Abstract: With respect to an armature for use in an electric motor of an engine starter, or the like, an annular steel-plate ring (11) is fitted on the outer circumference of commutator segments (2) through insulating paper (12) for the purpose of stably maintaining strength in the connection between each of the commutator segments (2) to which armature coils (6) are connected and a resinmolding member (3) fixed to an armature rotary shaft (5).

Abstract: A D.C. motor comprising an armature core, an armature coil, a commutator and an armature shaft is disclosed. The armature shaft is provided with a whirl-stop portion having a length sufficient to allow the armature core and the commutator to be fixed to it. The whirl-stop portion may be formed by rolling or knurling.

Abstract: A commutator 4 for a starter motor comprises a plurality of segments 4a electrically connected to armature coils 3, an insert member 4b fitted to an armature shaft 2, and a glass-fiber-reinforced phenol resin 4c molded to integrate the segments with the insert member, wherein mica particles in a range of 5%-10% by weight are uniformly contained in the resin.

Abstract: A small sized electric motor of a sectionally non-circular housing has a pair of magnets oppositely positioned therein in the lengthwise direction thereof and a rotor positioned between the magnets. A pair of fulcrums oppositely located on the housing in the length-wise direction thereof diametrically across the rotor, and a pair of elongated brushes, each has one end part secured to each of the fulcrums and the other end part elastically pressed against the commutator of the rotor at a point thereof to determine the electric displacement point with the angle 120.degree. displaced with respect to the magnetic line of force caused by the magnets. A winding is wound around the core structure of the rotor and has the initial ends and the wound ends each connected to a predetermined number of connector terminals of the commutator with the angle 120.degree. displaced with respect to the commutator.

Abstract: A rotary-type electromagnetic actuator for a toy includes a cylindrical magnetic stator fixedly mounted within a housing, a rotor arranged concentrically within the stator and rotatably supported by a shaft within the housing, a coil wound on the rotor for causing the rotor to rotate back and forth by being energized and deenergized, a commutator fixedly secured to the shaft of the rotor for supplying current to the coil, a rotor position-correcting member turnably provided inside the housing in opposition to the commutator and in concentric relation with respect to the housing, and a pair of current feeding brushes embracing the commutator from both sides thereof, each of the current brushes having one end secured to the position-correcting member and another end in pressing contact with a respective commutator bar of the commutator. The coil is energized by dc power supplied through the brushes and commutator.

Abstract: An assembled commutator for use in a fractional horsepower electric motor comprises a base moulded from thermoplastics and a commutator segment mounted on the base. The segment has a tang for electrical connection to an armature coil, the tang comprising a first radially extending portion and second portion connected to the first portion along a fold line.Wire of the coil is passed may be welded or forged to the tang by pressing together the first and second portions, and heating them electrically.

Abstract: A commutator for an electric machine with reciprocally insulated metal segments is described. At least one groove is made in the segments in the circumferential direction of the commutator and receives an insert formed from a precious metal or electrographite part. The axial width of the inserts only takes up part of the commutator contact surface. Alternatively two inserts are juxtaposed in each segment.

Abstract: A commutator for a rotary electric machine, and a method of manufacturing said commutator.The commutator comprises a ceramic support and metal segments, the support constituted by a plurality of parts (1, 2) which are mutually separated by respective axial gaps (8) and bonded to the ceramic by a copper oxide eutectic surface.The invention is applicable to electric machines including commutators.

Abstract: In order to insulate in a simple manner the basically bare rotor shaft region between the one rotor stack end face (21) and the oppositely located commutator end face (51) at its circumference, a sleeve (4) which can be upset (collapsed) axially, is fabricated as a unitized part and is formed preferably integrally at the collar (31) of the insulating end disc (3) covering the rotor stack end face (21), is put on before the commutator (5) is pressed on, and is axially collapsed when the commutator (5) is pressed on.

Abstract: A commutator is adapted for more consistent hot staking of the armature wire to the commutator hooks and easier armature winding with larger diameter wire by providing two commutator hooks on each of two diametrically opposed commutator bars and one hook on each of the others and spacing the commutator hooks circumferentially equally, so that there is a separate commutator hook for each armature wire needing commutator connection. The diametrically opposed commutator bars subtend the largest arcs to accommodate the two commutator hooks. The remainder of the commutator bars may subtend increasing arcs, progressing from one of the diametrically opposed bars to the other in opposite rotational directions, to spread the noise spectrum of brush noise during motor operation. Rotational orientation means for armature winding are provided in the form of indexing tabs projecting from the commutator bars having two hooks, in a direction axially opposite that of the hooks.

