WO2012031086A2

WO2012031086A2 - Electrical traction motor, a stator used in an electrical motor or generator and a method of manufacturing an electrical traction motor

Info

Publication number: WO2012031086A2
Application number: PCT/US2011/050157
Authority: WO
Inventors: Mark W. Mcpherson; Andrew D. Hirzel
Original assignee: Radam Motors, Llc
Priority date: 2010-09-03
Filing date: 2011-09-01
Publication date: 2012-03-08
Also published as: WO2012031086A3

Abstract

An electrical motor and method of manufacturing an electrical motor includes producing a stator having a yoke and a plurality of windings in magnetic communication with the yoke. A rotor is juxtaposed with respect to the stator to rotate in response to magnetic flux produced by the stator. A variable frequency power supply is connected with the windings in order to produce a magnetic flux with the stator. The yoke is made from a polymer bonded soft magnetic material that does not substantially increase in permeability in response to an increase in frequency of the power supply.

Description

ELECTRICAL TRACTION MOTOR, A STATOR USED IN AN ELECTRICAL

MOTOR OR GENERATOR AND A METHOD OF MANUFACTURING AN ELECTRICAL TRACTION MOTOR BACKGROUND OF THE INVENTION

The present invention is directed to an electrical motor and stator used in the electrical motor and, in particular, to a traction electrical motor that produces a high torque at low speeds and a lower torque at higher speeds. The invention may be used in a variety of applications, including electric and electric hybrid vehicles, such as a truck, an automobile, a cross-over vehicle, a bus, a train, or the like. The motor may be radial type or axial type. Certain aspects of the invention may also be used to manufacture a stator of an electrical generator.

The ideal vehicular traction motor performance is best described by a torque- power-speed curve in which high peak torque at low driving speeds is combined with less peak torque as travel speeds increase. The high peak torque is required for acceleration and for travel up steep grades, ramps, and the like. At a given performance point in the speed/torque curve, the vehicle should enter a constant power region, which is a region where the vehicle's peak tractive power needs generally remains constant throughout the remaining speed range. The constant power region can be defined by the constant power speed ratio (CPSR). CPSR is the ratio of the maximum vehicle speed divided by the speed at which maximum peak torque is developed.

Known methods and technologies to attempt to provide a permanent magnet motor capable of achieving a high-starting torque and also achieving a wide-ranging CPSR tend to fall into two categories. Broadly, these attempts are 1) based on motor software control schemes, which attempt to suppress the motor electromotive force (EMF) at high speeds, and 2) dynamic motor component relationship modification. The latter uses various mechanical devices, such as mechanical actuators, to weaken the field at increased speeds or gear trains to attempt to adjust the torque versus speed ratio.

Electrical motors and generators include a stator and rotor. The stator may be the armature and the rotor the electromagnetic field or vice versa. One common form of motor is a radial gap motor having an outer stator and a rotor inside of the stator. The stator is typically constructed of iron-based laminations that form the yoke and teeth as one piece. The tooth may be wider at the stator-rotor air gap than the main tooth body in order to better accumulate and focus the electromagnetic flux. The result may be very close spacing between adjacent teeth at the air gap.

Electrical conductors, such as insulated copper wires, are installed in slots between the teeth through the spacing between the teeth. Because of the narrow opening between teeth at the air gap, it is difficult to achieve precision in placement of the windings in the gap, resulting in non-uniform winding layers. This leaves available winding space that is not occupied by the winding, resulting in a device that achieves less than potential performance.

SUMMARY OF THE INVENTION

An aspect of the present invention is to provide an electric motor that is capable of functioning as a traction motor that overcomes the difficulties of proposed systems. An electrical motor and method of manufacturing an electrical motor, according to this aspect of the invention, includes producing a stator having a yoke and a plurality of windings in magnetic communication with the yoke. A rotor is juxtaposed with respect to the stator to rotate in response to magnetic flux produced by the stator. A variable frequency power supply is connected with the windings in order to produce a magnetic flux with the stator. The yoke is made from a material that substantially decreases in permeability in response to an increase in frequency of the motor synchronous frequency.

