WO2002015229A9

WO2002015229A9 - High performance slotless electric motor and method for making same

Info

Publication number: WO2002015229A9
Application number: PCT/US2001/025052
Authority: WO
Inventors: John Floresta; Xin-Tian Wang; Darius Bobelis
Original assignee: John Floresta; Xin-Tian Wang; Darius Bobelis
Priority date: 2000-08-10
Filing date: 2001-08-10
Publication date: 2003-04-03
Also published as: JP2004507197A; WO2002015229A1; AU2001286429A1

Abstract

A method of making a slotless motor using concentrated windings (18). The concentrated windings (18) are windings constructed from performed, multiple, non-overlaping, individual coils (32,33,34). Such coils (32,33,34) are generally concentric spiral coils with straight portions in the magnetic active region.

Description

High Performance Slotless Electric Motor And Method For Making Same

Related Applications This application is related to provisional U.S. Application Ser. No. 60/224,140 filed August 10, 2000, as well as an application entitled, "High Performance Slotless Motor" by John. Floresta et al. [0610-4012] filed on the even date herewith.

Object of the Invention

An object of this invention is to improve the torque/weight, torque/size, torque/volume , efficiency and motor constant Km performance of slotless type permanent magnet servo motors. Such motors are sometimes referred to as "ironless", "toothless", "coreless" or "wide airgap" motors. Another object is to achieve the high torque and high power required to drive traction- type vehicles. A further object is to provide an improved method for making slotless motors. Specifically, the motor according to the invention is significantly easier to manufacture, has superior performance characteristics and eliminates the need for expensive manufacturing tooling, winding techniques and/or elaborate production methods.

Background of the Invention

In designing permanent magnet servo motors attempts have generally been made to keep the flux density in the motor airgap as high as possible in order to achieve the highest possible torque and power according to the fundamental Lorentz relationships. The emphasis on higher flux densities has been facilitated by recent advancements in rare earth permanent magnets which are capable of producing high levels of flux. Improvements have also been made in manufacturing technology where smaller airgaps are made possible by more precise machining methods and tighter tolerances. Together, these improvements have encouraged the traditional approach of attempting to improve permanent magnet motor performance by increasing the flux density in the airgap.

Slotless motors are a class of permanent magnet motors where the airgap must be large and sufficient to accommodate the windings. In a slotless motor, the overall magnetic airgap consists of the mechanical clearance between the magnets and the coils plus the thickness of the winding coils (or "copper"). The absence of permeable material in the winding and consequently the absence of lamination slots automatically increases the overall magnetic airgap which, in turn, reduces the airgap flux density. The tradeoff that exists in such ironless designs achieves zero cogging and practically zero iron saturation. In this class of permanent magnet motors, as well as other types of motors, the general teaching has been to minimize the airgap to achieve higher performance.

The absence of iron within the winding removes one of the major limitations of conventional slotted motors, namely iron saturation. In slotted structures, in order to maximize the amount of copper in the winding to reduce heating and increase torque, the slotted teeth are typically made as narrow as possible. These narrow teeth tend to magnetically saturate as the coil current is increased which tends to place a physical limit on performance. This limit typically occurs when the motor current is increased to 2 or 3 times the rated value. In slotless motors, no such saturation limit is suffered, thus allowing for motor current to be increased to 5 to 10 times the rated value limited only by the thermal constraints of the motor and drive amplifier.

Notwithstanding these advantages, past designs of slotless motors have suffered disadvantages during manufacturing. The slots in conventional motors provide a convenient "holder" for the winding during manufacturing. In slotless motors, however, techniques have had to be developed to produce a self contained winding economically. The difficulty of doing this has led to many complicated proposals which have proven to be cumbersome, unreliable, or have required extensive tooling to accomplish.. Some examples of methods for producing slotless windings are as follows:

Kristiansen, U.S.P. 3,995,364; Nakamura, U.S.P. 4,110,901, MiyasakaU.S.P. 4,123,679; Faulhaber, U.S.P. 3,793,548; DiMeo, U.S.P. 4,271,370; Schultz, U.S.P. 4,679,313; and Lender U.S.P. 4,563,808. Summary of the Invention

According to the invention surprisingly significant improvements in performance are achieved in slotless motors by increasing the airgap and adding a higher copper volume to fill the increase in airgap. With a fixed mechanical clearance the increase in performance is optimized when the overall magnetic airgap is allowed to increase, and the copper fill increases until the copper fill exceeds 60% and preferably 70-90% of the overall magnetic airgap.

