CA1109915A - High power density brushless dc motor - Google Patents
High power density brushless dc motorInfo
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- CA1109915A CA1109915A CA299,981A CA299981A CA1109915A CA 1109915 A CA1109915 A CA 1109915A CA 299981 A CA299981 A CA 299981A CA 1109915 A CA1109915 A CA 1109915A
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Abstract
ABSTRACT OF THE DISCLOSURE
A multiple disk, pancake, self-fed brush less dc motor characterized by high power density has a variable number of interleaved, axially spaced annular rotor disks and annular stator disks. The annular rotor disks are made of high coercive force permanent magnets such as cobalt-samarium and fer-rites which do not demagnetize easily. Each annular stator disk is yokeless and consists of a laminated magnetic core made of spirally wound metal strip, with opposing sets of stator slots and windings on both sides of the core. The permanent magnet machine can be operated as a generator.
The Supplementary Disclosure describes a preferred embodiment of the machine in which the metal strip of the stator disks is low-loss magnetic amorphous metal ribbon.
A multiple disk, pancake, self-fed brush less dc motor characterized by high power density has a variable number of interleaved, axially spaced annular rotor disks and annular stator disks. The annular rotor disks are made of high coercive force permanent magnets such as cobalt-samarium and fer-rites which do not demagnetize easily. Each annular stator disk is yokeless and consists of a laminated magnetic core made of spirally wound metal strip, with opposing sets of stator slots and windings on both sides of the core. The permanent magnet machine can be operated as a generator.
The Supplementary Disclosure describes a preferred embodiment of the machine in which the metal strip of the stator disks is low-loss magnetic amorphous metal ribbon.
Description
~ g ~ ~ RD-3016 This invention relates -to brushless electric machines, and more particularly to a self-fed, high power density brushless dc motor with a multiple disk, pancake, rotor and stator structure~
There is an industrial and military need for high power density electric motors in a variety of applications such as adjustable speed motor drives and ordnance servo drives. The usual permanent magnet materials have sufficient flux density capability (for example, the remanent induction, Br, of Alnico lQ 57 is 12.9 kilogauss). However, armature reaction under overload or short circuit often exceeded the oerstead capability of the magnet, demagnetizing the magnet and resulting in degraded performance characteristics This is a well known permanent magnet problem. Hence, a well-designed machine incorporating such usual magnetic materials in the rotor does not result in a light weight motor~
Also, the yoke or stator iron behind the stator teeth in conventional motor structures serves no power producing func-tion and merely returns the magnetic flux to the roots of the teeth on the` next pole of the motor. Elimination of the excess stator yoke material would therefore contribute to the realization of increased power density in an electrical machine.
The exemplary embodiment of the multiple disk, high power density electric machine herein described is a self-fed brushless dc motor having a plurality of interleaved, axially spaced, alternating annular rotor disks and annular stator disks, the number of rotor disks and stator disks depending on the application. Each annular rotor disk comprises a plurality of clrcularly spaced, alternating polarity permanent magnet sectors made of high energy product permanent magnet materials with a high coercive force ~Hc) which do not easily demagnetize, while yet having satisfactory flux density capability (Br). These permanent magnet materials include rare earth, ferrite, and ~ RD-3016 Alnico types; ro-tor disks made with cobalt-rare earths and other such permanent magnets are light weight and contribute signific-antly toward the objective of high power density. Each annular stator disk located intermediate between two rotor disks consists of a laminated magnetic core made o~ spirally wound metal strip and having opposi.n~ sets of outwardly directed stator slots, one set on either core side, with each set of stator slots having a stator winding inserted therein. This configuration eliminates most of the yoke material and greatly improves efficiency.
The magnetic flux is axially directed in active areas of the machine, and a magnetic yoke is provided at each end of the plurality of interleaved rotor disks and stator disks for carrying peripheral flu~. A solid metal disk can be attached to an endmost rotor disk to function as a magnetic yoke, or an endmost stator disk can be made similar to the stator disks previously described, except that it has only one set of stator slots with one winding on one side of the endmost stator disk, the other side having no slots and functioning as a laminated magnetic yoke Two techniques are described for supporting the
There is an industrial and military need for high power density electric motors in a variety of applications such as adjustable speed motor drives and ordnance servo drives. The usual permanent magnet materials have sufficient flux density capability (for example, the remanent induction, Br, of Alnico lQ 57 is 12.9 kilogauss). However, armature reaction under overload or short circuit often exceeded the oerstead capability of the magnet, demagnetizing the magnet and resulting in degraded performance characteristics This is a well known permanent magnet problem. Hence, a well-designed machine incorporating such usual magnetic materials in the rotor does not result in a light weight motor~
Also, the yoke or stator iron behind the stator teeth in conventional motor structures serves no power producing func-tion and merely returns the magnetic flux to the roots of the teeth on the` next pole of the motor. Elimination of the excess stator yoke material would therefore contribute to the realization of increased power density in an electrical machine.