The rotor may be a permanent magnet rotor. The material in the yoke may be made of soft magnetic particles. The magnetic particles may be bonded with a polymer. The yoke may be manufactured by i) compression molding, ii) injection molding and/or iii) extrusion. The magnetic particles may be provided with a metal density of no more than approximately 95% in the yoke. The material in the yoke may decrease in permeability in response to an increase in frequency. The motor may have a generally constant power speed ratio that decreases in response to increasing frequency. The motor may be a traction motor, such as a traction motor used with i) a truck, ii) an automobile, iii) a cross-over vehicle, iv) a bus, or v) a train. The motor may operate without gear- changing between the motor and at least one traction wheel.

Another aspect of the present invention is to provide a stator for use with a rotor mounted for rotation within the stator thereby defining a radial air gap between the rotor and stator, an electrical motor or generator utilizing the stator and method of manufacturing a stator, according to an aspect of the invention, includes providing a plurality of spaced apart teeth thereby defining slots between the teeth and a yoke. The yoke physically mounts the teeth and provides a path of magnetic flux between the teeth. At least one winding is defined in the slots. The yoke is made of a polymer bonded soft magnetic material.

The polymer bonded soft magnetic material may be a blend of magnetic iron powder and a polymer. The yoke may be formed to the teeth. Each winding may be wound around one of the teeth prior to the yoke being formed to the teeth.

The teeth may be made from laminations. The teeth may be made from non- oriented iron laminations or from oriented iron laminations. The laminations may be made from silicon iron, nano-crystalline iron and/or amorphous iron ribbon. The teeth may include edge crenellations. Adjacent ones of the laminations may have different lengths. The laminations may have asymmetrical configurations and are stacked in a sequence that reverses orientation of at least some of the laminations. The teeth may have portions extending radially outward of the yoke.

The yoke may be formed by injection molding. The yoke may be formed by compression molding. The yoke may be formed of yoke pieces that are adhered together. The yoke may be formed by extrusion.

The stator may be used in a motor or generator having a fractional number of slots per phase per pole.

These and other objects, advantages and features of this invention will become apparent upon review of the following specification in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a stator according to an embodiment of the invention; FIG. 2 is an enlarged view of the area designated II in FIG. 1;

FIG. 3 is the same view as FIG. 1 without windings;

FIG. 4 is a diagram of typical vehicle tractive needs;

FIG. 5 is a series of curves illustrating materials that decrease in permeability as excitation frequency increases; FIG. 6 is an EMF and current versus speed curve achieved by the illustrated embodiments;

FIG. 7 is the same view as FIG. 3 of an alternative embodiment thereof;

FIG. 8 is an enlarged view of a yoke piece in FIG. 7;

FIG. 9 is the same view as FIG. 1 of an alternative embodiment thereof;

FIG. 10 is a sectional view taken along the lines X-X in FIG. 9;

FIG. 11 is a sectional view taken along the lines XI-XI in FIG. 10;

FIG. 12 is a sectional view taken along the lines XII-XII in FIG. 10;

FIG. 13 is the same view as FIG. 10 of an alternative embodiment thereof;

FIG. 14 is a sectional view taken along the lines XIV-XIV in FIG. 13;

FIG. 15 is a sectional view taken along the lines XV-XV in FIG. 13;

FIG. 16 is the same view as FIG. 9 illustrating flow of magnetic flux in the stator; and

