WO1996042132A1

WO1996042132A1 - High speed synchronous reluctance motor-generator

Info

Publication number: WO1996042132A1
Application number: PCT/US1996/006128
Authority: WO
Inventors: Harold A. Rosen; Thomas Hudspeth; David A. Stahl; Daniel Bakholdin
Original assignee: Rosen Motors, L.P.
Priority date: 1995-06-12
Filing date: 1996-05-07
Publication date: 1996-12-27
Also published as: AU5672396A

Abstract

A synchronous reluctance motor-generator exhibiting low eddy current and tooth harmonic losses, high bending stiffness and high radial strength, which can be manufactured at low cost, includes a stator and a conductively shielded solid rotor. The rotor for the synchronous reluctance machine is characterized as including at least one pair of magnetic material elements disposed at opposing ends of a line segment which line segment intersects the spin axis of the rotor and which line segment is included in a plane disposed substantially perpendicular to the spin axis, wherein the outer surface of the rotor, at least in the vicinity of the magnetic material elements, is covered with a layer of non-magnetic conducting material. A method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator and a clamping jig suitable for use with this method are also described.

Description

HIGH SPEED SYNCHRONOUS RELUCTANCE MOTOR-GENERATOR BACKGROUND OF THE INVENTION

1 . Field of the Invention

The present invention relates generally to synchronous reluctance motor-generators. More specifically, the present invention relates to the rotor design of a synchronous reluctance motor-generator providing high bending stiffness, high radial strength and low eddy-current and low hysteresis losses. It will be appreciated that these characteristics make a synchronous reluctance motor-generator employing such a rotor ideal for high speed applications. 2. Brief Discussion of Related Art

Synchronous reluctance machines controlled by variable frequency inverters offer advantages over induction motors and permanent magnet machines in a variety of applications. The lower rotor loss associated with a properly designed rotor can result in a higher efficiency than in a comparably sized induction ma- chine, while the lower material cost as compared to a similarly rated permanent magnet machine can make it less costly than those machines. For high speed applications, however, in which high centrifugal forces and shaft dynamic consid¬ erations can create problems, no previously described designs of synchronous reluctance machines have proven satisfactory. Synchronous reluctance machines are described in a paper entitled "Syn¬ chronous Reluctance Machines - A Viable Alternative for AC Drives" by T. A. Lipo, Electrical Machines and Power Systems, 1 991 , pp. 659-671 , and in "Design of a Synchronous Reluctance Motor Drive" by T. J. E. Miller et al. in IEEE Transactions on Industry Applications, Vol. 27, No. 4, July/August 1 991 . The design of the rotor for such a machine is specifically addressed by Lipo and Matsuo in "Rotor Design Optimization of Synchronous Reluctance Machine," IEEE Transactions on Energy Conversion, Vol. 9, No. 2, June 1 994, pp. 359-365. These papers describe configurations which lead to low rotor loss with an accept¬ able power factor. These designs, however, which feature axial laminations, would fly apart at high rotational speeds.

U.S. Patent No. 4,924, 1 30 entitled "Reluctance Synchronous Electric Ma¬ chine Having Intrinsic Phase Correction Means" describes, in passing, an arrange- ment of alternating ferro-magnetic and non-ferromagnetic axial laminations to obtain the anisotropy of reluctance necessary in these machines. SUMMARY OF THE INVENTION

The present invention was motivated by a desire to preserve the character- istics of previously described designs, while correcting the problems associated with high speed operation, for synchronous reluctance motor-generators.

The principal object of the present invention is to provide a synchronous reluctance machine that can be operated at high efficiency in high speed applica¬ tions. According to one aspect of the invention, several high speed synchronous reluctance motor-generators advantageously can be used in automotive applica¬ tions wherein shaft speeds exceeding 100,000 RPM and rotor surface speeds exceeding 300 meters per second may be encountered in flywheel motor-generator and turbo-generator applications, for example.

Another object of the present invention is to provide a synchronous reluc- tance machine having a rotor suitable for high operating speeds which can be manufactured at low cost.