The winding method according to this invention involves prewound concentrated windings. The term, "concentrated windings" refers to a winding constructed from multiple, non-overlapping, individual coils. Each tooth, or group of teeth, in a conventional slotted machine has an individual coil wound around it, and is thus "concentrated" around that tooth. The use of concentrated windings in slotted motors for reducing end turn losses is described in U.S.P. 5,442,250. In the absence of slots, inserting prewound individual coils into an open cavity to construct the motor stator is relatively simple. Complicated fixtures, winding machines, or time consuming methods of assembly are eliminated. With no stator teeth the coils side-by-side can be maximized to provide the highest copper volume. By prewinding the coils outside of the motor, perfect layer wound coils can easily be produced and optimized without concern for the stator teeth or for the method of coil insertion. General Description of the Drawings

The foregoing and other objects of the invention will become obvious from the following detailed specification which incorporates the drawings and wherein:

FIG. 1 is a cross-sectional view of the cylindrical motor made according to the invention using prewound concentrated windings;

FIG. 1A illustrates three concentrated coils of the prewound winding used in making the motor in FIG. 1;

FIG. IB illustrates concentrated coils of prewound windings with cooling tubes added;

FIG. 2 is a cross-sectional drawing of an embodiment of a cylindrical motor including embedded magnets;

FIG. 3 is an illustration of rotor magnets in a Halbach Array. Detailed Description of the Invention

The general structure of a cylindrical motor with prewound concentrated windings according to the invention is illustrated in FIG. 1.

Three coils of the prewound concentrated winding are shown in FIG. 1 A. and FIG. IB.

The motor includes a steel shaft 12 surrounded by a cylindrical iron sleeve 16 which provides the back iron for the rotor. Twelve permanent magnets (12 pole pairs) are mounted on shaft 12 extending radially and are magnetized to provide alternating north and south poles. The magnets are preferably high energy product magnets with energy products in excess of 26 MGOe (MegaGauss Oersteds) and preferably may be in excess of 30 MGOe. Suitable permanent magnets are those made from neodymium, iron and boron such as available from Sumitomo Special Materials Co. Ltd. of Japan under the trade name NEOMAX-30H. The magnets are mounted on the back iron sleeve surrounding the motor shaft. More recent magnet products are available under the trade name Crumax 4014.

A banding can surround the rotor structure to hold the magnets in place under high speed centrifugal force conditions. Banding is accomplished using high strength Kevlar filaments which are dipped in epoxy and then wound around the rotor including one or more helical layers followed by several hoop layers.

The stator structure includes a cylindrical outer shell 16 of laminated silicon steel which provides the outer back iron for the motor. The laminations can be assembled and then cast in an aluminum outer housing.

Three of the prewound coils, 32, 33 and 34, are coils of the concentrated windings for the motor shown in Fig. 1. As previously stated, "concentrated windings" are windings constructed from multiple, non-overlapping, individual coils. The coils are located side-by-side in the airgap and therefore there is no constraint on the coil geometry. Each coil can be several layers thick and has a generally spiral configuration with straight sections located in the active magnetic region. Adjacent coils abut one another which helps in maintaining the coils at the correct position. The preformed coils can be subjected to shaping operations to provide essentially perfect coils to achieve a high copper fill. Locating the coils with respect to each other is easily accomplished by designing the coils to maximum width, such that each coil is adjacent to and provides support for the next coil. In this way the copper volume is again maximized and no jigs or fixtures are required.

Coils 32, 33 and 34, are three of the nine coils making up winding 40. The coils are preformed and then mounted inside the cylindrical back iron shell. The stator structure is slotless and, therefore, the windings are located in the magnetic airgap between the permanent magnets of the rotor and the outer back iron shell. Since there are no teeth in the stator, the entire inner cylindrical surface can be used by the copper of the windings. If desired, small notches can be randomly located in the internal circumference of the laminations for better bonding to the winding against torque forces produced in the motor.

Given the absence of slots and any concern for tooth cross section, virtually any combination of coils versus rotating magnet poles can be used. However, in a three phase brushless motor according to the invention, certain combinations produce more optimal performance compared to other combinations. Since it is the goal of the invention to produce a low cost and high performance motor, these combinations are an integral part of the invention.

