US007067952B2
`
`(12) United States Patent
`Neal
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,067,952 B2
`Jun. 27, 2006
`
`(54) STATOR ASSEMBLY MADE FROM A
`MOLDED WEB OF CORE SEGMENTS AND
`MOTOR USING SAME
`
`(75) Inventor: Griffith D. Neal, Alameda, CA (US)
`(73) Assignee: Encap Motor Corporation, Alameda,
`CA (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 248 days.
`(21) Appl. No.: 10/383,219
`
`(*) Notice:
`
`(22) Filed:
`
`Mar. 5, 2003
`
`(65)
`
`Prior Publication Data
`|US 2004/0034988 A1
`Feb. 26, 2004
`
`Related U.S. Application Data
`(63) Continuation-in-part of application No. 09/798,511,
`filed on Mar. 2, 2001, now Pat. No. 7,036,207.
`
`(51) Int. Cl.
`(2006.01)
`H02K L/18
`(2006.01)
`H02K 15/02
`(2006.01)
`H02K 1.5/10
`(52) U.S. Cl. .......................... 310/259; 310/42; 310/45;
`310/218
`(58) Field of Classification Search ............ 310/42–43,
`310/45, 216–218, 254, 259: 244/432, 433,
`244/433.4; 29/596
`See application file for complete search history.
`
`(56)
`
`References Cited
`
`|U.S. PATENT DOCUMENTS
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`FOREIGN PATENT DOCUMENTS
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`BE
`
`870878
`
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`
`(Continued)
`OTHER PUBLICATIONS
`entitled
`LNP Engineering
`Plastics,
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`pages).
`
`(Continued)
`Primary Examiner—Burton Mullins
`(74) Attorney, Agent, or Firm—Steven P. Shurtz, Brinks
`Hofer Gilson & Lione
`
`(57)
`
`ABSTRACT
`
`A plurality of stator arc segments 20 are linked together by
`a phase change material 22 enabling simplified winding and
`higher slot fill. Once wound this continuous structure can be
`formed into a toroidal core 17 for a stator assembly 40 used
`to make a motor 100. In a preferred embodiment, a mono
`lithic body 42 of phase change material substantially encap
`sulates the conductors and holds the stator arc segments 20
`in contact with each other in the toroidal core 17. Hard disc
`drives using the motor 100, and methods of constructing the
`motor 100 are also disclosed.
`
`14 Claims, 5 Drawing Sheets
`
`Petitioners Exhibit 1001
`Page1
`
`

`

`5
`
`5
`
`Ol?o
`
`US 7,067,952 B2
`Page 2
`
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`...: º : Nº.
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`; #. º Nº.
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`
`BE
`DE
`º
`FR
`
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`
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`-
`-
`-
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`entitled “Epoxy Seal—Prevents Down Time and Keeps
`Equipment Running Longer,” Jul. 14, 1999, *http://www.
`bmwworks.com/VIP.htm>, 1 page.
`
`5
`
`5
`
`errot
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`
`Petitioners Exhibit 1001
`Page2
`
`

`

`US 7,067,952 B2
`Page 3
`
`The Epoxylite Corporation, article from the Internet entitled
`“Vacuum Pressure Impregnation (VPI) Systems”, Nov. 19,
`-
`-
`-
`1999,
`<http://www.epoxylite.com/EpoxyliteBquipment.
`htm>, 3 pages.
`
`Neeltran Inc., article from the Internet entitled “Vacuum
`Pressure Impregnation (VPI)”, Nov. 19, 1999, «http://www.
`neeltran.thomasregister.com/olc/neeltran?neel9.htm>
`pages.
`* cited by examiner
`
`2
`
`Petitioners Exhibit 1001
`Page3
`
`

`

`U.S. Patent
`
`Jun. 27, 2006
`
`Sheet 1 of 5
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`US 7,067,952 B2
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`F|G.
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`PRIOR ART
`
`Petitioners Exhibit 1001
`Page4
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`

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`U.S. Patent
`U.S. Patent
`
`J“ 27, 2006
`
`Sheet 2 of 5
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`US 7,067,952 B2
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`Petitioners Exhibit 1001
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`US 7,067,952 B2
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`Page6
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`Page6
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`U.S. Patent
`U.S. Patent
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`Sheet 4 of 5
`Sheet 4 of 5
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`US 7,067,952 B2
`US 7,067,952 B2
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`F|G. 8C
`FIG. 80
`
`CO!, N
`
`N
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`
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`Zaess:
`
`<No.Ñ
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`loners Exhibit 1001
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`P g 7
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`Petitioners Exhibit 1001
`Page7
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`

