DESC:FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See Section 10; rule 13]
"AN ARRANGEMENT FOR ALLOWING OPERATION OF DIRECT CURRENT (DC) MOTORS AT HIGH SPEEDS"
PORTESCAP INDIA PVT. LTD.
Unit No.2, SDF 1, SEEPZ - SEZ,
Andheri (E), Mumbai - 400096, India
The following specification particularly describes the invention and the manner in which it is to be performed
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FIELD OF INVENTION
The present invention generally relates to Direct Current (DC) motors. More specifically, the present invention relates to rotor design for DC motors.
BACKGROUND OF THE INVENTION 5
The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely 10 represents different approaches, which in and of themselves may also correspond to implementations of the claimed technology.
Referring now to Figures 1a and 1b illustrating exploded views of a conventional brushed Direct Current (DC) motor, arrangement of components present within the 15 brushed DC motor and limitations associated with such arrangement are described henceforth.
Within a permanent magnet brushed DC motor, a magnetic core of cylindrical form is present as a stator. The stator magnet 102 is glued or mechanically fitted into a 20 tubular housing i.e. outer tuber 104 and inner tube 116 by means of either over-molding the tubular housing and the stator magnet 102 or using a machined outer tubular structure providing base support to the stator magnet 102. The cylindrical tube-magnet assembly is then enclosed from the other end using a front plate 106.
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The tubular housing is chosen so as to provide a magnetic path to the flux lines, and hence closes the magnetic circuit. A rotor coil 108 of the brushed DC motor is a wound, multi-segment coil, generally comprising odd numbers of coil segments, with self-supporting winding pattern enveloping magnetic core of stator. A first end of the rotor coil 108 is supported by either a metallic disc or an over-molded plastic 5 body 110. Further, collectors 112 are electrically connected to open terminals of segments of the rotor coil 108, at the first end. A second end of the rotor coil 108 that is free is not supported, as a cylindrical cage inside the rotor houses the stator magnet 102. A rotor shaft 114 present within an inner tube 116 of the stator and passing through a longitudinal axis of the stator magnet 102 is supported at both 10 ends by bush or ball bearings 118. The rotor is made of individual loops of the coil with several turns and layers connected in series, parallel, or combination of series and parallel. Number of loops depend on design of a motor and can be present as any odd combination such as 3, 5, 7, 9, 13, 17, 21, and the like. The rotor is electrically connected with commutation segments by soldering, welding, or any 15 other electrical connection method.
Further, the brushed DC motor includes an end cap 120 having a plurality of brushes present in pairs, 180 electrical degrees apart, and equal to number of poles present in the stator. The number of poles present in the stator may be achieved using 20 permanent magnet in permanent magnet based brushed DC motors or coil windings in armature wound brushed DC motors. The brushes are generally made of copper, graphite, precious metal alloy, or a hybrid combination of said elements. Further,
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the brushes are designed in such a way that contact between static brushes and rotating commutation segment offers low friction and low electrical contact resistances. The brush-collector contact is designed to maximize the performance and life of the brushed DC motor; because dominant failure mode occurs when the life of either brush or the collector ends due to mechanical and electrical wear. 5
The second end of the rotor coil 108 that is free generally limits the maximum rotational speed of the rotor if the collector-brush design is suitable for high speed and the supported end of the rotor coil 108 is suitably designed. At operation beyond a certain speed or on prolonged operation at high speeds, the rotor coil 108 starts 10 flaring outwards, as illustrated in Figure 2a. While the brushed DC motor is run in such condition and at higher speeds than what is bearable based on the strength of the self-supported coil, the flared end touches the outer tube 104, as illustrated in Figure 2b. Some prior solutions tried in the past to resolve this issue include adding a wedge formation on the outer tube 104 close to the free end of the rotor coil 108. 15 However, with such designs, the maximum recommended speed of the rotor becomes limited to 10,000 – 15,000 RPM, depending on the motor design and few other electrical and mechanical parameters.
