Variable inductor

A variable inductor with a saturable core having three legs, including a center leg and two outer legs. A control winding is wound on the center leg and two outer windings are connected in parallel and wound on the outer legs. The inductances of the windings on the outer legs vary with the current through the control winding. The current through the control winding varies the saturation level of the outer legs. In one embodiment, the inductance of the control winding is substantially constant with a change in current in the control winding. In another embodiment, the outer legs are saturated and the center leg is not saturated. Portions of the core connecting the three legs are tapered down from the cross-section of the center leg to the cross-sections of the outer legs. The invention further includes methods of varying the inductance of an inductive circuit element in accordance with a control current.

FIELD OF THE INVENTION
 This invention relates to a variable inductor and a method of varying the
 inductance of an inductive circuit element. In particular, the invention
 relates to a variable inductor in which the inductance of an inductive
 circuit element is varied by means of an electrical signal.
 BACKGROUND OF THE INVENTION
 Variable inductors are of use in many circuit applications including
 magnetic amplifiers which vary the inductance of a circuit element to
 regulate power and resonant circuits which vary the inductance of circuit
 elements to vary the resonant frequency of the circuit. The simplest way
 to obtain a variable inductor is by mechanical movement of a connector
 along an inductive element. However, it is frequently desirable 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 may be employed to create a
 current controlled variable inductor such as that shown in prior art FIG.
 1. Variable inductors of this type typically have a limited variation
 range of 1 to 10 and suffer from parasitic effects such as capacitance and
 voltage across each series control winding that limit the quality factor
 of the inductor. Additionally, such current controlled variable inductors
 of the prior art typically require very high control currents in the range
 of 0 to 500 mA. FIG. 1 illustrates a current controlled variable inductor
 of the above-mentioned prior art in which the inductance L.sub.14 of
 center winding 14 is controlled by the current Ic delivered to outer
 control windings 12 and 13.
 More particularly, FIG. 1 shows a magnetic core 11, consisting of a
 magnetic material that can be saturated, with three legs 15, 16 and 17.
 The outer legs 15 and 17 have identical windings 12 and 13 that are
 connected in series as shown. Control windings 12 and 13 are wound and
 connected in such a way that the magnetic flux .phi..sub.c in respective
 legs 15 and 17 of the core arising from the control current Ic through the
 outer windings 12 and 13 is equal and points in opposite directions. The
 opposing magnetic flux .phi..sub.c results in cancellation in the center
 leg 16 of the core. The flux cancellation prevents coupling of AC signals
 between the center winding 14 and the series control windings 12 and 13.
 If an AC voltage were applied across the terminals of center winding 14, a
 voltage would be induced in both of the series windings 12 and 13 but the
 voltages in the control windings 12 and 13 would be of opposite polarity
 such that the voltage across the series connection of control windings 12
 and 13 would remain zero. The magnetic path for center winding 14 includes
 outer legs 15 and 17, center legs 16 and the connecting portions 18-21. If
 the control current Ic through windings 12 and 13 becomes large enough to
 saturate the legs 15 and 17 of the core, the inductance L.sub.14 of center
 winding 14 decreases because a portion of the magnetic path for the center
 winding 14 is saturated. The higher the control current Ic becomes, the
 lower the inductance L.sub.14 becomes. However, the center leg 16 will not
 be saturated due to the control current Ic.
 The inductance of an inductive circuit element is related to the
 permeability of the core and the number of turns:
 ##EQU1##
 where L is the inductance of an inductive circuit element;
 .mu..sub.0 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.
 In accordance with Equation 1 since the center leg is not saturated, the
 minimum inductance L.sub.14 is limited by the number of turns and the
 magnetic permeability of the core material of the center leg 16. Another
 undesirable side effect of the prior art circuit of FIG. 1 is that the
 inductance of each of the series connected control windings 12 and 13
 changes substantially with a change in the value of the control current
 Ic. In fact, the inductances of the control windings 12 and 13 change by a
 greater amount than the inductance of the center winding 14. This
 condition establishes significant limitations when the prior art variable
 inductor is part of a regulation loop. The inductor of the prior art
 circuit FIG. 1 has a limited variation range or requires a very high
 control current in the order of about 0 to 500 mA. Further, the voltage
 across each control winding 12 and 13 and the parasitic capacitances of
 control windings 12 and 13 limit the winding ratio and/or the operating
 frequency. The inductance of the control windings 12 and 13 changes
 substantially with the control current Ic.
