Inductor system for a submersible pumping system

An inductor assembly is disclosed for protecting electronic circuitry in a downhole equipment string. The inductor assembly includes a plurality of modular inductors coupled to one another in series to provide the desired inductance. The modular inductors are supported by a support structure in a protective housing, such as in a common housing with the electronic circuitry. The inductor assembly is electrically isolated from the housing. The support structure may include insulative end members and rail members extending between the end members to which the inductors are secured. One or more insulative covers are provided around the inductors to further isolate the inductors from the housing. The inductor assembly dissipates energy in the event of certain failure modes of power supply circuitry or lines extending from the earth's surface. The inductor may be secured electrically between a neutral node in a Y-wound motor to prevent high voltage ac waveforms from damaging the electronic circuitry. Insulation of the inductors inhibits arcing with the housing, thereby inhibiting damage to the inductors or the electronic circuitry during such failure modes.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to the field of submersible pumping
 systems of the type used in petroleum production and similar well
 applications. More particularly, the invention relates to a technique for
 protecting circuitry associated with such pumping systems, such as
 electronic circuitry for measuring or processing sensed or controlled
 parameters through the use of an inductor assembly.
 2. Description of the Related Art
 A variety of equipment is known and is presently in use for handling fluids
 in wells, such as petroleum or gas production wells. For example, a known
 class of such equipment includes submersible pumping systems, which
 typically comprise a submersible electric motor and at least one pump
 coupled to the electric motor. The pumping system may also include such
 equipment as motor protectors, fluid separators, and measuring or control
 equipment, such as digital or analog circuitry.
 The equipment may be deployed in a wellbore in a variety of manners. For
 example, a submersible pumping system may be lowered into a desired
 position within a wellbore via a cable coupled to a wire line or similar
 deployment device at the earth's surface. Power and data transmission
 lines are typically bound to the suspension cable for conveying power to
 the submersed equipment, as well as for conveying control signals to
 controllable components, such as valving, instrumentation, and so forth,
 and for transmitting parameter signals from the equipment to the earth's
 surface. In an alternative technique, the equipment may be coupled to a
 length of conduit, such as coiled tubing, and similarly lowered into a
 desired position within the well. In coiled tubing-deployed systems, power
 and data transmission cables may be positioned outside the coiled tubing,
 or may be disposed within the elongated bore defined by the coiled tubing.
 Once positioned in the well, circuits in the equipment are energized to
 perform desired functions. For example, in the case of submersible pumping
 systems, electrical power, typically in the form of three-phase
 alternating current power, is applied to the electric motor to drive the
 equipment in rotation. A pump thereby displaces wellbore fluids either
 through a stand of conduit to the earth's surface, or directly through a
 region of the well casing surrounding the cable or coiled tubing by which
 the equipment is deployed. Other well equipment may perform additional
 functions, such as reinjecting non-production fluids into subterranean
 discharge zones. In addition, powered well equipment may perform
 measurement functions, drilling functions, and so forth.
 In an increasing number of applications, rather sensitive electronic
 equipment is deployed in wells along with powered equipment. Electronic
 circuitry associated with the equipment will typically perform measurement
 or controlling functions, or both. In such cases, it is often necessary to
 provide a desired level of electrical power to the electronic circuitry.
 This is advantageously done by means of a common cable assembly used to
 supply power to the driven equipment. In the case of submersible electric
 motors, one technique for supplying power to measuring and control
 circuitry includes superimposing a desired power signal on the alternating
 current power used to drive the electric motor. At a Y-point of the motor
 windings, the power can be tapped and fed to the electronic circuitry.
 While it is advantageous to provide electrical power for monitoring and
 control circuitry by a power signal superimposed on drive power, this
 technique may call for protective circuitry in the event of certain
 failure modes. For example, where dc power is tapped from the Y-point of
 motor windings, a ground fault or loss of a phase in the motor drive
 circuitry can lead to referencing of the Y-point (i.e., a higher than
 desired power level at the Y-point). Such faults can cause damage to the
 downstream dc circuitry necessitating removal and servicing, and resulting
 in down time and maintenance costs. To protect the circuitry, inductors or
 chokes may be employed to prevent high voltage and current power from
 quickly entering the dc circuitry. However, existing choke structures do
 not typically provide sufficient protection for the circuitry. For
 example, in inverter motor drives, very high voltage spikes may occur at
 the Y-point of the motor windings, depending upon the failure mode. Such
 spikes can seriously damage conventional chokes. Larger or higher capacity
 choke structures may be provided, but these are typically limited by the
 dimensions of the wellbore, effectively limiting the options for
 increasing of the size or inductance of conventional choke structures.