`

`
` m\w\\,\\\flnhwwyw/
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`\§
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`Petitioners Exhibit 1001
`Page8
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`

`

`US 7,067,952 B2
`
`1
`STATOR ASSEMBLY MADE FROM A
`MOLDED WEB OF CORE SEGMENTS AND
`MOTOR USING SAME
`
`REFERENCE TO EARLIER FILED
`APPLICATION
`
`The present application is a continuation-in-part of appli
`cation Ser. No. 09/798,511, filed Mar. 2, 2001, now U.S. Pat.
`No. 7,036,207, and entitled Stator Assembly Made From A
`10
`Plurality Of Toroidal Core Arc Segments And Motor Using
`Same, which is hereby incorporated by reference.
`
`FIELD OF THE INVENTION
`
`The present invention relates generally to a stator assem
`bly used in a dynamoelectric machine such as a motor or a
`generator. It relates particularly to a spindle motor such as
`used in a hard disc drive, and to the construction and
`arrangement of a stator assembly made from a plurality of
`arc segments.
`
`15
`
`20
`
`BACKGROUND OF THE INVENTION
`
`2
`various components of the motor. Greater dimensional con
`sistency between components leads to a smaller gap between
`the stator 4 and the magnet 3, producing more force, which
`provides more torque and enables faster acceleration and
`higher rotational speeds.
`The conventional method of forming stators has a number
`of drawbacks. First, most steel is manufactured in rolled
`sheets and thus has a grain orientation. The grain orientation
`has an effect on the magnetic flux properties of the steel. In
`circular stamped pieces of steel, the grain orientation differs
`at different points around the circle. Compared from the
`radius line of the circle, the grain orientation is sometimes
`aligned along the radius, sometimes transverse to it, and
`mostly at a varying angle to the radius. The un-aligned grain
`structure of conventional stators causes the magnetic flux
`values to differ in parts of the stator, and thus the motor does
`not have consistent and uniform torque properties as it
`rotates.
`Another drawback with using circular steel pieces is that,
`especially for inward facing poles, it has been difficult to
`wind the wire windings tightly because of the cramped space
`to work inside of the laminated stator core. The cramped
`working space creates a lower limit on the size of the stator
`and thus the motor. The limited working space also results
`in a low packing density of wire. The packing density of
`wire coiled around the poles affects the amount of power
`generated by the motor. Increasing packing density increases
`the power and thus the efficiency of the spindle motor.
`An important factor in motor design is to reduce stack up
`tolerances in the motor. Stack up tolerances reduce the
`overall dimensional consistency between the components.
`Stack up tolerances refer to the sum of the variation of all the
`tolerances of all the parts, as well as the overall tolerance
`that relates to the alignment of the parts relative to one
`another. One source of stack up tolerances is from the
`circular stator body. Generally, the thickness of rolled steel
`is not uniform across the width of the roll. Sometimes the
`edges are thicker or thinner than the center. In a stator made
`from circular stamped pieces, the thicknesses of individual
`laminations are thus different from one side to the other.
`When stacked together, this creates a stack up tolerance
`problem. Furthermore, the circular stampings leave a lot of
`wasted steel that is removed and must be recycled or
`discarded.
`Another important factor in motor design is the lowering
`of the operating temperature of the motor. Increased motor
`temperature affects the electrical efficiency of the motor and