Abstract: A commutator for electric machines has a multiplicity of segments (2), which are provided with anchoring means (5) for securing in the insulating body (13), are joined at one end with associated winding ends (8) of a rotor, and are divided from one another in the circumferential direction by slots (10) which are filled with insulating material (15). The insulating material forming the insulating body (13) and filling the slots (10) as insulating lamellas (15) protrudes radially, in the form of widened supports (16), beyond the insulating lamellas (15). The supports (16) form the boundary in the circumferential direction for the terminal grooves (17) of the segments (2) for the rotor winding ends (8), which are welded to the segments (2).

Abstract: A shallow cup-shaped miniature motor having a motor case of a substantially shallow cup shape, fitted with a permanent magnet field; a motor case cover engaged with the open end face of the motor case; a rotor having a motor rotating shaft rotatably supported by a first bearing provided on the closed end face of the motor case and a second bearing provided on the motor case cover; brushes supported by the motor case cover for making sliding contact with commutator segments provided on the motor rotating shaft, in which an insulating core having wing portions substantially covering a laminated core comprising the rotor is provided on the outermost end face of the laminated core; the insulating core having at the center thereof a through hole through which the motor rotating shaft is passed, and a hollow cylindrical portion, provided around the through hole, having a plurality of notches for engaging with a plurality of the rising portions of the commutator segments; a commutator surface comprising the commutat

Abstract: A cylindrical outer surface of a commutator is formed with circumferencially extending striations separated by sharp-edged ridges which bite into the abutting surfaces of the brushes engaging the commutator to improve electrical conductivity and reduce static mechanical friction. These striations and the ridges may extend helically so that, on relative movement between the commutator and the brushes, the abutting surfaces of the brushes are scoured. There is also a tendency for the brushes to be moved axially of the commutator, against the restoring force provided by the brush leaf, until this restoring force rises sufficiently to cause the brush to return towards its original axial position so that the surfaces of the brushes and the commutator scrape against each other so as to present fresh surfaces for abutment.

Abstract: An improved commutator usable for an electric motor of the type adapted to be press fitted onto the support shaft of a rotor which is accommodated in the motor. The inner wall of the commutator comprises a plurality of larger diameter portions of which inner diameter is larger than the outer diameter of the support shaft and a plurality of press fitting portions of which inner diameter is smaller than the outer diameter of the support shaft. Press fitting of the commutator onto the support shaft is achieved by elastic bending deformation of the press fitting portions on the inner wall of the commutator in the outward direction. Typically, the inner wall of the commutator comprises three larger diameter portions and three press fitting portions. No preliminary machining is required on the inner wall of the commutator before press fitting is effected. The amount of press fitting allowance can be increased remarkably compared with any conventional commutator.

Abstract: A miniature motor having a plurality of commutator segments with commutator terminals which are connected to rotor windings. The commutator terminals are disposed on an insulating cylinder which is fixedly fitted to a motor rotating shaft, and a spark-quenching resistor connects each of the commutator segments. This resistor is a disc-shaped electrically conductive rubber ring which is fitted to the insulating cylinder so that the end face of the disc-shaped ring comes in contact with the commutator segment and commutator terminals and is brought into intimate contact with the commutator terminals by heating.

Abstract: The commutator of the motor is made up of three parts assembled coaxially and insulated from one another. Each part is connected to one of the windings of the rotor. The parts are blanked from a sheet of metal, preferably silver, with a flat annular zone, an inner lug, and radial fingers which are subsequently all bent at a 90.degree. angle at the same side of the annular zone. The parts are fitted into each other in such a way that the fingers of the different parts are seated around the circumference of the commutator.

Abstract: Brushgear for miniature motors comprising a pair of brushes each having brush shoes, made of electrically conduction metal strips, for making sliding contact with a miniature motor commutator, in which each brush shoe is bent into a V shape in cross section; ends of the feet of the V-shaped brush shoe being adapted to make sliding contact with the commutator, and the sliding positions of the brush shoes of the two brushes are disposed apart from each other by a predetermined distance in the axial direction of the commutator and/or in an inclined state in the axial direction of the motor rotating shaft at a predetermined angle with respect to the tangential direction of the commutator.