 U.K. Patent 715,610 discloses variable inductive elements having saturable
 cores. The U.K. '610 variable inductors are illustrated in FIGS. 2A and
 2B. The variable inductor of FIG. 2A has series windings on the outer legs
 of a three leg core, and accordingly is similar to FIG. 1 above. FIG. 2B
 illustrates parallel windings on the outer legs of a three leg core and a
 control winding on the center leg of the core. There is no teaching in the
 '610 U.K. Patent to set the magnetic cross-section of the center leg,
 relative to the magnetic cross-sections of the outer legs in a variable
 inductor so that the outer legs and the center leg have substantially
 equal levels of saturation, in order to obtain a substantially constant
 inductance of the control winding with a change in current of the control
 winding. Further, there is no teaching in the '610 U.K. Patent to taper
 the portions of a three leg core connecting the legs down from the
 cross-section of the center leg to the cross-sections of the outer legs in
 order to obtain the largest variation in inductance for a given control
 current. Further, the '610 U.K. Patent teaches the use of an additional
 body or additional lamination strips to add cross-sectional area to the
 center leg of the three leg variable inductor shown in FIG. 2A above. FIG.
 7 of the '610 U.K. Patent shows a perspective view of the three leg
 transductor of FIG. 2A above where additional cross-sectional area of the
 amount of a x e is added. The additional "bodies" make it difficult if not
 impossible to maintain a substantially constant inductance of the control
 winding.
 Magnetic amplifiers are known having cores with three legs, parallel
 windings on the outer legs and a separate winding on the center leg. U.S.
 Pat. No. 2,229,952 to Whiteley discloses magnetic amplifier embodiments of
 this type. However, magnetic amplifiers operate in accordance with
 different principles than variable inductors and have different inputs and
 outputs. For example, in the magnetic amplifiers by Whiteley mentioned
 above, the current in the control winding around the center leg biases the
 core magnetization and does not saturate the core. The core is saturated
 by the AC signal from the generator 4 and due to the action of diodes 5,
 only one outer leg is saturated at a time. Each outer leg is saturated at
 a different time. Each outer leg is alternately, saturated and then not
 saturated, every cycle of the AC signal. The current through the control
 winding 2 determines the part of the half cycle during which the core is
 in saturation. The average DC voltage output relates to the amount of
 current through the control winding 2. The operation of a magnetic
 amplifier is to control the DC output voltage in accordance with the
 control current in control winding 2.
 SUMMARY OF THE INVENTION
 It is an object of the invention to provide a variable inductor that is
 current controlled with a wide variation range.
 It is another object of the invention to provide a variable inductor which
 is current controlled and has a limited control current.
 A further object of the invention is to provide a variable inductor which
 is current controlled which does not suffer from parasitic effects.
 It is an additional object of the invention to provide a current controlled
 variable inductor wherein the core of the circuit element having an
 inductance that varies is saturated.
 It is still a further object of the invention to provide a variable
 inductor that is current controlled where the minimum inductance of the
 circuit element that has an inductance which varies is not limited by the
 magnetic permeability of the core material.
 It is yet another object of the invention to provide a current controlled
 variable inductor which eliminates steps in cross-sections of its core.
 Further it is an object of the invention to provide a current controlled
 variable inductor wherein the minimum inductance of the circuit element
 which has a variable inductance may be lower than permitted in the prior
 art.
 Additionally, it is an object of the invention to provide a variable
 inductor in which the capacitance of a control winding does not limit the
 winding ratio.
 Further, it is another object of the invention to provide a current
 controlled variable inductor in which a voltage across a control winding
 does not limit the operating frequency.
 It is yet a further object of the first embodiment of the invention to
 provide a current controlled variable inductor wherein the inductance of a
 control winding is substantially constant with a change in current in the
 control winding.
 These and other objects of the invention are accomplished by providing a
 variable inductor according to a first embodiment comprising a core formed
 of a saturable magnetic material, the core having three legs, including a
 center leg and two outer legs; a control winding on the center leg and
 windings on each of the outer legs connected in parallel and in such a way
 that the magnetic flux arising from currents through the windings on the
 outer legs is cancelled in the center leg; wherein a current through the
 control winding causes a changed inductance across the windings on the
 outer legs by changing the saturation level of the outer legs, wherein the
 inductance of the control winding is substantially constant with a change
 in current in the control winding; the magnetic cross-section of the
 center leg, relative to the magnetic cross-sections of the outer legs
 being such that the outer legs and the center leg have substantially equal
 saturation levels.