The exemplary embodiment of the multiple disk, high power density electric machine herein described is a self-fed brushless dc motor having a plurality of interleaved, axially spaced, alternating annular rotor disks and annular stator disks, the number of rotor disks and stator disks depending on the application. Each annular rotor disk comprises a plurality of clrcularly spaced, alternating polarity permanent magnet sectors made of high energy product permanent magnet materials with a high coercive force ~Hc) which do not easily demagnetize, while yet having satisfactory flux density capability (Br). These permanent magnet materials include rare earth, ferrite, and ~ RD-3016 Alnico types; ro-tor disks made with cobalt-rare earths and other such permanent magnets are light weight and contribute signific-antly toward the objective of high power density. Each annular stator disk located intermediate between two rotor disks consists of a laminated magnetic core made o~ spirally wound metal strip and having opposi.n~ sets of outwardly directed stator slots, one set on either core side, with each set of stator slots having a stator winding inserted therein. This configuration eliminates most of the yoke material and greatly improves efficiency.
The magnetic flux is axially directed in active areas of the machine, and a magnetic yoke is provided at each end of the plurality of interleaved rotor disks and stator disks for carrying peripheral flu~. A solid metal disk can be attached to an endmost rotor disk to function as a magnetic yoke, or an endmost stator disk can be made similar to the stator disks previously described, except that it has only one set of stator slots with one winding on one side of the endmost stator disk, the other side having no slots and functioning as a laminated magnetic yoke Two techniques are described for supporting the
2~ sta~or disks on the motor housing, one suitable for small motors and the other suitable for large motors. The permanent magnet machine can also be operated as a generator.
Applications for the high power density motor are in electric vehicles and industrial ad~ustable speed drives, and ~ :
as an ordnance servo drive motor and a starting motor.
In embodiments of the invention shown in the drawings:
FIG. 1 is a schematic vertical cross section through the high power density self fed brushless dc motor according to one embodiment suitable for small motors, FIG. 2 is a fragmentary vertical cross section through a motor similar to that in FIG.:l but with a modified ~ R~ 30~6 stator disk mounting for large motors;
FIG. 3 is a fragmentary front view of a single annular rotor disk mounted on the motor shaft;
FIGS. 4 and 5 are cross sections taken radially through the rotor disk in FIG. 3;
FIGS. 6-8 are plan views of the punched steel strip for winding laminated statox magnetic cores for small and large motors, and for an end core with a laminated yoke as in FIG. l;
FIG5. 9 and 10 are sketches depicting automatic assembly of spirally wound stator magnetic cores ~or small and large motors; and FIG. 11 is a fragmentary front view of a wound stator magnetic core illustrating diagrammatically in line form some of the coils of a conventional winding.
The improved, high power density, brushless dc motor is shown in FIG. 1 in a configuration for small motors.
For large motors a different technique for mounting the stator disk on the motor housing is preferred and is illus-trated in FIG. 2, although in other respects the motors are similar. The multiple disk, pancake, electric motor configuration permits stacking as many alternating rotor and stator dislcs as is necessary to mePt the requirements of an intended application. In FIG. 1, the brushless dc motor comprises a selected number of interleaved and alternating annular permanent magnet rotor disks 11 and annular, spirally w~ laminated stator disks 12 and 12' which~are axially spaced from one~another to establish axial air gaps 13 between adjacent disks. The permanent magnet rotor disks are made of high energy product permanent magne~
materials, and utilize both the high coercive force, Hc7 r~ e~
and high r~#~a~t lnduction, Br, characteristic of these
Applications for the high power density motor are in electric vehicles and industrial ad~ustable speed drives, and ~ :
as an ordnance servo drive motor and a starting motor.