Abstract: A by-pass vacuum motor assembly wherein the motor cooling fan is positioned in an opening at the bottom of the motor band and immediately above a flat plate portion of the fan housing bracket. The flat plate portion functions as a plate diffuser, redirecting air drawn through the commutator end bracket and across the motor internals into a radial exhaust between a bottom portion of the band and a top portion of the fan end bracket. The invention also includes a brush holder clip passing through a vertical opening in the commutator end bracket and engaging a collar on the brush mechanism. The brush mechanism passes through a horizontal opening in the commutator end bracket which intersects with the vertical opening. Armature grounding is achieved by a disk maintained in a recess in the commutator end bracket and urged by a spring against the bearing receiving the motor shaft.

Abstract: A rotating collector for electrical machines including a carrier bushing onto which a rim of collector segments is attached. The carrier bushing has first, swallowtail-type attachment devices in the form of circumferential crosspieces having slits formed therein and ring grooves, into which second swallowtail-type attachment devices formed on each collector segment are inserted at an angle to the carrier bushing axis. Each inserted collector segment is then oriented in the axial direction to create an intermeshing arrangement to complete the rim.

Abstract: A non-continuous brush and commutator segment torquer motor having a single radial air gap. The motor can be placed on the opposite side of the gimbal center line of a payload. This arrangement minimizes torquer mass, as well as providing greater clearance for the gimbal payload to travel through larger angular excursions. Hence, all payloads which are gimbal mounted, such as radar antennas, can be located closer to the gimbal center and can be driven through larger look angles.

Abstract: A miniature motor having a rotor comprising a commutator having locating projections formed on the commutator reference surface coming in contact with a rotor core, and a rotor core having thereon locating key holes for engaging with the locating projections; the commutator being fitted to the rotor core by engaging the locating projections with the locating key holes until the commutator reference surface comes in contact with the rotor core; in which each of the locating projections has a temporary insertion portion at the tip thereof, whose cross-section is smaller than the cross-section of the locating key hole, and a full insertion portion for engaging with the locating key hole; the full insertion portion has such a length that the commutator tongue of the commutator is located at a position higher than the height of the rotor windings in a state where the temporary insertion portion is inserted into the locating key hole.

Abstract: A collector includes a carrier bushing, onto which a rim of collector segments is attached by means of swallowtail-type attachment devices. An intermediate space remaining between the carrier bushing and the collector segments is filled with a casting material. An interlocking swallowtail arrangement is used, which fixes the collector segments radially to the carrier bushing in such a way that they cannot come loose. The carrier bushing has a T-profile groove, into which the collector segments can be inserted and pushed onto the carrier bushing in the circumferential direction. After the collector segments have been installed, the T-profile groove is closed with a wedge, which also has a swallowtail structure to hold the single remaining collector segment.

Abstract: A miniature D.C. motor for a toy allows the arrangement of a brush base in a housing of the motor to be readily and positively positioned to improve the manufacturing efficiency of the motor and to ensure the actuation of the motor without trouble for a long period of time.The motor includes a housing, a brush base which holds brushes therein and which detachably fits in the housing, and a detachable connection member for permitting a connection between the housing and the brush base. A motor further includes a commutator having an outer end formed into a truncated conical shape, wherein the brushes each are provided with a contact section having a convexly curved surface, to allow the outer end of the commutator to be pressedly interposed between the convexly curved surfaces of the brushes.

Abstract: A collector for electric machines, including a rotationally symmetrical sintered ceramic body and a plurality of radially disposed metallic segments which are separated from each other by one interspace each and which are bonded to the ceramic body via a eutectic intermediate layer. The segments are bonded to the ceramic body in accordance with the eutectic method by being surface-oxidized on their inside narrow side and radially pressed against the ceramic body with the totality being brought to the melting temperature corresponding to the metal/metal-oxide eutectic and subsequently being cooled down again. A preferred embodiment includes: copper segments on an Al.sub.2 O.sub.3 ceramic body.

