Noise suppressed commutator

Abstract: A commutator has a base 10, a plurality of commutator segments 20 mounted on the base and a noise suppression element 30. Element 30 is electrically connected to the segments by connecting means 40 in the form of an elastomeric ring with conductive portions 41 which is compressed between the element and the segments or terminals associated with the segments to avoid the need to solder the noise suppression element to the segments.

Description: This invention relates to miniature electric motors and in particular, to commutators for such motors incorporating a noise suppression element. Noise suppression elements are commonly used in electric motors having a commutator to reduce electrical noise produced by commutation. Noise suppression elements are commonly made of ceramic material which are soldered to the commutator segments. This is time consuming and care is required not to damage the element during soldering by excessive heat. Hence, ways to avoid soldering of the noise suppression element have been investigated. GB2202686 discloses a method of mounting a noise suppression element in the form of a ceramic varistor ring to segment tangs or terminals by the use of a clip on plastics clamping member which also acts as a fan. The clamping member allows the varistor to be resiliently supported against the commutator terminals by fingers which urge the varistor away from the clamping member. However, as the varistor is ceramic and has very little flexibility, the terminals must be all perfectly aligned in the same plane to enable all terminals to make proper contact with the varistor. Flexible or elastomeric noise suppression elements such as those disclosed in GB2000914 and GB2183933 have been developed. These are basically elastomeric rings which have been impregnated with a conductive material to produce a conductive elastic ring. However, the noise suppression characteristics of such elastic rings are not as good as those of a ceramic varistor. Hence, there is a need for a method of connecting noise suppression elements, particularly of the rigid type, to commutator segments without the use of heat. This is achieved in the present invention by providing a resiliently deformable connecting means to electrically connect the noise suppression element to the commutator segments. Accordingly, the present invention provides a commutator having a base, a plurality of commutator segments mounted on the base, noise suppression means, and connecting means resiliently electrically connecting the noise suppression means to the segments. Preferably, the connecting means is compressed against the noise suppression means to establish electrical contact therewith. This may mean that the connecting means is compressed directly between the noise suppression means and the segments, e.g., by using a clamping ring which supports the noise suppression means and urges it into contact with the connecting means to deform and clamp the connecting means between the noise suppression element and the segments directly or with other elements associated with and electrically connected to the segments such as separate terminal pieces used to connect the segments to respective rotor winding portions or armature leads. Preferably, the commutator has a crown which mates with the base and the noise suppression means and the connector means are located between the crown and the base. This would allow the mating of the crown and base to clamp the noise suppression means and connecting means together without the use of a separate clamping ring. Preferably, the segments are associated with respective insulation displacing terminals which straddle and grip armature leads held by the crown as the terminal and crown are brought together. Thus, the armature winding terminations may be completed prior to the fitting of the base with the segments to the crown. With the noise suppression means and the connecting means being sandwiched between the base and the crown, the connecting means may electrically connect the noise suppression means to the commutator segments by making direct electrical contact with the terminals as the connecting means is compressed between the noise suppression means and the terminals. Preferably, the connecting means is a resiliently deformable ring having one or more conductive portions which electrically connect the noise suppression means to the segments and most preferably, the connecting ring is formed from hollow elastomeric tubing with a surface coating of conductive material, for example, copper or carbon. If the surface coating is highly conductive, then it is important that the surface coating is intermittent to avoid short circuiting the commutator segments while connecting the noise suppression means to the respective segments. If the ring is not highly conductive, a single conductive portion may be used which may provide additional noise suppression in a manner similar to the elastomeric noise suppressors of the prior art. Alternatively, the connecting means may be an annular disc or ring formed from a sheet of elastomeric material which is conductive perpendicularly across the sheet but substantially non-conductive along the sheet. Such a sheet, also known