Yet another object of the present invention is to provide a synchronous reluctance machine having a rotor which is high in efficiency and low in cost for moderate speed applications. Still another object of the present invention is to provide a synchronous reluctance machine having a rotor which can be operated at high temperatures.

These and other objects, features and advantages according to the present invention are provided by a rotor for a synchronous reluctance motor-generator, the rotor being characterized in that the rotor includes at least one pair of mag- netic material elements disposed at opposing ends of a line segment, which line segment intersects the spin axis of the rotor and which line segment is included in a plane disposed substantially perpendicular to the spin axis, wherein the outside surface of the rotor, at least in the vicinity of the magnetic material ele¬ ments, is covered with a layer of non-magnetic conducting material. According to one aspect of the invention, the conducting material is copper. Alternatively, other conducting materials, e.g., silver, can be used in forming the outer conduct¬ ing layer, i.e., the shield layer. These and other objects, features and advantages according to the present invention are provided by a method for manufacturing a rotor for a synchronous reluctance motor-generator. The method includes steps for providing at least one pair of magnetic material elements, positioning the elements at opposing ends of a line segment, which line segment intersects the spin axis of the rotor and which line segment is included in a plane disposed substantially perpendicular to the spin axis, bonding the elements to the rotor, and coating the outside surface of the rotor, at least in the vicinity of the magnetic material elements, so as to form a layer of non-magnetic conducting material on the rotor. According to one aspect of the invention, a conducting material, e.g., copper or silver, can be formed into the conducting material layer by plating. Alternatively, the conducting materials can be formed into the conducting material layer by a process such as vapor deposition, cladding, brazing, casting, diffusion bonding or thermal spraying. BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiments are described with reference to the drawings wherein:

Fig. 1 A illustrates a side view of a cylindrical rotor while Figs. 1 B through 1 D illustrate rotor cross sections for 2, 4 and 8 pole rotors, respectively;

Fig. 2, by showing a step-wise approximation for sinusoidal spacial distribu- tion, is useful in understanding an air gap field excited by the stator for a typical synchronous reluctance machine;

Figs. 3A and 3B depict transversely and axially laminated rotor configura¬ tions, respectively, which can be used in a synchronous reluctance mo¬ tor-generator; Fig. 4 depicts an exemplary rotor with an integral conductive shield dis¬ posed within an exemplary stator of a synchronous reluctance motor-generator according to the present invention;

Fig. 5 is a chart comparing rotor loss for both an unshielded solid and conventional laminated rotors with that of the shielded solid rotor according to the present invention;

Figs. 6A and 6B are exemplary illustrations showing rotor components before and after bonding, respectively; and Fig. 7 illustrates an example of a fixture which finds application in the bonding of rotor segments during manufacture of the shielded rotor for a synchro¬ nous reluctance motor-generator according to the method of the present inven¬ tion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 A illustrates an exemplary rotor 10 while the rotor cross sections shown in Figs. 1 B to 1 D are typical of the current art for rotors of synchronous reluctance motor-generators. The alternation of magnetic material elements 1 and non-magnetic material elements 2 in the poles of the rotor 10 shown, for example, in Fig. 1 C, provide the required anisotropy needed for torque production. For clarity, the magnetic material elements 1 are designated by shading. More¬ over, while a variety of pole configurations is permitted, the description which follows treats the exemplary case of a four (4) pole rotor formed from 4N magnet¬ ic material elements and 4M non-magnetic material elements uniformly spaced about a cruciform rotor core 3. It will be noted that N is an integer greater than or equal to 1 while M is typically in the range of values between N-1 and N + 1 .

In synchronous reluctance machines, the magnetic field produced by the stator rotates in synchronism with the solid rotor 1 0, causing the field in the rotor to be substantially unvarying. Typical stator configurations are illustrated in the papers by T. J. E. Miller et al. and Lipo and Matsuo, which papers are discussed above and which papers are incorporated herein by reference for all purposes. It will be appreciated that the larger the volume ratio of non-magnetic to magnetic material, the better the power factor of the machine. However, it also should be noted that when this ratio becomes too large, the output torque of the motor- generator suffers.

