METHOD FOR MAKING A SERVOMOTOR

Abstract: The invention relates to permanent magnet rotor brushless high performance servo motors and to the design and manu­ facture of such motors making efficient use of high energy product magnets having an energy product in excess of 20 MGOe. The interpole distance to the radial air gap is greater than 1.3 and the ratio of permanent magnet length to air gap length is in the range of from 0.5 to 2.0. The winding is arranged in the air gap and secured to the inner surface of the slotless back iron cylindrical stator shell by encapsulation in a resin having a flexural strength of at least 140 kg/cm² at 250°C and high thermal conductivity and a coefficient of thermal expansion in the range of the other materials of the stator structure. The stator winding is formed on a cylindrical temporary support with reduced diameter on one side using preformed coils. The end turns at the end of the temporary support with reduced diameter flare inwardly, whereas the end turns at the other end flare outwardly. The support with the winding is inserted into the stator shell starting with the inwardly flared end turns and is withdrawn from the side of the stator shell with the outwardly flared end turns. The motor of the in­ vention, compared to a servo motor of approximately the same physical dimensions, produces an increase of about 70% in continuous performance and with respect to inter­ mittant performance an increase of 80% with only 30% in­ crease of the permanent magnet material.

Description: This invention relates to high performance servo motors, and, more particularly, to such motors which make effective use of very high energy product permanent mag­ nets. Recently, new types of permanent magnets have become available with significantly increased energy products. These new magnets comprise alloys of a rare earth (usually neodymium or praseodymium), iron and a promoter of metastable phases (such as boron or gallium). Prior alnico (aluminum, nickel, cobalt) magnets usually have an energy product in the range of 5-7 MGOe, samarium-cobalt SmCo₅, magnets have an energy product of about 17 MGOe and the more expensive samarium-cobalt Sm₂Co₁₇ magnets have an energy product of about 27 MGOe. By comparison, NeFeB (neodymium, iron, boron) magnets are now available, for example, with energy products in excess of 35 MGOe. A great many motor designs have been created in the past, many taking advantage of improved permanent magnet cha­ racteristics. Slotted motor structures have been the most common in which copper windings are placed in laminated iron slots. The slotted designs provide a motor with a re­ latively small air gap in the magnetic circuit to achieve a desired high permeance. Magnets with increased energy products (e.g., samarium-cobalt) have proportionately re­ duced the magnet mass and resulted in effective inside-out brushless designs with rotating magnets and windings on the stator. Non-slotted designs are also known where the windings are located in the air gap. Such slotless designs have proven effective primarily in large turbogenerators where expense is no object if it achieves improved operating efficiency. These turbogenerators employ sophisticated cooling systems and super-conducting materials to achieve high flux densi­ ties across a large air gap which accommodates the windings. Slotless designs in small motors have also been proposed, but these have usually been either special purpose (e.g., high speed toroidally wound motors) or low performance motors not suitable for servo applications. It is an object of the invention to provide a motor de­ sign which can make effective use of high energy product permanent magnet materials. A more specific object is to provide a motor design for effectively using permanent magnets like the available NeFeB magnets having an energy product above 26 MGOe and preferably above 30 MGOe. Still another object is to provide a method for making a high performance motor with windings located in the motor air gap. The dynamoelectric machine of this invention comprises a permanent magnet rotor, a cylindrical outer stator shell of magnetic material defining an air gap between said magnets and said stator shell, and a winding located in said air gap and affixed to said shell, characterized in that the permanent magnets of said permanent magnet rotor have an energy product in excess of 26 MGOe and that said winding is secured to the inside surface of said stator shell by means of an encapsulating resin having a flexural strength in excess of 140 kg/cm² at 250°C, and having a thermal conductivity in excess of 6 (cal)(cm)/(sec)(cm²)(°C). The dynamoelectric machine