Abstract:
A rotating power transformer comprises a primary magnetic core with a primary winding and a secondary magnetic core with a secondary winding. The magnetic cores preferably comprise a ferrite material and are U-shaped, and have a base connecting two legs. The width of the legs is significantly larger than the height of the base, resulting in a better magnetic coupling and a significantly improved tolerance to mechanical deviations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from and benefit of European Application No. 14198919.4 filed on Dec. 18, 2014, the disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to contactless rotary joints and, specifically, to contactless rotary joints configured to transfer high levels of electrical power, also called rotating power transformers. Such contact-less rotary joints may be used in CT scanners. 
         [0004]    2. Description of Relevant Art 
         [0005]    A contactless rotary joint comprising an inductive power coupler is disclosed in US 7,197,113 B1. Such a rotary joint is able to transfer power of more than hundred kilowatts from a stationary part to a rotating part. Such rotary joints have heavy iron or ferrite cores for guiding the magnetic fields. For example, in CT scanners a free bore diameter of more than one meter is required. Accordingly, the inner diameter of such a rotary joint may be more than 1 meter requiring large and massive mechanical support structures. 
         [0006]    The European patent publication EP 1 481 407 B1 discloses a rotating transformer with a winding form made of a plurality of shaped parts held within a U-shaped ring. 
         [0007]    An inductive rotary joint having U-shaped ferrite cores is further disclosed in U.S. Pat. No. 5,608,771. 
       SUMMARY 
       [0008]    The embodiments are directed to improving rotating power transformers by simplifying the mechanical design, and increasing robustness and the ability to withstand large centrifugal forces and reliability while decreasing weight. 
         [0009]    In an embodiment, a rotating power transformer has a stationary part and a rotating part. Herein the basic structure and function of such parts are shown. Generally it is preferred, when the rotating transformer is symmetrical, having basically identical stationary and rotating parts. Of course, these parts may differ to meet specific needs of the stationary or rotating parts like in the means for fixation to a machine. At least one of the stationary and rotating parts, preferably both are based on a body which preferably is ring shaped. It may have the shape of a disk or a drum. The main function of the body is to give a stable support to the electric and magnetic components of the rotating power transformer. The body may be further supported by parts of a machine, like a CT scanner, into which the power transformer is integrated. The body may be made of metal, like aluminum or of plastic material which preferably is further reinforced. It is preferred, to make the body from electrically isolating and non-magnetic material. The body preferably has circular shape. It may also have different shapes adapted to the machine. 
         [0010]    The rotating power transformer further has a primary side and a secondary side. The primary side may either be at the stationary side or at the rotating side, whereas the secondary side is at the other side. The primary side has at least a primary winding comprising at least one primary turn. The secondary side comprises at least one secondary winding having at least one secondary turn. For guiding the magnetic fields between the primary and secondary side, a magnetic material, preferably a soft magnetic material is provided. Most preferably, the magnetic material is a ferrite material. In an alternate embodiment, an iron powder material may be used. Whereas the ferrite material is preferred for higher frequencies, the iron powder material offer a higher Q at lower frequencies. Both materials offer high mechanical strength and can be machined precisely to desired dimensions. Most preferably, ferrite is used due to its high hardness and low thermal expansion. Ferrite materials may be precision machined by grinding, which allows a precise definition of an airgap between the rotating parts. 
         [0011]    As in large devices, like in CT scanners, the rotating power transformer has a diameter of about 0.5 m to 1.5 m, it is impossible to build a magnetic core of one piece. Instead, the magnetic cores for coupling the magnetic fields are assembled from a plurality of small segments. 
         [0012]    As known in the art, magnetic cores are designed such that the magnetic cross-section is approximately constant along the magnetic path, resulting in a constant flux density through the cores. In case of a non-constant cross-section, the section of a magnetic core having the smallest cross-section will be saturated first under high flux densities. This will lead to a reduced inductance and heat dissipation in the magnetic core. Therefore, it is desirable to have a constant magnetic cross-section to obtain the broadest operating range with minimized ferrite material requirement. Accordingly, standard U-shaped ferrite cores have a pair of legs and a base with the same cross section of ferrite material. 
         [0013]    Tests with a large number of magnetic materials and ferrite cores as well as magnetic simulations and calculations have shown that a magnetic core for a rotating power transformer should be designed differently. In contrast to the prior art, the legs have a significantly larger cross-section than the base. Preferably, assuming a constant thickness, the width of each leg is between 1.5- to 5-times the height of the base. Most preferably, the width of each leg is between 2- and 3-times the height of the base. In a most preferred embodiment, the width of each leg is about 2.5-times the base height. 
