APPARATUS AND METHOD FOR REDUCING NOISE AND VIBRATION IN AN ELECTROMOTIVE DEVICE

Abstract: An apparatus and method are provided for reducing noise and vibration in an electromotive device. In order to reduce the generation of vibrations and the resultant audible noise, vibration damping material is used to de-couple vibrations from portions of the electromotive device which would otherwise be transmitted to other parts of the electromotive device.

Description: APPARATUS AND METHOD FOR REDUCING NOISE AND VIBRATION IN AN ELECTROMOTIVE DEVICE CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation- in-part of application 09/211,021, filed December 14, 1998. TECHNICAL FIELD The present invention relates generally to vibration and noise damping in electromotive devices such as electric motors and generators . BACKGROUND OF THE INVENTION The components of a conventional electromotive device are rigid and are in direct contact . These rigid parts are typically metal. Having these rigid parts in direct contact is desirable in some respects: for example, the direct contact of metal parts facilitates heat transfer and allows the device to cool sufficiently during operation. In addition, the direct coupling of rigid parts helps to keep moving parts in proper alignment. However, the direct contact of rigid parts also facilitates the transmission of vibrations and consequently results in significant levels of noise. The conventional brushless direct current ("DC") motor illustrated in FIG. 2 provides an example of this direct contact. Because the rotor magnet 24 in the motor shown in FIG. 2 is inside the stator 20, the motor is called an "inside" design. In a conventional inside motor, the bearings 22 are in direct contact with the rotor shaft 18, and the motor housing 16 and the stator stack 20 are connected directly to the motor housing. The shaft 18 of the rotor is typically made of steel. The bearings 22 are also typically made of steel . The housing 16 is typically aluminum or steel. At rotational speed there will be some degree of residual rotor unbalance, even if a pre-balancing operation is performed prior to use. This unbalance causes vibration in the front and rear bearings. The vibration is conducted, through the metal-to-metal fit of the shaft bearing and housing, directly to the mounting surface. This vibration of the mounting surface will be transmitted to the machine that contains the motor, creating noise in the frame or other components of the machine. Any vibration that is generated in the rotor is transmitted to the motor stator by variation in the motor air gap. This variation in air gap changes the radial forces on the stator because of the change in magnetic attraction between the stator and rotor. Stator vibration is then transmitted to the motor housing and then to the frame of the machine to which the motor is fastened. Additional vibration in the stator is caused by magnetostriction in the stator laminations. When the motor in operation, these laminations are constantly being stressed from round to oval as each stator pole pair is electrically energized. The magnetic stress caused by the current flowing through successive coils 33 distorts the stator stack 20 as shown in FIG. 3. This variation in stack stress moves the motor housing, thereby generating audible noise as in an audio speaker. Conventional "outside" motor designs have analogous vibration and noise problems. FIG. 4 depicts a conventional outside motor with a rotating shaft 42. Rotor assembly 40 connects rotor magnet 44 and rotor shaft 42. Stator 46, bearings 50 and housing 38 are connected to support structure 51. Rotor shaft 42 is supported for rotation by bearings 50. FIG. 5 depicts a conventional fixed-shaft outside motor. Here, shaft 62 is directly affixed to base 72, cover 54, stator 68 and bearings 64. Rotor magnet 58 is directly attached to rotor 56, which is supported for rotation by bearings 64. All of the contacts indicated in FIGS. 4 and 5 are metal-to-metal and therefore tend to transmit noise and vibrations between connected parts. The inventor has previously used urethane bushings around the rotor bearings to dampen noise from the bearings. For example, Model No. D1H18-23- 004Z sold by BEI, Kimco Magnetics Division included such urethane bushings. However, the use of urethane bushings around the rotor bearings has met with limited success in that it did not result in a significant reduction in vibration or noise. SUMMARY OF THE INVENTION In accordance with the present invention, to reduce the generation of vibrations and the resulting generation of audible noise in an electromotive device, vibration damping material is used to decouple vibrations in the electromotive device which would otherwise be transmitted to other parts of the device. This vibration damping material may be composed of any substance which will cause a significant reduction in the transmission of vibrations, such as silicon rubber, urethane, the "ISODAMP" material manufactured by E-A-R Specialty Composites of Indianapolis, Indiana or material such as the "CHO-THERM" products manufactured by the Chomerics Division, Parker Hannifin PLC of Woburn, Massachusetts . Although the applications of this invention to various types of electric motors will be described in detail, the invention is equally applicable to other electromotive devices, such as generators, tachometers and alternators. The most dramatic improvements in noise suppression have resulted from de-coupling the stator vibrations from the motor housing and the other parts of the motor support structure. This is accomplished by using a layer of vibration damping material between the stator lamination and the housing or support