further includes a thermally conductive housing and wherein the resin encapsulating said winding is thermally coupled to said stator shell or in said shell to a separate housing. The ratio of the interpolar spacing between adjacent pole faces and the air gap length of the machine of this in­ vention is at least 1.3, and the ratio of the length of the permanent magnets forming the permanent magnet material of the rotor to the radial length of said air gap is se­ lected in the range of from 0.5 to 2.0. The invention further encompasses a new and efficient method of making the said dynamoelectric machine with the stator winding inside a cylindrical stator shell of mag­ netic material comprising the steps of creating a generally cylindrical support with a reduced diameter portion at one end; forming the winding around said support from pre­ formed coils such that the coil end turns at the reduced diameter portion of said support flare inwardly, and the coil end turns at the other end of said support flare outwardly; inserting said winding into said cylindrical stator shell starting with said inwardly flared end coils; and removing said support from said end of said outwardly flared end coils. Preferably, said winding is impregnated with resin prior to removing said support. The obvious first inclination of a servo motor designer with a new high energy permanent magnet material is to use it as a replacement for prior permanent magnets in con­ ventional designs and, after making design changes as dic­ tated by the different characteristics, hoping that the new motor will have improved operating characteristics. Surprisingly, a similarly designed motor replacing samarium-­ cobalt magnets with higher energy product NeFeB magnets results in a motor with significantly lower peak torque in a range unsatisfactory for high performance servo appli­ cations. At room temperature NeFeB magnets, like samarium-­ cobalt magnets, do not show any significant demagnetization characteristics. At elevated temperatures above 100°C, and particularly at temperatures above 140°C, however, the coercivity of the NeFeB magnet falls off rapidly beyond a "knee" and, hence, demagnetization can occur. Since the demagnetization force applied to the magnet is proportion­ al to armature current, a conventional design using NeFeB magnets will have limited peak current and, therefore, low peak torque despite the higher energy product magnets. Conventional slotted designs also impose limitations on the air gap flux density because of the saturating charac­ teristics of the iron on the teeth between slots. To in­ crease the flux density would require wider teeth, which in turn would result in narrower slots and fewer copper windings. Because of the tradeoff between iron teeth and copper in the slots, such designs usually limit the permanent magnet flux density in the air gap to about 7 kilogauss. The permeance of the magnetic circuit de­ termined by the magnet length compared to the air gap length is typically in the range of 4 to 6 in prior servo motor designs. Substitution of high energy product magnets is also likely to result in magnets that are im­ practical to make or handle during fabrication. According to the invention, however, it has been found that the benefits of the new high energy product magnets (above 26 MGOe and preferably above 30 MGOe) can be rea­ lized by using slotless design provided certain design parameters are observed. The stator winding is a multiphase winding contained wholly within the magnetic air gap so that there are no saturation constraints in the magnetic circuit and flux densities a­ bove 7 kilogauss in the air gap can be used. The ratio of the magnet length to the air gap length is in the range of from 0.5 to 2.0. The ratio of the interpolar distance to the radial gap length is greater than 1.3. By starting within these design parameters, motors can be designed using the high energy product magnets without danger of de­ magnetization and with significantly increased horsepower and continuous torque for a given size and weight. Further­ more, the motor of this invention has a reduced inductance which provides more power at high speeds and a lack of re­ luctance torque and cogging. A comparison of prior samarium-cobalt (Sm₂Co₁₇) magnet servo motors with motors of comparable size and weight made according to this invention indicates about a 70% increase in the dynamic continuous torque speed output performance and about 80% increase in the intermittent performance. In order to achieve the improved results, it is important to properly secure the winding within the surrounding back iron cylindrical shell which provides the flux return