         [0014]    When designing normal magnetic cores like for stationary transformers, an air gap must not be considered, as this is negligible. In rotating power transformers, there is at least a minimum air gap of 0.1 mm which may be up to 3 mm. Tests have shown, that this air gap is not constant. It changes with rotation due to asymmetries in the mounting. Furthermore, due to the large weight of the rotating part in CT scanners, there is a non-negligible effect of gravitational forces further deforming the parts and resulting in an air gap with non-constant size. Tests under large electrical powers have shown that there are also significant magnetic forces when operating the rotating transformer with high magnetic flux, which also leads to a further change of the geometry of the air gap. By using significantly broader legs, the magnetic coupling is improved. The magnetic flux is distributed over a larger area of the larger legs. This results in a better coupling, a larger main inductivity of the transformer, which further results in lower losses in an inverter driving the transformer. Due to the wider legs, the stray flux outside of the area between the legs is also significantly reduced. Therefore, the influence of and to external components is minimized and the losses in the windings of the rotating power transformer are also reduced as the magnetic field penetrating into the windings is reduced. Another benefit of the wide legs is an increased tolerance against displacement between the primary and secondary cores&#39; legs. 
         [0015]    In a further embodiment, the magnetic cores are held by a base body which may be a circular drum, cylinder, or disk. Preferably, the base body has a groove for holding the magnetic cores. It is further preferred, if the width of the groove is larger than the width of the magnetic cores, generating a gap between a side wall of the groove and a side wall of the cores. It is further preferred, if this gap is at least partially filled with a filler. This filler may be a soft or elastic material to compensate for forces generated by thermal expansion of the materials of the base and the cores. In a preferred embodiment, where the base is of a plastic material and the magnetic cores are of a ferrite material, both materials have a different thermal expansion coefficient resulting in different expansion when temperature changes. Therefore, the ferrite cores may expand differently than the gap in the base body, such that the gap in the base body may either be too narrow or too wide for the magnetic cores. An elastic filler in the gap may help to hold the magnetic cores in the gap. It is further preferred, with defined thermal expansion coefficients of the magnetic cores, the filler, and the base body, that the dimensions are selected such that the thermal expansion of the magnetic cores plus the thermal expansion of the filler is identical to the thermal expansion of the gap in the base body. 
         [0016]    In another embodiment, the ferrite cores have a winding space height and a winding space width which is adapted to the size of the wire wound into the winding space. In a preferred embodiment, a certain number of turns which may be between 2 and 10 turns, is wound into the winding space of the magnetic cores. Preferably, the turns are wound in one layer. This may help to keep parasitic capacitances low and simplify the mechanical assembly. It is now preferred, if the winding space width is approximately N-times the winding space height with a number N of the windings. 
         [0017]    According to a further embodiment, a shield is provided around the magnetic cores. The shield may be made of a thin layer of a conductive material, like aluminum or copper. The shield may be provided under the at least one magnetic core or mounted to the side of the body which is opposite to the groove. 
         [0018]    In a preferred embodiment, the shield is placed on the backside of the body forming a closed ring with roughly same dimensions as the ferrite but overlapping them (smaller inner and larger outer diameter). The ring may comprise overlapping ring segments, preferably of sheet metal bolted or soldered or welded together. The sheet metal can serve as magnetic shield reducing magnetic stray fields and as electric shield defining capacitive parasitic currents when connected (directly or through a capacitor) to a ground potential or a potential of the power circuitry. 
         [0019]    In a further preferred embodiment, the magnetic cores are U-shaped. According to another embodiment, there may also be used E-shaped ferrite cores, where the leg width is also larger than the base height. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In the following, the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment and with reference to the drawings. 
           [0021]      FIG. 1  shows a first embodiment of a disk-shaped rotating power transformer. 
           [0022]      FIG. 2  shows a second embodiment of a drum-shaped rotating power transformer. 
           [0023]      FIG. 3  shows a sectional view of a rotating power transformer. 
           [0024]      FIG. 4  shows dimensions of a magnetic core. 
           [0025]      FIG. 5  shows displaced primary and secondary cores. 
           [0026]      FIG. 6  shows a sectional view of a magnetic core mounted into a base. 
           [0027]      FIG. 7  shows a further sectional view of a magnetic core mounted into a base. 
           [0028]      FIG. 8  shows a CT scanner. 
       
    
    
       [0029]    While embodiments of the invention can be modified and altered, specific embodiments thereof are shown by way of example in the drawings and further described in detail. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0030]    In  FIG. 1 , a first embodiment of a disk-shaped rotating power transformer  100  is shown. A primary magnetic core  110  having a primary winding  111  is rotatable against a secondary magnetic core  120  having a secondary magnetic winding  121 . Between the primary magnetic core  110  and the secondary magnetic core  120  is an air gap  130  in a plane orthogonal to the rotation axis  150 . 