 There is a need, therefore, for an improved technique for protecting
 electronic circuitry supplied with power from powered equipment in well
 applications. In particular, there is a need for an improved structure
 which provides both dielectric strength as required by the anticipated
 level of voltage and current spikes, while providing sufficient inductance
 to dissipate power during such periods. There is also a need for a
 structure which can be manufactured and adapted to both new and existing
 applications, and which can be integrated into existing equipment
 envelopes, such as those dictated by the dimensions of conventional wells.
 SUMMARY OF THE INVENTION
 The invention provides a technique for inductively protecting electronic
 circuitry designed to respond to these needs. The technique may be
 employed in a variety of well environments, but is particularly well
 suited for use with equipment in petroleum, gas, and similar wells. The
 technique provides an electrical inductor structure which can be
 positioned between powered equipment and electronic circuitry to inhibit
 power spikes from being transmitted to the electronic circuitry which
 would otherwise cause damage. The inductor may be configured as a modular
 structure, such that an overall inductance level can be attained by
 associating a plurality of modules into a series arrangement. The
 technique is particularly well suited for use in systems wherein
 electronic circuitry is powered via a power signal superimposed over drive
 signals in a three-phase circuit. The inductor may also pass parameter
 signals back through the power circuitry to a surface location.
 Thus, in accordance with the first aspect of the invention, an inductor
 system is provided for an equipment string configured to be deployed in a
 well. The equipment string includes at least one powered component coupled
 to a power cable extending between the earth's surface and the equipment
 string. The inductor system is configured to be coupled between the
 powered component and a direct current circuit receiving power via the
 power cable. The system includes an inductor and an electrically
 insulative support structure. The inductor includes a conductive coil and
 a ferromagnetic core. The support structure includes a support portion
 configured to contact and retain the inductor, and an interface portion
 coupled to the support portion for supporting the inductor in a conductive
 housing. The support structure electrically isolates the inductor from the
 conductive housing. The support structure may include both conductive and
 insulative materials, such as end members made of an insulative material
 for mechanically supporting the inductor and for contacting conductive
 internal surfaces of the housing. The inductor may be formed of a
 plurality of inductor modules. The inductor is preferably covered by an
 insulative jacket or wrap to further electrically isolate it from
 conductive surfaces within the housing.
 In accordance with another aspect of the invention, an inductor assembly is
 provided for protecting an electronic circuit in a downhole tool. The
 assembly includes a plurality of modular, series-coupled inductors. An
 insulative support structure is coupled to the inductors and mechanically
 supports the inductors in a housing. The support structure electrically
 isolates the inductors from conductive surfaces within the housing. An
 insulative cover extends over the inductors to isolate the inductors from
 conductive surfaces within the housing. The support structure may include
 one or more insulative end members configured to support the inductors and
 to contact interior surfaces of the housing.
 In accordance with a further aspect of the invention, an electronic circuit
 module is provided for use in a downhole tool string. The module includes
 a housing configured to be secured to at least one other component in the
 tool string. An electronic unit is positioned within the housing. An
 inductor assembly is electrically coupled to the electronic unit and is
 supported within the housing. The inductor assembly includes an inductor
 and an insulative support for positioning the inductor assembly in the
 housing.
 In accordance with still another aspect of the invention, a submersible
 pumping system is provided for use in a well. The system includes a pump,
 a submersible electric motor drivingly coupled to the pump, and an
 electronic circuit module. The motor is configured to be coupled to a
 power cable assembly for providing electrical power from the earth's
 surface to the electric motor when the pumping system is deployed in the
 well. The electronic circuit module is powered by electrical energy
 transmitted through the cable. The electronic circuit module includes a
 conductive housing, an electronic circuit unit disposed in the housing,
 and an inductor assembly. The inductor assembly is electrically coupled to
 the electronic circuit unit in the housing and includes insulating members
 for electrically isolating the inductor assembly from conductive surfaces
 within the housing. The electric motor may be a polyphase motor, and the
 inductor may be electrically coupled to a junction point of phase windings
 so as to provide electrical power to the electronic circuit module via the
 phase windings.