Motor constant Km is a good measure of overall motor performance. Km indicates how much torque a motor can produce for a given amount of losses. An ideal motor would have a very high output torque with a small amount of losses. Improving the motor Km would also improve the motor's torque/weight, torque/volume and efficiency performances. Km is defined as:

Km - Output Torque/(I²R Power Losses)¹¹² Generally a motor with a higher Km is superior to a motor with a lower Km. One of the improvements of the invention is to maximize Km by optimizing the combination of coils and magnet poles. It can be seen in the following chart (Table 3) that Km is optimized for a three- phase brushless motor when the ratio of magnet poles to coils is 4:3.

TABLE 3

The motor in the first illustrative embodiment (Fig. 1) is a twelve pole three phase winding and therefore includes twenty-four (24) half coils in the winding. The winding includes nine coils (18 half coils). The ratio of poles to coils is the optimum 4:3 ratio. The coils are preformed and then placed in position inside the back iron surrounding the shaft. An inner Teflon sleeve can be used to define the inner diameter of the winding during assembly when needed.

When in place, the winding is impregnated with a suitable resin material to provide a rigid winding structure bonded to the back iron and housing of the stator shell. The winding, however, in some embodiments may be rigid enough so that the support may be removed prior to impregnating the winding with a suitable resin material.

The resin material must be carefully selected for the motor according to the invention. The resin should have a good flexural strength (i.e., compressive strength, tensile strength, tensile shear...) in order to rigidify the winding since any freedom of movement adversely affects the ability of the winding to produce torque. The motor is designed for continuous operation at 150° C and should be capable of withstanding peak termperatures of over 200° C. The thermal expansion of the resin must therefore be equal to or greater than the thermal expansion of the surrounding materials. The rating of the motor depends largely on the ability to dissipate heat from the windings and therefore the resin must also provide good thermal conductivity preferably in the range above 6 (BTU(in)/(hr)(ft )(F). This is particularly true with the compact motor design resulting from the invention. Ceramic fillers are preferably incorporated in the resin to improve thermal conductivity. However, the ceramic filler must be non-conductive and non-magnetic in order to avoid eddy current and iron losses. Furthermore, the resin must have a low viscosity, below 50,000cps in the uncured state, in order to properly impregnate the winding.

A suitable thermally conductive resin is Nordbak 7451-0148/7450-0027 epoxy available from Rexnord Chemical Products, Inc. Suitable resins are also specified in U.S.P. 4,954,739. Another suitable thermally conductive resin is Sytcast 2762 epoxy resin available from Emerson & Gumming, a division of W.R. Grace & Company.

When the winding is in place in the cylindrical outer shell of the stator, the epoxy is forced into the winding cavity at one end under pressure and is drawn through the winding by means of a vacuum applied at the other end. When the epoxy cures, the winding becomes rigid. The end surfaces can be machined to provide a flat surface for good thermal contact with the end bells of the motor (not shown). In most cases, however, good thermal contact between the resin and the housing provides adequate heat dissipation.

Since there are no slots and no teeth in the stator, the option for internal cooling of the coils through the use of a serpentine cooling tube 38 (FIG. IB) directly in contact with the coils can be provided. Many cooling designs have been conceived for conventional slotted motors. However, these methods have typically employed cooling "jackets", fans or heatsinks on the outside of the stator. Because the thermal time constant of the copper coils is much faster than the stator back iron, such external cooling is of limited effectiveness. The cooling according to the invention is much improved because the cooling is occurring directly at the coils The cooling tubes can be any diameter depending on the space available between coils. The width of the coils can be reduced to increase the available space between coils for the cooling tubes given that the increased cooling benefit will offset the loss in copper volume. Typical cooling tube diameter is in the range of 3 to 6mm. The tube must be non-magnetic to avoid cogging. Either copper, aluminum, or plastic are suitable.

In a further embodiment the same ease of manufacture and high performance structure can be enhanced further through the use of embedded magnet rotor designs such as illustrated in Fig. 2. Embedded magnet rotor designs are known (See, for example, Volkrodt U.S. 4,127,786.) in slotted motors for either producing flux focusing to enhance the magnetic field in the motor airgap, or for increasing the motor reluctance torque component. In addition, the embedded magnet is automatically restrained from centrifugal forces by virtue of being embedded in the rotor iron.

