Abstract:
A variable inductor which avoids electrical breakdown of the insulation in the control windings when used in high power applications includes a core formed of a permeable magnetic material, the core having three legs, including a center leg and two outer legs. A main winding element comprising a main conductor is wound around the center leg of the core. A control winding element comprising a control conductor is wound in a figure-eight configuration having a first winding and a second winding around respective outer legs, the winding configuration canceling induced voltages in the first and second windings, wherein a current through the control winding element causes a change in inductance of the main winding element.

Description:
RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/445,214, filed on Feb. 5, 2003, the entire teachings of which are incorporated herein by reference. 

   BACKGROUND 
   Variable inductors can be used in many circuit applications, such as resonant circuits which vary the inductance of circuit elements to vary the resonant frequency of the circuit. An example of a resonant circuit system is described in United States Patent Publication 2002/0121285, the entire teachings of which are herein incorporated by reference. 
   The simplest way to obtain a variable inductor is by mechanical movement of a connector along an inductive element. However, mechanical movement lacks the response time required for real time control. Further, mechanical movement-type variable inductors have a tendency to lock-up magnetically. Therefore, variable inductors have been designed to vary the inductance of a circuit element by means of an electrical signal rather than by mechanical movement. 
   The saturation effect of magnetic materials can be employed to create a current controlled variable inductor. These type of variable inductors typically have a limited variation range of 1 to 10 and suffer from parasitic effects such as capacitance and voltage across each control winding that limit the quality (Q) factor of the inductor. Additionally, such current controlled variable inductors require very high control currents in the range of 0 to 500 mA. 
   The inductance of an inductive circuit element is related to the permeability of the magnetic core and the number of turns: 
   
     
       
         
           
             
               
                 
                   L 
                   = 
                   
                     
                       μ 
                       o 
                     
                     ⁢ 
                     
                       N 
                       2 
                     
                     ⁢ 
                     
                       A 
                       l 
                     
                   
                 
                 ; 
               
             
             
               
                 equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 1 
               
             
           
         
       
     
   
   where L is the inductance of an inductive circuit element; 
   μ o  is the permeability of the magnetic core; 
   A is the cross-sectional area of the magnetic core; 
   N is the number of turns of the inductive element; and 
   l is the length of the inductive element. 
     FIG. 1  illustrates a current controlled variable inductor  10  in which the inductance L 20  of main winding  20  is controlled by the current (Ic) delivered to outer control windings  22  and  24 . Since the center leg  34  is not saturated, the minimum inductance L 20  is limited by the number of turns (N) and the magnetic permeability of the core material of the center leg  34 . The voltage across each control winding  22  and  24  and the parasitic capacitances of control windings  22  and  24  limit the winding ratio and/or the operating frequency. The inductance of the control windings  22  and  24  changes substantially with the control current (Ic). 
   A magnetic core  30  is shown consisting of a magnetic material which can be saturated, with three legs  32 ,  34  and  36 . The outer legs  32  and  36  have identical control windings  22  and  24  that are connected in series. The magnetic path for main winding  20  includes outer legs  32  and  36 , center leg  34  and the connecting portions  40 ,  42 ,  44 , and  46 . If the control current (Ic) through control windings  22  and  24  becomes large enough to saturate the outer legs  32  and  36  of the core  30 , the inductance L 20  of main winding  20  decreases because a portion of the magnetic path for the main winding  20  is saturated. The higher the control current (Ic) is made, the lower the inductance L 20 . However, the center leg  34  will not be saturated due to the control current (Ic). Control windings  22  and  24  are wound and connected such that the magnetic flux (Φ c1 , Φ c2 ) in respective legs  32  and  36  of the core  30  arising from the control current (Ic) through the outer control windings  22  and  24  is equal and points in opposite directions. The opposing magnetic flux (Φ c1 , Φ c2 ) results in cancellation in the center leg  34  of the core  30 . The flux cancellation prevents coupling of AC signals between the main winding  20  and the control windings  22  and  24 . AC voltage applied across the terminals of main winding  20  induces a voltage in both of the control windings  22  and  24 . 
   The induced voltage is related to the magnetic flux Φ c  and the number of turns: 
   