`bearing life. As temperature increases, resistive loses in wire
`increase, thereby reducing total motor power. Furthermore,
`the Arhennius equation predicts that the failure rate of an
`electrical device is exponentially related to its operating
`temperature. The frictional heat generated by bearings
`increases with speed. Also, as bearings get hot they expand,
`and the bearing cages get stressed and may deflect, causing
`non-uniform rotation, reducing bearing life. This non-uni
`form rotation causes a further problem of limiting the ability
`of the servo system controlling the read/write heads to
`follow data tracks on the magnetic media. One drawback
`with existing motor designs is their limited effective dissi
`pation of the heat, and difficulty in incorporating heat sinks
`to aid in heat dissipation. In addition, in current motors the
`operating temperatures generally increase as the size of the
`motor is decreased.
`Manufacturers have established strict requirements on the
`outgassing of materials that are used inside a hard disc drive.
`These requirements are intended to reduce the emission of
`materials onto the magnetic media or heads during the
`
`25
`
`30
`
`35
`
`45
`
`Computers commonly use disc drives for memory storage
`purposes. Disc drives include a stack of one or more
`magnetic discs that rotate and are accessed using a head or
`read-write transducer. Typically, a high speed motor such as
`a spindle motor is used to rotate the discs.
`In conventional spindle motors, stators have been made
`by laminating together stamped pieces of steel. These
`stamped pieces of steel are generally circular in nature, but
`also have “poles” extending either inwardly or outwardly,
`depending on whether the rotor is on the inside or surrounds
`the stator. The stamped pieces are laminated together and
`then coated with insulation. Wire is then wound around the
`poles to form stator windings.
`An example of a conventional spindle motor 1 is shown
`in FIG. 1. The motor 1 includes a base 2 which is usually
`made from die cast aluminum, a stator 4, a shaft 6, bearings
`40
`7 and a disc support member 8, also referred to as a hub. A
`magnet 3 and flux return ring 5 are attached to the disc
`support member 8. The stator 4 is separated from the base 2
`using an insulator (not shown) and attached to the base 2
`using a glue. Distinct structures are formed in the base 2 and
`the disc support member 8 to accommodate the bearings 7.
`One end of the shaft 6 is inserted into the bearing 7
`positioned in the base 2 and the other end of the shaft 6 is
`placed in the bearing 7 located in the hub 8. A separate
`electrical connector 9 may also be inserted into the base 2.
`Each of these parts must be fixed at predefined tolerances
`with respect to one another. Accuracy in these tolerances can
`significantly enhance motor performance.
`In operation, the disc stack is placed upon the hub. The
`stator windings are selectively energized and interact with
`the permanent magnet to cause a defined rotation of the hub.
`As hub 8 rotates, the head engages in reading or writing
`activities based upon instructions from the CPU in the
`computer.
`Manufacturers of disc drives are constantly seeking to
`improve the speed with which data can be accessed. To an
`extent, this speed depends upon the efficiency of the spindle
`motor, as existing magneto-resistive head technology is
`capable of accessing data at a rate greater than the speed
`offered by the highest speed spindle motor currently in
`production. The efficiency of the spindle motor is dependent
`upon the dimensional consistency or tolerances between the
`
`50
`
`55
`
`60
`
`65
`
`Petitioners Exhibit 1001
`Page9
`
`