Abstract: A disk armature for an electric, in-tank fuel pump motor is manufactured by laser welding a pair of annular disks in two concentric circles of spot welds, one near each of the inner and outer circumferences of the disks. One disk is made of malleable copper for the forming of commutator hooks and studs; the other is made of hardened copper alumina for superior wear characteristics in a sour gasoline environment. The welded disks are affixed to an insulating support and cut into segments, each having at least one weld from the inner circle and two from the outer circle. Thus the segments are each securely welded without deformation or degredation of the superior wear properties of the copper alumina disk.

Abstract: A molded commutator has conductive segments circumferentially arranged about a central axis so that their outer surfaces form an overall structure of cylindrical shape as the outer surface of the commutator. Inside of this structure is an annular core to which each commutator segment is anchored by a tang formed on the inner surface of each segment, which is mechanically interlocked with a pair of tangs on the core. The tangs are shaped and configured such that, after assembly, each segment tang is interlocked between the core tangs in a dove-tail-like relationship. A nonconductive matrix is molded between the segments and the core. A method of making the commutator is to form an annular segment pack containing the conductive segments, with each segment having a segment tang formed on its inner surface, and an annular core having the core tang for one axial end of the commutator formed on its outer surface.

Abstract: A molded commutator is made up of segments arranged in a ring with their brush contact surfaces facing inwardly and forming a cylindrical shape. A matrix of plastic is molded between and around the outside of the segment ring in order to separate the segments electrically and to hold them in the ring configuration. A reinforcing metal casing serves to contain the commutator against breaking up due to centrifugal forces experienced during rotation. An alternate embodiment uses metal bands embedded within the matrix for this purpose.

Abstract: The commutator comprises three conductive sectors 2 which are maintained against a cylindrical insulating core 4 by an insulating ring 3. Cut out from the rear end portion of each sector is a terminal 11 which defines two tabs 12 projecting axially at the rear end of the sector. Each pair of tabs straddles a parallelepipedic projection 5 of the core 4. The three sectors are maintained radially and axially in position by the ring 3 which comprises an annular portion 15 from which extend three claws 16 in the longitudinal direction, the claws being clipped behind a flange 6 which projects from the core. In this way, it is possible to mould the core and the ring 3 from a thermosetting material and to improve the maintenance of the sectors 2.

Abstract: To separate cooling fluid, such as water, from the commutator space of a dynamo electric machine, typically a motor for battery-energized vehicle, a disc (18) of insulating material is fitted on the commutator (5), terminating at its end portion in a slinger structure (20, 21) which is positioned in intergaging relation with respect to a labyrinth (21, 22) secured to the housing of the machine. Cooling fluid is admitted to the space within which all the electrical windings (3, 9) are located, so that the cooling fluid, for example water, can fill the space (28) of the housing, while being separated from the space (57) within which the commutator (5) and commutator brush holder (7) is located.

Abstract: The external segments of a commutator for rotating electrical machinery are mechanically interlocked to a molded matrix by a plurality of tang assemblies having protrusions at their ends for engagement deep within the internal matrix, to hold the segments against centrifugal forces. The tang assemblies also have anchoring bars shaped for interlocking engagement with the longitudinal channels located on the inner surfaces of the segments.

Abstract: A commutator has a plurality of spaced, electrically conducting segments, each of which has a physical dimension determining the relative time duration of brush contact with said segment during commutator rotation with audible brush noise being generated between the brush and said consecutive segments, the noise including a fundamental frequency component at a frequency related to the frequency at which the consecutive segments pass the brush. The physical dimension of the consecutive segments varies from each segment to the next according to a predetermined pattern effective to vary the relative time of brush contact from each segment to the next during rotation and thus reduce the amplitude of the fundamental frequency components to make the brush noise less tonal in character and thus less annoying.

Abstract: A commutator for a motor and a method of making the same wherein the motor has a stator and a rotor with a rotary shaft and the commutator consists of a plurality of commutator segments embedded in an insulating body, with each of the commutator segments having contact surfaces for engagement with a brush. The commutator is formed with a number of hemispherical shaped impressions so as to form a rough surface.A method of manufacturing the commutator comprises embedding a plurality of commutator segments in a body made of insulating material and the step of forming hemispherical impressions in the commutator segments by directing a jet of particles onto the surfaces of the commutator segments with a predetermined pressure so as to provide the desired roughened surface.