     
       
         
           
             
               
                 
                   
                     e 
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     N 
                     ⁢ 
                     
                       
                         ⅆ 
                         ϕ 
                       
                       
                         ⅆ 
                         t 
                       
                     
                   
                 
                 ; 
               
             
             
               
                 equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 2 
               
             
           
         
       
     
   
   where e(t) is the induced voltage as a function of time; 
   Φ is the magnetic flux 
             (       ⅆ   ϕ       ⅆ   t       )     ;         
and
 
   N is the number of turns of the inductive element. 
   Although the voltages in the control windings  22  and  24  have opposite polarity such that the voltage across the series connection of control windings  22  and  24  have a net zero voltage, the voltage with respect to ground increases with each respective turn of the control windings  22  and  24 . That is, the voltage at point B is greater than the voltage at point A. 
   SUMMARY 
   Although electrically variable inductors exist and provide a sufficient response time and a Q factor required for real-time control, these variable inductors do not perform as specified under high magnetic flux level operating conditions. These conditions produce a high magnetic flux density in the main winding which induces a voltage in the control windings proportional to the turns ratio between the control windings and the main winding. When used in high power applications, the induced voltage is of sufficient strength to result in the electrical breakdown of the insulation in the control windings, resulting in the catastrophic failure of the variable inductor. This effect can significantly limit the power handling capability in such applications. 
   In accordance with the present approach, there is provided a variable inductor which avoids electrical breakdown of the insulation in the control windings when used in high power applications. In one embodiment, the inductor includes a core formed of a permeable magnetic material, the core having three legs, including a center leg and two outer legs. The variable inductor further includes a main winding element comprising a main conductor wound around the center leg of the core and a control winding element comprising a control conductor wound in a figure-eight configuration having a first winding and a second winding around respective outer legs. The winding configuration cancels induced voltages in the first and second windings, wherein a current through the control winding element causes a change in inductance of the main winding element. 
   Various configurations of the variable inductor are contemplated by the present approach. In one embodiment, the variable inductor can include multiple cores magnetically coupled in series with each other. In another embodiment, the variable inductor can include an i-core magnetically coupled across the center leg and two outer legs of the core. 
   In another embodiment, the variable inductor can include an air gap provided in the center leg of the core. A non-magnetic spacer can be inserted in the air gap. In another embodiment, the main conductor and/or the control conductor can be made from Litz wire. 
   In another embodiment, the variable inductor can include a main core formed of a permeable magnetic material, the main core having three legs, including a center leg and two outer legs, a control core formed of a permeable magnetic material, the control core having three legs, including a center leg and two outer legs. The legs of the main core oppose the legs of the control core to provide a magnetic coupling between the legs. A main winding element comprising a main conductor is wound around the center leg of the main core and a control winding element comprising a control conductor is wound in a figure-eight configuration having a first winding and a second winding around respective outer legs of the control core. The winding configuration cancels induced voltages in the first and second windings, wherein a current through the control winding element causes a change in inductance of the main winding element. 
   The variable inductor can include multiple main cores magnetically coupled in series, and multiple control cores magnetically coupled in series. The legs of respective main cores oppose the legs of respective control cores to provide a magnetic coupling between the legs. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  shows a variable inductor according to the prior art; 