structure. This layer blocks the vibration from the stator magnetostriction and rotor pull effects from being transmitted to the motor housing or support structure . The most significant noise reduction is achieved when vibration damping material is used to isolate numerous rigid parts which were formerly in contact or otherwise vibrationally coupled, including: (1) the stator and housing or support structure, as described above; (2) the rotor bearings and housing; (3) the housing and front flange/end bell or other structural element; and (4) the rotor and the rotor magnet . These and other features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of certain preferred embodiments of the present invention and of the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a conventional motor 10 mounted to a frame. FIG. 2 is a partial cross-section of conventional motor 10 of FIG. 1, taken from the center line 18 of the shaft 12, radially to the mounting frame 14 of the motor 10. FIG. 3 is a radial cross-section of the stator 20 partially depicted in FIG. 2, which illustrates the magnetostrictive force caused by the sequential excitation of opposing stator windings. FIG. 4 is a partial cross-section of a rotating- shaft outside motor. FIG. 5 is a cross-section of a fixed-shaft outside motor. FIG. 6 is a partial cross-section of an inside motor illustrating bearing damping inserts 26 and 28, stator damping insert 30 and flange damping insert 32, in accordance with one embodiment of the present invention. FIG. 7 illustrates magnet damping insert 36 between rotor shaft 12 and rotor magnet 24, in accordance with one embodiment of the present invention. FIG. 8 is a partial cross-section of a rotating- shaft outside motor with housing damping insert 39, magnet damping insert 45, stator damping insert 48 and bearing damping inserts 52, in accordance with another embodiment of the present invention. FIG. 9 is a cross-section of a fixed-shaft outside motor with magnet damping insert 60, bearing damping inserts 66 and stator damping insert 70, in accordance with another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION In the preferred embodiment of the present invention, vibration damping material is used to damp vibrations of the stator, rotor magnet and bearings. Vibration damping material may also be placed between other parts of the electromotive device, such as the housing and support structure or between the housing and the front flange or end bell. When vibration damping material is used to damp vibrations to or from a particular part of an electromotive device, the vibration damping material so used forms a "vibration damping insert" which is named according to the part. For example, vibration damping material used to damp vibrations to or from a bearing is called a "bearing damping insert." These vibration damping inserts may be formed from any material suitable for damping vibrations, such as material which has the property of converting a substantial amount of the impinging mechanical energy into heat energy. Examples of such material include silicon rubber and urethane. "ISODAMP" material, manufactured by E-A-R Specialty Composites of Indianapolis, Indiana, has proven to be particularly suitable for fabricating vibration damping inserts. Vibration damping material composed of "SORBOTHANE" material, manufactured by Sorbothane, Incorporated of Kent, Ohio, has also proven to be useful for fabricating vibration damping inserts. Materials such as the "CHO-THERM" products manufactured by Chomerics Division of Parker Hannifin PLC of Woburn, Massachusetts, have a highly desirable combination of vibration damping and heat conduction properties which can make these materials well suited for use as vibration damping inserts. Referring to FIG. 6, elements 26 and 28 are vibration damping inserts which block vibrations from leaving the rotor and rotor bearings of an inside motor, which vibrations would otherwise enter the housing 16 or the flange 14. Elements 26 and 28 are positioned between the outer race of bearings 22 and the flange 14 and housing 16. Element 30 of FIG. 6 is a stator damping insert which provides a layer of vibration damping material between the stator lamination 20 and the housing 16. This layer blocks the vibration from the stator magnetostriction and rotor pull effects from being transmitted to the motor housing 16. When the invention is used with inside motors, the thickness of the layer of vibration damping material surrounding the stator can be important : the layer must be thin enough to fit into the space between the stator and the housing and to allow the motor to cool sufficiently during operation, yet thick enough to provide a significant degree of vibration damping. The layer thickness can be especially important for vibration damping materials which are inefficient in heat transfer. In this configuration, the need for heat dissipation must be balanced against the degree of noise suppression. The applicant has found that forming the stator damping insert 30 with a thickness in the range of 4 mils to 100 mils (.004 inches to .10 inches) allows for sufficient heat dissipation yet provides a significant level of noise suppression. However, other thicknesses may be used. The 4 mil to 100 mil thickness range is dependent on various factors, including: the type of material used for the stator damping insert; the size of the motor; the amount of heat produced by the motor during operation; and manufacturing considerations. In general, if the vibration damping material used also has the ability to efficiently conduct heat away from the stator, then a thicker layer can be used. Referring again to the embodiment shown in FIG. 6, the