Abstract: Commutator apparatus for electric motors includes electrical conductors making electrical contact with brushes through substantially three hundred sixty degrees of rotation of the commutator to provide current paths during time periods of current flow to prevent arcing. The commutator apparatus further includes cylindrical portions and blade portions which overlap and on which brushes are positioned to make the electrical contacts.

Abstract: The disclosed commutator electric machine comprises a field magnet structure electromagnetically connected with the armature having a winding; a commutator including main bars and at least one auxiliary bar for each main bar interconnected by an interbar current-conducting spacers, the electric contact between the interbar current-conducting spacers and the commutator bars taking place along the side surfaces of the bars and being ensured by compression effected in the process of the manufacture of the commutator structure and brushes of the commutator structure. The main commutator bars are also connected to the winding sections of said armature.

Abstract: The present invention relates to a commutator which is always exposed to gasoline in a motor to drive, for example, a fuel pump of an automobile or the like and the method of efficiently producing the commutator. A good conductive sheet, the wear of which is not promoted by oxidized gasoline, is fixed to the surface of each commutator segment made of copper or copper alloy at least at the part to contact a brush.

Abstract: A method for manufacturing commutator for use in rotary electric machines, in which a shaping material prepared from electrically conductive metal powder material or a mixture of the metal powder material and additive powder material such as carbon, molybdenum disulfide, etc. is placed in a shaping mold and subjected to compression-molding into annular or cylindrical shaped electrically conductive body, then an electrically insulative synthetic resin material is coated around, or filled in, the hollow cavity of the annular or hollow cylindrical body, and thereafter this annular or cylindrical body is machined to be divided into a plurality of segments for constituting the commutator.

Abstract: A commutator for electrical machines and a method of manufacture of the commutator is disclosed which consists of a lamination assembly held together by a pair of shrink-rings. One of the rings serves to support the commutator on a commutator hub and comprises first and second ring portions having between them a decoupling portion. The first ring portion is in the form of a shrink-ring and holds together the lamination assembly. The second ring portion is secured to the commutator hub. The other shrink-ring also holds together the lamination assembly. In the method of manufacture of the commutator, both the first and second ring portions are simultaneously shrunk on to the lamination assembly and commutator hub respectively.

Abstract: A segment of a commutator made by(a) placing a first layer of copper powder and on top thereof a second layer of a mixture of carbon powder and a binder into a die with a cavity shaped like the segment,(b) densifying the powder layers to produce a blank having the form of the segment, and(c) sintering the blank at an elevated temperature to cause the powder particles to cohere and the layers to firmly join together.

Abstract: A direct-current motor has a housing in which a stator and a rotor are mounted. A commutator is located in the housing but spaced from the stator and the rotor and subdivided from the same by a transverse housing wall. A magnetic coupling having two relatively rotatable elements is mounted in the housing extending through the transverse wall and the two elements define with one another an air gap in which a hood-like partition is located which is sealingly connected with the transverse wall so that the commutator is sealingly separated from the stator and rotor but is driven by one of the parts of the magnetic coupling which in turn is magnetically driven by the other part of the coupling that receives torque from the shaft of the motor.

Abstract: A dynamoelectric machine commutator structure having a plurality of conductive commutator segments secured in operating position by a commutator banding is characterized by including a flexible, thermally stable dielectric coating material between the outer edges of the commutator banding and the conductor segments in order to effectively seal the junction between those members, thereby preventing the deposit or buildup of electrically conductive contaminants at that junction so that the likelihood of electrically short circuiting the conductive segments of the commutator is effectively eliminated. According to the preferred method of the invention, a banded commutator is formed by applying such a coating to predetermined portions of the banding grooves so that the coating effectively seals the junctions between the banding and the conductive segments of the commutator, thereby preventing the accumulation of conductive contaminants at those junctions.

Abstract: An electrical commutator having a plurality of spaced apart commutator bars is disclosed. The commutator bars are formed from a composite of an electrically conductive copper portion bonded, for example by brazing or electron beam welding, at an interface to a steel body which is geometrically designed to reduce the bending and longitudinal stiffness of the body, whereby the shear stress at the interface is reduced during operation.

Abstract: A commutator including a support sleeve, a plurality of commutator segments equiangularly spaced around the support sleeve, said segments being electrically insulating from one another and each having a base region of dovetail form having its widest end presented to the sleeve. Integral with the sleeve adjacent one axial end thereof is a first retaining flange which defines a first internal frusto-conical surface coacting with one inclined face of the base region of each of said segments to locate the segments axially and raidally relative to the sleeve. The sleeve is deformed outwardly adjacent its axial end remote from said flange to resist movement of the segments axially of the sleeve away from said flange.