FIG. 17 is an enlarged portion of a stator according to yet another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and the illustrative embodiments depicted therein, a permanent magnet motor is typically characterized by a constant ratio of generated electromotive force (EMF) relative to operating rotational speed. This ratio is known as K_e. A motor controller 16 includes a variable frequency power supply that controls the motor at various speeds. This controller has a maximum voltage output that exceeds the motor generated EMF in order to force current into the motor. This maximum voltage output divided by the maximum speed limits the motor designer's choice of motor K_e (V/Hz). The result will be a certain K_e. At this maximum speed, the voltage is highest and, for constant power operation, the motor/controller traction system will require low current. At the base speed, which is a fraction of maximum speed (for example, 1/5 for a CPSR=5), the motor generated EMF will be the same fraction of that of the maximum EMF, if there is, by definition, a constant K_e motor. Therefore, to achieve a given base speed power that is equal to the maximum speed power, the current is increased by a factor of, for example, 5.0. Therefore, for any motor application with CPSR = 5, and true power motor output being generally constant for the entire speed range, the motor controller would be sized at maximum controller output voltage to the motor multiplied by the maximum controller output current. This product is typically five (5) times the base speed power. As with most electronic devices, the cost of the motor controller increases with both current and voltage capacity. A motor controller rated for five (5) times for a motor output power would be commercially unacceptable.

An electrical motor 15 includes a stator 20 is provided for use with a rotor (not shown) that is rotatably mounted within an interior 21 of stator 20. While illustrated for use as a traction motor, it should be understood that stator 20 may be used to define other types of radial air gap electrical machines, such as an electrical generator. Stator 20 further includes end-turn winding (not shown). As is understood by those skilled in the art, the same general structure for the electrical machine may be used as a motor, by supplying an electrical current to the stator and producing a torque output from the rotor, or as a generator by supplying a torque input to the rotor and producing an electrical current output from the stator. For simplicity, the illustrative embodiments will be described as electrical motors, but are intended to cover both. The rotor may be a multiple pole magnet defined by permanent magnets such as of the type disclosed in commonly assigned U.S. patent application Ser. No. 12/848,573 filed Aug. 2, 2010, by McPherson et al., the disclosure of which is hereby incorporated herein by reference. Alternatively, the rotor may be a multiple pole magnet defined by electromagnets. With an alternating current applied to armature coils on the stator, the motor is a synchronous motor that rotates at a design speed established by the frequency of electrical current applied to the armature coils as well as the number of magnetic poles on the field of the motor, as would be understood by the skilled artisan. Yet alternatively, the motor may be an asynchronous motor in which the rotor is an induction rotor of the type known in the art.

Stator 20 is made up of a plurality of teeth 22 that are spaced apart around the stator thereby defining slots 24 between the teeth. Stator 20 further includes a yoke 26. Yoke 26 physically mounts teeth 22 and provides a path of magnetic flux 30 between the teeth, as illustrated in FIG. 13. Stator 20 further includes a plurality of armature coils defined by a series of windings 28 of insulated electrical conductors 29 in slots 24. Yoke 26 is made up of a polymer bonded soft magnetic material, or PBSM, also known as a powdered metal-filled plastic material. The material may be a blend of magnetic iron powder with a polymer. The yoke may be formed in a mold and simultaneously over- molded onto teeth 22 by injection molding utilizing techniques known in the art. The teeth are first arranged in the mold in their intended pattern of the stator. The injection molding process adds a material that is a blend of plastic and magnetic iron powder to the regions in and around the tooth end that is opposite the air gap. In this manner, yoke 26 is formed to teeth 22. The yoke forms a magnetic path between the teeth and provides structural support for the teeth during motor operation. Also, yoke 26 may provide a thermally conductive path for heat to be removed from the motor.

One advantage of the structure of stator 20 is that a winding 28 may be wound around a tooth 22 prior to yoke 26 being formed to the teeth. The winding may first be formed on a bobbin (not shown) and slipped over the tooth or may be wound directly over an insulation layer on the tooth. As best seen in FIG. 2, the ability to apply the winding to the tooth without the necessity of passing the winding between gaps between the faces 23 of adjacent teeth allows a much more uniform winding 28 to be made. This increases the density of conductors 29, thereby increasing motor efficiency, as would be understood by the skilled artisan. Also, it may allow a relatively higher power output because the windings will have lower losses for a given current, thereby allowing winding current to be increased.