cross-sectional shape of the stator damping insert 30 resembles a railroad spike. However, the shape of the preferred embodiment of the entire stator damping insert 30 is similar to that of a cylinder with a tapered rim, a thicker wall in the bottom and a circular hole cut in the bottom. The tapered rim allows for the stator 20 to slide more easily into the stator damping insert 30 to permit an interference fit between the stator 20 and the housing 16. In the preferred embodiment of stator damping insert 30 for inside motors, the bottom wall is thickened to form a "shoulder" against which the end of stator 20 is pressed after stator 20 is inserted into stator damping insert 30. In FIG. 6, this shoulder is adjacent to the arrow indicating the position of stator 20. Element 32 of FIG. 6 is a flange damping insert formed of vibration damping material, the dimensions of which may be adjusted in accordance with the dimensions of the flange 14. The flange damping insert 32 is placed between the motor housing 16 and front flange 14 or end bell. This vibration damping insert blocks the axial vibration reaching the end bell and provides vibration damping for the flat end cap. Another useful vibration damping insert of the preferred embodiment, disposed between the rotor shaft 12 and the rotor magnet 24, is shown in FIG. 7. Magnet damping insert 36 can be attached to the shaft 12 by an interference fit or with epoxy. When epoxy is used, the magnet 24 slides on to the shaft 12 and epoxy and vibration damping material 36 are squeezed in between the magnet 24 and the shaft 12. FIG. 8 illustrates another embodiment of the present invention in which vibration damping material has been added to a rotating-shaft outside motor, as follows: bearing damping inserts 52 have been placed between bearings 50 and support structure 51; stator damping insert 48 has been placed between stator 46 and support structure 51; magnet damping insert 45 has been placed between rotor magnet 44 and rotor assembly 40; and housing damping insert 39 has been disposed between housing 38 and support structure 51. FIG. 9 illustrates yet another embodiment of the present invention in which vibration damping material has been added to a fixed- shaft outside motor, as follows: bearing damping inserts 66 have been placed between bearings 64 and fixed shaft 62; stator damping insert 70 has been placed between stator 68 and fixed shaft 62; and magnet damping insert 60 has been placed between rotor magnet 58 and rotor 56. Note that bearing damping inserts 66 and stator damping insert 70 may be formed as one piece. In addition, vibration damping material may be disposed between fixed shaft 62 and cover 54, and/or between fixed shaft 62 and base 72. This vibration damping material could be connected to bearing damping inserts 66 and/or stator damping insert 70, and may be formed as one piece which extends the length of shaft 62. Additional vibration damping material may be situated between bearings 64 and cover 54 and/or base 72. As a part of the best mode of practicing the invention, the factors which cause noise and/or vibration in the electromotive device should be reduced to a minimum: 1) the rotor balance should be improved, if possible; 2) the air gap distance should be held constant; 3) the windings should be impregnated; and 4) the best bearings and grease lubricants available should be used. After these vibration sources are minimized, the next step is to block the transmission of the vibration through various rigid barrier transitions of the electromotive device, using one or more of the following steps: A) The bearings should be supported on vibration damping material, blocking the vibration from leaving the bearings and entering the housing or support structure; B) The stator should be supported on a layer of vibration damping material between the stator and the housing or support structure, blocking the vibration from the stator magnetostriction and rotor pull effects from being transmitted to the housing or support structure; C) Vibration damping material should be placed between the rotor and the rotor magnet ; and D) Vibration damping material should be placed between the housing and front flange/end bell or support structure of rotating-shaft devices, or between the shaft and base and/or cover of fixed-shaft devices. Conventional inside motors of the type shown in FIGS. 1 and 2 had noise levels, when tested by a decibel ("dB") meter placed 6 inches from the motor shaft 12, of 60-70 dB in an ambient noise environment of 40 dB . The same motor stator and rotor, when assembled in accordance with steps A, B and D, above (FIG. 6) , had motor noise levels of 48-50 dB in an ambient noise environment of 40 dB . The noise level of a conventional motor, over ambient, was 20-30 dB . The noise level of the embodiment of the present invention shown at FIG. 6 was 8-10 dB over ambient, a ratio of about 3:1. The absolute improvement of 12-20 dB represents a reduction of noise energy to between 1/16 and 1/64 of the conventional design. The machine noise reduction because of this change is equally significant: the resulting noise level was from 1/16 to 1/64 of the original noise level. While the foregoing is a complete description of the preferred embodiments of the invention, various alternatives and modifications may be used. Although the foregoing specification has described embodiments of this invention in various types of electric motors, the invention is equally applicable to other electromotive devices, such as generators, tachometers and alternators. Therefore, the preferred embodiments should not be interpreted to limit the scope of the present invention, which is defined by the following claims.