It will also be appreciated that because the stator field is produced by a discreet arrangement of teeth, the desired sinusoidal spacial distribution of flux density can be approximated by the stepwise distribution shown in Fig. 2. As the rotor 10 rotates past stationary teeth 22 of stator 20, as illustrated, for example, in Fig. 4, the rotor 10 encounters flux variations at a frequency approxi¬ mately equal to (N x RPM)/60, where N is the number of teeth 22 in stator 20, and RPM is the shaft speed in revolutions per minute. For an exemplary shaft speed of 60,000 RPM in a machine having 36 teeth, this frequency, commonly referred to as the tooth harmonic, is 36 kHz. It will also be appreciated that in a well designed machine, the eddy currents caused by this high frequency flux variation are the primary cause of rotor loss. In most electrical machines, losses due to eddy currents can be reduced to acceptable levels by using thin laminations, i.e., a plurality of lamination layers, to break up the conducting path. The usual lamination arrangement consists of thin transverse slices of the ferro-magnetic material. However, since the rotor 10 of the synchronous reluctance machine consists of alternating magnetic material elements 1 and non-magnetic material elements 2, or other suitable materials, as shown in, for example, Fig. 1 C, the fabrication of transverse lamina¬ tions becomes complex and expensive. As can be seen readily from Fig. 3A, each of the plates used in forming the laminated rotor are bimetallic plates. For this reason, axial laminations, such as those illustrated in Fig. 3B, are preferred in modern synchronous reluctance motor-generators.

Axially laminated rotors can be incorporated into synchronous reluctance motor-generators which perform well at relatively low shaft speeds. However, this type of rotor is difficult to design for high speed operation, primarily due to fact that the relatively low tensile strength of the interlaminar insulation cannot cope with the high centrifugal forces encountered by the rotor 10. In extreme cases, the rotor flies apart. While the transverse laminations illustrated in Fig. 3A do not have this problem, this type of rotor is often prohibitively expensive to manufacture, especially in the ultra-thin sections desired for reducing losses resulting from the high frequencies of the tooth harmonics. As shown in Fig. 4, a relatively elegant and inexpensive solution to this dilemma is the use of a highly conductive coating on the exterior of an unlami- nated solid rotor, thereby producing a shielded solid rotor. Eddy currents pro¬ duced in this conducting shield layer 14 create a radial magnetic field which opposes the radial component of the inter-tooth flux, thereby preventing the inter- tooth flux from entering the lossier ferrous material sections of the rotor. Thus, even though the eddy currents in the conducting shield layer 14 produce losses, which losses are shown in Fig. 5, these losses are lower than those produced by either an unshielded solid rotor or a transverse laminated rotor in the tooth harmonic frequency range of interest, e.g., 20-40 kHz.

While still discussing Fig. 4, it should be noted that for operation in a vacuum, i.e., the normal operating environment of a high speed flywheel assem- bly, windage loss does not exist. Therefore, the outer surface need not be a smooth circular cylinder, thus permitting the non-magnetic material between the poles of rotor 10 to be eliminated.

Results of small scale tests comparing the temperature rise rates for an unshielded solid rotor, an unshielded laminated rotor and a shielded solid rotor 10 according to the present invention are denoted by Curves A through C, respectively, in Fig. 5 for various tooth harmonic frequencies. The temperature rise rates are used since the temperature rise rates are directly proportional to the rotor loss. The tooth excitation used to simulate tooth harmonic excitation were equivalent to the full range of operating speeds expected to be encountered by a rotor incorporated into a flywheel motor generator. The solid rotors tested were magnetic steel cylinders and laminated rotors were built up from 0.35 millimeter thick laminations of magnetic steel; the shielded solid rotors tested were magnetic steel cylinders plated with a 0.5 millimeter thick layer of silver.

