CA1046568A - Switching apparatus for a machine responsive to a source of dc power - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A switching apparatus for a machine responsive to a source of DC power, the apparatus having first and second annular brush rings associated with a rotor and first and second brush devices associated with a stator for respectively coupling the first and second brush rings to opposite polarities of a DC source. A first roller contact is associated with the rotor and is electrically connected to the first brush ring for progressively and sequentially engaging commutator bars to thereby momentarily couple the commutator bars to the first brush ring. A
second roller contact is associated with the rotor and is electrically connected to the second brush ring for progressively and sequentially engaging the commutator bars to thereby momentarily couple the commutator bars to the second brush ring. The first and second roller contacts have contoured outer surfaces adapted to mate with the complementary contoured inner surfaces of the commutator bars.

Description

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This invention relates to a switching apparatus for a machine responsive to a source of DC power, and more particularly the type of machine having interacting rotor and s-tator assemblies which rotate relative to each other wherein the stator has a winding having a plurality oE
energizeable coils, the coils having an annular array of commutator bars associated therewith for supplyina electrical power thereto, and the rotor having a `
magnetic assembly associated therewith. ~ -This is a division of copending Canadian Patant Application Serial Number 234,456, filed August 29, 1975.
According to the apparatus of the present invention, there is provided first and second annular brush rings associated with the rotor and first an~ second brush means associated with the stator for respectively coupling the first and second brush rings to opposite polarities of the DC source. First roller contact means is associated ;
with the rotor and is electrically connected to the first -brush ring for progressively and sequentially engaging the commutator bars to thereby momentarily couple the commutator bars to the first brush ring. Second roller contact means is assa~i~ted with the rotor and is electrically connected to the second brush ring for progressively and sequentially engaging the commutator bars thereby momentarily couple the -~
commutator bars to the second brush ring. The first and second roller contact means having contoured outer surfaces adapted to mate with the complementary contoured inner surfaces of the commutator bars.
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BRIE~ DFSCRIPTION OF THE DR~WINGS
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Figure 1 is a sec-tional view of a DC machine of the "inside-~ut" type;
Figure 2 is a partially sectionalized detail .
view of the roller contact assembly of Figure l;
Figure 2a is a partial end view of the roller contac-t assembly of Figure 2; . :-Figure 3 is an end view of the rotor permanent magnet assembly of Figure l;

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5~3 Figure 3a is a side view of the permanent magnet assembly of Figures 1 and 3 illustrating the manner of assembly;
Figure 4 is a partially sectionalized end view of a molded commu-tator array;
Figure 4a is a partial front view of the commutator bar array of Figure 4;
Figure 4b shows a detailed sectional view of one bar of the commutator bar array of Figure 4;
Figure 5 is a simplified diagrammatic view of a DC
machine of modular design;
Figure 6 is a simplified diagrammatic view of a DC
machine for use in high speed operation;
Figure 7 is a partially sectlonalized detail view of another roller contact assembly for use with the machine of the Figure l;
Figure 8a is a top view of a rotor lamination for use in the present invention, and ~ :
Figure 8b is a sectional view of a machined, cast rotor of the present invention employing the lamination of 20 Figure 8a. `~
DETAILED DESCRIPTION
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Figure 1 shows an inside-out DC motor 10 which is ~:
comprised of hous~ng members 11 ~nd 12 which are eachprovided :~ :
with openings lla and 12a for receiving bearings 13 and 14 which surround a rotatably mounted rotor shaft 15. The inner ends of housing members 11 and 12 are hollow and are contoured ~: ~
or otherwise formed to receive and suppor-t the stator and :
rotor assemblies.
The stator assembly is comprised of a laminated core 16 formed of individual laminations 16a. The stator winding is comprised of a plurality of coils 17 (not shown in detail for purposes of simplicity) which, when energized, create cb/.

