Rotor assembly for DC motor

Abstract

A rotor assembly for a DC motor is provided which includes a rotor having a rotor shaft, a commutator mounted to the rotor shaft, and a varistor which is connected to the commutator via an electrically-conductive solderless fixing means, such as an electrically-conductive adhesive.

Claims

1. A rotor assembly for a DC motor, the rotor assembly comprising: a rotor having a rotor shaft; a commutator mounted to the rotor shaft; and a varistor connected to the commutator via an electrically-conductive solderless fixing means, wherein the electrically-conductive solderless fixing means comprises electrically-conductive adhesive; wherein the varistor is provided as a looped element sleeved on the commutator, and the commutator extends through the varistor; a support element mounted between the commutator and a rotor-facing surface of the varistor to support the varistor, the varistor is provided with a plurality of electrodes on an opposite surface to the rotor-facing surface of the varistor, the electrically-conductive adhesive forms a plurality of adhesive joints between the electrodes of the varistor and the commutator.

2. The rotor assembly as claimed in claim 1, wherein the electrically-conductive adhesive comprises a metallic conductive component.

3. The rotor assembly as claimed in claim 2, wherein the metallic conductive component comprises silver and/or nickel.

4. The rotor assembly as claimed in claim 1, wherein the electrically-conductive adhesive comprises a non-metallic conductive component.

5. The rotor assembly as claimed in claim 4, wherein the non-metallic conductive component comprises graphite, graphene, and/or carbon nanotubes.

6. The rotor assembly as claimed in claim 1, wherein the electrically-conductive solderless fixing means comprises a flowable fixing material.

7. The rotor assembly as claimed in claim 1, wherein the electrically-conductive solderless fixing means comprises a curable material.

8. The rotor assembly as claimed in claim 1, wherein the electrically-conductive solderless fixing means comprises a non-reversibly-applicable fixing means.

9. A DC motor comprising a stator, at least one brush, and a rotor assembly as claimed in claim 1.

Description

ADVANTAGEOUS EFFECTS OF INVENTION

BRIEF DESCRIPTION OF DRAWINGS

Description of Drawings

(1) The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a side representation of one embodiment of a rotor assembly in accordance with the first aspect of the invention;

(3) FIG. 2 shows an exploded perspective representation of the commutator and varistor of the rotor assembly of FIG. 1, inclusive of an optional heat sink element;

(4) FIG. 3 shows a perspective representation of the rotor assembly of FIG. 1;

(5) FIG. 4 shows a side representation of a second embodiment of a rotor assembly in accordance with the first aspect of the invention; and

(6) FIG. 5 shows a side representation of a third embodiment of a rotor assembly in accordance with the first aspect of the invention.

MODE FOR THE INVENTION

Mode for Invention

(7) Referring firstly to FIG. 1, there is illustrated a rotor assembly, indicated globally at 10, suitable for a DC electric motor, which would ordinarily comprise a stator, preferably a plurality of brushes, and the rotor assembly 10.

(8) The rotor assembly 10 includes a rotor 12, typically formed via a plurality of rotor coils 14 wound around an armature 16. The rotor 12 includes a rotor shaft 18, via which rotational motion can be output from the DC electric motor, which is preferably a brushed DC electric motor, to which is mounted or mountable a commutator 20.

(9) A varistor 22, preferably formed as a looped element such as an annular varistor which is receivable around the commutator 20 over the rotor shaft 18. The varistor 22 here has a plurality of electrodes 24 on one of the planar surfaces 26 of the varistor material 28, via which an electrical connection can be made to the commutator. FIG. 2 shows an indicative exploded view of the commutator 20 and varistor 22. Three electrodes 24 are shown in the depicted embodiment, but the number of electrodes will be influenced by the number of rotor coils 14 forming the rotor 12. A heat sink element 30 may also be provided, which can improve the stability of the varistor 22. Here, the heat sink element 30 incorporates a plurality of fan elements 32 to improve radiative distribution of thermal energy generated at the varistor 22. Note that the heat sink element 30 is omitted from FIG. 1.

(10) The electrical connection is made using an electrically-conductive fixing material or similar fixing means and is devoid of solder. This arrangement eliminates the risks involved with mechanical or chemical failure of a solder connection. Preferably, the electrically-conductive solderless fixing means may be devoid of or not primarily include metal-metal (metallic) bonding between the constituent parts, as is the case for solder.

(11) The term solder is intended to refer to a fusible metal alloy which is used to create a permanent or semi-permanent bond between conductive, typically metallic, components, most commonly in electrical circuitry. The most common form of solders are alloys of lead and tin, but solder will often also include other, or use only other, metals, such as antimony, bismuth, copper, germanium, nickel, indium, silver, and zinc. The term solderless, by contrast, is intended to exclude any of these fusible metal alloys.

(12) Furthermore, the electrically-conductive solderless fixing means may preferable be formed from a material which does not undergo a reversible transformation under the application of thermal energy. Solder can be melted and re-melted to regain flowability so as to be re-used or modified, which can be useful in some circumstances, but during high motor operation temperatures, this becomes deleterious to operation. The electrically-conductive solderless fixing means is intended to avoid this behaviour.

(13) In the depicted embodiment, and as best illustrated in FIG. 3, the electrically-conductive solderless fixing means comprises an electrically-conductive material such as an electrically-conductive adhesive, here formed as a plurality of adhesive joints 34 which are in physical and electrical communication with the electrodes 24 of the varistor 22. The form of the adhesive joints 34 corresponds with those which would be formed by a solder weld and is influenced by the viscosity or fluid flow properties of the adhesive used. For this reason, it may be preferred that a flowable electrically-conductive fixing material is used to secure the varistor 22 to the commutator 20. Such a material may then be able to set in a secure configuration to maintain the position of the varistor 22 relative to the commutator 20.