The effectiveness of the shield in reducing power loss results from its low resistance to the eddy currents. Since the eddy currents are confined to a thin layer near the surface approximately one skin depth in thickness, it is the resistivi¬ ty characteristic of this layer that is important. This resistivity is proportional to the square root of the product of the electrical resistivity and the relative perme¬ ability, e.g., relative with respect to the permeability of free space, of the material. Therefore, a low resistivity, low relative permeability (i.e., non-magnetic) material is required. Any non-magnetic metal used as a shield will be effective; low resistivity non-magnetic metals used as a shield will be more effective. The most effective configuration of the shielded solid rotor for reducing losses will satisfy the condition that the product of magnetic material's electrical resistivity and relat- ive permeability > > the product of the non-magnetic material's electrical resistivi¬ ty and relative permeability.

The Table immediately below provides a non-limiting list of non-magnetic materials and their respective electrical resistivity ranges which are suitable for use as the conducting shield layer 14.

MATERIAL ELECTRICAL RESISTIVITY

HIGH LOW

Series 3000 Aluminum Alloys 4.2 //Ω-cm 1 .71 //Ω-cm

Coppers, wrought 3.82 //Ω-cm 2.8 //Ω-cm

Series 1000 Aluminum Alloys 3.0 /Ω-cm

Gold Plating 2.4 //Ω-cm

Silver Plating 1 .6 //Ω-cm

Silver 1 .47 //Ω-cm

Shielded solid rotors, which are suitable for a flywheel motor-generator capable of sustained rotational speeds in excess of 60,000 RPM, can be manufac¬ tured using one of the exemplary methods described immediately below. First, the various magnetic and non-magnetic elements can be prepared using conventional fabrication techniques. In the exemplary case under discus¬ sion, the magnetic material 1 ' and non-magnetic material 2' used are 41 30 alloy steel and Inconel 71 8, respectively, while the braze alloy 4 used is B Ni-2 nickel braze strip. The magnetic and non-magnetic materials 1 ' and 2' are machined to fit and then assembled as shown in Fig. 6A. This assembly advantageously can be restrained in a brazing fixture or jig 200, as shown in Fig. 7, and brazed. In view of the fact that forms 202 and associated bolts 206 are formed from materials with relatively low coefficients of thermal expansion, e.g., 8.3 x 10^"6/°F, while spacers 204 are formed from a material having a relatively high coefficient of thermal expansion, e. g., 9.6 x 10^"6/°F, fixture 200 creates symmetrical clamp¬ ing forces. More importantly, these symmetrical clamping forces increase during the furnace brazing cycle, which is typically at temperatures in excess of 1 950° F. Advantageously, the generated clamping forces eliminate voids in the braze joints. It will be appreciated that the corners of the forms 202 act as indexing positions with respect to the elements 1 '. It will also be appreciated that the arrangement of Fig. 7 is an exemplary configuration and that other configurations will occur to those of ordinary skill in the art.

After the brazing step is completed, the solid rotor piece thus formed is machined on its outside diameter and then coated with a non-magnetic conduct- ing material, e.g., silver, on all exterior surfaces to thereby form the shielded solid rotor 10 for the synchronous reluctance motor-generator according to the present invention. In an exemplary case, the conducting material is applied to the solid rotor by plating. It will be appreciated that other coating processes such as vapor deposition, cladding, brazing, casting and thermal spraying can also be employed. Advantageously, shielded solid rotors can be manufactured by the general steps of casting, forming or machining precisely interlocking elements of magnetic and non-magnetic materials 1 ' and 2', bonding these elements to one another in an alternating layer pattern and then coating the majority of the solid rotor with a conducting material. It will be appreciated that the materials must be chosen to meet the magnetic, non-magnetic and strength requirements of the intended application. While an exemplary bonding process of brazing individual elements into a solid rotor was previously described, the solid rotor advantageously can be formed using a selected one of the processes including diffusion bonding, soldering, casting, forging, welding and explosive forming. Moreover, the solid rotor can also be formed using compacting, sintering and consolidating, i.e., the so-called powder metallurgy processes. It will also be appreciated that the fabrication process chosen must be compatible with the materials used and be strong enough to sustain the stresses associated with the high spin speeds.