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magnetic fields in -the stator core which interact with the magnetic fields set up ;n the rotor assembly to effect rotor rotation.
Housin~ member 12 is further adaptecl to receive the commutator assernb]y 18 whieh includes a plurality of comrnu-tator bars 19 mounted in radial fashion (see Figures 4-4b) within annular-shaped molded insulating ma-terial 42, Selected ones of the commutator bars are electrically connected to the encl terminals of associated stator coils. As shown in Figure ~a, the commutator bars 19 are skewed at a small angle ~ so that a roller contact moving left to right with respect to Figure 4a will effect a make-before-break contact with the commutator `~
bars. In addition, skewing the commutator bars provides à
smooth rolling surfaee for the roller contact so that it w~ill mate smoothly with the inside surface of the commutator.~
As shown in Figure 1, the rotor assembly comprises .~
a permanent magnet arrary secured to shaft 15. The outer peri-phery of the permanent magnet array lies a small, spaced dis- -~
tanee from the interior periphery of the stator core 16 to form a hollow, annular-shaped air gap G therebetween Referring now to Figures 2-2a, the details of roller eontaet assemblies 22 will be explained. Unitary insulating sleeve 20 is mounted on rotor shaft 15 (see also Figure 1) - and has an annular reeess 20a for positioning floating roller platform 30 whieh is resiliently mounted to sleeve 20 by springs 31 and 31'. Support arms 30a and 30b respectively support roller shaft 32 and contact 33. Spring 3~ urges -roller 35 towards eontact 33. In the event of excessive wear, that portion of roller 35 bearing against contact 33 may be fitted with a plug of highl~ conductive, low resistance material.
The arms 30a and 30b are slidably supported by the slo-ts 20c provided in upright supports 20b arranged at spaced intervals about sleeve 20.