 A method is also provided for protecting an electronic circuit in a tool
 string submersible in a well. In accordance with the method an inductor
 assembly is provided including at least one inductor for dissipating
 electrical energy. The inductor assembly is mounted in a protective
 housing configured to be assembled in the tool string. The inductor
 assembly is electrically insulated to inhibit arcing between the inductor
 assembly and conductive elements within the housing. The inductor assembly
 is electrically coupled between the electronic circuit and a source of
 electrical energy.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
 Turning now to the Figures, and referring first to FIG. 1, an equipment
 string 10 is illustrated in the form of a submersible pumping system
 deployed in a well 12. Well 12 is defined by a wellbore 14 which traverses
 a number of subterranean zones or horizons. Fluids 16 are permitted to
 flow into and collect within wellbore 14 and are transmitted, via
 equipment string 10, to a location above the earth's surface 18 for
 collection and processing. In the embodiment illustrated in FIG. 1, the
 pumping system is positioned adjacent to a production horizon 20 which is
 a geological formation containing fluids, such as oil, condensate, gas,
 water and so forth. Wellbore 14 is surrounded by a well casing 22 in which
 perforations 24 are formed to permit fluids 16 to flow into the wellbore
 from production horizon 20. It should be noted that, while a generally
 vertical well is illustrated in FIG. 1, the equipment string 10 may be
 deployed in inclined and horizontal wellbores as well, and in wells having
 one or more production zones, one or more discharge zones, and so forth,
 in various physical layouts and configurations.
 In the embodiment illustrated in FIG. 1, equipment string 10 includes a
 production pump 26 configured to draw wellbore fluids into an inlet module
 28 and to express the wellbore fluids through a production conduit 30 to
 the earth's surface. Pump 26 is driven by a submersible electric motor 32.
 A motor protector 34 is preferably provided to prevent wellbore fluids
 from penetrating into motor 32 when deployed in the well. An electronic
 module, represented generally at reference numeral 36, is coupled to motor
 32 and may include a variety of electronic circuitry for executing
 monitoring and control functions. In particular, electronic module 36 may
 include circuitry for monitoring operating parameters within well 12, such
 as temperatures, pressures, and so forth. In addition, the module may
 include circuitry for carrying out in situ control functions, such as for
 controlling operation of motor 34. Moreover, as discussed in greater
 detail below, module 36 preferably includes circuitry for encoding or
 encrypting digital data for retransmission to the earth's surface.
 Finally, module 36 includes an inductor assembly as described in greater
 detail below for protecting electronic circuitry from damage due to
 certain failure modes or anomalies in the electrical supply circuitry
 associated with equipment string 10.
 In the illustrated embodiment motor 32 receives electrical power from a
 surface location via a multi-conductor cable 38. Cable 38 is routed beside
 equipment string 10 and production conduit 30 and terminates at power
 supply and monitoring circuitry above the earth's surface, as represented
 by generally by the reference numeral 40. In operation, power supply and
 monitoring circuitry 40 transmits electrical power, preferably three-phase
 alternating current power, to motor 32 via cable 38. Circuitry 40 also
 preferably applies a direct current voltage, such as a 78 volt dc
 regulated power signal, over the alternating current power applied via
 cable 38. The direct current voltage passes through motor 32 and is
 transmitted therefrom to electronic module 36. Parameter signals for
 monitoring or controlling equipment within string 10 are transmitted back
 to circuitry 40 along cable 38.
 As will be appreciated by those skilled in the art, electronic module 36
 may be incorporated in a variety of equipment strings, such as that
 illustrated in FIG. 1, as well as alternative equipment strings. Such
 equipment strings may include additional or other components, such as
 injection pumps, fluid separators, fluid/gas separators, packers, and so
 forth. Moreover, while in the embodiment described below power is applied
 to electronic module 36 via cable 38, various alternative configurations
 may be envisaged wherein power applied to electronic module 36 does not
 pass through windings of motor 32 as described below. Similarly,
 electronic module 36 may be configured to transmit parameter signals to
 the earth's surface via alternative techniques other than through cable
 38, such as via radio telemetry, a separate communications conductor, and
 so forth.
 A presently preferred configuration for supplying power to circuitry within
 module 36 through motor 32 is illustrated in FIG. 2. In general, the
 technique employed for applying power and transmitting signals to and from
 the electronic module may conform to the technique described in U.S. Pat.