In the illustrated embodiment, teeth 22 are made from a plurality of laminations that are stacked on top of each other. Each lamination would have the overall shape of the tooth including one slice of face 23. The laminations may be non-oriented iron laminations or oriented iron laminations. The concept of "oriented" and "non-oriented" refers to the crystalline structure of the metal, as is known in the art. The laminations may be made from silicon iron, nano-crystalline iron, amorphous iron ribbon, or the like.

PBSM material 32 making up yoke 26 substantially decreases in permeability in response to an increase in frequency. The iron particles are commercially available from various sources, such as by Hoeganaes Corp. and marketed as Anchor Steel lOOOC. The iron particles may be of various sizes and shapes, including flakes, spheres, or the like. Yoke 26 may be manufactured by compression molding or injection molding of material 32. Teeth 22 may be made of various materials and retained by yoke 26 using various techniques, such as disclosed in commonly assigned U.S. patent application Ser. No. 61/422,458 filed on Dec. 13, 2010, and U.S. patent application Ser. No. 61/469,894 filed on Mar. 31, 2011, by Mark W. McPherson et al., the disclosures of which are hereby incorporated herein by reference. In the illustrated embodiment, the iron particles of material 32 may make up a metal density of no more than approximately 95% in yoke 26.

Referring to FIG. 6, not only does PBSM material 32 not substantially increase in permeability in response to an increase in frequency of the current produced by power supply 30, PBSM material 30 decreases in permeability in response to an increase in frequency of power supply 30. FIG. 6 illustrates the permeability versus frequency of material 32 for various densities of particles. The material denoted with open circles best exemplifies a preferred embodiment material.

Motor 15 has a generally constant power speed ratio that decreases in response to increasing frequency of power supply 30. This achieves the overall results required of a traction motor. Such traction motor may be used with a vehicle, such as a truck, an automobile, a cross-over vehicle, a bus, a train, or the like. Motor 15 can achieve the general properties illustrated in FIG. 4 without gears or over-sizing of power supply 30. When the PBSM is applied to manufactured yoke 26 of motor 15, at stall speeds and low speeds, the motor will experience low frequency and thus the PBSM will work at its higher permeability, conduct more magnetic flux, and provide for higher K_e. Flux is directly proportional to IQ. At higher speeds and higher frequencies, the PBSM will have lower permeability and conduct less flux, and, hence, provide a lower ¾. The effect of the varying Ke to the motor controller will be of a motor that, although frequency increases, the amount of generated voltage to overcome will not substantially increase and power supply 30 of the controller will control the motor over a much greater speed range. As the Ke decreases, the motor torque also will decrease, naturally providing for a constant power.

Motor 15 is capable of providing extensive advantages. CPSR motors can be designed with maximum Ke possible at the base speed (low base frequency). As the speed slightly increases, the generated EMF will begin to increase as a function of increasing frequency. The skilled artisan can choose the motor pole count to achieve the correct frequency that, at the speed point that the motor voltage might be a limiting factor, the PBSM permeability decreases, the flux decreases and, hence, the Ke decreases. At this speed, the PBSM material, if carefully selected by the skilled artisan, "naturally" reduces the Ke and the motor controller will be capable of continuing to control the motor up to whatever speed is desired.

Use of the PBSM will require an understanding of the motor application on the part of the skilled artisan, as the PBSM material may introduce an additional level of complexity to the design process. This complexity will be worth the additional design effort, in providing industry solutions that are presently impractical. Ultimately, the motor controller may be designed at a lower Volt- Ampere product and lower cost. No moving parts will be necessary to achieve a variable Ke, nor will complex control schemes be required.