`

`3
`operation of the drive. Of primary concern are glues used to
`attach components together, varnish used to insulate wire,
`and epoxy used to protect steel laminations from oxidation.
`In addition to such outgassed materials, airborne particu
`late in a drive may lead to head damage. Also, airborne
`particulates in the disc drive could interfere with signal
`transfer between the read/write head and the media. To
`reduce the effects of potential airborne particulate, hard
`drives are manufactured to exacting clean room standards
`and air filters are installed inside of the drive to reduce the
`contamination levels during operation.
`An example of a spindle motor is shown in U.S. Pat. No.
`5,694,268 (Dunfield et al.) (incorporated herein by refer
`ence). Referring to FIG. 5 of this patent, a stator of the
`spindle motor is encapsulated with an overmold 42. The
`overmolded stator 40 contains openings through which
`mounting pins 44 may be inserted for attaching the stator
`200 to a base. U.S. Pat. No. 5,672,972 (Viskochil) (incor
`porated herein by reference) also discloses a spindle motor
`having an overmolded stator. One drawback with the stators
`described in these patents is this difficulty in winding wire
`on the poles. Another drawback is the height of the lami
`nation stacks. Further, the overmolds shown in these patents
`are not effective in dissipating heat or dampening some
`vibrations generated by energizing the stator windings.
`U.S. Pat. No. 5,806,169 (Trago) (incorporated herein by
`reference) discloses a method of fabricating an injection
`molded motor assembly. However, neither the Trago design
`nor the other prior art designs address the problems of
`winding wire, variation in the thickness of steel used to
`30
`make the stator cores and the non-uniform grain structure in
`the steel compared to the magnetic flux in the stator during
`operation of the motor.
`Some of these problems have been addressed by motor
`manufacturing methods in which individual stator arc seg
`ments are made and wound with wire to form poles, and
`these segments are then assembled to form a complete stator.
`While this process allows for higher packing density, it has
`several drawbacks. Somehow the individual segments have
`to be assembled and held in place to form the stator. In
`addition, the individual wires of the different poles have to
`be connected together for the poles that are of the same
`phase. These numerous wires tend to get in the way during
`the assembly process, slowing down the manufacturing
`process.
`45
`U.S. Pat. No. 6,049,153 to Nishiyama describes the use of
`crimping or welding to attach segments together. This pro
`cess deforms the steel and reduces the level of magnetic flux
`produced by the laminations. The process also requires
`numerous wire interconnections when the poles are wound
`50
`as discrete components, and it does not offer improvements
`in wire routing.
`U.S. Pat. No. 5,729,072 to Hirano describes the use of
`welding or an adhesive to hold the segments together. A
`disadvantage of this approach is that the stator poles must be
`55
`handled as separate elements during stator construction.
`This requires complicated assembly equipment and a slow
`manufacturing process.
`U.S. Pat. No. 6,265,804 to Nitta describes the use of
`plastic insulation in combination with segmented stators.
`This approach does not improve on the problem of how to
`assemble and hold the individual segments in place, nor does
`it aid in connecting the various wires.
`U.S. Pat. No. 6,167,610 to Nakahara describes a method
`of making a rotary motor where a length of steel strip has
`thin portions between blocks of pole teeth. Wire is wound on
`the pole teeth while the steel strip is straight. Later the thin
`
`4
`sections are bent to allow the poles to form a stator. One
`problem with this design is that when the thin portions are
`bent, the stress on the steel reduces the flux capacity of the
`connecting steel, forming the back iron. Also, the stamping
`of such a length of steel strip would be expensive and result
`in large amount of scrap. Thus, a need exists for a method
`of making motors overcoming the aforementioned prob
`lems.
`
`BRIEF SUMMARY OF THE INVENTION
`
`A method of making stator assemblies has been invented
`which overcomes many of the foregoing problems. In addi
`tion, unique stator assemblies and other components of a
`motor have been invented. In one aspect, the invention is a
`stator assembly comprising a plurality of discrete stator
`segments each at least partially encased with a phase change
`material, wherein the phase change material also comprises
`a bridge between adjacent segments to link adjacent seg
`ments into a continuous strip; and the linked stator segments
`being arranged and secured together to form the stator
`assembly.
`In a second aspect, the invention is a combination of stator
`arc segments and a flexible carrier used to link said stator arc
`segments during a winding operation comprising: a) a
`plurality of stator arc segments; and b) a phase change
`material constituting said flexible carrier adhered to the
`stator arc segments which links said segments in a uniform
`and predetermined position with respect to one another.
`In another aspect the invention is a method of making a
`stator assembly comprising: a) providing at least two stator
`arc segments linked together by a phase change material and
`each constituting a pole and having a first side surface and
`a second side surface; b) winding wire on the poles; c)
`aligning said stator arc segments to form a toroidal core,
`wherein each said side surface of one segment is in contact
`with an opposing side surface of another segment; and d)
`substantially encapsulating said toroidal core with a mono
`lithic body of phase change material to form said stator