 No. 5,515,038, issued to Alistair Smith on May 7, 1996 and assigned to
 Camco International Inc. of Houston, Tex., which is hereby incorporated
 into the present disclosure by reference. As illustrated in FIG. 2,
 circuitry 40 generally comprises monitoring and control circuitry 42
 configured to generate signals for prompting transmission of information
 from the tool string when deployed. Circuitry 42 may also generate control
 signals for commanding operation of components of the equipment string,
 such as the speed of the electric motor, position of control valves (not
 shown), and so forth. Monitoring and control circuitry 42 is coupled to
 power supply circuitry 44 which generates power needed for operation of
 the equipment string. Power supply circuitry 44 may be of a generally
 known configuration, and will typically include switch gear for connecting
 the equipment to a source of three-phase electrical power, as well as
 circuit protective devices, overload protective devices, and so forth. In
 the presently preferred embodiment, power supply circuitry 44 also
 provides a fixed direct current voltage of 78 volts dc, which is
 superimposed over alternating current power applied to the equipment via
 cable 38.
 In the diagrammatical representation of FIG. 2, cable 38, including three
 phase conductors, extends from the location of circuitry 44 above the
 earth's surface, as represented by reference numeral 46 in FIG. 2, to the
 location of the electric motor 32 below the earth's surface, as
 represented by reference numeral 48 in FIG. 2. Motor 32 is then coupled,
 such as via a sealed electrical coupling (not shown) to the conductors of
 cable 38. Stator windings 50 are coupled in a Y-configuration as
 illustrated in FIG. 2 to drive a rotor of the motor in rotation, thereby
 driving pump 26 (see FIG. 1). Stator windings 50 join one another at a
 Y-point 52, which defines a neutral node of the motor windings. This node
 point will, during normal operation, have a neutral relative potential.
 However, when a direct current power signal is superimposed over the
 conductors of cable 38, this direct current potential difference will
 result at node point 52 during normal operation. Power from node point 52
 is transmitted to circuitry within electronic module 36 via a jumper
 conductor 54.
 Within module 36, power incoming from motor 32 is routed through protective
 filtering circuitry, including a diode 56, an inductor 58 and a Zener
 diode 59. Power is thus transmitted to instrument circuitry 60 to provide
 power for operation of the circuitry. Circuitry 60 may include dc power
 supplies, voltage regulators, current regulators, microprocessor
 circuitry, solid state memory devices, and so forth. Instrument circuitry
 60 is coupled to a ground potential as represented generally at reference
 numeral 62 in FIG. 2. This ground potential will normally be provided by
 the housing of module 36 as described more fully below.
 As mentioned above, during normal operation of the circuitry as configured
 in FIG. 2, neutral node 52 will remain at the direct current voltage
 desired to be applied to instrument circuitry 60 through diode 56,
 inductor 58 and Zener diode 59. However, in the event of a ground fault,
 loss of phase or similar fault condition within motor 32 or within the
 circuitry applying power to motor 32, neutral point 52 may experience
 spikes in potential, including sizable alternating current spikes of a
 voltage level capable of damaging or crippling instrument circuitry 60.
 Upon the occurrence of such spikes, diode 56 serves to clip alternating or
 pulsed waveforms, such as to limit such waveforms applied to inductor 58
 to unidirectional voltage pulses. Inductor 58, which may be a 10,000 volt
 diode, then dissipates energy from the pulses due to its high inductance
 level so as to prevent damage to circuitry 60. Zener diode 59, which may
 be a 68 volt diode, regulates dissipation of the energy. In a presently
 preferred embodiment, inductor 58 is a 200 Henry inductor, comprised of a
 series of modular inductors coupled to one another in series.
 FIG. 3 illustrates an exemplary physical configuration for electronic
 module 36, including electronic circuitry, parameter measurement
 circuitry, and an inductor assembly for protecting the circuitry from
 power spikes during certain types of failure modes. While the electronic
 circuitry and the inductor assembly may be provided in separate component
 modules, in a presently preferred configuration illustrated in FIG. 3,
 these are housed in a common elongated housing 64 formed of a metal shell
 66 surrounding an internal cavity 68 in which the components are disposed.
 As will be appreciated by those skilled in the art, the housing is sized
 to permit its insertion into a petroleum production well or a similar
 well, in conjunction with associated equipment. Within internal cavity 68,
 module 36 thus includes an electronic unit 70, and an inductor assembly
 72. Moreover, because the illustrated embodiment is a measurement or
 sensing device, a sensor assembly 74 is also provided within housing 64.