 In a preferred embodiment, the magnetic cross-section of the center leg is
 equal to or somewhat larger than the sum of the magnetic cross-sections of
 the outer legs. Additionally, in a preferred embodiment, the portions of
 the core connecting the three legs are tapered down from the cross-section
 of the center leg to the cross-sections of the outer legs. Further, in a
 preferred embodiment, the center leg is formed by a single magnetic
 element. In still a further preferred embodiment, the portions of the core
 which connect the legs and the legs of the core may have circular
 cross-sectional areas.
 Also disclosed is a method of varying the inductance of an inductive
 circuit element in accordance with a control current comprising: a)
 obtaining a three leg core of saturable magnetic material, a magnetic
 cross-section of a center leg of the core, relative to magnetic
 cross-sections of two outer legs of the core set so that the outer legs
 and the center will have leg substantially equal saturation levels during
 step d) below; b) winding parallel windings on the outer legs of the core
 in such a way that the magnetic flux arising from current through the
 parallel windings is cancelled in the center leg of the core; c) winding a
 control winding on the center leg of the core; and d) varying the control
 current on the control winding to change the saturation level of the outer
 legs of the core to vary the inductance of each of said parallel windings;
 the inductance of the control winding on the center leg of the core
 remaining substantially constant with changes in the current on the
 control winding.
 It is yet another object of the second embodiment of the invention to
 provide a current controlled variable inductor which obtains the largest
 variation of inductance with the minimal control current by tapering the
 cross-sections of the portions of a three leg core that connect the legs
 of the core down from the cross-section of a center leg to the
 cross-sections of outer legs in order to channel all flux lines in the
 center leg to the outer legs.
 These objects are accomplished by providing a variable inductor comprising
 a core formed of a saturable magnetic material, the core having three
 legs, including a center leg and two outer legs; a control winding on the
 center leg and windings on each of the outer legs connected in parallel
 and in such a way that the magnetic flux arising from currents through the
 windings on the outer legs is cancelled in the center leg; wherein a
 current through the control winding causes a changed inductance across the
 windings on the outer legs; the magnetic cross-section of the center leg,
 relative to the magnetic cross-sections of the outer legs, being such that
 the outer legs are saturated, and the center leg is not saturated and
 portions of the core connecting the three legs are tapered down from the
 cross-section of the center leg to the cross-sections of the outer legs.
 Also contemplated is a second method of varying the inductance of an
 inductive circuit element in accordance with a control current comprising:
 a) obtaining a three leg core of saturable magnetic material, a magnetic
 cross-section of a center leg of the core, relative to magnetic
 cross-sections of two outer legs of the core set so that the outer legs
 are saturated, the center leg is not saturated and portions of the core
 connecting the three legs are tapered down from the cross-section of the
 center leg to the cross-sections of the outer legs; b) winding parallel
 windings on the outer legs of the three leg core in such a way that the
 magnetic flux arising from current through the parallel windings is
 cancelled in the center leg of the core; c) winding a control winding on
 the center leg of the core; d) varying the control current on the control
 winding to saturate the outer legs of the core to vary the inductance of
 each of the parallel windings; and e) performing the step of varying the
 control current to saturate the outer legs of the core while not
 saturating the center leg.
 The above and other objects, aspects and features and advantages of the
 invention will be more readily apparent from the description of the
 preferred embodiments thereof taken in conjunction with the accompanying
 drawings and appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 3, a variable inductor is shown in accordance with the
 invention.
 The variable inductor 40 of the invention includes a core 41 formed of a
 saturable magnetic material. The core 41 has three legs 42, 43 and 44. Leg
 43 is the center leg and legs 42 and 44 are outer legs. The center leg 43
 is a single magnetic element. In other words, center leg 43 is a single
 piece of magnetic material throughout a cross-section cut perpendicular to
 the direction of the flow of flux .phi. along the length of the leg
 induced by a control current Ic during operation. A control winding 45 is
 wound about the center leg 43. Identical outer windings 46 and 47,
 respectively, are wound about the outer legs 42 and 44, respectively. The
 outer windings 46 and 47 are connected in parallel across a source of an
 AC signal and are wound in such a way that the magnetic flux .phi..sub.c
 induced by currents through the outer windings 46 and 47 is cancelled in
 the center leg 43 as shown.