In embodiments of the invention shown in the drawings:
FIG. 1 is a schematic vertical cross section through the high power density self fed brushless dc motor according to one embodiment suitable for small motors, FIG. 2 is a fragmentary vertical cross section through a motor similar to that in FIG.:l but with a modified ~ R~ 30~6 stator disk mounting for large motors;
FIG. 3 is a fragmentary front view of a single annular rotor disk mounted on the motor shaft;
FIGS. 4 and 5 are cross sections taken radially through the rotor disk in FIG. 3;
FIGS. 6-8 are plan views of the punched steel strip for winding laminated statox magnetic cores for small and large motors, and for an end core with a laminated yoke as in FIG. l;
FIG5. 9 and 10 are sketches depicting automatic assembly of spirally wound stator magnetic cores ~or small and large motors; and FIG. 11 is a fragmentary front view of a wound stator magnetic core illustrating diagrammatically in line form some of the coils of a conventional winding.
The improved, high power density, brushless dc motor is shown in FIG. 1 in a configuration for small motors.
For large motors a different technique for mounting the stator disk on the motor housing is preferred and is illus-trated in FIG. 2, although in other respects the motors are similar. The multiple disk, pancake, electric motor configuration permits stacking as many alternating rotor and stator dislcs as is necessary to mePt the requirements of an intended application. In FIG. 1, the brushless dc motor comprises a selected number of interleaved and alternating annular permanent magnet rotor disks 11 and annular, spirally w~ laminated stator disks 12 and 12' which~are axially spaced from one~another to establish axial air gaps 13 between adjacent disks. The permanent magnet rotor disks are made of high energy product permanent magne~
materials, and utilize both the high coercive force, Hc7 r~ e~
and high r~#~a~t lnduction, Br, characteristic of these
- 3 S
magnet materials, Thus, the disk rotor is not easily demagnetized, has good flux density capability, and can be made light weight. The annular laminated stator disks 12 are essentially yokeless and have a paix of opposing stator windings 14, one at each side of the spirally ~ound laminated magnetic core.
The rotor disks 11 are attached, either individually or in combination, for rotation with motor shaft 15 and, as here illustrated, the individual disks are secured to spokes 16 fixed to a hub member 17 which in turn is clamped to shaft 15. Bearings for the motor shaft are indicated at 18.
Stator disks 12 and 12', on the other hand, are supported or mounted on the motor housing 19. To facilitate easy assembly, the outer periphery and outside stator winding end turns are formed with a larger diameter, encapsulating resin washer 20.
A suitable process is to cast the outer periphery reyion in epoxy resin, and this serves also to insulate the coil end turns from the housing. In assembling the motor, spacer rings 21 are alternated with stator disks 12 and 12' so that resin washers 20 are retained between two spacer rings, one at either side. From the standpoint of cooling the motor, the selected resin should be a heat-conducting iller. For larye motors, the technique in FIG. 2 may be required for mountiny the annular ~tator disks on the motor housing. As will be explained in detail later, each laminated stator disk 12 or 12' has a radial hole between the two windings 14 in which is received an insulating stud 22, the outer ends of the stud being retained in holes ln motor housiny 19. In both embodiments, heat transfer from the stator winding and core are improved by the radLal configuration. The machine is also suitable for ventilation by an ex~ernal blower or fan whlch may be requlred for some large motors.
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magnet materials, Thus, the disk rotor is not easily demagnetized, has good flux density capability, and can be made light weight. The annular laminated stator disks 12 are essentially yokeless and have a paix of opposing stator windings 14, one at each side of the spirally ~ound laminated magnetic core.
The rotor disks 11 are attached, either individually or in combination, for rotation with motor shaft 15 and, as here illustrated, the individual disks are secured to spokes 16 fixed to a hub member 17 which in turn is clamped to shaft 15. Bearings for the motor shaft are indicated at 18.
Stator disks 12 and 12', on the other hand, are supported or mounted on the motor housing 19. To facilitate easy assembly, the outer periphery and outside stator winding end turns are formed with a larger diameter, encapsulating resin washer 20.
A suitable process is to cast the outer periphery reyion in epoxy resin, and this serves also to insulate the coil end turns from the housing. In assembling the motor, spacer rings 21 are alternated with stator disks 12 and 12' so that resin washers 20 are retained between two spacer rings, one at either side. From the standpoint of cooling the motor, the selected resin should be a heat-conducting iller. For larye motors, the technique in FIG. 2 may be required for mountiny the annular ~tator disks on the motor housing. As will be explained in detail later, each laminated stator disk 12 or 12' has a radial hole between the two windings 14 in which is received an insulating stud 22, the outer ends of the stud being retained in holes ln motor housiny 19. In both embodiments, heat transfer from the stator winding and core are improved by the radLal configuration. The machine is also suitable for ventilation by an ex~ernal blower or fan whlch may be requlred for some large motors.