In an alternative embodiment, a stator 120 includes a plurality of teeth 122 that are mounted to a yoke 126 (FIGS. 7 and 8). Yoke 126 is made up of a series of yoke pieces 40 that are made from a polymer bonded soft magnetic material (PBSM). In the illustrated embodiment, yoke pieces 40 may be formed by compression molding, utilizing techniques known in the art. The yoke pieces are then assembled together, using an adhesive, or the like, in a manner that retains teeth 122. Alternatively, the yoke can be manufactured in one piece using compression molding or injection molding and the teeth be inserted into the yoke with manufacturing techniques, such as using pressure, heat, mechanical vibration, spinning, or the like. In yet another alternative embodiment, the yoke can be manufactured by extrusion or other type of plastic forming technique.

Another alternative embodiment of a stator 220 includes a plurality of spaced apart teeth 222 that define slots 224 between the teeth and a yoke 226 (FIGS. 9-16).

Yoke 226 physically mounts teeth 222 and provides a path of magnetic flux 30 between teeth 222. Stator 220 further includes winding 228 formed in slots 224. Teeth 222 include edge crenellations 234. The purpose of the crenellations is to increase the surface area of teeth 222 that engages yoke 226. This reduces magnetic flux conduction losses. It may also increase mechanical bond strength between the teeth and the yoke.

In particular, teeth 222 may be made up of laminations 232a and 232b that define crenellations 234. Laminations 232a have different lengths from lengths 232b. Thus, when laminations 232a are alternated with laminations 232b, the surface area of laminations 232a that contact yoke 226 is further increased. Alternatively, teeth 322 may be provided that are made up of asymmetrical laminations 332 that define crenellations 334. Laminations 332 may be divided into laminations 332a and laminations 332b that are generally identical in structure, but are rotated with respect to each other when stacked in a tooth 322. This increases the surface area of tooth 322 in contact with the yoke without requiring two different configurations of laminations. It should be understood that a strict A-B-A-B... sequence in lamination stacking is not required. Other sequences are possible, such as A-A-B-A-A-B or A-A-B-B-A-A, or the like. Also, other techniques may be used to provide crenellations, such as holes or other features in the laminations.

In yet another alternative embodiment, a stator 420 includes a plurality of spaced apart teeth 422 that define slots 424 between the teeth and a yoke 426 (FIG. 17). Yoke 426 physically mounts teeth 422 and provides a path of magnetic flux between teeth 422. Stator 420 further includes winding 428 formed in slots 424. Teeth 422 have portions 438 that extend radially outward of yoke 426. This allows heat to be conducted from the interior of the motor, such as generated by windings 428 and the like, more efficiently to the outside of stator 420. Thus, teeth 422 act as heat pipes to conduct heat from a hotter interior environment of the motor to a cooler exterior embodiment. Teeth portions 438 may be contacted by ambient air, a heat sink, a cooling liquid, or the like, to further assist in removing heat from the interior of the motor.

While stator 20, 120, 220 and 420 has many potential configurations, it is especially suitable for use with motors having fractional number of slots per phase per pole (SPP). The reason is that the yoke must carry magnetic flux between teeth. The teeth pass the flux produced by the windings. In fractional SPP configurations, each tooth carries flux that is not in phase (either directionally or in magnitude) with its neighbor. Therefore, the yoke is required to carry less flux than in an integral SPP configuration. Therefore, the yoke set forth in the illustrated embodiments can be made relatively small in size in fractional SPP configurations to compensate for increase in material costs over iron laminations that make up conventional motor yokes. Such conventional yokes may cost less, but have disadvantages to overall motor performance. Also, the use of relatively high surface area connections between the teeth and yoke material reduces magnetic flux conduction losses.

Also, fractional SPP configurations are better suited for higher frequency of drive current with higher pole count. This is because a greater number of slots are typically used for high pole devices thus making use of the ability to manufacture the stator utilizing the techniques disclosed herein. However, it should be understood that the illustrated embodiments can also be used with designs having a relatively fewer slots.