Abstract: A small electric motor having a plurality of commutator segments disposed at equal intervals in the angular positional direction on an insulating cylinder fixedly fitted to the rotating shaft of a motor rotor, and brushes for making electrical contact with the commutator segments, wherein reservoirs for holding an electrically conductive mixture are provided by boring the insulating cylinder in such a manner that the reservoirs extend to the adjoining commutator segments, the electrically conductive mixture being injected into the reservoirs so that the mixture is deposited in such a manner that the mixture bridges each commutator segments and covers the intervening portions between the commutator segments, the reservoirs being disposed in the locations at which the commutator segments and the brushes do not contact.

Abstract: A small electric motor having a plurality of commutator arcuate segments equally spaced in the direction of angular position on the rotating shaft of motor rotor, wherein conductive grease is coated on the commutator arcuate segments and the equally spaced gaps, and edges provided on the sliding portions of brushes are caused to press and contact the commutator by cutting the conductive grease film.

Abstract: A commutator including a support sleeve, a plurality of commutator segments equiangularly spaced around the support sleeve, said segments being electrically insulating from one another and each having a base region of dovetail form having its widest end presented to the sleeve. Integral with the sleeve adjacent one axial end thereof is a first retaining flange which defines a first internal frusto-conical surface coacting with one inclined face of the base region of each of said segments to locate the segments axially and radially relative to the sleeve. The sleeve is deformed outwardly adjacent its axial end remote from said flange to resist movement of the segments axially of the sleeve away from said flange.

Abstract: The present invention relates to a distributor for an electric D.C. machine comprising a commutator associated with two elastically deformable conductor rings, each forming a wing placed above the commutator on the path of at least one roller carried by a moving element, each roller establishing, by elastic deformation of the corresponding ring, a mobile electrical contact between said ring and the commutator. The invention is more particularly applied to rotary machines.

Abstract: An improved small electric motor for use as a power source for toys, small electric instruments, and portable electric instruments, and a method for manufacturing rotors for such motor. The small electric motor is provided on a rotary shaft with a commutator provided in parallel and feeder brushes in resilient contact with the commutator. The commutator is fitted to the rotary shaft in a manner providing simplicity of assembly, adjustable positioning of the commutator on the rotary shaft, standardization of a magnet case, and favorable performance in the small motor.

Abstract: An axial-air-gap type semiconductor motor has a disc-shaped rotor, a pair of disc-shaped armatures coaxially located adjacent the opposite faces of the rotor with air-gaps therebetween, a system for detecting the rotational position of the rotor for generating an output proportional to the rotating speed of the rotor, and a semiconductor circuit operable in response to the output from the position-detecting system for controlling the armature current to effect constant-speed control of the rotation of the rotor. The rotor includes a field magnet. Two groups of armature coils are mounted on the face of each of the armature discs facing the rotor. The position-detecting system includes a control band which includes a plurality of control sections and which is synchronously rotatable with the rotor, and detectors which are located at fixed positions along the control band for generating an output in response to the control sections of the control band.

Abstract: A rotor which utilizes winding guide discs to enable the slots of any rotor to be completely filled with wire without thereby increasing the diameter of the rotor by bulging out of the end windings. Winding guide discs having substantially the same cross section as the rotor are spaced a sufficient distance from the ends of the rotor to contain that portion of the end windings which will fill the space between the outside diameter of the rotor shaft and the minimum radial dimension of the slots in the rotor. After this space is filled, further end windings are wound from the rotor slots about corresponding apertures in the winding guide discs within the corresponding space on the disc. In the event that this corresponding space is filled further winding guide discs may be used as required to prevent the bulging of the end windings sought to be avoided.

Abstract: A commutator or slip-ring has a contact face formed of at least two dissimilar electrically conductive materials which have different values of electrical resistance. The two materials each occupy different discrete areas with a brush bearing on both areas simultaneously.According to a further feature a commutator comprises pairs of segments of alternately high resistance and low resistance mounted in a machine which is arranged to rotate so that brushes bear firstly on a low resistance segment and then on a high resistance segment.