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~ ~ ~ RD 3016 The path oE magnetic ~lux in active areas of the machine is in the axial direction, with the flu~ being turned around at the ends o~ the machineO To this end, magnetic yoke means is provided at both ends of the plurality of interleaved rotor disks and stator disks for carrying peripheral flux from one pole to the neYt pole. ~wo dif~erent yoke struc-tures for accomplishing this are shown at the left and right of FIG. 1. At the left side a solid dis]c 23 made of steel or other metal, which need not be laminated because it is not cutting ~lux, is attached to the outer face of the endmost rotor disk 11 At the right side, as is further clarified in the discussion of FIG. 8, spirally wound stator disk 12' has a set of stator slots at only one side of the laminated magnetic core and consequently only one winding, the other side functioning as a continuous laminated yoke. In a practical motor, there can be a rotor disk at one end of the machine and a stator disk at the other end, or rotor disks at both ends, or stator disks at both ends, and thus the two ~;
return flux magnetic yoke structures can be intermixed or one or the other used exclusively.
The struc-ture of an individual permanent magnet rotor disk 11 is shown in greater detail in FIGS. 3-5. The rotor disk comprises a plurality of circularly spaced, alternating polarity, permanent magnet sectors or sections 24 whose circular spacing ~s typically 50 to 60 electrical degrees.
As was previously discussed, the permanent magnet material for this application is a high energy product permanent magnet material such as cobalt~samarium and various ferrites characterized by a high coercive force and good flux density capability, so that there is a high resistance to demagnetization and the required volume of magnet material is relatively low. Additional suitable permanent magnet ,~0 . i .
11~9~5 RD 3016 materials are other cobalt-rare earths and Alnico 9, whose composition is 7 percent aluminum, 15 percent nickel, 35 percent cobalt, 34 percent iron, 4 percent copper, and 5 percent titanium. Alnico 9 has the same composition as Alnico 8 but is processed differently to enhance the coercive force characteristic. In FIG. 3, spokes 16 pro-jecting from hub member 17 are extended radially and have a cross piece 25 at the outer ends, the permanent magnet sectors being inserted between adjacent spokes and held from outward movement during rotation by the cross pieces. The rotor support just described is constructed of a nonmagnetic me-tal such as aluminum or an aluminum alloy. In the axial `~
direction, each magnet sector can have a single permanent magnet 24 as in FIG. 4, or can have a pair of axially spaced sector-shaped permanent magnets 26 and 27 as in FIG. 5.
The spirally wound, laminated magnetic cores for the stator disks are fabrica-ted from a continuous, thin strip of electrical grade steel or other metal. The preferred lamination punchings for double winding, yokeless, ;
lamina~ed stator disks located intermediate between -two rotors are shown in FIGS. 6 and 7 for small and large motors. The steel strip is punches to define opposing stator teeth 30 and 31, alternating with opposing, outwardly directed stator slots 32 and 33, the teeth being connected by a thin web member 34. In order to have the slot openings line up as the strip is wound circularly to form an annular magnetic core, the tooth width is varied either continuously or as a step function. Thls is essential to accommodate diametrical increments as the strip is wound where the stator outside diameter/inslde diameter ratio is large. Passage of flux through the core lamination is primarily in the direc-tion of arrow 35, lenythwise along the sta-tor teeth with ~ RD 3016 little diversion of ~lux through the narrower web members 34, which functlon primarily to hold the teeth together. For large motors, the preferred punching pattern (see FIG. 7) includes a hole 36 in web member 34' for radial ventilation and/or to receive ~he insulated mounting studs 22 shown in FIG. 2.
FIG. 8 depicts the preferred punching of a stator core lamination for an endmost stator disk 12' having only one winding and a combined laminated yoke for carrying peripheral flux such as is shown at the right side of FIG. 1. One side of the steel strip is punched to define a single stator tooth 30 alternating with a slot opening 32, the other side being continuous and unpunched.
Automatic assembly of a spirally wound stator magnetic core for a small motor can be, for example, as illustrated in FIG. 9~ A reel 37 of unpunched slit steel is unwound and fed through a punch press 38, and the punched strip is continuously wound on a bobbin 39 to form the annular magnetic core. The assembly of stator magnetic cores for large motors can be as shown in FIG. lO and uses a reel 40 of prepunched strip. As the stacking of the laminated core proceeds, studs 22 are driven into holes 36 in the punched stripO
The stator winding is normally a conventional polyphase winding, but some savings of end turns may result with concentric windinys. A third and potentially attractive idea which results in considerable savings of coppar wire is a semitoroidal winding for random stators. A wound stator with a conventional winding is illustrated schematically in FIG. llo Thre~ coils 41a, 41b, and 41c are indicated ~4~ icl ~ diagramatically in ~a~ outlines, inserted in the radial ; and outwardly directed stator slots 42 at one side of annular ~ 9 ~ 5 RD 3016 magnetic cor~ 43. The second stator winding in the opposing set of stator slots at the other side of the core is a separate stator circuit and can be connected in parallel or in series with the first winding.