`assembly.
`In another aspect the invention is a method of making a
`stator assembly comprising: a) providing at least two stator
`arc segments linked together by a phase change material and
`each providing a pole and having a first side surface and a
`second side surface; b) winding wire on each pole of each
`arc segment; c) aligning said stator arc segments to form a
`toroidal core, wherein each said side surface of one segment
`is in contact with an opposing side surface of another
`segment; and d) placing a retaining member on the exterior
`of the toroidal core to unitize the structure.
`In yet another aspect, the invention is a series of discrete
`stator segments each substantially encapsulated with, and
`linked together by bridges made from, an injection molded
`thermoplastic material.
`With the unique linked but discrete segment assemblies,
`wire can be wound around the poles with a high packing
`density, yet at the same time the segments can be maintained
`in their proper order so that one continuous piece of wire can
`be used to wind all poles in the same series or phase, making
`it unnecessary to later connect wires from individual wind
`ings to one another. The invention provides the foregoing
`and other features, and the advantages of the invention will
`become further apparent from the following detailed
`description of the presently preferred embodiments, read in
`conjunction with the accompanying drawings. The detailed
`description and drawings are merely illustrative of the
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`US 7,067,952 B2
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`US 7,067,952 B2
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`invention and do not limit the scope of the invention, which
`is defined by the appended claims and equivalents thereof.
`BRIEF DESCRIPTION OF SEVERAL VIEWS OF
`THE DRAWINGS
`
`FIG. 1 is an exploded, partial cross-sectional and perspec
`tive view of a conventional prior art high speed motor.
`FIG. 2 is perspective view of a stator arc segment being
`loaded into an injection mold prior to injecting a phase
`change material to make a limited series of stator arc
`segments of the present invention.
`FIG. 3 is a perspective, partial cross-sectional view of an
`encapsulated stator arc segment of FIG. 2.
`FIG. 4 is a perspective view of the encapsulated stator arc
`segment of FIG. 2.
`FIG. 5 is a perspective view of a series of encapsulated
`stator arc segments of FIGS. 2–4 linked together by a
`thermoplastic webbing.
`FIG. 6 is a perspective view of the series of stator arc
`segments of FIG. 5 during wire winding.
`FIG. 7 is a perspective view of an injection molded stator
`assembly using the linked serial of webbed stator arc seg
`ments of FIG. 6.
`FIG. 8a is a cross-sectional view of a toroidal core made
`from the linked series of stator arc segments after the wire
`winding shown in FIG. 5 in an injection mold assembly,
`prior to injecting a phase change material.
`FIG. 8b is a cross-sectional view of the toroidal core of
`FIG. 8a in an injection mold assembly after injecting a phase
`change material, resulting in the stator assembly of FIG. 7.
`FIG. 9 is an exploded, partial cross-sectional and perspec
`tive view of a motor using the encapsulated webbed stator of
`FIG. 7.
`FIG. 10 is a perspective view of a stator assembly of a
`second embodiment of the present invention using a steel
`band to unitize the webbed stator arc segments.
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`DETAILED DESCRIPTION OF THE DRAWINGS
`AND PREFERRED EMBODIMENTS OF THE
`INVENTION
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`ing inwardly. The stamped pieces are then coated with
`encapsulating material 22 which provides electrical insula
`tion and laminates the pieces together to form a stator arc
`segment 20, and links other arc segments into a continuous
`strip via webbing 23.
`The encapsulating material 22 is preferably formed of a
`phase change material, meaning a material that can be used
`in a liquid phase to envelope the stator, but which later
`changes to a solid phase. There are two types of phase
`change materials that will be most useful in practicing the
`invention: temperature activated and chemically activated. A
`temperature activated phase change material will become
`molten at a higher temperature, and then solidify at a lower
`temperature. However, in order to be practical, the phase
`change material must be molten at a temperature that is low
`enough that it can be used to encapsulate a toroidal core.
`Preferred phase change materials will be changed from a
`liquid to a solid in the range of about 200°F. to about 700°
`F., more preferably in the range of about 550° F to about
`650° F. The most preferred temperature activated phase
`change materials are thermoplastics. The preferred thermo
`plastic will become molten at a temperature at which it is
`injection-moldable, and then will be solid at normal oper
`ating temperatures for the motor. An example of a phase
`change material that changes phases due to a chemical
`reaction, and which could be used to form the body, is an
`epoxy. Other suitable phase change materials may be clas
`sified as thermosetting materials.
`As shown in FIG. 2 the segments 20 can be placed in a
`multi-cavity mold 28 to increase productivity. In the pre
`ferred embodiment the individual laminations 11 making up
`the segments are not interconnected but loosely stacked
`together before insertion into the mold 28. After the ther
`moplastic solidifies, the overmolded segments are ejected
`from their cavities. New laminations are inserted into the
`cavities and the process repeats. In the preferred embodi
`ment, a continuous strip of segments is formed by linking
`the webbing from successive molding operation. This is