 At a lower end of housing 64, shell 66 is terminated by a lower end cap 76
 in which sensor assembly 74 is installed. In the illustrated embodiment
 sensor assembly 74 includes circuitry for measuring temperatures and
 pressures within a wellbore. Accordingly, end cap 76 includes a plurality
 of openings or apertures 78 for permitting wellbore fluids penetrate into
 end cap 76 for measurement by assembly 74. Sensor assembly 74 is coupled
 to electronic unit 70 via a jumper or conductor set 80.
 An upper end of housing 64 is provided with an upper end cap 82 permitting
 the module to be coupled to additional components within an equipment
 string, such as to an electric motor 32 as illustrated in FIG. 1. Thus,
 upper end cap 82 includes a flanged interface 84 for receiving fasteners
 (not shown) for securing the components of the equipment string to one
 another. As will be appreciated by those skilled in the art, upper end cap
 82 may either be open to the interior cavity of an adjacent component or
 may be sealed. For example, where desired, the interior of module 36 may
 be in fluid communication with the interior of an electric motor coupled
 adjacent to it in the equipment string, and may share a common internal
 fluid with the motor, such as a high grade mineral oil. Alternatively, end
 cap 82 may provide a sealed interface between the motor and the components
 within housing 64. In such cases, a sealed electrical connection may be
 provided in end cap 82 in a manner generally known in the art, to permit
 the exchange of electrical power and signals between circuitry within
 module 36 and electrical conductors within a motor or other component.
 Also, electronic circuitry housed within module 36 may be conveniently
 provided in an electronic circuit enclosure 86. In a presently preferred
 embodiment, electronic circuitry housed within enclosure 86, and sensor
 circuitry in assembly 74 may be of the type commercially available in a
 measurement module from Reda of Bartlesville, Okla. under the commercial
 designation Downhole Measurement Tool.
 In the embodiment of FIG. 3, inductor assembly 72 includes a support
 structure, represented generally by reference numeral 88, and series of
 modular inductors 90. Support structure 88 mechanically supports the
 inductors within housing 64, while electrically isolating the inductors
 from conductive surfaces within housing 64. In prior art systems, it has
 been found that grounding between inductors within a conductive housing
 can lead to failure of the inductors through short circuits produced
 either between the inductors and the housing or within the inductor units
 themselves. The support structure provided for inductors 90 inhibits such
 contacts by providing a non-conductive barrier between the inductors and
 the housing. In particular, support structure 88 includes a lower
 insulative end member 92 and an upper insulative end member 94 which
 position inductors 90 in a desired location within housing 64, while
 providing a non-conductive interface between the inductors and the
 housing. The support structure further includes mechanical supports, such
 as in the form of rails 96 extending between lower and upper insulative
 end members 92 and 94. In the illustrated embodiment, inductors 90 are
 secured to rails 96 via bolts or similar fasteners 98. Rails 96 may be
 made of a conductive material, or an insulative material, where desired.
 An insulative jacket 100, represented generally by a dashed line in FIG.
 3, and described more fully below, is preferably provided around inductors
 90. Although jacket 100 may be provided within housing 64 separate from
 the inductor assembly, it is preferably secured directly to the inductor
 assembly to facilitate preconfiguring of the assembly and insertion of the
 assembly into housing 64.
 As best illustrated in FIG. 4, the support structure 88 for inductor
 assembly 72 both supports the inductors and isolates the inductors
 electrically from adjacent components. As shown in FIG. 4, end members 92
 and 94 serve as interface members between the inductors and other
 components. Thus, lower insulative end member 92 includes a central wiring
 aperture 102 through which a conductor can be passed after wiring of the
 inductors as described below. Moreover, rail mounting apertures 104 are
 provided in both lower and upper end members 92 and 94 to receive
 fasteners for securing rails 96 to the end members. Additional mounting
 apertures, such as apertures 106 in lower insulative end member 92 may be
 provided, such as for supporting circuit enclosure 86 (see FIG. 3).