The motor structure for a self-fed brushless dc motor as here described is particularly advantageous for a four-pole or six-pole motor. Most commonly, the control circuitry includes an inverter fed from a dc source whose frequency is determined by a shaft posi~ion sensor. Excita-tion of the windings is therefore self-synchronous with shaft position. There are many applications for the high power density, self fed brushless dc motor including, among others, a motor for an electric vehicle or industrial adjustable speed drive, an ordnance servo drive motor' and a starting motor.
Some comments on efficiency are important because ~ it is a substantial factor in power density capability. The ; permanent magnet rotor, of course, has very little loss except surface losses, thereby contri~uting substantially to improved efficiency. Also the absence of yoke material in the stakor disk greatly improves efficiency. Heat transfer from the stator windings and core are improved by the radial configuration of the stator, which can be ventilated by an external fan if required.
As will be understood by those skilled in the art, the permanent magnet machine herein described can also be operated as a generator, in which case the terminal voltage is the output voltage.
While the invention has been particularly shown and described with reference to several preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
~99~5 RD-3016 Supplementary ~isclosu.re In a preferred embodiment of the machine, the metal strip of the stator disks 12 and 12' is low loss magnetic amorphous metal ribbon (such as Fe80B20).
Amorphous metals are known also as metallic glasses and exist in many different compositions including a variety of magnetic alloys which include iron group elements and boron or phosphorus. Metallic glasses are formed from alloys that can be ~uenched without crystallization, and these solids are mechanically stiff, strong and ductile, but more importantly they have low losses and are low in cost~
Amorphous metal motor laminations differ from -those made of steel strip in that the amorphous metal is very thin, :
with a practical limitation at present of about 2 mils in thickness, and furthermore the amorphous metal is brittle ~like glass) so that toothed laminations cannot be made by punch-ing. Its natural configuration is a longr narrow and thin ribbon, and this suggests an application for the flat pancake .
motor. Ribbon in uniform widths of one-half inch or greater ..
- can be processed by directing liquid alloy onto a rapidly rotating cold chill cylinder, the liquid alloy being changed into a solid ribbon in a short time (measured in microseconds) before it becomes crystalline. The cooling rate is of the order of 106 C/sec, and the thickness limitation is set by the rate of heat transfer through the already solidified material. This heat transfer must be rapid enough that the : last increment to solidify still avoids crystallization. Any of the magnetic alloys can be ut.ili2ed, but the ~referred composition, because o~ its high induction characteristics, is the Fe~OB20 alloy. AnoLher suitable amorphous metal is Fe4~Ni40P14B6 or the variation of this material sold as METGLAS~ ) ~lloy Ribbon 2826 MB by Allied Chemical Corp.
- 9 ~
- . - .:
~ RD-3016 Amorphous metal ribbon of the Fe80B20 alloy has one-fourth the losses, at a given induc-tion for sinusoidal flux, of the best oriented Fe-Si sheet steel. Additional information is given in the article "Potential of Amorphous Metals for Application in Magnetic Devices" by F~E. Luborsky et al, Journal of Applied Physics, Vol. 49, No. 3, Part II, March 1978, pp. 1769-177~.
Etching or chemical milling can be used on amorphous metal to make motor laminations, and while the process can be automated, the cost o~ chemicals may make the process expensiveO A more effective technique to put slots in the ribbon is to draw the ribbon past a cutting laser beam, which can be controlled by a micxoprocessor. In FIG. 9, punch press 38 is replaced by a laser beam station, and amorphous metal tape is drawn past the cutting beam as slots are cut out by the beam. Spacing of the slots is controlled so they will fall into a line when the ribbon is rolled up. This material i5 ; mechanically strong and can be pulled as the laminated stator core is wound spirally, resulting in a magnetic core with an improved stacking factor or packing fraction~ That is, more flux is carried by a given volume of the core material and this contributes to the objective of a high power density machine. An alternative technique~ one which makes amorphous metal shaped laminations directly from the alloy melt, is disclosed and claimed in United States patent 4,155,397 issued May 22, 1979 to V.B. Honsinger and R.E. Tompkins, entitled "Method and Apparatus for Fabrica-ting Amorphous Metal Laminations for Motors and Transformers" and assigned to the present assignee.