Therefore, existing design of brushed DC motors suffer from several drawbacks, 20 such as mechanical friction occurring within the sliding contacts, and the mechanical friction resulting in gradual wearing of the commutation segments and the brushes. In addition, electro-erosion occurs due to voltage sparks that take place
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while contacts are made and separation occurs between the commutation segments and the brushes. The electro-erosion, hence, is also a function of maximum rotor speed for brushed DC motors. Moreover, since the rotor has a coreless design and is supported from one end only, centrifugal force acting on free end of the rotor coil 108 tend to flare the free end of the rotor coil 108 outwards. Outward flaring of the 5 free end of the rotor coil 108 results in making contact with an enveloping magnetic tube i.e. outer tube 104, by covering available definite airgap. Such contact between the free end of the rotor coil 108 and the enveloping outer tube 104 increases friction that produces heat, and thereby resulting in mechanical failure of the brushed DC motor. This frictional contact eventually also removes the insulation layer from the 10 rotor coil 108, thereby, short-circuiting segments of the rotor coil 108.
Further, a limited airgap of only 100 to 200 microns is available between an inner diameter of the outer tube 104 and an outer diameter of the rotor coil 108. Presence of such limited airgap makes it even difficult to accommodate any solution to the 15 problem of flaring of the rotor coil 108. Thus, there remains a need of a mechanism to prevent flaring of rotor coils of coreless brushed DC motors when running at very high speeds, particularly 30,000 to 50,000 Rotations Per Minute (RPM).
OBJECTS OF THE INVENTION 20
A general objective of the invention is to prevent damage of components present in motors.
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Another objective of the invention is to prevent flaring of rotor coils in motors.
Yet another general objective of the invention is to prevent flaring of rotor coils of coreless brushed Direct Current (DC) motors.
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Still another objective of the invention is to prevent flaring of rotor coils of coreless brushed DC motors when running at very high speeds.
SUMMARY OF THE INVENTION
This summary is provided to introduce aspects related to an arrangement for 10 allowing operation of Direct Current (DC) motors at high speeds, and the aspects are further described below in the detailed description. This summary is not intended to identify essential features of the claimed subject matter nor is it intended for use in determining or limiting the scope of the claimed subject matter.
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In one embodiment, the retainer ring may be either press-fitted or slide fitted and glued or assembled in other possible ways, on an unsupported end of rotor coil of a brushed DC motor. The retainer ring may prevent outward flaring of the unsupported end of the rotor coil caused due to prolonged usage or performing operation at high speeds. The retainer ring may be made of a thin and non-magnetic 20 material. The retainer ring may be attached on an outer perimeter of the unsupported end of the rotor coil. The retainer may be attached using a suitable joining technique such as gluing or press-fitting. This design is not obvious in the sense that any
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metallic disc inside a coreless rotor would increase the eddy current losses, however, this design enables the motor to run at higher speed than the designated speed for a brushed DC miniature motor.
In another embodiment, the retainer ring may be hollow and open at one edge and 5 closed at another edge. The unsupported end of the rotor coil may slide into a hollow edge of the retainer ring until the rotor coil touches closed edge of the retainer ring. The rotor coil may slide into the retainer ring such that an inner component of the retainer ring may come into contact with the inner perimeter of the rotor coil and an outer component of the retainer ring may come into contact with the outer 10 perimeter of the rotor coil. Said surfaces of the rotor coil and the retainer ring may be glued or press-fitted or attached by any other methods to prevent displacement and rotation of the retainer ring.
Selection of the retainer ring’s material and thickness would depend upon factors 15 including but not limited to the speed at which the motor is running, amount of centrifugal force acting on the rotating body, ambient temperature, humidity, and air gap available in the space between outer tube and the rotor coil. This selection would also depend on the allowable range of eddy current due to the presence of metallic ring rotating in the magnetic field. 20
In another embodiment, the retainer ring supports the free end of the rotor coil against centrifugal force and hence, the overall length of the rotor coil can be
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increased which is currently limited due to the over-hanged rotor. This will enable increasing the length of the overall motor, and hence the performance in same diameter of the motor.
In one embodiment, a Direct Current (DC) motor may comprise a hollow 5 cylindrical rotor coil supported at a first end with a metallic disc or an over-molded plastic body. Further, a stator magnet may be housed inside the rotor coil. A rotor shaft (114) may pass through a longitudinal axis of the stator magnet and may be supported at both ends by bush or ball bearings. Collectors may be electrically connected to open terminals of segments present at the first end of the rotor coil. 10 An end cap may be present at the first end of the rotor coil and may have a plurality of brushes present in pairs and equal to number of poles present in a stator. A retainer ring may be joined over and/or within a second end of the rotor coil for providing mechanical support.