 The DC control current Ic is input to the control winding 45. A change in
 the control current Ic causes a change in the inductances L.sub.46 and
 L.sub.47 across the outer windings 46 and 47. In a first embodiment of the
 invention, the inductance L.sub.45 of the control winding 45, however,
 remains substantially constant with a change in the control current Ic in
 the control winding 45.
 In the first embodiment of the invention, the magnetic cross-section of the
 center leg 43, relative to the magnetic cross-sections of the outer legs
 42 and 44, is such that the outer legs (42, 44) and the center leg (43)
 have substantially equal levels of saturation. In a preferred version of
 the first embodiment, substantially equal levels of saturation are
 achieved by making the magnetic cross-section of the center leg 43 equal
 to or slightly larger than the sum of the magnetic cross-sections of the
 outer legs 46 and 47. The magnetic cross-section of the center leg 43 may
 be somewhat larger than the sum of the magnetic cross-sections of the
 outer legs 42 and 44, however, the difference in magnetic cross-sections
 must be small enough that the center leg 43 and the outer 42 and 44 legs
 have substantially equal levels of saturation. Thus, if the center leg is
 about 80% saturated, the outer legs 42 and 44 must also be about 80%
 saturated.
 In a second embodiment of the invention, the magnetic cross-section of the
 center leg 43, relative to the magnetic cross-sections of the outer legs
 42 and 44, is such that the outer legs 42 and 44 are saturated, the center
 leg 43 is not saturated, and portions 48-51 of the core connecting the
 three legs are formed so that the connecting portions 48-51 are tapered
 down from the cross-section of the center leg 43 to the cross-sections of
 the outer legs 42 and 44 as further described with respect to FIG. 4B. In
 the second embodiment of the invention, the inductance L.sub.45 will not
 remain constant with a change in the control current Ic in the control
 winding 45.
 In the first embodiment of the invention, wherein the outer legs 42 and 44
 and the center leg 43 have substantially equal levels of saturation and
 where the inductance of the control winding L.sub.45 is substantially
 constant with a change in current in the control winding 45, a larger
 variation of the inductance L.sub.46 and L.sub.47 with a smaller control
 current Ic may be obtained by tapering the cross-sections of the portions
 48-51 of the three leg core that connect the legs of the core, down from
 the cross-section of the center leg 43 to the cross-sections of the outer
 legs 42 and 44 in order to channel all of the flux lines in the center leg
 43 to the outer legs 42 and 44. The cross-sections of the portions 48-51
 may be circular or oval. Alternatively, the cross-sections of the portions
 may be another shape, for example rectangular. The tapering from the
 relatively large cross-section of the center leg 43 to the smaller
 cross-sections of the outer legs 42 and 44 may be formed in only one or in
 both dimensions of the cross-sectional shape. The greatest variation of
 inductance with the minimal control current Ic may only be obtained as in
 the second embodiment of the invention, by tapering the cross-sections of
 the connecting portions 48-51 of the core where the outer legs 42 and 44
 are saturated and the center leg 43 is not saturated. However, as
 mentioned before, in the second embodiment of the invention, the
 inductance of the control winding L.sub.45 does not remain substantially
 constant with a change in current in the control winding 45.
 For both the first and the second embodiments of the invention, the entire
 core 41 may be formed as an integral piece of magnetic material as shown
 in FIG. 3 or as two E-shaped magnetic sections as shown in FIG. 4A.
 More particularly, FIG. 4A illustrates a variation of the core 41 employed
 in the invention shown in FIG. 3 above. The core 41 has three legs 42, 43
 and 44 as in FIG. 3. Additionally, the core 41 includes connecting
 portions 48, 49, 50 and 51 between the three legs of the core. However, in
 the embodiment illustrated in FIG. 4A, the core 41 is composed of two
 E-shaped sections of magnetic material 41a and 41b. The E-shaped sections
 of magnetic material 41a and 41b are clamped together such that their
 faces meet at boundary 52 to form the three-leg core.