 Moreover, one or both end members may include seals or gaskets for
 securing the insulator assembly within the housing in a relatively
 resilient manner. In the illustrated embodiment, for example, lower end
 member 92 includes an annular gasket groove 108 in which an elastomeric
 ring or gasket 110 is positioned to maintain radial alignment of the end
 member within housing 64 (see, e.g., FIGS. 5 and 6). Also as illustrated
 in FIG. 4, in the present embodiment, rails 96 include bent end portions
 114 through which fasteners are positioned for securing the rails to end
 members 92 and 94.
 FIGS. 5 and 6 illustrate the components of the inductor assembly in
 somewhat greater detail. In particular, as shown in FIGS. 5 and 6, four 50
 Henry inductors 90 are coupled to one another in series to form the 200
 Henry inductor desired for protection of the electronic circuitry. As will
 be appreciated by those skilled in the art, other inductor ratings and
 combinations may be foreseen to provide an overall inductance as needed
 for protection of particular circuits. A lead 116 extends from lower end
 member 92 and, in the assembled module, is coupled to a Zener diode and,
 therethrough, to electronic circuitry as illustrated diagrammatically in
 FIG. 2. Between each adjacent pair of inductors 90, leads are coupled to
 one another in series as indicated at reference numeral 118. Splices
 between the leads may be covered with a heat shrink insulative jacket of a
 type well known in the art. A diode subassembly 120 is preferably provided
 on the last inductor 90 adjacent to upper end member 94, and includes a
 diode for clipping negative-going pulses as discussed above with regard to
 diode 56 of FIG. 2. From diode assembly 120, an input lead 122 extends
 through upper end member 94 (see FIG. 6) for coupling to a source of
 electrical power, such as a neutral node point of the motor windings as
 illustrated in FIG. 2.
 In the presently preferred embodiment illustrated in FIGS. 5 and 6,
 inductors 90 are further isolated from conductive components by a series
 of insulative panels or covers 124, 126 and 128. A first insulative panel
 124 is provided directly adjacent to sides of the inductors, such as below
 leads 118. Although a single panel 124 is illustrated in FIG. 6, similar
 panels may be provided around all sides of the inductor assembly. A
 further insulative panel 126 is provided above panel 124 to further
 insulate the leads and inductors from surrounding components. Finally, an
 insulative wrap 128 (see FIG. 5) is provided around panels 124 and 126. In
 the preferred embodiment, insulative cover 128 extends between shoulders
 130 provided on end members 92 and 94, to define a structure in which
 substantially all conductive components are insulated from the internal
 surfaces of housing 64 when installed therein as illustrated in FIG. 3.
 Any suitable material may be used for insulating inductors 90 from
 conductive surfaces within housing 64. In a presently preferred
 embodiment, for example, end members 92 and 94 are constructed of a high
 temperature engineering plastic, such as a plastic material available
 under the commercial designation Ultem 2300. Moreover, in the present
 embodiment, insulative panels 124 and 126 and insulative cover 128 are
 constructed of an insulative plastic material commercially available under
 the name Nomex from DuPont. Additional insulative materials, such as
 tetrafluoroethylene tubes may be provided around at least a portion of
 insulative cover 128, where desired.
 FIG. 7 illustrates a typical configuration for each inductor module 90
 shown in vertical section. As shown in FIG. 7, the modules include a core
 assembly 132 and windings 134 of an electrically conductive material, such
 as copper. Core 132 is preferably made of a ferromagnetic metal, such as
 steel, and includes an "E" section 136 designed to receive windings 134,
 and an "I" section 138 which serves to cover and enclose the windings.
 Sections 136 and 138 are secured to one another during assembly of the
 inductor. Moreover, the windings 134 are insulated turn-to-turn, and are
 further insulated from the core in a conventional manner. Core sections
 136 and 138 maybe constructed of plate-like steel laminations in a manner
 generally known in the art. Apertures 142 are provided through core 132
 for receiving fasteners used for securing the inductor modules to the
 support structure described above (see FIGS. 4, 5 and 6). Leads (not shown
 in FIG. 7) extend from windings 134 to the outside of the core 132 to
 permit the windings to be electrically coupled in series between a source
 of electrical power and a protected circuit as described above.
 While the invention may be susceptible to various modifications and
 alternative forms, specific embodiments have been shown by way of example
 in the drawings and have been described in detail herein. However, it
 should be understood that the invention is not intended to be limited to
 the particular forms disclosed. Rather, the invention is to cover all
 modifications, equivalents, and alternatives falling within the spirit and
 scope of the invention as defined by the following appended claims.