`done by designing the tool to insert a section of the plastic
`webbing of the outermost segment molded in the prior cycle
`with the new laminations to be molded. When the plastic
`encapsulates the new segments it can mechanically lock
`with or, depending upon design, re-melt, the webbing from
`the prior cycle, thus making a continuous strip, as shown in
`FIG. 5. The series has segments 20 with poles 21A, 21B and
`21C arranged next to one another as they will be in the
`finished stator assembly.
`The stator arc segments 20 are preferably molded into a
`continuous strip where the webbing acts as a carrier to link
`the segments together. In the preferred embodiment the
`encapsulating material 22 forms wire retaining flanges 24 to
`prevent wire from slipping off the pole. In a preferred
`embodiment, winding posts 25 as well as webbing 23 allow
`orientation of wire as it transfers across multiple poles.
`By precisely aligning the stator arc segments 20, the
`webbing 23 can also be used to guide the wire between
`common phase poles, thus eliminating the need for inter
`connections commonly used on segmented Stator motors.
`This greatly enhances the efficiency for winding wire 15
`around the poles 21 and significantly reduces the cost.
`The webbing can be deflected to allow the gap between
`adjoining poles to be increased as is shown in FIG. 6. This
`allows wire 15 to be wound around the poles 21 of the stator
`arc segments 20 using a fly winder 34 that has a set of
`needles 35. The wire 15 is wound around one pole 21 and is
`then wound around another pole 21 in its phase until all
`poles 21 in the same phase are wound with the same wire 15.
`
`A preferred embodiment of a motor of the present inven
`tion and portions of the motor at different stages of manu
`facture are shown in FIGS. 2–7 and 9. The spindle motor
`45
`100 (FIG. 9) is designed for rotating a disc or stack of discs
`in a computer hard drive. Motor 100 is formed by using an
`injection molded stator assembly 40, that is formed by
`injection molding a plurality of stator arc segments 20 (FIG.
`2) aligned to form a toroidal core 17 (FIG. 7). Although the
`embodiment described here uses individual arc segments,
`one of ordinary skill in the art will understand that groups of
`two, three or any greater number of arc segments may be
`used. The preferred motor of the present invention may be
`smaller, has a grain structure that is more uniformly aligned,
`and allows for greater packing density of wire and reduces
`waste of steel in the manufacturing process, as compared
`with conventional motors, thereby increasing power and
`reducing stack up tolerances and manufacturing costs and
`producing other advantages discussed below.
`Referring to FIG. 2, a stator arc segment 20 is first
`constructed, using steel laminations 11. The stator arc seg
`ment 20 is made of steel pieces that are stamped out of rolled
`steel. The stamped steel pieces are arc segments, but also
`have a pole 21 extending inwardly or outwardly depending
`on whether the rotor is inside or surrounds the stator. In the
`embodiment shown in FIG. 2, the pole 21 is shown extend
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`Poles 21 in other phases are also similarly wound. Having
`only arc segments, rather than a full toroidal core, and
`spreading the spacing between the adjoining segments for
`needle 35 to wind wire 15 around poles 21, allows a wire
`packing density of more than 80 percent to be achieved.
`A length of connected stator segments 20 corresponding
`to the number of poles 21 required to produce the motor are
`cut from the continuous strip. The strip is then rolled into a
`magnetically inducible toroidal core 17 having a plurality of
`poles 21 thereon, and wire windings 15 which serve as
`conductors. To form the toroidal core 17, a side surface 16
`of each stator arc segment 20 is aligned and brought into
`contact with a corresponding side surface of another stator
`arc segment 20. In certain embodiments where a reduction
`in eddy currents is desirable, it may be preferable to separate
`faces 16. This could be done by using a thin film of
`encapsulation material 22 over the side surfaces 16, or the
`edges 19 of the insulator end surface (FIG. 4) could be used
`to create the gap. The wire 15 between the poles 21 of
`different stator arc segments 20 is also aligned in the toroidal
`20
`core 17, following the arc of the stator arc segments 20. As
`a result, the wire in the toroidal core 17 is taught.
`As shown in FIG. 7, the toroidal core 17 is then encap
`sulated in a body 42. Together the toroidal core 17 and the
`body 42 make up an injection molded stator assembly 40.
`The body 42 is preferably a monolithic body. Monolithic is
`defined as being formed as a single piece. The body 42
`substantially encapsulates the toroidal core 17. Wires 44
`extend out of the body 42 for connection to the power source
`used to supply the motor. Substantial encapsulation means
`that the body 42 either entirely surrounds the toroidal core
`17, or surrounds almost all of it except for minor areas of the
`toroidal core 17 that may be exposed. However, substantial
`encapsulation means that the body 42 and toroidal core 17
`are rigidly fixed together, and behave as a single component
`with respect to harmonic oscillation vibration.
`The preferred method of developing the monolithic body
`42 comprises designing a phase change material to have a
`coefficient of linear thermal expansion such that the phase
`change material contracts and expands at approximately the
`same rate as the metal laminations of the toroidal core 17.
`For example, the preferred phase change material should
`have a CLTE of between 70% and 130% of the CLTE of the
`core of the stator. The phase change material should have a
`CLTE that is intermediate the maximum and minimum
`CLTE of t