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Conductive roller 35 is provided with hollow elongat-ed passageways 35a parallel to the rotating axis and which extend to both sidewalls of the roller. The passageways communicate with radially aligned passageways 35b which open onto the cylindrical surface of the roller.
Fan blades 36 are molded into sleeve 20 and are interspersed between adjacent pairs of rollers.
Sleeve 20 supports continuous brush rings 23 and 24, each of which is electrically connected to selected ones of 10 the roller assemblies 22 by flexible conductors 37, 37a Brushes 25 and 26 are spring loaded and mounted upon the stator assembly. They engage brush rings 23 and 24 respectively.
The brushes are, in turn, connected to opposite polarities -~
of a DC power source ~not shown). While depicted in Figure 1 as disposed to one side of the roller contact assemblies 22, brush rings 23 and 24 may be positioned on opposite sides of the roller contact assemblies, if desired.
The rolling engagement between rollers 35 and the bars 19 of the commutator array rotates the rollers creating a centrifugal force which causes air to be drawn into the side openings of passageways 35a and urged out of radial passageways 35b. The rapidly moving air blows dust and/or other conductive particles away from the commutator array. ~;
Blades 35 serve the same function.
Rollers 35 of roller contact assemblies 22 progress-ively rollingly engage the commutator bars 19 while brushes 25 and 26 wipingly engage conductive rings 23 and 24, whereby the electrical path extends from one terminal of the DC source to brush 25, brush ring 23, flexible conductor 37, contac-t 33, conducting roller 35 and the commutator bars 19. The opposite polarity of the DC source is coupled to brush 26, brush ring 24, flexible conductor 37a, roller (spaced from roller 35 of .
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Figure 2) and commuta-tor bars 19. The end terminals of the stator winding are thus progressively energized and the mag-netic field generated thereby interacts with the magnetic field of the rotor permanent magnet structure to sustain rotation.
The commutator bar array of Figures 4-4b contributes ,!
to the blower action by providing gaps 40 between adjacent bars 19. Each commutator bar has a roller contact portion l9a and upright arms l9b and l9c. Arm l9b extends inwardly at l9d to secure commutator bar l9 to insulating material 42, Terminals 10 of the stator windings are connected to commutator bars l9 at ~ -l9e. The bars l9 are embedded in an insulating material 42 which extends partially into each gap 40 and engages one wall of each bar. For example, molded portion 40a engages one sidewall of bar l9' and is spaced from the adjacent sidewall of bar l9''.
Particles falling into gap 40' and collecting in the bottom-most portion are prevented from creating an electrical path between bars l9' and 13'' due to the presence of molded insulat-ing portion 40a. The blower action created by fan blades 38 and~or rollers 35 keeps gaps 40' clear of particles.
Figures 3 and 3a show the rotor permanent magnet structure in greater detail. The permanent magnet structure -;
comprises a plurality of laminated ironpole piece assemblies 44 each having individual pole pieces 45 ~see also Figure l) and each having an arcuate outer periphery and radially aligned s~des 44b and 44c. Each pole piece is provided with an opening 44e. The s~dewalls of laminated assemblies 44 are em~raced by solid rectangular-shaped permanent magnets 46.
- Rotor shaft 15 has a hexagonal-shaped cross section extending the length of the permanent magnet assembly and is preferably formed of a magnetic material such as, for example, soft iron. Elongated rectangular-shaped permanent magnets 47 are positioned in pole pieces 45 and an associated surface lSa cb/ - 6 -'' ,, ' ' ~ ~.
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oE rotor shaEt 15. The magnets 46 are preEerably rare-earth magnets whlch resist demagneti~ation, provide be-tter impedance matching and serve to increase flux density across -the air gap G ~see Figure 1). ~lagne-ts ~7 are preferably Alnico-8 magne-ts.
T~le magnet members 46, 47 and 46 embrace pole pieces 45 and serve to concentrate the flux density in the air gap G.
Figure 3a shows the manner of assembly of the rotor permanent magnet structure. End caps 49 and 50, rods 51 and fastening nuts 52 hold the permanent magnet assembly together.
Set screws 53 and 54 engage tapped openings in collar portions 49a and 50a of the end caps 49 and 50 to lock the assembly to shaft 15.
Figure 5 shows a DC machine 60 of modular design comprised of a hermetically sealed housing 61 having internally mounted bearings 62 and 63 for rotatably mounting shat 15.
The permanent magnet assembly 23, which is preferably of the type shown in Figures 3 and 3a, is mounted upon rotor shaft 15.
The stator assembly has a laminated core 16 comprised of individual laminations 16a. The stator winding is comprised of a plurality of coils 17 ~not shown in detail) which, when energized, create magnetic fields in the stator core which interact with the magnetic fields set up by the rotor permanent magnet assembly to effect rotor rotation.
The end terminalsl7a and 17b of the stator coils are led out of the hermetically sealed housing and terminate at a hermeti-cally sealed terminal assembly 64 molded into side face 61a of housing 61.
A second housing 65 has molded or otherwise provided along one side wall 55a a mating terminal assembly 66 which is releasably inserted into terminal assembly 64. Housing 65 is provided with bearing assemblies 67 and 68 for rotatably mounting shaft 69. Roller contact assemblies such as, for -cb/ ~ 7 -. . .. . .
' ~ ~4~613 example, 70 and 70' and brush rings 71 and 72 are mounted on insulating sleeve 73 which encircles shaft 69. S-tationary mounted brushes 74 and 75 are secured within housing 65 and respectively wipingly engzge rings 71 and 72. Opposite polarities of a DC source are electrically connected -to brushes 74 and 75 by conductors 76 and 77 which extend between brushes 74 and 75 and the e~terior of housing 65. Flexible conductors 78 and 79 electrically connect brush rings 74 and 75 to spring loaded con-tacts 80 and 81 which engage roller contacts 82 and 83 respect-ively.
Roller contacts 82 and 83 are preferably of the typeshown in Figures 2 and 3 and operate to sequentially rollingly engage stationary mounted commutator bars 84 mounted within housing 65 and which are preferably of the type shown in Figures 4-4b. The commutator bars are selectively coupled to the stator coils 17 through conductors 85j terminal assemblies 66 and 64 and conductors 17a-17b. A permanent magnet member 86 is secured to rotor shaft 15 and is positioned in housing 61 immediately adjacent side wall 61a. A second permanent magnet member 87 is secured to shaft 69 and is positioned in housing 65 immediately adjacent side wall 65a and ad~acent to member 86.
In operation, the DC source is progressively and sequentially coupled to coils of the stator-mounted hermeti-cally sealed housing 61 through leads 76-77, brushes 74-75, brush rings 71-72, conductors 78-79, contacts 80-81, conduct-ive rollers 82-83, commutator bars 84, leads 85-86, terminaI
assemblies 66 and 64 and conductors 17a-17b. The magnetic fields created by coils 17 interact with the fields of the permanent magnet structure 23 in air gap G to effect rotation.
The rotation of shaft 15 rotates magnetic member 86. The magnetic coupling between members 86 and 87 imparts rotation ~b/ ~ 8 -, . . . .. ~ ... ..... ... ...
, ' ,, ' to shaf-t 69 causiny the roller contac~ assemblies 70-70' and commutator bars 84 to progressively switch DC power to successive stator coils.
Hermetically sealed housing 61 keeps the rotor and stator assemblies, which are practically wear-free, safe from contamination by dust or dirt. Housing 65, ~owever, is design-ed to be accessible for servicing. Alternatively, housing 65 may comprise a replaceable unit. Whlle housing 61 is shown as ~`
containing the load 88 driven by rotor shaft 15, rotor shaft 15 may extend beyond the left-hand side ~all 61b of housing 61 and an appropriate seal may be provided to ~eep the housing -~
interior hermetically sealed. The magnetic member 86 may also be eliminated and lnstead rotor shaft 15 extended beyond the right-hand side wall 65a upon providing a similar seal. The magnet member 87 of shaft 69 may then be eliminated and replaced by keying means on the left-hand end of shaft 69 for locking shaft 69 to rotate with rotor shaft 15.
Figure 6 shows a DC motor 100 adapted ~or use in ~
high speed applications. It comprises a housing 101 which con- ~ `
tains bearings 62 and 63 for rotor sha~t 15 which has a perma-nent magnet assembly 23 mounted thereon. The stator comprises a laminated core 16 having lndi~idual laminations 16a. The stator coils 17 are electrically connected to selected commu-tator bars 84 by leads 17a-17b. A pair of brush rings 71 and 72 are ~ounted upon rotor shaft 15 and are respectively wipingly engaged by brushes 74 and 75. Leads 76 and 77 electrically connect brushes 74 and 75 to opposite polarities of a DC source.
P~oller support assemblles 102 and 103 support roller contact shafts 104 and 105 which rotatably mount conductive rollers 106 and 107. Spring mounted contacts 108 and 109 are secured to supports 102 and 103 and electrically connect brush rings 71 a~d 72 to rollers 106 and 107 by conductors ~b/ - 9 -`, , ' ` ' , ., ' ', .