path. Since the stator teeth are eliminated, the winding must be secured to the stator structure with sufficient adhesion to withstand the maximum motor torque force throughout a range of operating temperatures. The winding must be rigid since movement of the conductors adversely affects the ability to generate torque. Also, heat must be dissipated from the windings. According to the invention, the winding is encapsulated and bonded to the cylindrical stator shell by a ceramic filled epoxy selected to provide (1) a high flexural strength of at least 140 kg/cm² at 250°C and preferably above 280 kg/cm² at 250°C, (2) good thermal conductivity, and (3) a coefficient of thermal expansion in the range of other material in the stator structure. One such suitable ma­ terial is Stycast® 2762. The invention further includes a method for assembling a motor with the winding in the air gap. The winding is formed using a cylindrical support with a reduced diameter section at one end. A fiberglass sleeve is placed around the cylin­ drical support in the uniform diameter portion and, there­ after, preformed coils are placed in position. As is usu­ ally the case, the end turn portion of the winding is thicker because of crossing conductors. Using the method of this invention with a reduced diameter at one end of the support, the end turns at one end of the winding flare in­ wardly whereas the end turns at the other end flare out­ wardly. The winding can then be inserted into the cylin­ drical back iron shell starting with the inwardly flared end of the winding. The support can thereafter be withdrawn from the outwardly flared end of the winding leaving the fiberglass sleeve in as part of the stator structure. The winding is preferably encapsulated using a suitable resin after the winding is inserted into the stator shell. Notched laminations can be used in the stator shell to im­ prove the shear strength of the bond between the winding and the stator shell. The notches are randomly distributed along the axial length of the machine to eliminate any appearance of a reluctance effect. The foregoing and other objects of the invention will be­ come obvious from the following detailed description which incorporates the drawings and wherein Fig. 1 is a cross-sectional end view of the motor of this invention; Figs. 2A and 2B are a side and end view, respectively, of the rotor portion of the motor; Fig. 3 is a cross-sectional view of the stator of the motor. Fig. 4 is an illustration showing the lapped winding structure in the motor of the invention; Fig. 5 is a diagram showing the demagnetization curves of a high energy product permanent magnet of the type used in the motor according to the invention; Fig. 6 shows the improved operating performance of a motor made in accordance to the invention as compared to a prior art motor with samarium-cobalt magnets having a comparable size; and Figs. 7A to 7D are a series of illustrations showing the method for forming the winding for the motor. The general structure of the motor according to the inven­ tion is shown in Figures 1 to 3. The motor includes a steel shaft 10 (Figs. 2A and 2B) surrounded by a cylindrical iron sleeve 12 which provides the back iron for the rotor. Six permanent magnets 14-19 are mounted on sleeve 12 extending radially and are magne­ tized to provide alternating north and south poles, as shown in Fig. 1. The magnets 14-19 are high energy product magnets with energy products in excess of 26 MGOe (MegaGauss Oersteds) and preferably in excess of 30 MGOe. Suitable permanent magnets are those made from neodymium, iron and boron (NEOMAX-30H ®). The magnets are pressed arcuate shaped magnets and are mounted on the back iron sleeve 12 surrounding shaft 10. A banding 20 surrounds the rotor structure to hold the magnets in place under high speed centrifugal force con­ ditions. 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 hooped layers. The rotor can be constructed using six magnets each ex­ tending the full length of the rotor, or the magnets can be segmented as shown in Fig. 2A. An advantage to the seg­ mented magnets is that a single motor design can produce motors of different horsepower ratings by simply changing the motor length and the number of magnet segments. The stator structure includes a cylindrical outer shell 30 of laminated silicon steel which provides the outer back iron for the motor. The laminations are assembled and then cast in an aluminum outer housing 32. The individual laminations can include one or more notches on the inner diameter and can be assembled to randomly distribute the notches on the inner cylindrical surface to provide a better bond with the motor