In the embedded magnet embodiment the stator structure including back iron 30 and coils 40 are essentially the same as in FIG. 1. The magnets 42 are embedded and not mounted on the rotor surface. As a result the magnetic flux path passes through the rotor iron to increase the motor reluctance torque. In slotless motors the primary advantage of an embedded magnet design is to enhance the magnetic field in the motor airgap, and to increase the motor output torque without adding any additional magnetic material.

In addition, as an alternative to a rotor with embedded magnets, surface magnets can be positioned in a so-called Halbach array illustrated in Fig. 3. Halbach arrays are known for enhancing the magnetic field in the motor airgap. In the Halbach array the magnets are surface mounted on the rotor iron. The magnets 20 are poled so that the magnetic flux flows radially outwardly from the rotor iron through magnets 20 and through the magnetic airgap. Magnets 22 are poled so that magnetic flux flows radially inwardly. Magnets 21 create a surface flux from magnet 20 toward magnet 22 and magnets 23 create a surface flow from magnets 20 to magnets 22. In combination with the construction according to the invention, the Halbach array rotor can increase the torque output even further; typically in the range of an additional 10-30% improvement.

While the preferred embodiment of the present invention has been shown and described, it will be understood that this is not intended to limit the disclosure, but rather it is intended to cover all modifications and alternative methods and apparatus as falling within the sphere and scope of the invention as defined in the appended claims or their equivalents.

Claims

ClaimsWhat is claimed is

1. A method for making a high performance slotless electric motor having at least one permanent magnet, a winding, an airgap, and back iron for completing a magnetic path through the magnet, winding, airgap, and back iron, including the steps of:

(1) forming a plurality of generally concentric spiral coils for said winding;

(2) placing said coils within said back iron in a non-overlapping concentrated fashion; and

(3) impregnating said winding with a resin to secure said winding in place in said back iron.

2. The method according to claim 1 wherein said resin is selected to provide good electrical insulating properties and good heat conductivity.

3. The method according to claim 2 wherein resin flows into stationary recesses to prevent rotation of said winding relative to said back iron.

4. The method according to claim 1 further providing a path for a coolant to remove heat from said winding.

5. The method according to claim 1 further including surface mounting said permanent magnets.

6. The method according to claim 1 wherein said permanent magnets are embedded.

7. The method according to claim 1 wherein said permanent magnets are arranged in a Halbach configuration.

8. A method for making a high performance slotless electric motor having at least one permanent magnet, a winding, an airgap, and back iron for completing a magnetic path through the magnet, winding, airgap, and back iron, including the steps of:

(1) forming a plurality of generally concentric spiral coils with straight portions in the magnetically active region for said winding;

(2) applying heat and pressure to said formed coils to improve the ideal configuration for maximum copper fill;

(3) placing said coils within said back iron in a non-overlapping concentrated fashion; and

(4) impregnating said winding with a resin when in place in said back iron.

9.. The method according to claim 8 wherein said copper fill is at least 70%

10. The method according to claim 8 wherein said copper fill of said airgap is 70-90%.

11. A method for making a high performance slotless electric motor having at least one permanent magnet, a winding, an airgap, and back iron for completing a magnetic path through the magnet, winding, airgap, and back iron, including the steps of:

(1) forming a plurality of generally concentric spiral coils for said winding; (2) placing said coils within said airgap in a non-overlapping concentrated fashion to achieve a copper fill in said airgap of 70-90%; and

(3) impregnating said winding with a resin when in place is said back iron.

(4) applying pressure to said formed coils to obtain the ideal configuration for maximum copper fill.

12. The method according to claim 11 wherein said copper fill is at least 70%.

13. A method according to claim 12 wherein said coils are placed within said airgap in a non- overlapping concentrated fashion to achieve a copper fill in said airgap of 70-90%.

14. A method for making a high performance slotless electric motor including the steps of:

(1) forming a winding of concentrated non-overlapping coils;

(2) placing said winding inside the back iron of the motor with said coils being placed side- by-side with no overlap;

(3) impregnating said winding with a resin having a good electrical insulating qualities and good heat conducting properties.

15. A slotless electric motor comprising:

(1) at least one permanent magnet

(2) back iron

(3) a copper winding made up of concentrated non-overlapping coils

(4) an airgap (5) a magnetic path through said back iron, said permanent magnet, said copper winding and said airgap; and

(6) said winding being impregnated with resin.