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110 and 111. The bars 8~ of the co~nutator array are secured in stationary fashion and are selectively connected to end terminals of the stator coils 17 by leads 17a-17b. Conductive rollers 106 and 107 which may be of the type shown in detail in Figures 2 and 2a, rollingly engage the outer peripheries of commutator bars 84.
The diameter of rotor shaft 15 is made as small as practical under the commutator array while the outer diameter of conductive rollers 106 and 107 is made as large as practical.
This arrangement, in one preferred embodiment, reduces the angular velocity of the conductive rollers to less than one-half the angular velocity of rotor shaft 15, thereby providing a motor design which is advantageous for use in applications requiring high speed rotation. Since the distance traveled rollers 106 and 107 during one revolution of rotor shaft 15 is 2 ~f times the outer diameter of the commutator bar array, then for the angular velocity of rollers 106 and 107 to be less than the angular velocity of rotor shaft 15 the diameters of rollers 106 and 107 must be greater than the outer diameter of the commutator bar array.
It has been discovered in connection with the roller commutating systems of the type disclosed in my U, S. Patent No. 3,819,964 and in Figures 1, 2, 5 and 6 herein that if the longitudinal axis of the roller contact is not parallel to the longitudinal axis of the rotor then substantial forces are generated which tend to drive the roller along its longi-tudinal axis until slippage occurs be-tween the roller contact and the commutator bar array. Indeed, at higher rotational speeds when the centrifugal force on the roller is greater and the roller is pressed harder against the commutator bar array the forces tending to drive the roller along its axis are increased. To overcome this difficulty a novel roller ,~

