Silencer for an internally-ventilated electric motor
BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference may be made to the following description of an exemplary embodiment, taken in conjunction with the figures of the accompanying drawings, in which: 
     FIG. 1 is a plan view of two silencers according to the present invention mounted on the drive motors for a truck of a transit car; 
     FIG. 2 is an end view of one of the silencers and one of the motors of FIG. 1, taken along line 2-2 of FIG. 1; 
     FIG. 3 is a side sectional view of the silencer and electric motor of FIG. 2, taken along line 3-3 of FIG. 1; 
     FIG. 4 is a graphical representation of the sound power levels generated by the motor of FIGS. 2 and 3 at various operating speeds; and 
     FIG. 5 is a graphical representation of the noise spectra generated by the motor of FIGS. 2 and 3. 
    
    
     DESCRIPTION OF AN EMBODIMENT 
     FIG. 1 of the drawings illustrates, in plan, a truck 10 for a rapid transit vehicle such as a subway car. The truck 10 includes two laterally spaced apart and parallel side frames 12 and 14 joined together by a bolster 16 that extends between the two side frames intermediate their ends. In the illustrated embodiment, the side frames and bolster are formed in one piece of cast steel, but in other constructions the side frames and bolster may be three separate members, for example. Two axles 18 and 20 also extend across the truck 10 between the side frames 12 and 14, one adjacent each end of the truck. Mounted at each end of each axle 18 and 20 and outside the adjacent side frame 12 or 14 is a wheel 22. Between the bolster 16 and each of the axles 18 and 20 is an electric traction motor 24 or 26 and a gear box 28 or 30. Since the motors 24 and 26 are identical in structure and function, as are the gear boxes 28 and 30, the motors and gear boxes will be described with particular reference to the motor 26 and the gear box 30. 
     The motor 26 has a generally cylindrical motor housing or frame 32 (see FIGS. 2 and 3) and is mounted to extend transversely of the truck 10 and parallel to the bolster 16 and the axle 20. The associated gear box 30 is oriented with its major axis extending parallel to the side frames 12 and 14 and transverse to the longitudinal axis of the motor 26. Adjacent one end, the gear box 30 is connected to the motor 26. Adjacent its other end, the gear box 30 is connected to and supported by the axle 20. The end of the gear box 30 attached to the motor 26 and the adjacent end of the motor are secured to the bolster 16 of the truck 10 by a mounting mechanism 34. The opposite end of the motor 26 is coupled to the truck 10 by mounting mechanisms 36 and 38. The three mounting mechanisms 34, 36, and 38 are generally similar in structure and function and are best shown in FIG. 2. Refering to mechanism 38 as typical of all three, the mechanism includes as elongated metal bar 40 threaded at both ends. The metal bar 40 extends through aligned openings formed in two parallel and spaced apart lugs 42 and 44. The lug 42 is secured to the bolster 16 of the truck 10, while the lug 44 is integral with the frame 32 of the motor 26. On each side of the lug 42, an annular elastomeric mounting element 46 encircles the rod 40. Similar annular elastomeric mounting elements 48 encircle the rod 40 on either side of the lug 44. Flanges 50 are formed on the rod 40 between the lugs 42 and 44 to hold adjacent mounting elements 46 and 48 against movement along the rod 40. On the other side of each of the lugs 42 and 44, a washer 52 encircles a threaded end of the rod 40 and a nut 54 is screwed onto the threaded end of the rod. The nuts 54 and washers 52 hold the entire mounting mechanism 38 in place. The threaded rod 40 of the mounting mechanism 38 is horizontally aligned, while the corresponding rods of mounting mechanisms 34 and 36 are vertically aligned. The lugs of mechanisms 34 and 36 which correspond to the lug 42 are secured to the bolster 16 and to the side frame 14 of the truck 10, respectively. 