Another advantage of stator 20, 120, 220 and 420 is that the material used for the yoke may have significantly lower losses than conventional iron laminations at frequencies that are higher than for non-conventional motors. As the frequency of the excitation current rises, the eddy current losses increase because they are frequency dependent. Thus, the disclosed embodiments assist in lowering the difficulties associated with employing higher frequencies in motor design. Any resulting loss in flux-carrying capacity by the materials used in the disclosed embodiments can be compensated for by increasing yoke volume. Also, the use of oriented material for tooth laminations increases efficiency at higher frequencies in comparison to non-oriented materials

While the foregoing description describes several embodiments of the present invention, it will be understood by those skilled in the art that variations and

modifications to these embodiments may be made without departing from the spirit and scope of the invention, as defined in the claims below. The present invention

encompasses all combinations of various embodiments or aspects of the invention described herein. It is understood that any and all embodiments of the present invention may be taken in conjunction with any other embodiment to describe additional embodiments of the present invention. Furthermore, any elements of an embodiment may be combined with any and all other elements of any of the embodiments to describe additional embodiments.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrical motor, comprising:

a stator, said stator comprising a yoke and a plurality of windings in magnetic communication with said yoke;

a rotor adapted to rotate in response to magnetic flux produced by said stator; and a power supply connected with said windings in order to produce a magnetic flux with said stator;

wherein said power supply comprises a variable frequency power supply and wherein said yoke is made from a material that does not substantially increase in permeability in response to an increase in frequency of said power supply.

2. The motor as claimed in claim 1 wherein said rotor comprises a permanent magnet.

3. The motor as claimed in claim 1 wherein said material comprises soft magnetic particles.

4. The motor as claimed in claim 3 wherein said magnetic particles are bonded with a polymer.

5. The motor as claimed in claim 4 wherein said yoke is manufactured by at least one chosen from i) compression molding, ii) injection molding and iii) extrusion.

6. The motor as claimed in claim 3 wherein said magnetic particles comprise a metal density of no more than approximately 95% in said yoke.

7. The motor as claimed in claim 1 wherein said material decreases in permeability in response to an increase in frequency.

8. The motor as claimed in claim 1 wherein said motor has a generally constant power speed ratio that decreases in response to increasing frequency.

9. The motor as claimed in claim 1 comprising a traction motor.

10. The motor as claimed in claim 9 used as a traction motor used with a vehicle comprising one chosen from i) a truck, ii) an automobile, iii) a cross-over vehicle, iv) a bus, and v) a train.

11. The motor as claimed in claim 9 wherein said motor operates without gear- changing between the motor and at least one traction wheel.

12. The motor as claimed in claim 1 wherein said stator comprises a plurality of spaced apart teeth thereby defining slots between said teeth, said yoke physically mounting said teeth and providing a path of magnetic flux between said teeth and at least one winding defining in said slots.

13. The motor as claimed in claim 2 wherein said material comprises a polymer bonded soft magnetic material.

14. The motor as claimed in claim 13 wherein said yoke is formed to said teeth.

15. The motor as claimed in claim 14 wherein said at least one winding is wound around at least one of said teeth prior to said yoke being formed to said teeth.

16. The motor as claimed in claim 15 wherein said teeth comprise laminations.

17. The motor as claimed in claim 16 wherein said teeth are made from at least one chosen from i) non-oriented iron laminations, ii) oriented iron laminations, iii) silicon iron laminations, iv) nano-crystalline iron laminations and v) amorphous iron ribbon.

18. The motor as claimed in claim 12 wherein said teeth include edge crenellations.

19. The motor as claimed in claim 18 wherein adjacent ones of said laminations have different lengths.

20. The motor as claimed in claim 18 wherein said laminations have asymmetrical configurations and are stacked in a sequence that reverses orientation of at least some of said laminations.

21. The motor as claimed in claim 12 wherein said teeth have portions extending radially outward of said yoke.

22. An electrical motor or generator, comprising:

a stator;

a rotor mounted for rotation within said stator, thereby defining a radial air gap between said rotor and said stator;

said stator comprising a plurality of spaced apart teeth thereby defining slots between said teeth and a yoke, said yoke physically mounting said teeth and providing a path of magnetic flux between said teeth; and

at least one winding defining in said slots;

wherein said yoke comprises a polymer bonded soft magnetic material.