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The retainer ring may be made using non-magnetic metals, ferro-magnetic metals, non-metals, alloys, plastics, ceramics, and composite materials. In one implementation, the retainer ring may be open and hollow at one edge and closed at another edge for joining with both an inner perimeter and an outer perimeter of the rotor coil. In another implementation, the retainer ring may be present as a 20 helical spring type ring, manufactured through deep drawing of wires, and glued after being wound over the rotor coil. In alternate implementations, the retainer ring may be present as a hollow cylindrical ring, a helical spring, a cylindrical retainer
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ring having one or more radial holes, and a split-piece design retainer ring. Such retainer ring may be manufactured using a mechanical process including machining, stamping, metal injection molding, extrusion, casting, plastic or injection molding, and sintering. The retainer ring may be joined with the rotor coil by gluing, spraying, press-fitting, or mechanical locking. 5
In different implementations, an outer diameter of the retainer ring may range from 3mm to 100mm, and an inner diameter of the retainer ring may range from 2mm to 90mm. Further, a thickness of the retainer ring may range from 0.05mm to 2mm, and a length of the retainer ring may range from 0.5mm to 50mm. 10
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
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BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings constitute a part of the description and are used to provide a further understanding of the present invention.
Figures 1a and 1b illustrate exploded views of a conventional brushed Direct 20 Current (DC) motor, in accordance with prior art.
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Figures 2a and 2b illustrate exploded views of the conventional brushed DC motor having rotor coil with flared end, in accordance with prior art.
Figure 3a illustrates a perspective view of a retainer ring, in accordance with an embodiment of the present invention. 5
Figure 3b illustrates an exploded perspective view of a brushed DC motor adapted to be fitted with the retainer ring, in accordance with an embodiment of the present invention.
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Figure 4a illustrates a perspective view of a retainer ring, in accordance with another embodiment of the present invention.
Figure 4b illustrates an exploded perspective view of a brushed DC motor adapted to be fitted with the retainer ring, in accordance with another embodiment of the 15 present invention.
Figure 5 illustrates different shapes of retainer rings, in accordance with one or more embodiments of the present invention.
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DETAILED DESCRIPTION OF THE INVENTION
The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is
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not intended to represent the only embodiments in which the present invention may be practiced. Each embodiment described in this disclosure is provided merely as an example or illustration of the present invention, and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough 5 understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.
In miniature brushed Direct Current (DC) motors having coreless rotors, a major limitation is imposed on rotor speed due to electro-erosion, mechanical friction and 10 centrifugal force causing unsupported ends of rotor coils to flare outward. Mechanical friction and electro-erosion are physical phenomena where the friction and sparking between brush-collector configuration occurs at the time of commutation. However, there are many methods and processes available to ensure that friction and sparking are reduced to affect the collector wear significantly. 15 Thus, the limiting factor, that still remains, is the unsupported end of a rotor coil which is unable to sustain high speed. A retainer ring 300, as illustrated in Figure 3a, is used to retain the rotor coil from spreading out. Further details related to the retainer ring 300 are provided in successive sections of the description.
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The unsupported end of the rotor coil also limits total length of the rotor coil that can be suitably designed for a particular size of the motor. By adding the retainer ring 300, at the unsupported end, the total length of the rotor coil can be increased
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significantly, thereby increasing the overall motor length and power density in the same diameter of the motor.
Figure 3b illustrates an exploded view of a brushed DC motor. Exploded view of the brushed DC motor is provided to illustrate a rotor shaft 302, rotor coil 304, stator 5 inner tube 306, magnetic stator 308, stator outer tube 310, endcap 312, terminal 314, and a cover 316. The rotor coil 304 may be present as a hollow-cylindrical outer rotor winding. The winding may be composed of a plurality of coil segments forming loops. The loops may overlap each other in an imbricated manner.
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The endcap 312 may include a set of brush pairs depending on number of pole pairs formed by a stator magnetic circuit. Further, the brushed DC motor may comprise a commutation system including a set of collectors disposed around a cylindrical lamella and electrically connected to the winding terminations of the rotor coil 304. Connection between coil loops and collector segments are done with the help of 15 annular groves in a molding where end terminals of each loop are soldered, brazed, or welded with the collector segments. For higher torque application, plastic molding is replaced with a metallic plate welded in the shaft and connected or bonded to the rotor coil 304 by some means. Towards the collectors’ end, the rotor coil 304 is supported with help of a plastic molding or a metal disc welded on the 20 rotor shaft 302. Another end of the rotor coil 304 is generally overhanging and remains present in an airgap between the magnetic stator 308 and the stator outer
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tube 310 which completes a magnetic circuit. The airgap between the magnetic stator 308 and the stator outer tube 310 generally ranges from 100 to 200 microns.