     FIG. 3 of the drawings shows the upper half of the motor 26 in section and on an enlarged scale. As previously indicated, the motor 26 includes a generally cylindrical motor frame or housing 32 that encloses most of the other components of the motor. The frame 32 includes a tubular main body portion 56 and a flange 58 that is formed in one piece with one end of the body portion. The flange 58 is annular in shape and extends radially inwardly from the end of the body portion 56 of the housing 32 toward a shaft 60 that extends lengthwise through the motor 26. Interposed between the radially inner circumference of the flange 58 and the outer circumference of the adjacent end of the shaft 60 is a ball bearing assembly 62. The outer race 64 of the bearing 62 is disposed adjacent to the interior circumferential surface of the flange 58 and is held against movement axially or longitudinally of the frame 32. In a similar manner, the radially inner race 66 of the ball bearing 62 is mounted on the shaft 60 so as not to move axially of the shaft 60. The bearing assembly 62 permits the shaft 60 to rotate freely relative to the flange 58 of the motor housing 32. 
     Adjacent the end of the main body portion 56 of the motor frame 32 opposite the flange 58 is an annular flange plate 68. The flange plate 68 is spaced axially from the end of the main body portion 56 of the frame 32 and extends radially inwardly relative to the main body portion toward the shaft 60. The flange plate 68 is joined to and spaced from the main body portion 56 of the frame 32 about its periphery by a plurality of circumferentially spaced apart ribs 70. The ribs 70 are integral with and extend axially from the end of the main body portion 56 of the motor frame 32. Axially extending, threaded bores are formed in the ribs 70 to receive the threaded ends of lug bolts 72. The bolts 72 pass through openings circumferentially spaced apart about the outer periphery of the flange plate 68 and are screwed into the threaded bores to hold the flange plate in place relative to the main body portion 56 of the motor frame 32. Along its radially inner circumference, the flange plate 68 engages the radially outer race 74 of a roller bearing 76. The race 74 is held against axial or longitudinal movement relative to the flange plate 68. The radially inner race 78 of the bearing 76 is mounted and held axially in place on the shaft 60 adjacent the shaft end 80 opposite the end that mounts the ball bearing 62. The end 80 of the shaft 60 is splined and extends beyond the roller bearing 76 and the flange plate 68 to engage an appropriate member (not shown) in the gear box 30. 
     Inside the motor frame 32 immediately adjacent the flange plate 68, several radially extending fan blades 82 are mounted on the shaft 60 to rotate with the shaft. The radially outermost end 84 of each fan blade 82 lies adjacent the ribs 70 that join the flange plate 68 to the main body portion 56 of the frame 32. The width of the radially outermost end 84 of each blade 82 is about equal to the length of each rib 70. To enable air to reach the fan blades 82, a section of the main body portion 56 of the frame 32 adjacent the flange 58 is omitted to form an inlet opening 86 into the interior of the motor 26. An air-permeable filter 88 prevents contaminants from entering the motor 26 through the inlet 86. As can be seen in FIG. 3, the circumferential spacing between the ribs 70 provides a plurality of outlet openings 90 in the motor frame 32 to permit air to flow from the fan blades 82. 
     Also mounted on the shaft 60 and disposed between the fan blades 82 and the ball bearing 62 is the rotor 92 of the electric motor 26. Spaced closely to the rotor 92 and mounted on the interior circumferential surface of the main body portion 56 of the frame 32 is an annular stator 94. The stator 94 is secured to the frame 32 by lug bolts 96. the bolts 96 extend from the outside of the main body portion 56 of the frame 32 into threaded bores formed in the stator 94. A narrow annular gap 98 between the rotor 92 and the stator 94 provides a space through which air may flow to cool the interior of the motor 26. The electrical current to operate the motor 26 is conducted to the motor by electrical leads 99 that pass through the main body portion 56 of the frame 32. 