       FIG. 2A  shows a perspective view of outer legs of a variable inductor according to the principles of the present invention; 
       FIG. 2B  shows a cross-sectional view of outer legs of the variable inductor of  FIG. 2A ; 
       FIG. 3A  shows a perspective view of another embodiment of the invention; 
       FIG. 3B  shows an exploded view of the embodiment of  FIG. 3A ; 
       FIG. 4A  shows a perspective view of a control winding wound in a yoke configuration on bobbins; 
       FIG. 4B  shows a top view of the control winding of  FIG. 4A ; 
       FIG. 4C  shows a perspective view of the control winding positioned on a magnetic e-core; 
       FIG. 5  shows a perspective view of another embodiment of the invention including multiple magnetic e-cores; 
       FIG. 6  shows a perspective view of another embodiment of the invention; and 
       FIG. 7  shows a perspective view of another embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   A description of preferred embodiments of the invention follows. 
   An ultrasonic continuous processing system is described in detail in United States Patent Publication 2002/0121785, the entire teachings of which are herein incorporated by reference. Generally, the system comprises a processing chamber having an outer wall and an inner wall, the inner wall defining a volume of the processing chamber. The outer wall of the chamber can be constructed of glass, metal, or other suitable material with a piezoelectric actuator mounted on the outer wall. The chamber can be filled with a gas, fluid, or slurry. The piezoelectric actuator (a capacitive element) when coupled with an inductive element forms a series resonant tank circuit. 
   In operation, the series resonant tank circuit of 2002/0121785 can be electrically driven via an oscillator to produce an acoustical wave front within the processing chamber when operated at or near resonant frequency of the container walls. It was observed that the resonant tank circuit could be initially configured to produce a power factor near unity. However, during operation of the processing system, the power factor dropped and the energy efficiency declined because operating conditions of the system components changed. These changes caused component parameter variations which included but were not limited to fluctuations in output frequency of the oscillator; changes in fluid pressure on the chamber walls; and temperature dependent changes in the piezoelectric film, the series inductor and the electrical driver circuit. These changes in system parameters also resulted in a reduction of the power factor and the loss of system efficiency. 
   It became apparent that a control device would be required to maintain a unity power factor while changes occurred in the operating conditions of the ultrasonic processing system. Electrically efficient operation of the resonant circuit occurs when the voltage and current are in phase. When this situation occurs, the circuit is said to have a power factor of unity. A series resonance circuit is produced by a connection of an inductor with a current lag relationship compared to an applied voltage to a capacitor that behaves as a current lead device. When the capacitor and the inductor are out of balance there is a net lag or lead between the phase relationship of the applied voltage to the current in the resonant circuit. This situation is said to have a power factor of less than unity. 
   The present invention provides an electrically controlled variable inductor that is suitable for use, for example, as a control device in high magnetic flux (high power), high Q factor (minimal loss), series resonant tank circuits.  FIGS. 2A and 2B  show a variable inductor  100  according to the principles of the present invention. For illustration purposes only, a main winding about the center leg is not shown in  FIG. 2A  and the center leg of the magnetic core is not shown in  FIG. 2B . A magnetic core  110  is shown consisting of a magnetic material which can be saturated, having three legs  112 ,  114  and  116 . Control windings  120 ,  122  are formed simultaneously on legs  112  and  116  respectively by winding an insulated control conductor in a figure-eight configuration as shown in  FIG. 2B . One revolution around legs  112  and  116  is equal to one-turn (N) of the control windings  120 ,  122 . This step is repeated until a desired number of (N) turns are completed. Typically, several hundred to several thousand turns are used to create the variable inductor  100 . 
   The control conductor can be made from Litz wire. Litz wire consists of a number of insulated strands of individual wires twisted together and electrically connected to each other only at the ends. The use of Litz wire provides a current load capacity to carry the load through the inductor  100 . However, because the wires are insulated from each other they do not have the effective Eddy current losses of a single large wire, or multiple strands of non-insulated wires, that will have greater losses in an alternating magnetic field. 