In one embodiment, the retainer ring 300 may be slide-fitted on the free end of rotor coil 304. The retainer ring 300 may be made of a non-magnetic material. In one 5 case, the retainer ring 300 may be glued on an outer perimeter of unsupported end of the rotor coil 304 to prevent any displacement of the retainer ring 300. Other joining techniques such as press-fitting could also be used for joining the retainer ring 300 with the rotor coil 304. Length of the retainer ring 300 can be as small as 5% of total length of the rotor coil 304, to cover only an edge of the rotor coil 304. 10 This design can be varied in terms of the retainer ring 300 materials, length, and thickness based on the speed at which the rotor is operated. Upon integration of the retainer ring 300 at the free end of the rotor coil 304, the rotor could be utilized at speeds higher than self-supporting capacity of the rotor.
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In an alternate embodiment, a retainer ring 400 may be joined with both an inner perimeter and an outer perimeter of the rotor coil 304. In such case, the retainer ring 400 may be open and hollow at one edge and closed at another edge, as illustrated in Figure 4a showing a perspective view of the retainer ring 400. Further, Figure 4b illustrates an exploded perspective view of a brushed DC motor adapted to be fitted 20 with the retainer ring 400. The unsupported end of the rotor coil 304 may slide into a hollow edge of the retainer ring 400 until the rotor coil 304 touches closed edge of the retainer ring 400. The rotor coil 304 may slide into the retainer ring 400 such
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that an inner component of the retainer ring 400 may come into contact with the inner perimeter of the rotor coil 304 and an outer component of the retainer ring 400 may come into contact with the outer perimeter of the rotor coil 304. Said surfaces of the rotor coil 304 and the retainer ring 400 may be glued to prevent displacement of the retainer ring 400. 5
Apart from having a cylindrical shape like the retainer ring 300 or being open and hollow at one edge and closed at another edge for joining with both an inner perimeter and an outer perimeter of the rotor coil 304 like the retainer ring 400, retainer rings of other shapes could be used in different implementations. Figure 5 10 illustrates a few alternate shapes of retainer rings, such as a retainer ring 500 having a helical spring shape, a cylindrical retainer ring 502 having one or more radial holes 504, a split-piece design retainer ring 506, and a helical spring type ring 508.
In one embodiment, the retainer rings 300, 400, 500, 502, 506, and 508 may be 15 fabricated using non-magnetic metals or alloys, such as stainless steel, steel, brass, copper, and aluminum. In such case, the retainer rings 300, 400, 500, 502, 506, and 508 may be manufactured using mechanical processes including, but not limited to machining, metal injection molding, extrusion, and casting, and may be affixed onto the rotor coil 304 using glue, press-fitting, or mechanical locking. Further, outer 20 diameter of such retainer rings 300, 400, 500, 502, 506, and 508 may range from 3mm to 100mm, inner diameter may range from 2mm to 90mm, thickness may range from 0.05mm to 2mm, and length may range from 0.5mm to 50mm.
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In another embodiment, the retainer rings 300, 400, 500, 502, 506, and 508 may be fabricated using non-metals, such as plastic, ceramic, and composites. In such case, the retainer rings 300, 400, 500, 502, 506, and 508 may be manufactured using mechanical processes including, but not limited to machining, plastic or injection 5 molding, and sintering, and may be affixed onto the rotor coil 304 through spraying. Further, outer diameter of such retainer rings 300, 400, 500, 502, 506, and 508 may range from 3mm to 100mm, inner diameter may range from 2mm to 90mm, thickness may range from 0.05mm to 2mm, and length may range from 0.5mm to 50mm. 10
In yet another embodiment, the retainer rings 300, 400, 500, 502, and 506 may be fabricated using laminated or non-laminated magnetic materials, such as silicon iron, and cobalt iron. In such case, the retainer rings 300, 400, 500, 502, and 506 may be manufactured using mechanical processes including, but not limited to 15 machining and stamping, and may be affixed onto the rotor coil 304 using glue. Further, outer diameter of such retainer rings 300, 400, 500, 502, and 506 may range from 3mm to 100mm, inner diameter may range from 2mm to 90mm, thickness may range from 0.05mm to 2mm, and length may range from 0.5mm to 50mm.
