Abstract:
A plasma generating apparatus includes a plurality of discharge cells in which a gas is excited by a high frequency excitation signal produced at an inverter. Each of a plurality of transformers couples the excitation signal from the inverter to one of the discharge cells, thereby forming a separate resonant circuit that has a resonant frequency. A gap in the transformer core creates a stray magnetic field outside the transformer. The plurality of transformers are in close proximity to each other so that the stray magnetic field from one transformer is coupled to at least one other transformer. Coupling the stray magnetic fields between transformers results in each resonant circuit resonating at the same frequency, thereby compensating for manufacturing tolerances and changes in operating conditions of the discharge cells that otherwise affect the resonant frequency of a given circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to plasma discharge devices, such as for generating ozone, for example; and more particularly to the high voltage power supply for such plasma discharge devices. 
         [0005]    2. Description of the Related Art 
         [0006]    High energy plasmas are used for a variety of purposes, such as ionizing gas for the generation of ozone or to reduce undesirable nitrogen oxide automobile emissions.  FIG. 1  shows a block diagram of a conventional apparatus for generating ozone and is typical of most equipment for generating a plasma with different types of gases. The high volume plasma generator  10  comprises a plurality of plasma discharge cells  12 ,  13 , and  14  each having the schematic design shown for the first cell  12 . The plasma discharge cell includes a chamber  16  containing the gas that is to be excited to produce the plasma. The chamber may be closed or, as is the case for an ozone generator, may have a passageway into which oxygen enters and the generated ozone exits. A pair of electrodes  17  and  18  are spaced apart on opposite sides of the chamber  16 . When a high voltage is applied across the electrodes, the gas within the chamber  16  is excited, thereby producing the plasma that coverts the incoming oxygen (O 2 ) into ozone (O 3 ). Each plasma discharge cell exhibits a large capacitance load. 
         [0007]    The plasma discharge cells  12 - 14  are driven by a power supply which receives alternating electric current at an input to an inverter  20 . The inverter  20  converts the line frequency of the input electric current to a higher frequency suitable for exciting the gas of interest. The output of the inverter  20  is coupled by an inductor/choke  22  to a set of high voltage transformers  24 ,  25 , and  26  connected in parallel. Each transformer  24 ,  25 , and is associated with a different one of the plasma discharge cells  12 ,  13 , and  14 , respectively. 
         [0008]    The capacitive load of each plasma discharge cell  12 - 14  is reflected through the respective high voltage transformer  24 - 26  and the choke  22  to the electronics of the inverter  20 . That capacitive load can vary dynamically due to manufacturing tolerances of the plasma generator, as well as variation of the pressure, temperature, and flow rate of the gas being excited. The combination of that capacitive load along with the inductance and resistance of the associated power supply branch form a separate series resonant circuit for each plasma discharge cell. Although those resonant circuits have identical designs to theoretically resonant at the same frequency, the manufacturing tolerances and dynamic gas parameter variations cause each circuit branch to have a different resonant frequency. Nevertheless a single inverter  20  is employed to simplify tuning of the resonance and to eliminate beat frequencies that would exist if multiple inverters were employed in the same plasma generator. 
         [0009]    A disadvantage with such conventional power supplies for multiple plasma discharge cells is the relatively large size of the magnetic components, i.e. the choke  22  and transformers  24 - 26 , which significantly add to the cost and weight of the apparatus. 
         [0010]    Furthermore, conventional design practice dictates that each transformer for a multiple cell plasma generator be constructed so that its primary and secondary coils are tightly coupled magnetically to reduce stray magnetic fields by minimizing the internal flux leakage. The sum of the transformer leakage inductance and the external choke inductance create an aggregate inductance that ultimately balances the capacitance of the associated plasma discharge cell. In other words, each transformer has a core that maximizes the conductance of magnetic flux between the primary and secondary coils. 
         [0011]    Furthermore, standard engineering practice is to physically separate the transformers  24 - 26  and the choke  22  by an amount that minimizes the stray magnetic field coupling between those components and to the enclosure of the power supply. Metal objects within such stray magnetic fields become heated to undesirable temperatures. However, separating the magnetic components from each other and from other metal objects within the apparatus has the drawback of requiring a significant amount of empty space within the device. Therefore, conventional design practice dictates that it is desirable to tightly couple the primary and secondary coils of each transformer so as to minimize the stray fields originating from the component. 
       SUMMARY OF THE INVENTION 
       [0012]    A plasma generator includes a plurality of plasma discharge cells for exciting a gas to produce a plasma. A signal generator produces an excitation signal having a high frequency, which is between 2 kHz and 30 kHz for ozone generators. The excitation signal is applied to a separate transformer for each plasma discharge cell. 
         [0013]    Each transformer has a ferromagnetic core on which is wound a primary coil that is connected to the generator. Also wound on the core is a secondary coil connected to one of the plasma discharge cells, thereby forming a resonant circuit having a resonant frequency. Considered individually, each resonant circuit typically has a different resonant frequency due to component manufacturing tolerances and variation in the dynamic operating conditions of the respective plasma discharge cell. The core has at least one gap, thereby producing a stray magnetic field outside the transformer. The transformers are placed in close proximity to each other so that the stray magnetic field from one transformer is coupled to at least one other transformer. 