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commutating syste~ has been devised whereby both the roller and the commu-tator bar array are contoured in complementary fashion so as to contain the travel of the roller while ensur-ing continued electrical contact therebetween.
This novel commutating system is illustrated in Figure 7 wherein the same numerals designate the same parts previously described in connection with Figures l, 2 and 4.
Attached to sleeve 20 is a flat, conducting spring 115 which may be formed from a beryllium-copper alloy. Flexible conductor 37 connects spring 115 with brush ring 23. Attached to spring 115, e.g., by soldering, is a conducting tube 116 which also may be formed from a beryllium-copper alloy. Roller contact 35 is fitted over conducting tube 116 and is free to rotate thereon.
The outer surface of roller 35 is curved and mates with the complementary curved inner surface of commutator bars 19. In this manner the roller is free to "rock" about its axis while maintaining contact with the commutator bars. Although not shown for purposes of simplicity, roller 35 has passageways for blowing dust and dirt from the commutator bars as shown -i~ Figures 2 and 2a and the commutator bars 19 have the confi-guration shown in Figures 4, ~a, and 4b. In addition, Figure 7 illustrates but one of several pairs of similar roller commutating assemblies which sequentially couple opposite polarities of the DC sourcs to pairs of commutator bars connect-ed to the ends of individual windings. ;
Turning now to Figure 8a there is shown full scalea rotor lamination 200 preferably of 1010-1020 steel and about 0.02 inches thick. Lamination 200 has a first set of equally .
spaced openings 201 adapted to receive pieces of rare earth magnet material. Lamination 200 also has a second set of equally spaced openings 202 adapted to receive non-ferrous casting material, preferably molten aluminum, during the ;;-~-~
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In manu~acturina the rare earth magnet rotor of the present invention a stack of laminations 200 of desired size, for example 4 inches, is assembled in a mold. ~ext, the pieces of rare earth magnet material are inserted into the slots 201 to build up a column the height of the stack of the laminations. The pieces of rare earth magnet material are typically 0.5 inches by 0.8 inches by 0.4 inches so that, in the present illustrative embodiment, 10 pieces of the rare earth magnet material would be required to fill each of the slots 201. The rare earth magnet material employed is prefer-ably a cobalt-rare earth intermetallic compound, the preparation of such compounds and magnets therefrom being disclosed in Benz U. S. Patent ~os. 3,655,~63, issued April 11, 1972, 3,655, 46~, issued April 11, 1972 3,695,945, issued October 3, 1972 and Benz et al U. S. Patent No. 3,684,593, issued ~ugust 15, 1972 and all granted to General Electric Company. The rare earth magnet material for use in the present invention must be "virgin", i.e., it must be processed so as not to contain any significant magnetic field. It is permissible, however, if the rare earth magnet material possess very weak residual magnetic fields due, for example, to the presence of the earth's magnetic field during formation of the virgin rare earth magnet material. If the rare earth magnet material is not virgin then the extxemely high coercive force of this material renders impractical developing sufficient flux densities to alter the magnetiZation thereof in situ, after casting.
~ather than assembling a complete stack of laminations followed by filling up the slots 201 with pieces of rare earth magnet ma-terial, the formation of the stack and insertion of c~/ - 12 -r~ ~

tne pleces of rare ear-th magnet ma-terial may proceed in-ter-mit-tently. The order is not critical and is merely a matter of choice.
~ fter tlle laminationsand pieces of rare earth magnet material have been assembled a shaft is preferably positioned at the center of the array, the longitudinal axis of the shaft being coincident with the longitudinal axis o the stack of laminations. The shaft may be of ferrous material such as steel.
The assembly comprising the laminations, the pieces of virgin rare earth magnet material and the rotor shaft is now ready for castiny using a non-ferrous material, preferably aluminum because of its light weight, low cost, high strength and high melting point. The molten aluminum flows into the center section of the stack of laminations and into the second set of slots 202, thereby rendering rigid the rotor structure.
It also locks the pieces of virgin rare earth magnet material into the slots 201 and has the effect of reducing any retained residual magnetism possessed by the rare earth magnet material.
Finally, it makes the rotor shaft an integral part of the rotor structure.
After casting, the rotor is machined so as to remo~e those portions of the laminations radially aligned with the slots 201, thereby forming a plurality of separate pole pieces equally spaced around the rotor. Figure 8b shows in cross section the machined, cast rotor~ The slots 201 are now filled with pieces of virgin rare earth magnet material 203 while the slots 202 are filled with aluminum, which also surrounds the rotor shaft 204.
The cast rotor with the virgin rare earth magnet ~;~
material is now ready for magnetization. This virgin rare earth magnet material can be saturated with between about 12,000 and about 18,~00 oersteds to develop a field strength of c~ 13 -,, .
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be-tween about 9,000 and about 12,000 gauss. These low values may be employed because the rare earth magnet material is in a virgin state. The particular values selected depend on the type of rare earth magnet material used. The fac-t tha-t the rare earth material is magneti~e~ in situ in the cast rotor places certain constraints on the design of the rotor structure.
For example, the rotor pole pieces should be formed fromlamina-tions, otherwise the eddy current losses during magnetization would be too great. Moreover, -there must be sufficient iron available to carry the flux so as to saturate the pieces of rare earth magnet material. Thus, the poles must contain more iron than is necessary to carry the flux out from the rare earth magnets into the stator during operation of the motor.
The amount of iron required in the poles also depends upon the geometry of the pieces of rare earth magnet material.
The longer the dimension of the rare earth magnet material in the radial direction, the more pole section area that is requir-ed. Accordingly, to ensure proper magnetization of the rare earth magnet material the rotor sections should have an area adequate to provide full saturation to the innermost radial por~ions of the rare earth magnet material. Thus, the rotor pole section area may be 1.5 times that necessary to carry the flux out from the rare earth magnets into the stator during motor operation~ Depending on the geometry of the rare earth magnet material, the area may be 2 to 3 times gréater, or more.
After magnetization the field strength of the rare earth permanent magnets may be adjusted or "trimmedl' by heating the rotor structure in the absence of significant amounts of ferrous material so that flux path will not be completed.