Claims: WHAT IS CLAIMED IS: 1. An electromotive device, comprising: a housing; a flange attached to the housing; a rotor including a rotor shaft and a magnet, the rotor shaft being supported for rotation at a first end by the housing and at a second end by the flange; a stator disposed between the housing and the rotor, the stator having a length and a circumference; and a stator damping insert disposed between the housing and the stator, extending substantially the length of the stator and enclosing substantially the entire circumference of the stator. 2. The electromotive device of claim 1, further comprising: a first bearing disposed between the rotor shaft and the housing; and at least one bearing damping insert disposed between the first bearing and the housing. 3. The electromotive device of claim 1, further comprising : a second bearing disposed between the rotor shaft and the flange; and at least one bearing damping insert disposed between the second bearing and the flange. 4. The electromotive device of claim 1, further comprising at least one flange damping insert disposed between the flange and the housing. 5. The electromotive device of claim 1, further comprising at least one vibration damping insert disposed between, and in contact with, both the rotor shaft and the magnet . 6. The electromotive device of claim 1, wherein the stator damping insert is formed in the shape of a cylinder with an open top end, a bottom end and side walls, wherein the side walls have a side wall thickness which decreases in the direction of the open end. 7. The electromotive device of claim 6, wherein the bottom end of the stator damping insert is formed in the shape of a shoulder against which an end of the stator may be placed. 8. An electromotive device, comprising: a housing; a flange attached to the housing; a rotor including a rotor shaft and a magnet, the rotor shaft being supported for rotation at a first end by the housing and at a second end by the flange; a first bearing disposed between the rotor shaft and the housing; a second bearing disposed between the rotor shaft and the flange; a stator disposed between the housing and the rotor, the stator having a length and a circumference ; at least one bearing damping insert disposed between the first bearing and the housing; at least one bearing damping insert disposed between the second bearing and the flange; at least one flange damping insert disposed between the flange and the housing; a stator damping insert disposed between the housing and the stator, extending substantially the length of the stator and enclosing substantially the entire circumference of the stator; and at least one vibration damping insert disposed between, and in contact with, both the rotor shaft and the magnet . 9. An electromotive device, comprising: a support structure; a housing; a first bearing supported by the support structure; a second bearing supported by the support structure; a rotor comprising a rotor shaft, a rotor assembly and a rotor magnet, the rotor shaft being supported for rotation by the first bearing at a first end and being supported for rotation by the second bearing at a second end; a stator supported by the support structure and disposed between the attachment member and the rotor magnet, the stator having a length; and a stator damping insert disposed between the attachment member and the stator. 10. The electromotive device of claim 9, further comprising bearing damping inserts disposed between the support structure and the first bearing and between the support structure and the second bearing. 11. The electromotive device of claim 9, further comprising magnet damping inserts disposed between the rotor magnet and the rotor assembly. 12. The electromotive device of claim 9, further comprising a housing damping insert disposed between the housing and the support structure. 13. The electromotive device of claim 9, wherein the stator damping insert substantially prevents the stator from rotating relative to the support structure . 14. The electromotive device of claim 9, wherein the stator damping insert extends substantially the length of the stator. 15. An electromotive device, comprising: a base; a cover ; a fixed shaft supported by the base at a first end and supporting the cover at a second end; a first bearing supported by the fixed shaft; a second bearing supported by the fixed shaft; a rotor comprising a rotor assembly and a magnet, the rotor being supported for rotation at a first end by the first bearing and supported for rotation at a second end by the second bearing; a stator supported by the fixed shaft and disposed between the fixed shaft and the rotor magnet, the stator having a length; and a stator damping insert disposed between the fixed shaft and the stator. 16. The electromotive device of claim 39, further comprising bearing damping inserts disposed between the fixed shaft and the first bearing and between the fixed shaft and the second bearing. 17. The electromotive device of claim 39, further comprising magnet damping inserts disposed between the rotor magnet and the rotor assembly. 18. The electromotive device of claim 39, further comprising a shaft damping insert disposed between the fixed shaft and the base. 19. The electromotive device of claim 39, further comprising a shaft damping insert disposed between the fixed shaft and the cover. 20. The electromotive device of claim 39, wherein the stator damping insert substantially prevents the stator from rotating relative to the fixed shaft. 21. The electromotive device of claim 39, wherein the stator damping insert extends substantially the length of the stator.

Inventors: MORREALE ANTHONY PETER, MORREALE, ANTHONY, PETER
Applicants: BEI KIMCO MAGNETICS DIVISION, BEI, KIMCO MAGNETICS DIVISION
Application year: 1999
Main classification: H02K5
IPC codes: H02K1, H02K5, H02K11