The speed range of the brus~less dc motor can be increased by rotating two stator disks relative to each other.
In an operating motor, the resulting phas~ shift reduces the ~ -~
- 10 - ' ' ., .. ~ .
. . .
series sum of back emf, so that the motor speeds up and lncreases its back emf until a new stable operating condition is reached at a higher speed. When the machine is operating as a permanent magnet generator, this feature of relative rotation of two stator disks can provide voltage regulation.
,-.': ' , ~ ' .
~ ~ ~ RD 3016 The path oE magnetic ~lux in active areas of the machine is in the axial direction, with the flu~ being turned around at the ends o~ the machineO To this end, magnetic yoke means is provided at both ends of the plurality of interleaved rotor disks and stator disks for carrying peripheral flux from one pole to the neYt pole. ~wo dif~erent yoke struc-tures for accomplishing this are shown at the left and right of FIG. 1. At the left side a solid dis]c 23 made of steel or other metal, which need not be laminated because it is not cutting ~lux, is attached to the outer face of the endmost rotor disk 11 At the right side, as is further clarified in the discussion of FIG. 8, spirally wound stator disk 12' has a set of stator slots at only one side of the laminated magnetic core and consequently only one winding, the other side functioning as a continuous laminated yoke. In a practical motor, there can be a rotor disk at one end of the machine and a stator disk at the other end, or rotor disks at both ends, or stator disks at both ends, and thus the two ~;
return flux magnetic yoke structures can be intermixed or one or the other used exclusively.
The struc-ture of an individual permanent magnet rotor disk 11 is shown in greater detail in FIGS. 3-5. The rotor disk comprises a plurality of circularly spaced, alternating polarity, permanent magnet sectors or sections 24 whose circular spacing ~s typically 50 to 60 electrical degrees.
As was previously discussed, the permanent magnet material for this application is a high energy product permanent magnet material such as cobalt~samarium and various ferrites characterized by a high coercive force and good flux density capability, so that there is a high resistance to demagnetization and the required volume of magnet material is relatively low. Additional suitable permanent magnet ,~0 . i .
11~9~5 RD 3016 materials are other cobalt-rare earths and Alnico 9, whose composition is 7 percent aluminum, 15 percent nickel, 35 percent cobalt, 34 percent iron, 4 percent copper, and 5 percent titanium. Alnico 9 has the same composition as Alnico 8 but is processed differently to enhance the coercive force characteristic. In FIG. 3, spokes 16 pro-jecting from hub member 17 are extended radially and have a cross piece 25 at the outer ends, the permanent magnet sectors being inserted between adjacent spokes and held from outward movement during rotation by the cross pieces. The rotor support just described is constructed of a nonmagnetic me-tal such as aluminum or an aluminum alloy. In the axial `~
direction, each magnet sector can have a single permanent magnet 24 as in FIG. 4, or can have a pair of axially spaced sector-shaped permanent magnets 26 and 27 as in FIG. 5.
The spirally wound, laminated magnetic cores for the stator disks are fabrica-ted from a continuous, thin strip of electrical grade steel or other metal. The preferred lamination punchings for double winding, yokeless, ;
lamina~ed stator disks located intermediate between -two rotors are shown in FIGS. 6 and 7 for small and large motors. The steel strip is punches to define opposing stator teeth 30 and 31, alternating with opposing, outwardly directed stator slots 32 and 33, the teeth being connected by a thin web member 34. In order to have the slot openings line up as the strip is wound circularly to form an annular magnetic core, the tooth width is varied either continuously or as a step function. Thls is essential to accommodate diametrical increments as the strip is wound where the stator outside diameter/inslde diameter ratio is large. Passage of flux through the core lamination is primarily in the direc-tion of arrow 35, lenythwise along the sta-tor teeth with ~ RD 3016 little diversion of ~lux through the narrower web members 34, which functlon primarily to hold the teeth together. For large motors, the preferred punching pattern (see FIG. 7) includes a hole 36 in web member 34' for radial ventilation and/or to receive ~he insulated mounting studs 22 shown in FIG. 2.
FIG. 8 depicts the preferred punching of a stator core lamination for an endmost stator disk 12' having only one winding and a combined laminated yoke for carrying peripheral flux such as is shown at the right side of FIG. 1. One side of the steel strip is punched to define a single stator tooth 30 alternating with a slot opening 32, the other side being continuous and unpunched.