windings. The windings 40 are formed and then mounted inside the cylindrical back iron shell 30. The stator structure is slotless and, hence, the windings 40 are located in the motor air gap between the permanent magnets of the rotor and the outer back iron shell. Since there are no teeth in the stator, the entire inner cylin­ drical 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 bond­ ing to the winding against torque forces produced in the motor. The motor in the illustrative embodiment is a six pole three phase winding and, therefore, includes eighteen (18) coils in the winding. The coils are pre-formed and then placed in a lapped configuration as shown in Fig. 4. The coils of one phase are shaded in the illustration to show the relative orientation of the coils. A coil 41 of phase A is followed by a coil 42 of phase B which, in turn, is followed by a coil 43 of phase C, and then the sequence repeats. The longitudinal conductors 44 of one side of a coil are on the outside of the winding whereas the longi­ tudinal conductors 45 of the other side of the same coil are on the inside of the winding beneath the conductors 46 of the next coil of the same phase. The coils are lapped in this fashion to provide a balanced three-phase six pole winding 40. The winding 40 is formed on a temporary cylindrical support 50 (Fig. 7) which is of a constant diameter starting from one end (to the left of Fig. 7A) and includes a reduced diameter portion 52 at the other end. A fiberglass sleeve 54 is placed surrounding the winding support 50 (Fig. 7A) and the preformed coils 40 are then placed in position surrounding the sleeve 50 (Fig. 7B). When the coils are in place, the end turns 48 at one end of the winding tend to flare inwardly as permitted by the reduced diameter portion 52 of the support 50, whereas the end turns 49 at the other end of the winding flare outwardly. The winding thus formed is then inserted into the stator structure 30, 32, as is shown in Fig. 7C, starting with the end at which the end turns 49 flare inwardly. Once the winding has been located within the stator shell as shown in Fig. 7D, support 50 can be removed from the outwardly flared end leaving the fiber­ glass sleeve 54 in place as part of the final stator structure. With the method described, it is important that the pre­ formed winding includes inwardly flared end turns at one end and outwardly flared end turns at the other end. The in­ wardly flared end turns permit insertion of the preformed winding into the cylindrical stator shell. The outwardly flared end turns permit removal of the support after the winding is in place in the stator shell. When the winding is in place, it is impregnated with a suitable resin ma­ terial to provide a rigid winding structure bonded to the back iron and housing of the stator shell. The resin material must be carefully selected for the motor of this invention. It must have a high flexural strength in order to rigidify the winding since any freedom of movement adversely affects the ability of the winding to produce torque. A flexural strength of at least 140 kg/cm² at 250°C, and preferably 280 kg/cm² at 250°C is required. The motor is designed for continuous operation at 150°C and must be capable of withstanding peak temperatures of over 200°C. The thermal expansion of the resin must, therefore, be equal to or somewhat greater than the thermal expansion of the surrounding materials. The thermal coefficients of expansion for silicon steel, copper and aluminum are 10.8 × 10⁻⁶, 16.8 × 10⁻⁶ and 23 × 10⁻⁶ per °C, respective­ ly. For these materials, the coefficient of thermal ex­ pansion for the resin should be in the range of 23 to 30 × 10⁻⁶/°C. The rating of the motor depends largely on the ability to dissipate heat from the windings and, there­ fore, the resin must also provide good thermal conductivi­ ty, preferably in the range above 6(cal)(cm)/(sec)(cm²)(°C). This is particularly true with the compact motor design re­ sulting from the invention. Ceramic fillers are preferably incorporated in the resin to improve thermal conductivity. However, the ceramic fillers 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 50 000 cps in the uncured state in order to properly im­ pregnate the winding. A suitable thermally conductive resin is Stycast® 2762 epoxy resin. The typical properties for this resin are as follows: 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 and is securely bond­ ed to the stator laminations. The ends of the winding cavity preferably flare out at both ends in the region of the end turns to increase the surface area. 