous machine rotor winding  116  through a rotary transformer  112  and rectifier/filter module  114 . Thus, assembly  110  implements a DC-to-DC converter in which stationary components  130  are electrically coupled to rotating components  140  via rotary transformer  112 , as described in further detail below. 
     Inverter  104 , which may be a conventional switched power supply inverter known in the art, is coupled to a DC input  102 —e.g., DC power from a traction bus of the type used in connection with hybrid electric vehicles. Inverter  104  also accepts rotor current commands  108  from, and sends status reports  110  to, an inverter control processor  106 . Processor  105  receives the current command  108 , controls the power conversion process, achieves supervisory and protection functions, and provides status reports  110  back to inverter control processor  106 . Thus, the received rotor current command  108  is impressed upon the field windings of rotor  116  (through rotary transformer  112  and module  114 ). 
     Referring to the conceptual cross-sectional view shown in  FIG. 2 , a rotary transformer  112  in accordance with one embodiment of the invention will now be described. As shown, rotary transformer  112  includes a generally disc-shaped primary component  212  having primary transformer winding  230  (collectively referred to herein as the “primary”), and a corresponding secondary component  214  having secondary transformer winding  232  (collectively referred to herein as the “secondary”). As a gap is provided between primary  212  and secondary  214  in the axial direction (i.e., along rotational axis  205  of motor shaft  206 ), the embodiment illustrated in  FIG. 2  is generally referred to as an “axial-gap” rotary transformer. It will be understood that  FIG. 2  is a simplified, schematic illustration that is not necessarily drawn to scale and which in practical embodiments might include additional conventional motor components. 
     With continued reference to  FIG. 2 , primary  212  is mechanically coupled to the stator (not shown) as illustrated. Secondary  214 , on the other hand, is coupled to a rotor  208 —e.g., a rotor stack having corresponding rotor windings  210 . In alternate embodiments, primary  212  may be coupled to the stator, while secondary  214  is coupled to rotor  208 . Electrical contacts  202  provide connections from primary winding  230  to the stationary switched-mode power supply (i.e., inverter  104  of  FIG. 1 ). A conventional rectifier/filtering circuit  216  (corresponding to block  114  in  FIG. 1 ), is also mechanically coupled to rotor  208  and is electrically coupled between transformer windings  232  and rotor winding  210 . During operation, rotor  208 , rectifier/filtering circuit  216 , secondary  214 , and motor shaft  206  rotate with respect to primary  212  and the associated stator (not shown). As a result, a flux path  204 , independent of the rotor speed or position is generated by via windings  230  and  232 , thereby providing the commanded power to winding  210 . 
     Rotary transformer  112  may be fabricated in a variety of ways and using a variety of known materials. In one embodiment, for example, rotary transformer  112  comprises a ferrite rotary transformer. The segmentation of the core of rotary transformer  112  as shown improves robustness, preventing the magnetic material of the core from fracturing under vibration if a brittle material (such as ferrite) is used. The size of transformer  112  may be selected to achieve the desired performance based on rotor size, stator size, etc. 
     Referring now to  FIG. 3 , an alternate embodiment of rotary transformer  112  will now be described. Unlike the embodiment shown in  FIG. 2 , the illustrated embodiment includes a radial-gap between the transformer&#39;s primary and secondary components. More particularly, rotary transformer  112  in this embodiment includes a primary component  312  having a primary transformer winding  332  (collectively referred to herein as a “primary”), and a corresponding secondary component  314  having a secondary transformer winding  330  (collectively referred to herein as a “secondary”). A gap is provided between primary  312  and secondary  314  in the radial direction (i.e., extending radially from rotational axis  305 ). The embodiment illustrated in  FIG. 3  is generally referred to as a radial-gap rotary transformer. 
     Primary  312  is mechanically coupled to a stator  308  having stator windings  310 , as illustrated. Secondary  314  is mounted within a rotor hub  320 , and rotates therewith. In alternate embodiments, primary  312  may be coupled to rotor hub  320 , while secondary  314  is coupled to stator  308 . Electrical contacts  302  provide connections from primary winding  332  to the stationary switched-mode power supply (e.g., inverter  104  of  FIG. 1 ). A suitable rectifier/filtering circuit is incorporated into rotary transformer  112  adjacent the secondary core of the transformer. During operation, rotor hub  320 , secondary  314 , and rectifier/filter rotate with respect to primary  312  and stator  308 . As a result, a flux path  304  is generated by via windings  330  and  332 , thereby providing the commanded power to rotor winding. 
     It will be appreciated that, in accordance with the embodiment shown in  FIG. 3 , nesting rotary transformer  112  within motor rotor hub  320  saves space by reducing the total length of the electrical machine. That is, rotary transformer  112  does not extend, in the axial direction, beyond rotor hub  320  itself. Furthermore, since the outer portion of transformer  112  is coupled to the rotor, the resulting centrifugal forces exerted on the rotor winding tends to push the winding inside the structure. In this way, winding retention at high rotor speeds is achieved automatically. 
     It is desirable that the magnetic flux ( 304 ,  204 ) in the core of rotary transformer  112  be independent of the angular position between the transformer stationary part (stator, or primary) and rotating part (rotor, secondary). In accordance with the embodiments of  FIGS. 2 and 3 , when the rotor of the transformer rotates with the rotor of the motor at any speed, the voltage induced into it by the primary does not change, regardless of the relative speed between the primary and secondary. 
     In various embodiments, to achieve high power density, the rotating transformer is preferably cooled with a fluid such as a conventional oil. For example, oil provided from an automotive transmission may be introduced between the moving surfaces of rotary transformer  112 . Oil passages may then be provided into the rotor and/or stator for winding cooling. As depicted in  FIG. 3 , an oil path  350  may be provided for lubricating the respective surfaces of rotary transformer  112 . 
     In accordance with one embodiment, in order to compensate for any axial play in the motor rotor  320 , which might bring misalignment between the components of transformer  112 , one of the components is preferably configured to be thicker in the axial direction by an amount equal to the maximum axial play value. In this way, the flux ( 204 ,  304 ) through the transformer  112  will be substantially invariant within the axial play limits of the rotor. 
     It will be appreciated that the rotary transformer  112  illustrated in  FIGS. 2 and 3  is a high frequency transformer typically on the order of tens or hundreds of kilohertz or higher. This is in contrast to large, low frequency transformers that operate at a frequency of on the order of 60 Hz. 
     In accordance with the illustrated embodiments, the windings  230  and  232  of  FIG. 2 , and the windings  330  and  332  of  FIG. 3  consist of continuous toroids, rather than being segmented windings as in many prior art transformers. 
     In summary, what has been described is an improved rotary transformer design to power the field winding of wound rotary synchronous machines. By using segmented primary and secondary transformer components as shown, a very compact, light, and manufacturable high frequency power supply is provided. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.