Automatic assembly of a spirally wound stator magnetic core for a small motor can be, for example, as illustrated in FIG. 9~ A reel 37 of unpunched slit steel is unwound and fed through a punch press 38, and the punched strip is continuously wound on a bobbin 39 to form the annular magnetic core. The assembly of stator magnetic cores for large motors can be as shown in FIG. lO and uses a reel 40 of prepunched strip. As the stacking of the laminated core proceeds, studs 22 are driven into holes 36 in the punched stripO
The stator winding is normally a conventional polyphase winding, but some savings of end turns may result with concentric windinys. A third and potentially attractive idea which results in considerable savings of coppar wire is a semitoroidal winding for random stators. A wound stator with a conventional winding is illustrated schematically in FIG. llo Thre~ coils 41a, 41b, and 41c are indicated ~4~ icl ~ diagramatically in ~a~ outlines, inserted in the radial ; and outwardly directed stator slots 42 at one side of annular ~ 9 ~ 5 RD 3016 magnetic cor~ 43. The second stator winding in the opposing set of stator slots at the other side of the core is a separate stator circuit and can be connected in parallel or in series with the first winding.
The motor structure for a self-fed brushless dc motor as here described is particularly advantageous for a four-pole or six-pole motor. Most commonly, the control circuitry includes an inverter fed from a dc source whose frequency is determined by a shaft posi~ion sensor. Excita-tion of the windings is therefore self-synchronous with shaft position. There are many applications for the high power density, self fed brushless dc motor including, among others, a motor for an electric vehicle or industrial adjustable speed drive, an ordnance servo drive motor' and a starting motor.
Some comments on efficiency are important because ~ it is a substantial factor in power density capability. The ; permanent magnet rotor, of course, has very little loss except surface losses, thereby contri~uting substantially to improved efficiency. Also the absence of yoke material in the stakor disk greatly improves efficiency. Heat transfer from the stator windings and core are improved by the radial configuration of the stator, which can be ventilated by an external fan if required.
As will be understood by those skilled in the art, the permanent magnet machine herein described can also be operated as a generator, in which case the terminal voltage is the output voltage.
While the invention has been particularly shown and described with reference to several preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
~99~5 RD-3016 Supplementary ~isclosu.re In a preferred embodiment of the machine, the metal strip of the stator disks 12 and 12' is low loss magnetic amorphous metal ribbon (such as Fe80B20).
Amorphous metals are known also as metallic glasses and exist in many different compositions including a variety of magnetic alloys which include iron group elements and boron or phosphorus. Metallic glasses are formed from alloys that can be ~uenched without crystallization, and these solids are mechanically stiff, strong and ductile, but more importantly they have low losses and are low in cost~
Amorphous metal motor laminations differ from -those made of steel strip in that the amorphous metal is very thin, :
with a practical limitation at present of about 2 mils in thickness, and furthermore the amorphous metal is brittle ~like glass) so that toothed laminations cannot be made by punch-ing. Its natural configuration is a longr narrow and thin ribbon, and this suggests an application for the flat pancake .
motor. Ribbon in uniform widths of one-half inch or greater ..
- can be processed by directing liquid alloy onto a rapidly rotating cold chill cylinder, the liquid alloy being changed into a solid ribbon in a short time (measured in microseconds) before it becomes crystalline. The cooling rate is of the order of 106 C/sec, and the thickness limitation is set by the rate of heat transfer through the already solidified material. This heat transfer must be rapid enough that the : last increment to solidify still avoids crystallization. Any of the magnetic alloys can be ut.ili2ed, but the ~referred composition, because o~ its high induction characteristics, is the Fe~OB20 alloy. AnoLher suitable amorphous metal is Fe4~Ni40P14B6 or the variation of this material sold as METGLAS~ ) ~lloy Ribbon 2826 MB by Allied Chemical Corp.
- 9 ~
- . - .:
~ RD-3016 Amorphous metal ribbon of the Fe80B20 alloy has one-fourth the losses, at a given induc-tion for sinusoidal flux, of the best oriented Fe-Si sheet steel. Additional information is given in the article "Potential of Amorphous Metals for Application in Magnetic Devices" by F~E. Luborsky et al, Journal of Applied Physics, Vol. 49, No. 3, Part II, March 1978, pp. 1769-177~.