 FIG. 4B is a cross-sectional view of the core 41 taken along line 4B-4B in
 FIG. 4A. The cross-section of each leg 42, 43 and 44 is circular as shown
 in FIG. 4B. The cross-section of each leg 42, 43 and 44 is constant across
 the length of the leg as better seen in FIG. 4A. Referring to FIG. 4B, the
 cross-sectional area of center leg 43 is equal to or larger than the
 cross-sectional areas of the outer legs 42 and 44. More particularly, in
 the first embodiment of the invention, the magnetic cross-section of the
 center leg 43 is equal to or somewhat larger than the sum of the magnetic
 cross-sections of the outer legs 42 and 44. In the second embodiment of
 the invention, the magnetic cross-section of the center leg 43 may be
 substantially larger than the sum of the magnetic cross-sections of the
 center legs 42 and 44.
 Further, for both the first and second embodiments of the invention, as
 shown in FIG. 4B, the portions 50 and 51 of the core connecting the three
 legs 42, 43 and 44 are tapered down from the center leg 43 toward the
 outer legs 42 and 44. Accordingly, the cross-sectional areas of the
 connecting portions 50 and 51 (as well as similar connecting portions 48
 and 49) vary along the length of the connecting portions, with the
 cross-sectional area of each connecting portion taken close to the center
 leg 43 being greater than the cross-sectional area of the same connecting
 portion taken close to one of the outer legs 42 and 44.
 FIG. 4C is a cross-sectional view of the core 41 illustrated in FIG. 4A
 taken along line 4C-4C. A similar cross-sectional view would be obtained
 if taken along a symmetrical horizontal line through connecting portions
 51 and 49 and looking toward leg 44. (Further, similar views could be
 obtained for the core 41 of FIG. 3 but these views do not have the
 boundaries 52.) As shown in FIG. 4C, leg 42 (as well as legs 43 and 44,
 not shown in FIG. 4C) has a constant cross-sectional area along the length
 of the leg 42. Further, connecting portions 50 and 48 have circular
 cross-sectional areas. As shown in FIG. 4B, the circular cross-sectional
 areas of connecting portions 48 and 50 vary along the length of the
 connecting portions with the cross-sectional area of each of the
 connecting portions taken close to the outer legs 42 and 44 being smaller
 than the cross-sectional area of each of the connecting portions taken
 close to the center leg 43.
 FIG. 4D is a view similar to FIG. 4C of an alternate embodiment of the core
 41 illustrated in FIG. 4A showing rectangular cross-sections for the
 connecting portions 48 and 50. Other shapes may also be employed for the
 cross-sections of the connecting portions 48-51. The cross-sectional areas
 of the connecting portions 48-51 may vary in one or two dimensions along
 the length of the connecting portions, with the cross-sectional area of
 each connecting portion taken close to the center leg 43 being greater
 than the cross-sectional area of the same connecting portion when close to
 one of the outer legs 42 and 44.
 Though center leg 43 shown in FIG. 4A comprises a first part 43a which is
 part of section 41a of magnetic material and a second part 43b which is
 part of section 41b of magnetic material, center leg 43 is still
 considered a "single magnetic element" as defined herein. The language "a
 single magnetic element" is meant to designate a magnetic element (such as
 a leg of a core) with a continuous single piece of magnetic material
 throughout at least one cross-section cut perpendicular to the direction
 of the flow of flux .phi. during operation. The magnet flux .phi. flows
 along the length of the center leg 43 during operation. The cross-section
 taken perpendicular to the flow of flux .phi. is not composed of multiple
 magnetic elements such as lamination strips or additional bodies added to
 increase the cross-section of the element. The division between sections
 41a and 41b of magnetic material illustrated by boundaries 52 does not
 increase the cross-sectional area of the center leg 43 and does not mean
 that center leg 43 is composed of more than a "single magnetic element",
 because the cross-section, cut perpendicular to the flow of flux .phi.
 during operation, shows only a single magnetic element. The same magnetic
 material is employed throughout the cross-section.