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 aluminum housing 32 will provide adequate heat dissipation. The demagnetization curves of a suitable magnet material, for example NEOMAX-30H®, are shown in Fig. 5. For tempe­ ratures below 100°C the properties are substantially linear and, hence, no demagnetization is likely to occur when operating in this temperature range. At elevated tempera­ tures above 100°C, however, there is a "knee" in the curve which, at 140°C, occurs at Bd = 3 500 Gauss and Hb = 6 000 Oersted. The rapid falloff of the coercivity at field strengths higher than 6 000 Oersteds can cause significant demagnetization of the magnets. The permeance P is the operating slope of the magnet in a given circuit. The slope is given by where Lm is the magnet length in orientation direction; Lg is the length of the magnetic gap; Am is the area of magnet; and Ag is the area of gap. The allowable demagnetization field Ha is given by a line having slope P and passing through (Hd, Bd) at the knee in the curve. This can be written as Substituting for P and simplifying the equation becomes Thus, the maximum allowable demagnetization field Ha can be calculated for a given demagnetization characteristic and operating permeance P. For design comparison purposes, the worst case demagneti­ zation field is when the stator currents are arranged such that the stator MMF directly opposes the magnet MMF. This is a realistic case since many servos are braked by short­ ing phase leads together, thus giving such a field align­ ment. Current in phase A is peak and current in phases B and C is 1/2 the peak current value. By symmetry, the armature H field is radial at the centreline of the magnet. Taking this path, the enclosed ampere turns per pole is where C is series conductors per phase. From Ampere's law Thus, for a given combination of poles, gap length, magnet length, conductors, and current, the applied demagneti­ zation field H can be calculated. Solving equation (2) for Ipeak and setting the allowable applied demagnetization field Ha equal to the applied field H, gives Substituting equation (1) for H gives Therefore, the maximum allowable peak current before de­ magnetization is expressed as a function of magnet material (Bd, Hd) and magnetic circuit design (poles, Lm, Lg, Am, Ag, C). The various parameters of equation (3) for the conventional slotted design and the air gap winding design of the in­ vention, both using the NdFeB magnet material shown in Fig. 5 with (Hd, Bd) of (6000, 3500) are as follows: As can be seen from Table I, the air gap winding design allows more than 3 1/2 times the peak torque allowed by the cinventional slotted design. With a maximum of 34.8A RMS current before demagnetization, the conventional slotted design does not offer the needed peak torque for a high response servo motor. If the air gap is made relatively large, such as 0.3 inches in the illustrative embodiment of the invention, the re­ luctance of the magnetic path for flux generated in the stator is sufficiently high such that the flux, as seen by the magnets, remains below the level at which demagneti­ zation is likely to occur. The ratio of the air gap length (Lg) to the magnet length (Lm), Fig. 1, must be in the range between 0.5 and 2.0. The use of permeances in the range of 4 to 6, common in slotted motor structures, is undesirable since it results in either an excessively large amount of expensive permanent magnet material or a small air gap inadequate to hold the desired number of windings required for a high performance motor. The ratio of the interpolar distance (Lip) to the radial gap length (Lg), as seen in Fig. 1, should be greater than 1.3. With high energy product magnets this ratio becomes important since a lower ratio results in ineffective use of the expensive permanent magnet materials due to increasing­ ly high leakage flux. Figure 6 is a diagram illustrating the dynamic comparison of two motors with approximately the same outside physical dimensions. Curves 60 and 61 are for a conventional slotted structure with samarium-cobalt (Sm₂Co₁₇) magnets having an energy product of about 27 MGOe, whereas curves 62 and 63 are for a motor according to the invention including per­ manent magnets of the NeFeB type with an energy product of about 35 MGOe. Area A in Fig. 6 represents an increase of about 70% additional continuous performance while area B shows about an 80% increase in the intermittent per­ formance. These improvements in the operating character­ istics are achieved with an increase of only about 30% in the energy product of the magnets. Although only one illustrative embodiment of the invention has been described in detail, there are numerous variations within the scope of this invention.