     To attenuate noise that is generated by the fan blades 82 and the related flow of air, as will be explained hereinafter, a silencer 100 partially encircles the motor frame 32 adjacent the end of the frame at which the fan blades are located. As best shown in FIG. 2, the silencer 100 includes two arcuate silencer halves 102 and 104 of similar construction. Looking to the silencer half 102 shown in FIG. 3, it can be seen that the primary component of the silencer half is an arcuate and elongated main acoustical panel member 106. The main panel member 106 extends from adjacent the flange plate 68 of the motor frame 32 to the edge of the inlet opening 86 formed in the motor frame adjacent its other end. The curvature of the main acoustical panel 106 is such as to be coaxial with the curvature of the main body portion 56 of the motor frame 32. The radii of curvature are sufficiently different, however, that a space occurs between the outer circumferential surface of the frame 32 and the inner circumferential surface of the acoustical panel 106. As best shown in FIG. 1, the silencer half 102 includes a removable subsection 107 that is releasably secured to the remainder of the silencer half. The subsection 107 permits the silencer half to be installed on and removed from the motor 26 without having to disconnect the electrical leads 99. 
     The main panel member 106 of the silencer half 102 comprises an arcuate, perforated sheet of galvanized steel 108 that is presented to the exterior circumferential surface of the main body portion 56 of the motor frame 32. Spaced from the perforated sheet 108 and parallel to it is a longer, solid sheet of steel 110. The sheet 110 may be galvanized steel or painted cold-rolled steel. A layer of mineral wool 112, which is permeable to air, is disposed between the sheets 108 and 110 to act as sound absorbing material. The sheets 108 and 110 are joined together at one end of the silencer half 102 by an annular flange 114 that extends radially of both sheets. The flange 114 is solid galvanized steel sheet and may be fabricated in one piece with the sheet 110. The flange 114 may also be a separate element, as shown in FIG. 3, which is welded or otherwise secured to both sheets 108 and 110. As best shown in FIG. 2, the longitudinal edges of the panel 106 are formed of turned down portions of the sheet 110. The turned down portions of the sheet 110 are also bent to form flanges to secure the silencer half 102 to the motor frame 32. Screws (not shown) pass through holes in the flanges formed by the sheet 110 and are screwed into threaded bores in the motor frame. 
     At the end of the silencer half 102 opposite the flange 114 is a similar flange 116. The flange 116, however, extends from the outer, impermeable steel sheet 110 to a point approximately even with the exterior circumferential surface of the flange plate 68 of the motor frame 32. Spaced from and parallel to the flange 116 is a flange 118 of perforated steel sheet. The perforated flange 118 extends from adjacent the exterior circumferential surface of the motor frame 32 radially outwardly until it meets the perforated sheet 108. The radial flanges 116 and 118 are joined by an axially extending annular metal strip 120 that is contiguous with the motor frame 32. The space between the perforated flange 118 and the solid flange 116 is filled with sound absorbing material 122 so as to form a radially extending acoustical panel. The result is a barrier-like element that completely blocks off the space between the main panel member 106 of the silencer half 102 and the motor frame 32 at one end. Thus, if any air flows out of the outlet openings 90 in the motor frame 32, the air must flow along the length of the motor frame between the silencer half 102 and the exterior circumferential surface of the motor frame toward the inlet opening 86. To prevent heated air from the silencer 100 from being immediately drawn back into the inlet opening 86, an annular baffle 123 is welded to and extends radially outwardly from the motor frame 32 between the inlet opening and the adjacent end of the silencer. 
     In operation of the motor 26, a direct electric current is transmitted to the motor through the electrical leads 99. The direct current tends to cause the rotor 92 to move relative to the stator 94. The force acting on the rotor 92 drives the shaft 60, on which the rotor is mounted, to rotate in the frame 32 on the ball bearing 62 and the roller bearing 76. As the shaft 60 rotates, the fan blades 82 also rotate. The rotating fan blades 82 draw air in through the filter 88 and the inlet opening 86 in the motor frame, as generally indicated by arrows 124. The air from the inlet opening 86 flows through the interior of the motor frame 32 and between the rotor 92 and the stator 94 to the fan blades 82. During its flow through the interior of the motor frame 32, the air cools the interior of the motor 26. Air that reaches the fan blades 82 is exhausted in a radially outwardly direction through the outlet openings 90. As the fan blades 82 rotate to exhaust air from the motor frame 32, the blades move in close proximity past the ribs 70 of the motor frame. The air that is being exhausted by the fan blades 82 is compressed between the radially outward ends 84 of the blades 82 and the adjacent surfaces of the ribs 70. The compression of the air produces a fairly continuous and often high pitched whine that resembles a siren, for example. The whine produced by the motor 26 is audible almost continuously while the motor is in operation. 