         [0031]    In  FIG. 2 , a second embodiment of a drum-shaped rotating power transformer is shown. A primary magnetic core  110  having a primary winding  111  is rotatable against a secondary magnetic core  120  having a secondary magnetic winding  121 . Between the primary magnetic core  110  and the secondary magnetic core  120  is an air gap  130  on a cylinder surface coaxial with the rotation axis  150 . 
         [0032]    In  FIG. 3 , a sectional view of a rotating power            
         [0033]               d described herer transformer is shown. A primary magnetic core  110  having a primary winding  111  is rotatable against a secondary magnetic core  120  having a secondary magnetic winding  121 . Between the primary magnetic core  110  and the secondary magnetic core  120  is an air gap  130 . 
         [0034]    In  FIG. 4 , dimensions of a magnetic core are shown. The primary magnetic core  110  has essentially the same dimensions as the secondary magnetic core  120 . Therefore, reference is only made to the primary magnetic core  110 . It has two legs  118  and a base  119  connecting both legs. The legs  118  have a leg width  152 , which is larger than the base height  151 . Preferably, the width of each leg  152  is between 1.5- to 5-times the height of the base. Most preferably, the width of each leg  152  is between 2- and 3-times the height of the base. In a most preferred embodiment, the leg width  152  is about 2.5-times the base height  151 . The magnetic core has a winding space with a winding space height  161  and a winding space width  162 . It is preferred, if the winding space height and the winding space width are adjusted to the diameter  171  of the winding wires forming the individual turns of the winding. In this embodiment, four turns are shown. Accordingly, the winding space width  162  is about 4-times the winding space height  161 . 
         [0035]    In  FIG. 5 , displaced primary and secondary cores are shown. Here, the primary magnetic core  110  is displaced against the secondary magnetic core  120  about a displacement  153  and an air gap width  155 . Due to the large leg widths as shown in the previous figure, there remains still a significant overlap between the primary magnetic core  110  and the secondary magnetic core  120  having an overlap length  154 , which is still larger than the base height  151 . If a regular core as known from prior art would have been used, having a leg width which is the same as the base height, there would be no overlap between the legs of the primary core and the secondary core. Therefore, there would be a very low coupling, which would render the rotating power transformer useless. 
         [0036]    In  FIG. 6 , a sectional view of a magnetic core  120  mounted into a base body  200  is shown. The base body  200  may be a disk- or cylinder-shaped body which preferably has a groove  201  for holding a magnetic core  120 . It is preferred, if the groove  201  is wider than the width of the magnetic core  120 , resulting in a gap. It is further preferred to have the gap filled with a filler  210  which preferably is an elastic material to compensate for different thermal expansions of the magnetic core and the base. 
         [0037]    In  FIG. 7 , a further sectional view of a different embodiment with magnetic core  120  mounted into a base body  200  is shown. Here, there are gaps on both sides of the core, filled with a filler  210 . This results in a better distribution of pressure. In this figure, also a shield  250  is provided. 
         [0038]      FIG. 8  shows schematically a CT (Computed Tomography) scanner gantry. The stationary part is suspended within a massive frame  810 . The rotating part  809  of the gantry is rotatably mounted with respect to the stationary part and rotates along the rotation direction  808 . The rotating part may be a metal disk which supports an X-ray tube  801 , a detector  803  and further electronic and mechanic components. The X-ray tube is for generating an X-ray beam  802  that radiates through a patient  804  lying on a table  807  and which is intercepted by a detector  803  and converted to electrical signals and imaging data thereof. The data obtained by the detector  803  are transmitted via a contactless rotary joint (not shown) to an evaluation unit  806  by means of a data bus or network  805 . Electrical power from a stationary power supply unit  811  may be transmitted by an inductive power coupler  800  to the rotating part. 
         [0039]    It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention is believed to provide a rotating power transformer. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           100  rotating power transformer 
           110  primary magnetic core 
           111  primary winding 
           118  leg 
           119  base 
           120  secondary magnetic core 
           121  secondary winding 
           130  air gap 
           150  rotation axis 
           151  base height 
           152  leg width 
           153  displacement 
           154  overlap length 
           155  air gap width 
           161  winding space height 
           162  winding space width 
           171  wire diameter 
           200  base body 
           201  groove 
           210  filler 
           250  shield 
           800  inductive power coupler 
           801  x-ray tube 
           802  x-ray beam 
           803  x-ray detector 
           804  patient 
           805  network 
           806  evaluation unit 
           807  patient table 
           808  rotation direction 
           809  rotating part 
           810  frame 
           811  power supply unit  10  Gantry