         [0014]    During operation of the plasma generator, the leaky coupling of a given transformer allows the stray magnetic fields from the adjacent transformers to influence the resonant frequency of the resonant circuit containing the given transformer. The present invention intentionally cross couples the stray magnetic fields among the plurality of transformers which results in circuits resonating at substantially the same frequency. This enables a common signal generator to produce a single excitation frequency that efficiently drives all the plasma discharge cells. 
         [0015]    In the preferred embodiment of each transformer, the ferromagnetic core is annular with opposing first and second side legs and first and second cross legs providing separate flux paths between the side legs. The primary coil is wound around the first side leg and the secondary coil is wound around the second side leg, which separates the coils and further increases the loose magnetic coupling there between. 
         [0016]    Preferably the transformer core is formed by a pair of U-shaped sections. The first U-shaped section includes a first leg and a second leg, parallel to each other. The second U-shaped section has a third leg in a spaced apart alignment with the first leg and having a fourth leg in a spaced apart alignment with the second leg. Thus two gaps are created between the legs of the first and second U-shaped sections. The first and third legs combine to form the first side leg of the core, while the second and fourth legs combine to form the second side leg. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic electrical diagram of a previous plasma discharge device; 
           [0018]      FIG. 2  is a schematic electrical diagram of a plasma discharge device incorporating the present invention; 
           [0019]      FIG. 3  is a top view of a transformer used in the present power supply for a plasma discharge device; 
           [0020]      FIG. 4  is a side view of the transformer; 
           [0021]      FIG. 5  is a cross sectional view along line  5 - 5  in  FIG. 3 ; 
           [0022]      FIG. 6  illustrates one arrangement of three transformers according to the present invention; 
           [0023]      FIG. 7  is a second arrangement of three transformers; and 
           [0024]      FIG. 8  illustrates a third arrangement of a plurality of transformers. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    With reference to  FIG. 2 , a plasma generator  30  according to the present invention has a conventional inverter  28  with a high frequency output (e.g. 2 kHz to 30 kHz) that is connected directly to the primary coil of a separate transformer  34 ,  35 , and  36  for each of three plasma discharge cells  37 ,  38 , and  39 , respectively. It should be understood that the present invention has applicability to a plasma discharge system having two or more plasma discharge cells and thus could have a different number of cells and transformers than is shown in the drawings. The term “directly connected” as used herein means that the associated components are electrically connected to one another without the intervention of any impedance, other than that inherently present in any conductor or cable. Each transformer  34 - 36  couples the inverter  28  to the electrodes  41  within one of the plasma discharge cells  37 - 39 . As noted previously, each plasma discharge cell  37 - 39  exhibits a significant capacitive load. The combination of a transformer  34 ,  35 , and  36  and the associated plasma discharge cell  37 ,  38 , and  39 , respectively, forms a branch  31 ,  32  and  33  of the electrical circuit for the plasma generator  30 . Each branch  31 ,  32  and  33  is a separate resonant circuit. 
         [0026]      FIGS. 3 ,  4  and  5  depict the first transformers  34  with the understanding that the other transformers  35  and  36  have an identical construction. The first transformer  34  comprises a rectilinear, annular core  40  on which a primary coil  42  and a secondary coil  44  is mounted. The turns ratio of the primary and secondary coils is selected to increase the voltage of the excitation signal from the inverter to the level necessary to excite the gas and produce a plasma in the respective discharge cell. The core  40  has a first side leg  51  and second side leg  52  parallel to each other on opposite sides of the core with one end of those first and second side legs being connected by a first cross leg  53  and the other ends of the side legs being connected by a second cross leg  54 . The first and second cross legs  53  and  54  provide flux paths between the first and second side legs  51  and  52 . 
         [0027]    With particular reference to  FIG. 5 , the core  40  comprises first and second U-shaped sections  48  and  49 , respectively, both of which are fabricated of a ferromagnetic material commonly used in transformer cores. The upper, first section  48  comprises the first cross leg  53  and first and second substantially parallel section legs  55  and  56 . The lower, second section  49  comprises the second cross leg  54  and third and fourth substantially parallel section legs  57  and  58 . When the core  40  is assembled the core sections are placed facing each other with the first section leg  55  aligned with the third section leg  57  and the second section leg  56  aligned with the fourth section leg  58 . 
         [0028]    The first side leg  51  extends the primary coil  42  while the second side leg  52  extends the secondary coil  44 . Preferably the side legs have a circular cross section to facilitate winding the wires of each coil. One end of the wire forming the secondary coil  44  terminates at a high voltage terminal  46  for connection an electrode in the plasma discharge cell. In the exemplary transformer, the other end of the wire for the secondary coil  44  is attached to the transformer core  40 , which is connected to the circuit ground of the plasma generator. The other plasma discharge cell electrode also is connected to the circuit ground. In an alternative embodiment, a second terminal is provided for the other end of the secondary coil. 