     FIG. 2B  shows the resulting current flow in the conductor  130  as denoted by current arrow  132 . The current flow creates an opposing magnetic flux Φ in each leg  112  and  116  as denoted by symbols  140 ,  142  respectively. One skilled in the art should understand that if current flowed in the opposite direction from that shown, the resulting magnetic flux Φ would also reverse direction. The figure-eight configuration allows for a turn-by-turn cancellation of induced voltages, i.e. zero volts on the control windings  120 ,  122  when an AC voltage is applied to the main winding (not shown). That is, each successive one-half of a coil turn of the winding has an induced voltage, due to the main winding, in the opposite polarity from its paired half. The induced inter winding voltage between any two loops on a respective leg is also near zero volts. It should be understood by one skilled in the art that the figure-eight configuration can be accomplished by taking a flat wound coil and giving it a 180 degree twist. 
     FIGS. 3A and 3B  show another embodiment of the present invention. A variable inductor  200  includes a main magnetic e-core  202  and a control magnetic e-core  204 . Main e-core  202  includes three legs  206 ,  208 ,  210  and control e-core  204  includes legs  212 ,  214 ,  216 . A magnetic shunt bar or i-core  218  is magnetically coupled to legs  212 ,  214 ,  216  of e-core  204 . A non-magnetic spacer  220  is coupled between the i-core  218  and legs  206 ,  208 ,  210  of e-core  202 . The spacer  220  provides an air gap to reduce the permeability and inductance in the inductor  200 , thereby increasing the magnetizing current in the main winding  222 . Optionally, the air gap can be provided by shortening the leg  208  by grinding or any other known means. A main winding  222  is wound around the leg  208  of e-core  202 . A control winding  224  is wound around legs  212 ,  216  of e-core  204  in a figure-eight configuration as described above. The e-cores  202 ,  206 , i-core  218 , and spacer  220  can be mechanically coupled using a compression assembly consisting of a bottom bar  230 , threaded-rods  232 , top bar  234  and lock down nuts  236 , although it should be understood by one skilled in the art that any suitable means may be used to couple these elements. 
   The magnetic shunt bar  218  includes a smooth surface in contact with the surfaces of the legs  212 ,  214 ,  216  of the control core  204 . The magnetic shunt bar  218  can be notched to accommodate the threaded rods  232  in the compression assembly. The notches assist in the alignment of the magnetic shunt bar  218 . The voltage applied to the control winding  224  attracts the magnetic shunt bar  218  and controls the magnetic flux density and related permeability within the magnetic shunt bar  218 , thereby reducing or increasing the effective permeability of the main e-core  202 . 
     FIG. 4A-4C  show a technique for forming control winding  224 . The control winding  224  can be formed on a bobbins  300 ,  302  as described above. Once formed, the control coil  224  and bobbins  300 ,  302  can be place over legs  212 ,  216  of the control core  204 . The control coil  224  can be held in place by an insulated wire wrapping device, such as tie-wraps, string, or any other suitable device known in the art. 
     FIG. 5  shows another embodiment of a variable inductor  400  including multiple main cores  202   a  . . .  202   n  and multiple control cores  204   a  . . .  204   n . Optional magnetic shunt bar  218   a  . . .  218   n  and non-magnetic spacers may be used. A main winding  222  is wound around the legs  208   a  . . .  208   n  of main e-cores  202   a  . . .  202   n . A control winding  224  is wound around legs  212   a  . . .  212   n ,  216   a  . . .  216   n  of control e-cores  204   a  . . .  204   n  in a figure-eight configuration as described above. 
     FIG. 6  shows another embodiment of a variable inductor  410  according to the principle invention. The variable inductor  410  is similar to inductor  200  of  FIG. 3A and 3B  but without the non-magnetic spacer  220 . 
     FIG. 7  shows another embodiment of a variable inductor  420  according to the principle invention. The variable inductor  420  is similar to inductor  200  of  FIGS. 3A and 3B  without the non-magnetic spacer  220  and without the magnetic shunt bar  218 . 
   It should be understood that embodiments can be provided with or without a non-magnetic spacer, with or without a magnetic shunt bar, and with or without multiple e-cores. 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.