23. The motor or generator as claimed in claim 22 wherein said metal-filled plastic material comprises a blend of magnetic iron powder.

24. The motor or generator as claimed in claim 22 wherein said yoke is formed to said teeth.

25. The motor or generator as claimed in claim 24 wherein said at least one winding is wound around at least one of said teeth prior to said yoke being formed to said teeth.

26. The motor or generator as claimed in claim 24 wherein said teeth comprise laminations.

27. The motor or generator as claimed in claim 26 wherein said teeth are made from at least one chosen from i) non-oriented iron laminations, ii) oriented iron laminations, iii) silicon iron laminations, iv) nano-crystalline iron laminations and v) amorphous iron ribbon laminations.

28. The motor or generator as claimed in claim 22 wherein said teeth include edge crenellations.

29. The motor or generator as claimed in claim 28 wherein adjacent ones of said laminations have different lengths.

30. The motor or generator as claimed in claim 28 wherein said laminations have asymmetrical configurations and are stacked in a sequence that reverses orientation of at least some of said laminations.

31. The motor or generator as claimed in claim 16 wherein said teeth have portions extending radially outward of said yoke.

32. The motor or generator as claimed in claim 22 wherein said yoke is formed by injection molding.

33. The motor or generator as claimed in claim 22 wherein said yoke is formed by compression molding or extrusion.

34. The motor or generator as claimed in claim 33 wherein said yoke is formed of yoke pieces that are adhered together.

35. The motor or generator as claimed in claim 22 that has a fractional number of slots per phase per pole.

36. A method of manufacturing an electrical motor, comprising:

producing a stator, said stator comprising a yoke and a plurality of windings in magnetic communication with said yoke;

producing a rotor and juxtaposing said rotor with respect to said stator to rotate in response to magnetic flux produced by said stator;

connecting a variable frequency power supply with said windings in order to produce a magnetic flux with said stator; and

producing said yoke from a material that does not substantially increase in permeability in response to an increase in frequency of said power supply.

37. A method of manufacturing a stator for an electric motor or generator having a stator and a rotor mounted for rotation within said stator, thereby defining a radial air gap between said rotor and said stator, said method comprising:

providing a plurality of spaced apart teeth thereby defining slots between said teeth;

providing a yoke, said yoke physically mounting said teeth and providing a path of magnetic flux between said teeth;

providing at least one winding defining in said slots;

wherein said providing a yoke comprises making said yoke from a polymer bonded soft magnetic material.

38. The method as claimed in claim 37 wherein said metal-filled plastic material comprises a blend of magnetic iron powder.

39. The method as claimed in claim 37 including forming said yoke to said teeth.

40. The method as claimed in claim 39 wherein said providing at least one winding comprising winding a conductor around at least one of said teeth prior to said forming said yoke to said teeth.

41. The method as claimed in claim 39 wherein said teeth comprise laminations.

42. The method as claimed in claim 41 including making said teeth from at least one chosen from i) non-oriented iron laminations, ii) oriented iron laminations, iii) silicon iron laminations, iv) nano-crystalline iron laminations and v) amorphous iron ribbon laminations.

43. The method as claimed in claim 37 including making said teeth with edge crenellations.

44. The method as claimed in claim 43 including making said teeth with adjacent ones of said laminations having different lengths.

45. The method as claimed in claim 43 including making said teeth with laminations having asymmetrical configurations and are stacked in a sequence that reverses orientation of at least some of said laminations.

46. The method as claimed in claim 37 including making said teeth with portions extending radially outward of said yoke.

47. The method as claimed in claim 37 including forming said yoke by injection molding.

48. The method as claimed in claim 37 including forming said yoke by compression molding or extrusion.

49. The method as claimed in claim 48 wherein said forming said yoke mcludes forming yoke pieces and adhering said yoke pieces together.