as an AF-type interconnector, is more fully described in US 4199537. Preferably, the noise suppression means is an annular ceramic varistor ring as this provides superior noise suppression. The invention also provides a method of connecting a noise suppression element, including a rigid element such as a ceramic varistor ring, to commutator segments of an electric motor. The method comprises the steps of providing a commutator with a plurality of commutator segments mounted on a base, fitting the element to the base, fitting an elastomeric ring with conductive portions to the commutator adjacent the element, and deforming the ring to urge it into resilient contact with the element to establish electrical contact between the element and the segments. Preferably, the step of deforming the ring includes compressing the ring between the element and terminals associated with the segments. The method allows a rigid noise suppression element to be connected to the commutator segments without the use of heat, for example, as associated with soldering which may lead to damage of the element if not applied correctly. The invention will now be more particularly described by way of example only with reference to the accompanying drawings in which: Figure 1 is a sectional side elevational view of a commutator according to the present invention having a single piece housing; Figures 2 and 3 are similar views to Figure 1 of commutators with a two part housing; and Figure 4 is a perspective view of an elastomeric connecting ring forming a part of the commutators of Figures 1 and 2. The commutator of Figure 1 has a base 10 supporting a number of commutator segments 20. The segments have a brush contacting portion 21, a radial extension 22, an axial extension 23 terminating in a tang or terminal 25 for connecting to an armature lead 26. A clamping ring 11 is used to hold the segments 20 to the base 10 while urging a ceramic annular varistor element 30 into contact with a connecting ring 40 with sufficient force to deform the ring as shown. The ring 40 is formed from a hollow elastomeric tube which is readily and resiliently deformable. The tubing has an intermittent conductive coating as more easily seen in Figure 4 providing a number of conductive portions 41. Alternatively, the ring could be impregnated with a conductive material, e.g., carbon, to form a conductive ring or a ring with conductive portions. In Figure 1, the ring has been compressed resulting in a flattened ring. The resilient urgings of the ring bring the conductive portions 41 into electrical contact with respective commutator segments and the varistor. The varistor 30 is a ceramic varistor of the type generally known in the prior art and will not be described further here save to mention that it has terminal portions formed on its surface for being electrically connected to the commutator segments, in this case, via the ring 40. The clamping ring 11 has a small ledge or seat 12 on which the varistor sits. The conductive portions 41 of the connecting ring 40 bear against respective varistor contact portions and segment radial extensions 22 to electrically connect the varistor to the segments. In Figure 2, the commutator has a two part housing comprising a base 10 and a crown 15. The crown has a number of recesses 16 for receiving insulation displacing terminals 25. Slots 17 pass across the recesses 16 for receiving armature leads 26. The terminals 25 are plate like with two slots 51 and 52. Slot 51 straddles and grips an armature lead held in slot 17 as the terminal 25 is pressed into the recess 16 in the crown 15. As the terminal is forced over the armature lead 26, any insulation on the armature lead is displaced by the edges of the slot to establish direct electrical connection between the armature lead and the terminal. Slot 52 in the terminal is arranged to receive an axial extension 23 of the commutator segment as the base 10 is fitted to or mated with the crown 15 to establish electrical contact between the armature lead and the commutator segment. In the embodiment of Figure 2, the varistor ring 30 is mounted against a rear surface of the commutator base 10 with the ring 40 being clamped against the varistor by radially extending portions 22 of the segments. In this particular arrangement, the commutator segments would be inserted into the base 10 through apertures 13 from the rear or crown end of the base and then secured to the base by deforming the distal end of the brush contacting portion of the segments into a recess 14 formed in the front end of the base. In the embodiment of Figure 3, a cylindrical carbon segment commutator having a two part housing is shown. The commutator segments have a brush contacting portion 21 formed from a carbon based material. A copper bar 24 is embedded in the carbon material and extends through a slot in the base to mate with the terminal 25. A short radially extending portion 22 provides a butt for the copper bar 24 to ensure it is correctly inserted. In this embodiment, the connecting ring 40 illustrated is an annular disc formed from an anisotropically conductive elastomeric material sheet which is conductive across the sheet but not along the sheet. This type of connecting ring is shown as an