(14) Any appropriate electrically-conductive adhesive may be considered. An adhesive having a metallic conductive component could be used, such as a silver glue, paste or epoxy resin. Alternatively, an adhesive having a non-metallic conductive component could be used, such as a graphene glue, paste or epoxy resin. The choice of conductive component could, for example, be one or a combination of metallic materials such as silver and/or nickel, or could be formed from non-metallic materials such as carbon-containing conductive materials, such as graphite, graphene, or carbon nanotubes.

(15) The electrically-conductive adhesive does not exhibit the thermal degradation experienced by solder and will therefore remain thermally stable across the operating temperature range of the DC motor, preferably being thermally stable to at least 200° C., and more preferably thermally stable to at least 250° C. or 300° C. Furthermore, the electrically-conductive adhesive may be thermally conductive without exhibiting a propensity towards eutectic reaction with the varistor electrode. The issue with lead-free solder is that the melting point thereof is typically less than 220° C. As such, there is a clear threshold maximum operating temperature for a motor, at the point at which operating temperature exceeds the melting point of the solder. In the present invention, electrically-conductive fillers in the adhesive, such as silver and/or nickel, have melting points which far exceed that of lead-free solder. Metallic filler melting would not occur even when the motor is running in severe high-temperature conditions.

(16) As illustrated, the varistor 22 may be preferably arranged such that the plurality of electrodes 24 of the varistor 22 are provided on an opposite surface to a rotor-facing surface of the varistor 22. This may allow the varistor 22 to be readily located around the commutator 20 during assembly, providing the greatest amount of access for an adhesive applicator to be introduced at or adjacent to the varistor 22 to ensure that fixing can occur.

(17) An alternative rotor assembly, indicated at 110 in FIG. 4, is shown, in which an alternative varistor 122 mounting arrangement is provided. Identical or similar reference numerals to those previous used above will be used to refer to identical or similar components, and further detailed description is omitted for brevity.

(18) In this rotor assembly 110, the plurality of electrodes may be provided on said rotor-facing surface of the varistor 122, which may simplify construction if there is a ledge or lip 136 in communication with the commutator 120 against which the adhesion may occur to form the electrical connection, preferably via a plurality of adhesive joints 134.

(19) The ledge or lip 136 associated with the commutator 120, or indeed any similar support element which preferably extends in a radial direction away from the rotor shaft 118, provides a physical support for the varistor 122 and the electrically-conductive solderless fixing means, thereby improving a stability of the rotor assembly 110.

(20) The ledge or lip 136 may be a continuous lip which extends around a complete perimeter or circumference of the commutator 120, or could be discontinuous, so as to be oriented appropriately to the positions of the electrodes of the varistor 122.

(21) It is thus possible to provide a method of forming a rotor assembly for a DC motor comprising the steps of: providing a rotor having a rotor shaft, and a commutator mounted to the rotor shaft, and then connecting a varistor to the commutator via an electrically-conductive solderless fixing means, such as the electrically-conductive adhesive as described above.

(22) Where such an electrically-conductive adhesive is provided, it may be applied using any one of: dispensing the electrically-conductive adhesive via an adhesive dispenser; screen printing of the electrically-conductive adhesive; 3D printing of the electrically-conductive adhesive; air spraying of the electrically-conductive adhesive; wet dipping of the varistor and/or commutator into the electrically-conductive adhesive; and tape casting of the electrically-conductive adhesive.

(23) To solidify the adhesive once applied, it is preferably cured using any one of: addition curing; heat curing; radiation curing; anaerobic curing; and moisture curing, thereby ensuring that a secure bond is made to the varistor and commutator. A curable electrically-conductive solderless fixing means is useful since, once cured, the reversibility of the application of the fixing means is removed, and therefore will not become flowable once again during the operational temperature of the motor.

(24) There may be alternative methods by which an electrically-conductive solderless fixing means could be achieved. One possibility is indicated in FIG. 5. Again, identical and/or similar reference numerals will be used to refer to identical and/or similar components to those previously described in the above embodiments, and further detailed description is omitted for brevity.

(25) The rotor assembly, indicated at 210, could be by forming a casing 238 for the varistor material 228 from an at least in part electrically-conductive material, thereby forming a varistor 222 which could be aligned to the electrodes and then coupled to the commutator 220 around the rotor shaft 218. One option might be to use an electrically-conductive plastics material which can be overmoulded to the varistor material 228, and which has a high thermal stability over at least the operating temperature of the DC motor. Preferably, the casing 238 may serve to hold the varistor 222 in place relative to the commutator 220, as well as providing the necessary electrical conductivity.

(26) Whilst the present invention is intended for use in the context of a DC motor, it may be desirable to utilise a varistor in combination with an AC commutator motor, in which case, the solderless solution presented would be equally applicable.

(27) Similarly, whilst the present description refers to a solderless electrically-conductive fixing means, it will be apparent that this need not necessarily be formed from a single type of material and could be a composite material or several different materials which are interconnected to provide the electrical connection between the commutator and varistor to form an appropriate solderless electrically-conductive fixing means.

(28) The provision of a solderless electrically-conductive fixing means for a DC motor, to permit connection of the varistor to the commutator, advantageously can improve the mechanical and chemical stability of the motor, which can significantly enhance the operational lifespan thereof.

(29) The words ‘comprises/comprising’ and the words ‘having/including’ when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

(30) It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

(31) The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined herein.