     It has been found that the noise produced by the compression of air between the ribs 70 of the motor frame 32 and the radially outermost ends 84 of the fan blades 82 is characterized by a single dominant tone at each operating speed of the motor 26 and the fan blades. Th dominant pure tone characteristic is illustrated by the solid line or curve 126 in FIG. 5. The line or curve 126 represents the noise spectrum for a motor similar to the motor 26 when operating at a rotational speed of about 4,000 revolutions per minute. The curve 126 is characterized by a single major peak 128 which is located at about 830 hertz and which represents a noise level of about 105 decibels. The peak 128 represents a pure tone that dominates the noise spectrum. On either side of the peak 128, the noise level drops off rapidly and substantially. As is suggested by the dashed curve 130 in FIG. 5, a dominant pure tone such as that represented by the peak 128 of the curve 126 will occur at other frequencies, depending upon the rotational speed of the motor 26 and the fan blades 82. The curve 130 represents a projected noise spectrum for the same motor when operating at about 6,000 rpm. The dominant pure tone peak 132 of the curve 130 occurs at 1245 hertz and 125 decibels. The directly proportional relationship between the maximum sound power level generated by the motor 26 and the rotational speed of the motor is shown in FIG. 4. The maximum noise or sound power levels represented by the discrete points on the curve of FIG. 4 were measured at a distance of fifteen feet from the motor. As suggested by FIG. 5, the maximum sound power level at each operating speed occurs at the frequency of the dominant pure tone of the noise spectrum for the operating speed. 
     The noise characteristics of a direct current motor, such as the motor 26 shown in FIGS. 1 to 3, which are discussed in the preceding paragraph present a problem in terms of attempting to silence the motor. Rather than generating a relatively constant level of noise throughout all frequencies and all speed ranges, the motor 26 generates noise at power levels that vary with the rotational speed of the motor. For each different operating speed, there is also a different dominant frequency at which the sound power level is much greater than at any other frequency. One approach for attenuating the noise generated by the motor 26 might be to provide a device with a series of resonant cavities, each tuned to a different frequency. Such as solution would, of course, be too bulky to be feasible. Another approach, which has been tried and is used on the motor 26, is to space the fan blades 82 circumferentially in an irregular manner about the shaft 60 on which the blades are mounted. The irregular spacing technique does tend to spread the sound power over additional frequencies, as represented by the two &#34;knees&#34; 134 and 136 in the curve 126. Nonetheless, irregular spacing of the fan blades 82 does not provide an especially effective solution to the problem. 