alternative to the hollow or tubular ring shown in the other embodiments. One such material manufactured by Shin-Etsu Polymer Company of Japan for use in electronic watches and calculators is described as an anisotropically conductive composite wafer or an electrically insulating elastomeric matrix embedded with aligned conductive fibers. In a simple form, it is a rubber sheet with embedded carbon filaments extending perpendicular to the plane of the sheet giving good conduction across the sheet while being substantially non-conductive along the sheet. As in the embodiment of Figure 2, the varistor is located against the rear wall of the base 10 but in this case, the connecting ring is not sandwiched by or in direct contact with the segments. As the base is fitted to the crown, the connecting ring is compressed between the varistor on the one side, and the crown and terminals on the other, with the conductive portions of the connecting ring bearing on respective terminals and connection portions of the varistor to electrically connect the varistor to the terminals and thus, to the commutator segments. Figure 4 illustrates the preferred elastomeric ring shown in perspective with a part cut away to reveal the hollow interior and to show, diagramatically, the conductive surface layer, such as a flexible copper based coating. Being hollow, the ring can be compressed further using a relatively light force resulting in greater reliability of the contact between the ring and the varistor and segment/terminal even under adverse operating conditions. Thus, it can be seen that the noise suppression element can be fitted to the commutator without the application of heat in a reliable yet convenient manner and thus allowing the use of rigid and/or brittle noise suppression elements such as ceramic varistors in a mechanical connection type commutator. Although only cylindrical commutators have been illustrated in the embodiments, it will be readily apparent to those working in this area of technology that this invention is equally applicable to planar or face plate type commutators.

Claims: A commutator comprising a base (10) and a plurality of commutator segments (20) mounted on the base, and noise suppression means (30) characterised by connecting means (40) resiliently electrically connecting the noise suppression means (30) to the segments (20). A commutator as defined in Claim 1 wherein the commutator has a crown (15) which mates with the base (10) and the noise suppression means (30) and the connecting means (40) are located between the crown (15) and the base (10). A commutator as defined in Claim 2 wherein the segments (20) are associated with insulation displacing terminals (25) which straddle and grip respective armature leads (26) held by the crown (15) as the terminals and crown are brought together. A commutator as defined in any one of the preceding claims wherein the connecting means (40) is a resiliently deformable ring having conductive portions (41) which electrically connect the noise suppression means (30) to the segments (20). A commutator as defined in Claim 4 wherein the resiliently deformable ring is formed from hollow elastomeric tubing with an intermittent surface coating of conductive material. A commutator as defined in Claim 4 wherein the resiliently deformable ring is formed from an anisotropically conductive elastomeric sheet material. A commutator as defined in any one of the preceding claims wherein the noise suppression means (30) is an annular ceramic varistor ring. A commutator as defined in Claim 2 wherein the segments (20) are connected to armature leads (26) by way of separate intermediate insulation displacing terminals (25) which straddle and grip respective portions of the armature leads (26) held by the crown (15), and releasably contact the segments to establish electrical contact between the segments (20) and the armature leads (26), the noise suppression means (30) is an annular ceramic varistor ring, and the connecting means (40) is a resilient ring formed from an elastomeric material with conductive portions, the varistor ring and the resilient ring being sandwiched between the crown (15) and the base (10) with the resilient ring being urged against the terminals (25) and the varistor to electrically connect the varistor to the segments (20). A method of connecting a noise suppression element (30) to commutator segments (20) comprising the steps of: providing a commutator with a plurality of commutator segments (20) mounted on a base (10), and fitting the element (30) to the base (10), characterised by fitting an elastomeric ring (40) with conductive portions to the commutator adjacent the element (30), and deforming the ring (40) to urge it into resilient contact with the element (30) to establish electrical contact between the element (30) and the segments (20). A method as defined in Claim 9 wherein the step of deforming the ring (40) includes compressing the ring (40) between the element (30) and terminals (25) associated with the segments (20).

Inventors: WANG PATRICK SHUI-CHUNG, WANG, PATRICK SHUI-CHUNG
Applicants: JOHNSON ELECTRIC SA, JOHNSON ELECTRIC S.A.
Application year: 1995
Main classification: H02K13
IPC codes: H02K13, H01R39, H02K11, H01R13