         [0029]    The core  40  is intentionally designed to provide a loose electromagnetic coupling between the first and section sections  48  and  49 , and between the primary and secondary coils  42  and  44 . Specifically, those core sections are spaced apart by bodies  50  of electrical insulating material, that is up to one-quarter inch thick, for example. In should be understood that at very high frequencies, the gap can be reduced in thickness and even eliminated if sufficient leakage flux and significant stray magnetic fields still exist. This creates a gap between the two core sections  48  and  49  around which the magnetic fields must bridge to couple the two core sections  48  and  49 . This construction thereby creates the electrical equivalence of a choke in the circuit of the transformer, thus providing a high leakage inductance. Whereas conventional design wisdom dictates that the transformer core not have gaps in order to provide a tightly coupled transformer with minimum flux leakage, the present design intentionally incorporates gaps to create inductance leakage or leakage flux to balance the capacitance of the associated plasma discharge cell. As a result of that leakage flux, a significant stray magnetic field is generated outside the transformer. 
         [0030]    Conventional design practice also is contradicted with respect to positioning a plurality of transformers in a plasma generator with multiple discharge device cells, as shown in  FIG. 2 . Specifically, standard engineering practices dictate that transformers, which are loosely coupled and thus produce large stray magnetic fields, should be spaced far apart from each other and from other metal objects. That practice prevents the stray magnetic fields emitted by one transformer from being coupled to another transformer or metal component. 
         [0031]    Instead, as shown in  FIG. 6 , the three transformers  34 ,  35 , and  36 , for the present plasma generator  30  in  FIG. 2  are placed close together so that their stray magnetic fields are coupled into one or more adjacent transformer. Specifically, the transformers are aligned so that their secondary coils  44  are adjacent each other and face in the same direction (e.g. upward in the drawing), and the primary coils  42  are adjacent each other facing in the opposite direction. Preferably the primary coils  42  are spaced apart by the same distance as the secondary coils  44 , but that does not have to be the case. Because of the different diameters of the primary and secondary coils, the array of transformers forms an arc, which is even more pronounced in a plasma generator with additional transformers. As noted previously, the transformers  34 - 36  are placed sufficiently close together so that the leakage flux from one transformer is coupled into the adjacent transformer or transformers. For example, the spacing can vary from zero, where the coils contact each other, up to one inch, for example; with the range 0.0″ to 0.3″ being preferred where each circuit branch is rated up to 600 watts with a 4 kilovolt secondary. The distance depends upon the power levels and the number of transformers so that even greater distances may be possible with transformers for larger power plasma generators. Due to this relatively close spacing, the fields generated by the primary coils interact with each other and the separate fields generated by the secondary coils interact with each other. 
         [0032]    During operation of the plasma generator  30  shown in  FIG. 2 , the leaky coupling of the transformers aids in tuning the entire system to resonate a single frequency. Considered individually, each circuit branch  31 ,  21  and  33  of the plasma generator circuit typically has a different resonant frequency due to component manufacturing tolerances and variation in the dynamic operating conditions of the respective plasma discharge cell. Such resonant frequencies can differ by 15%-20% in the same plasma generator. However, the loose coupling of a given transformer allows the stray magnetic fields from the adjacent transformers to influence the resonant frequency of the circuit branch  31 - 33  containing the given transformer. In other words, the intentional cross coupling of the stray magnetic fields among the transformers  34 - 36  causes all the circuit branches  31 - 33  to resonate at substantially the same frequency. This enables a common inverter which produces a single excitation frequency to drive all the plasma discharge cells  37 - 39  efficiently, without requiring a large external choke. Therefore, the cross flux leakage coupling provided in the present invention not only compensates for manufacturing tolerance variation among the different transformers and plasma discharge cells, it also compensates for dynamic variance of the effective capacitance of each plasma discharge cell  37 - 39  due to fluctuations in the pressure, temperature, or flow rate of the gas being excited. That coupling also enables the use of smaller transformers for the same power rating as compared with a conventional plasma discharge devices that employ tightly coupled transformers spaced significantly apart. 
         [0033]      FIG. 7  illustrates an alternative device placement in which the three transformers  37 - 39  nest into each other with the primary coils  42  facing in one direction and the secondary coils  44  facing in an opposite direction. Specifically, a separate recess  60  is created between the primary and secondary coils  42  and  44  on both sides of each transformer  34 ,  35 , and  36 . When the array of transformers is assembled, the secondary coil  44  of the middle transformer  35  is arranged so as to nest into the recesses  60  provided in the outside transformers  34  and  36 . In addition, the primary coils  42  of those outside transformers  34  and  36  nest in the recesses  60  provided on opposite sides of the middle transformer  35 . This cross couples the leakage flux among the transformers. 
         [0034]    A further alternative arrangement is shown in  FIG. 8 , in which the outer transformers  34  and  36  are inverted with respect to the middle transformer  35 . In this arrangement, the larger secondary coil  44  of each transformer fits into the recess  60  in the adjacent transformer. This third alternative, while theoretically possible, has several practical disadvantages as it requires phase compensation of the electrical signals. In addition, this structure creates a power supply that is more sensitive to the load power factors and is more difficult to manage electrically. 
         [0035]    The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.