     The silencer 100 of the present invention is intentionally designed to provide a resonant cavity between its perforated members, such as sheet 108, and its impermeable members, such as sheet 110. The resonant cavity or cavities so formed are all tuned, subject to space limitations, to attenuate sound at a single predetermined frequency. The frequency selected is the frequency of the dominant pure tone of the noise spectrum generated at the highest or maximum operating speed of the motor 26 under normal conditions. The tuning of the silencer 100, which involves selecting the spacing between the sheet 108, for example, and both the sheet 110 and the outer surface of the motor frame 32, is done in accordance with design criteria such as those set out in the article entitled &#34;Theoretical Analyses of Sound Attenuation in Acoustically Lined Flow Ducts Separated By Porous Splitters (Rectangular, Annular and Circular Ducts)&#34; prepared by S. H. Ko and published in the Journal of Sound and Vibration (1975) at pages 471 to 487 . Tuning the resonant cavity defined between the sheets 108 and 110, for example, will provide very effective attenuation of noise generated at the maximum operating speed of the motor 26. In order to attenuate noise at other operating speeds of the motor 26, the space between the sheet 108 and 110, for example, is filled with sound absorbing material, such as mineral wool 112, that is placed and packed in accordance with the general principles set out in the previously mentioned article in the Journal of Sound and Vibration. The effect of the porous sound absorbing material is to diminish the effectiveness of the silencer 100 at the frequency of the dominant pure tone of the highest operating speed at which the motor 26 will turn. On the other hand, the sound absorbing material will also afford noise attenuation at frequencies above and below the frequency to which the cavity of the silencer 100 is tuned, but to progressively decreasing degrees. As a result, the silencer 100 will be most effective at the frequency of the dominant pure tone of the highest operating speed of the motor 26 and will be effective to an extent that decreases with both increasing and decreasing frequency at not too steep a rate for frequencies above and below the frequency for which the silencer is tuned. The decrease in effectiveness of the silencer 100 with decreasing and increasing frequency, when filled with sound absorbing material, is more gradual than the decrease in effectiveness of the silencer&#39;s resonant cavity alone with decreasing and increasing frequency. Because both the maximum sound power levels generated by the motor 26 and the dominant frequencies of the maximum sound power levels decrease at a fairly rapid rate with decreasing operating speed, as shown in FIGS. 4 and 5, the functional characteristics of the silencer 100 result in a silencer that is effective over most of the operating speed range of the motor 26. 
     As explained above, the silencer 100 operates as a resonant cavity for the dominant pure tone of the highest operating speed of the motor 26, and otherwise acts as a dissipative silencer or absorber with respect to sound that travels along the outer surface of the motor frame 32 between the motor frame and the panel member 106. The effectiveness of the silencer 100 as a sound absorber is improved by having the air and airborne noise that is exhaused by the fan blades 82 flow toward the main acoustical panel member 106 adjacent the outlet openings 90 in the motor frame 32. At least part of the airborne noise will thus be carried through the perforated sheet 108 directly into the sound absorbing material 112. The sound absorption of the silencer 100 is also improved by the generally smooth outer circumferential surface of the main body portion 56 of the motor frame 32. The smoothness of the surface of the motor frame 32 permits the air and airborne noise exhausted by the fan blades 82 to travel naturally in a spiral about the motor frame between the outer circumferential surface of the frame and the main acoustical panel 106 of the silencer 100. As a result, the effective length of the silencer 100 is increased, with respect to airborne noise from the motor, as compared to a silencer through which air flows in a generally straight path. 
     The silencer 100 will, to some extent, restrict the flow of air from the motor 26. Nonetheless, the reduction in air flow is acceptable in view of the compact size and shape of the silencer 100, which minimizes the increase in the outer dimensions of the motor 26. The need to minimize the space occupied by the silencer 100 is dictated by the limited clearances provided between the motor 26 and the adjacent portions of the transit car body, the transit car truck, and the road bed. Space limitations require the bottom portion of the lower silencer half 104 to be flattened, rather than arcuate. The resulting resonant cavity defined by the flattened portion of the silencer half 104 is not tuned to precisely the same frequency as the rest of the absorber. Nonetheless, the frequency to which the flattened portion of the lower silencer half 104 is tuned is as close as possible to the frequency of the dominant pure tone at the highest operating speed of the motor. Space limitations, such as the need for axle clearance, also require the silencer to be in two halves 102 and 104. 
     It will be understood that the embodiment described above is merely exemplary and that persons skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. Such modifications may include variations in the thicknesses of the layers of sound absorbing material 112 and 122, the use of different type of sound absorbing material, the use of polyethylene bags to protect the sound absorbing material against contamination, and changes in the thicknesses of and the materials used in the various sheets of the silencer. All such modifications and variations are intended to be within the scope of the invention as defined in the appended claims.