During the selected forming or bonding process, pressure advantageously may be applied to hold the alternating magnetic and non-magnetic materials 1 ' and 2' in place while preventing the formation of voids in the bond zone. After bonding, the solid rotor thus formed is subsequently machined and then coated or plated with a relatively low electrical resistivity coating on substantially all of the exposed exterior surface of the solid rotor, thus forming the shielded solid rotor.

Claims

WHAT IS CLAIMED IS:

1 . A synchronous reluctance motor-generator comprising: a stator; and a conductively shielded solid rotor.

2. The synchronous reluctance motor-generator recited in claim 1 , wherein the shielded solid rotor is un-laminated.

3. The synchronous reluctance motor-generator as recited in claim 1 , wherein said conductively shielded solid rotor comprises: a solid rotor including a plurality of magnetic material elements disposed around the spin axis of said solid rotor; and a layer of conducting material coating substantially the entire outer surface of said solid rotor, at least in the vicinity of said magnetic material elements, wherein said magnetic material elements have a first electrical resistivity and a first relative permeability, wherein said conducting material has a second electrical resistivity and a second relative permeability, and wherein said magnetic material elements and said conducting material satisfy the condition that the product of said first electrical resistivity and said first relative permeability > > the product of said second electrical resistivity and said second relative permeability.

4. The shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 3, wherein said conducting material is non-magnetic and has an electrical resistivity < 5.0 microohm-cm.

5. A shielded solid rotor for a synchronous reluctance motor-generator, wherein said shielded solid rotor is characterized in that said shielded solid rotor includes at least one pair of magnetic material elements which elements are dis¬ posed at opposing ends of a line segment, which line segment intersects the spin axis of said rotor and which line segment is included in a plane disposed substan¬ tially perpendicular to the spin axis, wherein an outside surface of said shielded solid rotor, at least in the vicinity of said magnetic material elements, is covered with a layer of non-magnetic conducting material.

6. The shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 5, wherein said magnetic material elements have a first electrical resistivity and a first relative permeability, wherein said conducting material has a second electrical resistivity and a second relative permeability and wherein said magnetic material elements and said conducting material satisfy the condition that the product of said first electrical resistivity and said first relative permeability > > the product of said second electrical resistivity and said second relative permeability.

7. The shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 6, wherein said conducting material is copper.

8. The rotor for a synchronous reluctance motor-generator as recited in claim 6, wherein said conducting material is silver.

9. The shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 5, wherein said conducting material has an electrical resistivity < 5.0 microohm-cm.

10. A method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator, said method comprising the steps of:

(a) providing a solid rotor including at least one pair of magnetic material elements at opposing ends of a line segment intersecting the spin axis of said solid rotor and which line segment is included in a plane disposed substantially perpendicular to the spin axis of said solid rotor;

(b) coating substantially all outside surfaces of said solid rotor, at least in the vicinity of said magnetic material elements, so as to form a layer of non¬ magnetic conducting material on the rotor to thereby form a shielded solid rotor.

1 1 . The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (a) compris¬ es the steps of:

(a)(i) providing said at least one pair of magnetic material elements and a rotor core;

(a)(ii) positioning said elements at opposing ends of a line segment inter- secting the spin axis of said rotor core and which line segment is included in a plane disposed substantially perpendicular to the spin axis; and

(a)(iii) brazing all of said elements to said rotor core to thereby form said solid rotor.

1 2. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (a) compris¬ es the steps of: (a)(i) providing a plurality of magnetic material elements and a cruciform rotor core;

(a)(ii) positioning pairs of said elements at opposing ends of respective line segments intersecting the spin axis of said cruciform rotor core and which line segments are included in respective planes disposed substantially perpendicular to and equidistantly spaced around the spin axis;

(a) (Hi) clamping said elements in respective fixed positions with respect to said cruciform rotor core using a plurality of spacers having a relatively high coefficient of thermal expansion disposed around a jig having a relatively low coefficient of thermal expansion; and (a)(iii) brazing all of said elements to said cruciform rotor core to thereby form said solid rotor.

13. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (b) compris¬ es the step of plating substantially all outside surfaces of said solid rotor, at least in the vicinity of said magnetic material elements, with copper to thereby form a shielded solid rotor.

14. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (b) compris¬ es the step of plating substantially all outside surfaces of said solid rotor, at least in the vicinity of said magnetic material elements, with silver to thereby form a shielded solid rotor.

1 5. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said magnetic material elements have a first electrical resistivity and a first relative permeability, wherein said conducting material has a second electrical resistivity and a second relative permeability and wherein said magnetic material elements and said conducting material satisfy the condition that the product of said first electrical resistivity and said first relative permeability > > the product of said second electrical resistivity and said second relative permeability.

16. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (a) further comprises forming said solid rotor including at least one pair of magnetic material elements at opposing ends of a line segment intersecting the spin axis of said solid rotor and which line segment is included in a plane disposed substantially perpendicular to the spin axis of said solid rotor by a casting process.

17. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (a) further comprises forming said solid rotor including at least one pair of magnetic material elements at opposing ends of a line segment intersecting the spin axis of said solid rotor and which line segment is included in a plane disposed substantially perpendicular to the spin axis of said solid rotor by a diffusion bonding process.

18. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (a) further comprises forming said solid rotor including at least one pair of magnetic material elements at opposing ends of a line segment intersecting the spin axis of said solid rotor and which line segment is included in a plane disposed substantially perpendicular to the spin axis of said solid rotor by a powder metallurgy process.

19. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (a) further comprises forming said solid rotor including at least one pair of magnetic material elements at opposing ends of a line segment intersecting the spin axis of said solid rotor and which line segment is included in a plane disposed substantially perpendicular to the spin axis of said solid rotor by a process selected from a group consisting of forging, welding, thermal spraying and explosive forming.

20. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (b) further comprises plating substantially all outside surfaces of said solid rotor, at least in the vicinity of said magnetic material elements, so as to form said layer of said conducting material on the solid rotor to thereby form said shielded solid rotor.

21 . The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (b) further comprises vapor depositing said layer of said conducting material over substan¬ tially all outside surfaces of said solid rotor, at least in the vicinity of said magnet- ic material elements, so as to form said shielded solid rotor.

22. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (b) further comprises cladding over substantially all outer surfaces of said solid rotor, at least in the vicinity of said magnetic material elements, so as to form said layer of said conducting material on the solid rotor to thereby form said shielded solid rotor.

23. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (b) further comprises brazing said layer of said conducting material over substantially all outer surfaces of said solid rotor, at least in the vicinity of said magnetic material elements, so as to form said shielded solid rotor.

24. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (b) further comprises casting said layer of said conducting material over substantially all outer surfaces of said solid rotor, at least in the vicinity of said magnetic material elements, so as to form said shielded solid rotor.

25. The method for manufacturing a shielded solid rotor for a synchronous reluctance motor-generator as recited in claim 10, wherein said step (b) further comprises thermal spraying said layer of said conducting material over substan¬ tially all outer surfaces of said solid rotor, at least in the vicinity of said magnetic material elements, so as to form said shielded solid rotor.

26. A clamping apparatus for holding a plurality of elements including N magnetic material elements at fixed positions with respect to a non-uniform radius rotor core during manufacture of a solid rotor for a synchronous reluctance mo¬ tor-generator, said clamping apparatus comprising: at least one form member defining an area capable of accommodating the rotor core, said form member including N indexing positions and first and second ends, said form member having a first coefficient of thermal expansion; an attaching element, operatively connected to said first and said second ends of said form member, capable of varying the separation between said first and said second ends, said attaching element having a second coefficient of thermal expansion; and N spacers for connecting a selected indexing position of said form member with a corresponding one of said N elements, each of said N spacers having a third coefficient of thermal expansion, wherein N is an integer greater than 1 , wherein said first and said second coefficients of thermal expansion are substantially equal to one another, and wherein said third coefficient of thermal expansion is greater than that of said first and said second coefficients to thereby permit an increase in tempera¬ ture during a process step to produce a resultant increase in clamping forces symmetrically applied to said N elements and said rotor core.