 In operation, for the first embodiment of the invention, where the magnetic
 cross-section of the center leg 43, relative to the magnetic
 cross-sections of the outer legs 42 and 44 is set such that the outer legs
 and the center leg have substantially equal levels of saturation, the
 control current Ic through center windings 45, causes the inductances
 L.sub.46 and L.sub.47 across the outer windings 46 and 47 to vary by
 changing the saturation levels of the outer legs 42 and 44. However, the
 inductance L.sub.45 of the control winding 45 remains substantially
 constant with the change in the control current Ic through control winding
 45. The control current Ic through winding 45 is large enough that the
 outer legs 42 and 44 of the core 41 saturate, which lowers the magnetic
 permeability of the core and results in a decrease in the inductances
 L.sub.46 and L.sub.47 of the parallel outer windings 46 and 47. The higher
 the control current Ic becomes, the lower the inductances L.sub.46 and
 L.sub.47 become.
 Additionally, for the first embodiment, the connecting portions 48-51 of
 the core 41 may be tapered to make sure that there is a minimum
 incremental increase or "step" in cross-sectional area from the legs to
 the connecting limbs. "Steps" in cross-sectional area will leave parts of
 the core material non-saturated around the "steps". This limits the
 minimum inductance and the variation range of the inductance. The tapered
 connecting portions permit a greater variation in the inductances L.sub.46
 and L.sub.47 for a given control current Ic through control winding 45.
 A circular cross-section for the core 41 as opposed to another type of
 cross-section such as a rectangular cross-section, provides a more
 homogenous spread of the magnetic flux through the cross-section which
 results in a more homogenous saturation of the magnetic material of the
 legs and connecting portions. Thus, a greater variation in inductance for
 a given control winding current Ic may be obtained. Additionally, a circle
 has the shortest possible perimeter for a given cross-section.
 Accordingly, a given number of turns around a circular leg of the core 41
 will require a shorter total length of wire than that required for legs
 having a different cross-sectional area such as a rectangle. The shorter
 total length of wire results in lower power loss in the variable inductor
 40.
 Other shapes for the cross-sections of the legs and connecting portions may
 be employed, however. Further, tapering of a cross-section of one of the
 connecting portions 48-51 may be in one or two dimensions. A practical
 embodiment employs a rectangular cross-section for the connecting portions
 48-51 with tapering in only one dimension as shown in the perspective view
 of FIG. 5. FIG. 5 is a perspective view of the invention shown in FIG. 3.
 Employing a single magnetic element (throughout a cross-section taken
 perpendicular to the flow of flux during operation) for the center leg
 permits a constant control winding inductance L.sub.45. For the inductance
 of the control winding 45 to be exactly constant the following conditions
 are required:
 1. The outer windings 46 and 47 on the outer legs 42 and 44 are connected
 in parallel;
 2. The magnetic cross-section of the center leg 43 is equal to the sum of
 the magnetic cross-sections of the outer legs 42 and 44; and
 3. The connecting portions 48-51 are not completely saturated. If the
 connecting portions are not completely saturated, the coupling of the
 windings stays in tact as the legs are going into saturation. The
 connecting portions are maintained non-saturating due to the fact that the
 connecting portions have larger cross-sections than the cross-sections of
 the outer legs. This is true when the connecting portions are tapered as
 illustrated.
 The inductance of the control winding 45 will be substantially constant, if
 the magnetic cross-section of the center leg 43 is somewhat larger than
 the sum of the magnetic cross-sections of the outer legs 42 and 43
 provided that the outer legs and the center leg have substantially equal
 levels of saturation.
 In operation of the invention as shown in FIG. 3, the inductance L.sub.45
 of the control winding 45 on the center leg 43 is of a low value in
 comparison to the inductances L.sub.12 and L.sub.13 in the prior art
 circuit of FIG. 1. Further, the inductance of control winding 45 does not
 change significantly with a change in control current Ic. In fact, as
 stated above, if the magnetic cross-section of the center leg 43 is
 exactly equal to the sum of the magnetic cross-sections of the outer legs
 42 and 44, the inductance of control winding 45 will not change at all
 with the control current Ic. Outer windings 46 and 47 act as short
 circuited secondary windings on a transformer having winding 45 as the
 primary winding. Thus, the inductance L.sub.45 of control winding 45 is
 only determined by the leakage inductance of the transformer. Accordingly,
 the inductance L.sub.45 of the control winding 45 may be a low and
 substantially constant value.