Etching or chemical milling can be used on amorphous metal to make motor laminations, and while the process can be automated, the cost o~ chemicals may make the process expensiveO A more effective technique to put slots in the ribbon is to draw the ribbon past a cutting laser beam, which can be controlled by a micxoprocessor. In FIG. 9, punch press 38 is replaced by a laser beam station, and amorphous metal tape is drawn past the cutting beam as slots are cut out by the beam. Spacing of the slots is controlled so they will fall into a line when the ribbon is rolled up. This material i5 ; mechanically strong and can be pulled as the laminated stator core is wound spirally, resulting in a magnetic core with an improved stacking factor or packing fraction~ That is, more flux is carried by a given volume of the core material and this contributes to the objective of a high power density machine. An alternative technique~ one which makes amorphous metal shaped laminations directly from the alloy melt, is disclosed and claimed in United States patent 4,155,397 issued May 22, 1979 to V.B. Honsinger and R.E. Tompkins, entitled "Method and Apparatus for Fabrica-ting Amorphous Metal Laminations for Motors and Transformers" and assigned to the present assignee.
The speed range of the brus~less dc motor can be increased by rotating two stator disks relative to each other.
In an operating motor, the resulting phas~ shift reduces the ~ -~
- 10 - ' ' ., .. ~ .
. . .
series sum of back emf, so that the motor speeds up and lncreases its back emf until a new stable operating condition is reached at a higher speed. When the machine is operating as a permanent magnet generator, this feature of relative rotation of two stator disks can provide voltage regulation.
,-.': ' , ~ ' .
Claims (5)
1. A high power density electric machine comprising:
a housing having a shaft journalled for rotation within said housing, a plurality of interleaved and alternating annular rotor disks and annular stator disks spaced from one another to establish an axial air gap between adjacent disks, and magnetic yoke means at both ends of said plurality of interleaved rotor disks and stator disks for carrying peripheral flux, mounting means attaching said rotor disks to said shaft for rotation with said shaft, and mounting means attaching said stator disks to said housing, each rotor disk comprising a plurality of circularly spaced alternating polarity cobalt-rare earth permanent magnet sectors, each stator disk intermediate between two rotor disks comprising a laminated magnetic core made of spirally wound metal strip, each such stator disk having opposing sides with each side having a set of radial and outwardly-directed stator slots containing a corresponding stator winding.
a housing having a shaft journalled for rotation within said housing, a plurality of interleaved and alternating annular rotor disks and annular stator disks spaced from one another to establish an axial air gap between adjacent disks, and magnetic yoke means at both ends of said plurality of interleaved rotor disks and stator disks for carrying peripheral flux, mounting means attaching said rotor disks to said shaft for rotation with said shaft, and mounting means attaching said stator disks to said housing, each rotor disk comprising a plurality of circularly spaced alternating polarity cobalt-rare earth permanent magnet sectors, each stator disk intermediate between two rotor disks comprising a laminated magnetic core made of spirally wound metal strip, each such stator disk having opposing sides with each side having a set of radial and outwardly-directed stator slots containing a corresponding stator winding.
2. The electric machine of claim 1, wherein said magnetic yoke means for at least one of said ends comprises a solid metal disk attached to an end most rotor disk.
3. The electric machine of claim 1, wherein said magnetic yoke means for at least one of said ends comprises an endmost stator disk consisting of a laminated magnetic core made of spirally wound metal strip and having opposing sides, one of said sides of said endmost stator disk facing an adjacent rotor disk and having a set of radial and outwardly-directed stator slots containing a stator winding, and the other of said sides of said endmost stator disk functioning as a magnetic yoke.
Claims Supported by Supplementary Disclosure
Claims Supported by Supplementary Disclosure
4. The electric machine of claim 1 or 2, wherein said metal strip of said each stator disk intermediate between two rotor disks comprises magnetic amorphous metal ribbon.
5. The electric machine of claim 3, wherein said metal strip of said each stator disk intermediate between two rotor disks comprises magnetic amorphous metal ribbon, and said metal strip of said endmost stator disk comprises magnetic amorphous metal ribbon.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA299,981A CA1109915A (en) | 1978-03-29 | 1978-03-29 | High power density brushless dc motor |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA299,981A CA1109915A (en) | 1978-03-29 | 1978-03-29 | High power density brushless dc motor |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1109915A true CA1109915A (en) | 1981-09-29 |
Family
ID=4111103
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA299,981A Expired CA1109915A (en) | 1978-03-29 | 1978-03-29 | High power density brushless dc motor |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1109915A (en) |
-
1978
- 1978-03-29 CA CA299,981A patent/CA1109915A/en not_active Expired
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