Thus, the rotor s-tructure is in an air-stabilized condition with as low a B/H as practical, e.g., 0.5. This is because cb/

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the per cent of irre~ersible magnetic loss in rare earth magnets with increasing temperature is a function of the B/H
slope and the lower the slope the sharper the fall-off of both B and H wi.th temperature. The rare earth magnets in the rotor structure can be magnetized to increase their magnetic field up to any portion oE the maximum flux by remagnetizing, pro-vided that the magnetizing fixture is positioned so that it energizes the poles in the same direction as during initial magnetization.
The invention disclosed and claimed herein is not limited to the specific mechanism and techniques herein shown and described since modifications will undoubtedly occur to those skilled in the art. Hence, departures may be made from the form of the instant invention without departing from the principles thereof.

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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Switching apparatus for a machine responsive to a source of DC power and having interacting rotor and stator assemblies which rotate relative to each other wherein said stator has a winding having a plurality of energizable coils, said coils having an annular array of commutator bars associat-ed therewith for supplying electrical power thereto, and said rotor has a magnetic assembly associated therewith, said appar-atus comprising:
a) first and second annular brush rings associated with said rotor;
b) first and second brush means associated with said stator for respectively coupling said first and second brush rings to opposite polarities of said DC source;
c) first roller contact means associated with said rotor and electrically connected to said first brush ring for progressively and sequentially engaging said commutator bars to thereby momentarily couple said commutator bars to said first brush ring; and d) second roller contact means associated with said rotor and electrically connected to said second brush ring for progressively and sequentially engaging said commutator bars to thereby momentarily couple said commutator bars to said second brush ring, said first and second roller contact means having contoured outer surfaces adapted to mate with the complementary contoured inner surfaces of said commutator bars.

2. Apparatus according to claim 1 wherein said first and second roller contact means each includes a convex roller mounted to rotate about its axis and to move radially with respect to said rotor so as to engage the concave commutator bars.

3. Apparatus according to claim 2 wherein said roller is provided with passageways communicating between a sidewall and the cylindrical surface thereof for blowing air upon said commutator bars.

4. Switching apparatus for a machine responsive to a source of DC power and having interacting rotor and stator assemblies which rotate relative to each other wherein said stator has a winding having a plurality of energizable coils, said coils having an annular array of commutator bars associat-ed therewith for supplying electrical power thereto, and said rotor has a plurality of permanent magnets, said apparatus comprising:
a) first and second annular brush rings associated with said rotor;
b) first and second brush means associated with said stator for respectively coupling said first and second brush rings to opposite polarities of said DC source;
c) first roller contact means associated with said rotor and electrically connected to said first brush ring for progressively and sequentially engaging said commutator bars to thereby momentarily couple said commutator bars to said first brush ring; and d) second roller contact means associated with said rotor and electrically connected to said second brush ring for progressively and sequentially engaging said commutator bars to thereby momentarily couple said commutator bars to said second brush ring, said first and second roller contact means each including a convex roller mounted to rotate about its axis and to move radially with respect to said rotor so as to engage the concave commutator bars, said roller having passageways commun-icating between a sidewall and the cylindrical surface thereof for flowing air upon said commutator bars.