Claims: 1. A dynamoelectric machine comprising a permanent magnet rotor, a cylindrical outer stator shell of magnetic material defining an air gap between said magnets and said stator shell, and a winding located in said air gap and affixed to said shell, characterized in that the permanent magnets of said permanent magnet rotor have an energy product in excess of 26 MGOe and that said winding is se­ cured to the inside surface of said stator shell by means of an encapsulating resin having a flexural strength in excess of 140 kg/cm² at 250°C, and having a thermal conducti­ vity in excess of 6 (cal)(cm)/(sec)(cm²)(°C). 2. The dynamoelectric machine of claim 1 wherein said resin encapsulates said winding. 3. The dynamoelectric machine of claim 2 wherein the motor further includes a thermally conductive housing and wherein said resin encapsulating said winding is thermally coupled to said housing. 4. The dynamoelectric machine of claim 2 wherein said resin has a viscosity below 50 000 cps in the uncured state. 5. The dynamoelectric machine of claim 1 wherein said resin is a ceramic filled epoxy and preferably is non-mag­ netic and non-conductive. 6. The dynamoelectric machine of claim 1 wherein said winding consists of a plurality of lapped flat coils. 7. The dynamoelectric machine of claim 1 wherein said permanent magnets have an energy product in excess of 30 MGOe. 8. The dynamoelectric machine of claim 7 wherein said permanent magnets are made from neodymium, iron and boron. 9. The dynamoelectric machine of claim 1 wherein said resin has a thermal expansion coefficient equal to or greater than that of surrounding stator materials, and pre­ ferably in the range of 23 to 30 x 10⁻⁶/°C. 10. A dynamoelectric machine comprising a rotor in­ cluding a plurality of radially extending permanent mag­ nets each having an energy product in excess of 26 MGOe; a cylindrical outer stator shell of magnetic material de­ fining an air gap between said magnets and said stator shell; the ratio of the interpolar spacing between ad­ jacent pole faces and the air gap length being at least 1.3; and a multiphase winding in said air gap secured to the inner surface of said cylindrical stator shell. 11. The dynamoelectric machine of claim 10 wherein said winding is encapsulated and secured by epoxy resin, preferably a ceramic filled epoxy resin. 12. The dynamoelectric machine of claims 1 to 11 where­ in the ratio of the length of said permanent magnets to the radial lenth of said air gap is in the range of from 0.5 to 2.0 and the winding is a multiphase winding. 13. A method for making a dynamoelectric machine with the stator winding inside a cylindrical stator shell of magnetic material comprising the steps of creating a gene­ rally cylindrical support with a reduced diameter portion at one end; forming the winding around said support from preformed coils such that the coil end turns at the re­ duced diameter portion of said support flare inwardly, and the coil end turns at the other end of said support flare outwardly; inserting said winding into said cylindrical stator shell starting with said inwardly flared end coils; and removing said supprt from said end of said outwardly flared end coils. 14. The method of claim 13 wherein said winding is impregnated with resin prior to removing said support. 15. The method of claim 14 wherein said resin has a flexural strength in excess of 140 kg/cm², and preferably in excess of 280 kg/cm² at 250°C. 16. The method of claim 14 wherein said resin has a thermal conductivity in excess of 6 (cal)(cm)/(sec)(cm²)(°C). 17. The method of claim 14 wherein said resin has a coefficient of thermal expansion equal to or greater than that of the cylindrical stator shell. 18. The method of claim 14 wherein said resin has a viscosity below 50 000 cps in the uncured state. 19. The method of claim 14 wherein said resin is an epoxy filled with a non-conductive and non-magnetic ceramic.

Inventors: ALTIZER, CLARK C., ENGLAND, THOMAS R., SCHULTZ, ROY D.
Applicants: KOLLMORGEN CORPORATION
Application year: 1986
Main classification: H02K3
IPC codes: H02K15, H02K3, H02K29, H02K1, H02K21, H02K37