 The greatest variation in inductance as a result of a given control current
 Ic is obtained where the cross-section of the center leg 43 is
 substantially larger than the sum of the cross-sections of the outer legs
 42 and 44 and the connecting limbs are tapered down from the center leg 43
 to the outer legs 42 and 44 since only the outer legs 42 and 44 will be
 saturated due to the control current Ic. The center leg 43 is not
 saturated. This describes the second embodiment of the invention. In the
 second embodiment of the invention, however, the inductance of the control
 winding 45 is not substantially constant.
 In operation, if the sum of the magnetic cross-sections of the outer legs
 42 and 44 is smaller than the cross-section of the center leg 43, the
 outer legs 42 and 44 saturate before the rest of the core. When only the
 outer legs 42 and 44 are saturated a greater change in inductances
 L.sub.46 and L.sub.47 with the same change in control current Ic, or a
 smaller change in control current Ic for the same change of inductance
 L.sub.46 and L.sub.47, is obtained. The magnetic material in outer legs 42
 and 44 becomes saturated by the control current Ic. Accordingly, minimum
 inductances L.sub.46 and L.sub.47 are determined by the number of turns
 and not by the permeability of the magnetic material.
 In contrast, for the prior art circuit of FIG. 1, the minimum inductance of
 winding 16 depends upon the permeability of the magnetic material because
 in the prior art variable inductor, the magnetic material in center leg 16
 does not become saturated. Accordingly, Equation 1 above, which depends
 upon permeability, applies limiting the minimum inductance in that case.
 The invention further contemplates a first method of varying the inductance
 L.sub.46 or L.sub.47 of an inductive circuit element 46 or 47 in
 accordance with a control current Ic. In the first method, a three leg
 core of saturable magnetic material is obtained, the magnetic
 cross-section of a center leg 43 of the core 41, relative to magnetic
 cross-sections of two outer legs 42 and 44 of the core 41 set so that the
 outer legs 42 and 44 and the center legs 43 have substantially equal
 saturation levels when the control current is varied the parallel windings
 46 and 47 are wound on the outer legs 42 and 44 of the three-leg core in
 such a way that the magnetic flux .phi. arising from current through the
 parallel windings 46 and 47 is cancelled in the center leg 43 of the core.
 Further, a control winding 45 is wound on the center leg 43 of the core.
 The control current Ic of the control winding 45 is varied to change the
 saturation level of the outer legs 42 and 44 of the core 41 to vary the
 inductance L.sub.46 and L.sub.47 of each of the parallel windings 46 and
 47 on the outer legs 42 and 44. The inductance L.sub.45 of the control
 winding 45 on the center leg 43 of the core is maintained substantially
 constant with changes in the current Ic on the control winding 45.
 The invention further contemplates a second method of varying the
 inductance L.sub.46 or L.sub.47 of the inducted circuit element 46 or 47
 in accordance with a control current Ic. In the second method, a three leg
 core 41 of saturable material is obtained. The magnetic cross-section of
 the center leg 43 of the core 41, relative to magnetic cross-sections of
 two outer legs 42 and 44 of the core 41 set so that outer legs 42 and 44
 are saturated, the center leg 43 is not saturated, and portions 48-51 of
 the core 41 connecting the three legs 42, 43 and 44 are tapered down from
 the cross-section of the center leg 43 to the cross-sections of the outer
 legs 42 and 44. The parallel windings 46 and 47 are wound on the outer
 legs 42 and 44 of the three leg core 41 in such a way that the magnetic
 flux arising from the current through the parallel winding 46 and 47 is
 cancelled in the center leg 43 of the core. A control winding 45 is wound
 on the center leg 43 of the core. The control current Ic of the control
 winding 45 is varied to saturate the outer legs of the core 41 to vary the
 inductance L.sub.46 and L.sub.47 of each of the parallel windings 46 and
 47.
 The control current Ic is varied to saturate the outer legs 42 and 44 of
 the core 41 while not saturating the center leg 46. The connecting
 portions 48-51 are not completely saturated. In fact, only the parts of
 the connecting portions that are closest to the outer legs are saturated.
 Although the invention has been described with reference to the preferred
 embodiments, it will be apparent to one skilled in the art that variations
 and modifications are contemplated within the spirit and scope of the
 invention. The drawings and description of the preferred embodiments are
 made by way of example rather than to limit the scope of the invention,
 and it is intended to cover within the spirit and scope of the invention
 all such changes and modifications.