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In yet another embodiment, the retainer ring 508 may be fabricated using non-magnetic metals and alloys, such as stainless steel, steel, brass, copper, and aluminum. In such case, the retainer ring 508 may be manufactured using a
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mechanical process including, but not limited to deep drawing of wires having thickness ranging from 0.05mm to 2mm. Further, outer diameter of the retainer ring 508 may range from 3mm to 100mm, inner diameter may range from 2mm to 90mm, thickness may range from 0.05mm to 2mm, and length may range from 0.5mm to 50mm. The retainer ring 508 may be cylindrical wound or helical spring 5 type wound with a wire of circular, square, or rectangular shape. The retainer ring 508 may be may be wound over the rotor coil 304 and may then be glued.
It must be understood that materials, lengths, and thickness of the retainer ring 300, 400, 500, 502, and 506 could be varied based on the environment in which the 10 retainer ring 400 is required to operate, such as for different pressures, humidity, and temperatures. Factors such as eddy current losses, air gap between magnet-coil and coil-tube, and centrifugal forces acting on the rotor coil 304 should also be considered to design the retainer ring 300, 400, 500, 502, and 506.
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The above described retainer ring could be easily integrated in existing DC motor design without making any major design changes. With the implementation of the retainer ring, speed of the rotor can be safely increased to about 20,000-50,000 RPM for usages where low-cost DC solution is needed.
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In view of the above provided embodiments and their explanations, it is evident that the retainer ring and its method of installation, as described in the present disclosure, prevents flaring of unsupported ends of rotor coils of DC motors. Instead
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of the conventional approach, currently proposed retainer ring does not limit speed of operation of the DC motors. The proposed retainer ring could also be applied to other motor technologies and can be used into motor of any size, specifically into miniature motors where reliability is important. ,CLAIMS:We Claim:
1. A Direct Current (DC) motor comprising:
a hollow cylindrical rotor coil (108) supported at a first end with a metallic disc or an over-molded plastic body (110);
a stator magnet (102) housed inside the rotor coil (108); 5
a rotor shaft (114) passing through a longitudinal axis of the stator magnet (102) and supported at both ends by bush or ball bearings (118);
collectors (112) electrically connected to open terminals of segments present at the first end of the rotor coil (108);
an end cap (120), present at the first end of the rotor coil (108), having a 10 plurality of brushes present in pairs and equal to number of poles present in a stator; and
a retainer ring (300, 400, 500, 502, 506, and 508) joined over and/or within a second end of the rotor coil (108) for providing mechanical support.
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2. The Direct Current (DC) motor as claimed in claim 1, wherein the retainer ring (300, 400, 500, 502, 506, and 508) is made using one of non-magnetic metals, ferro-magnetic metals, non-metals, alloys, plastics, ceramics, and composite materials.
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3. The Direct Current (DC) motor as claimed in claim 1, wherein the retainer ring (400) is open and hollow at one edge and closed at another edge for joining with both an inner perimeter and an outer perimeter of the rotor coil (304).
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4. The Direct Current (DC) motor as claimed in claim 1, wherein the retainer ring (508) is present as a helical spring type ring, manufactured through deep drawing of wires, and glued after being wound over the rotor coil (304).
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5. The Direct Current (DC) motor as claimed in claim 1, wherein the retainer ring (300, 500, 502, and 506) is present as one of a hollow cylindrical ring, a helical spring, a cylindrical retainer ring having one or more radial holes (504), and a split-piece design retainer ring.
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6. The Direct Current (DC) motor as claimed in claim 1, wherein the retainer ring (300, 400, 500, 502, and 506) is manufactured using a mechanical process including machining, stamping, metal injection molding, extrusion, casting, plastic or injection molding, and sintering.
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7. The Direct Current (DC) motor as claimed in claim 1, wherein the retainer ring (300, 400, 500, 502, and 506) is joined with the rotor coil (108) by gluing, spraying, press-fitting, or mechanical locking.
8. The Direct Current (DC) motor as claimed in claim 1, wherein an outer 20 diameter of the retainer ring (300, 400, 500, 502, 506, and 508) ranges from 3mm to 100mm, and an inner diameter of the retainer ring (300, 400, 500, 502, 506, and 508) ranges from 2mm to 90mm.
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9. The Direct Current (DC) motor as claimed in claim 1, wherein a thickness of the retainer ring (300, 400, 500, 502, 506, and 508) ranges from 0.05mm to 2mm, and a length of the retainer ring (300, 400, 500, 502, 506, and 508) ranges from 0.5mm to 50mm. 5
Dated this 17th day of September, 2020
[JAYANTA PAL]
IN/PA 172
Of REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]