Switch for a brush wear recording circuit

A switch for a recording circuit used to determine brush wear of an electrical brush comprises a flag having a first end and a second end defining distal ends of the flag. The flag is mounted on the electrical brush at the first end so as to move in unison with brush wear while being in electrical communication with the electrical brush and has a piercing feature on the second end. The switch further comprises a contact plate arrange relative to the flag so as to bring the flag in contact with the contact plate at a predetermined brush wear and includes an inner conductive core encased by a cover forming an outer layer of the contact plate, wherein the piercing feature is configured to penetrate the cover and to contact the conductive core to cause electrically continuous contact between the flag and the contact plate and close the switch.

Priority is claimed to European Application No. EP 08155159.0, filed on Apr. 25, 2008, the entire disclosure of which is incorporate by reference herein.

The invention relates to brushes used for transferring current in electrical machines, such as electric motors or alternators, and to recording systems used to measure or determine the wear of these brushes. More specifically, the invention relates to switches for brush wear recording systems that rely on brush flags to initiate brush wear alarms through electrical contact.

BACKGROUND

Brush gear mechanisms are used extensively in certain motors, generators and other rotating electrical machinery to conduct current between stationary and rotating parts (commutators or slip rings). To avoid or at best minimise the wear of the rotating parts, which are typically constructed of copper or brass, brushes are made of soft materials, such as carbon and graphite, so as to preferentially wear. It is important that brush wear is monitored to ensure that the brushes do not wear beyond a minimum level as a completely worn brush can lead to electrical arcing resulting in machine part failure.

There are a number of techniques and methods known for the measurement and determination of brush wear. One such technique involves the closing of a switch, by flags or projections located on the brush, by movement resulting from the brush wear. An example is provided in the German Patent No. DE 34 17 711 A1. Another example is provided in German Patent No. DE 86 00 934 U1. A problem with brush wear monitoring methods utilising brush flag switches is that dust created by the wearing of brush creates a dirty environment in the brush housings in which brush are located. This dust can interfere with the operation and reliability of the switching operation.

SUMMARY OF THE INVENTION

An aspect of the present invention is to provide a means to overcome the problem of dust fouling in brush wear houses affecting operation of brush wear switches that forms part of brush wear recording circuit.

The invention is based on the general idea of providing a switch that includes a covered contact plate. The switch is closed by penetration of the cover by a flag thereby creating electrically continuous contacts between the contact plate and flag. The cover ensures the reliably of the switching independent of the degree of brush dust foulant in the brush housing.

Accordingly, provided is a switch for a recording circuit used to determine brush wear of an electrical brush wherein the switch comprises a flag and a contact plate. The flag has a first end and a second end defining distal ends of the flag and is mounted on a brush at the first end so as to move with brush wear while being in electrical communication with the brush. The mounting may be such that the flag moves by the amount the brush is worn i.e. in unison. Towards the second end of the flag is a piercing feature. The contact plate is configured and arranged relative to the flag such that at a predetermined brush wear the flag is brought in contact with the contact plate by brush wear thereby closing the switch. The contact plate further comprises an inner conductive core that is encased by a cover that forms an outer layer of the contact plate. The arrangement of flag and contact plate is such that the piercing feature of the flag is capable of penetrating the cover so as to cause electrically continuous contact between the flag and the contact plate by contacting of the conductive core.

Preferably the cover also includes a sealable slot arranged such that the piercing feature penetrates the cover through the sealable slot so by reducing the stress imposed on the flag by the piercing action.

Typically the cover is made of deformable material wherein the deformable property of the material provides the sealable slot with the capability to seal. The deformable property of the cover may also enable the non-destructive penetration of the slot by the piercing feature whereby the slot is capable of resealing, through memory properties of the cover, when the penetrating feature is withdrawn from the slot. This resealing enables the reuse of the contact plate.

In a further aspect the switch comprises a plurality of flags wherein the contact plate and the plurality of flags are configured and arranged such that the plurality of flags are capable of electrically continuous contact with the conductive core of a single contact plate. This provides a simplified arrangement wherein a plurality of flags can be monitored by a single circuit without interference from dust accumulation in the brush housing.

Other objectives and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings wherein by way of illustration and example, an embodiment of the invention is disclosed.

DETAILED DESCRIPTION

Preferred embodiments of the present invention are now described with reference to the drawings, wherein like reference numerals are used to refer to the elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It may be evident, however, that the invention may be practiced without these specific details.

FIGS. 1, and2show details of a switching system suitable for use in a recording circuit used to determine the brush wear of an electrical brush2a,2b.

FIG. 1shows two brushes2a,2bof an electrical machine, in different wear states. The brushes2a,2bare typical of those used extensively in certain motors, generators and other rotating electrical machinery to conduct current between stationary and rotating parts4(commutators or slip rings). The blushes2a,2b, which are stationary are worn by frictional contact with the rotating parts4. This results in a decrease in the length of the brush2a,2bin the radial direction RD.

A flag18is mounted onto each brush2a,2bat a first end of the flag, in such a way that the flag18moves, preferably in unison, with the brush2a,2band the flag18is in electrical communication with the brush2a,2b. Unison, in the sense of this specification means moved in the radial direction RD by the same amount the brush2a,2bwears. For example, if the brush2a,2bwears 1 mm, the radial distance between the flag18and rotating part4decreases by the same 1 mm. While the flag18is shown as being mounted on top of a brush2a,2b, it could equally be mounted on the side of the brush2a,2bor along the length of the brush2a,2bin the radial direction RD or any other conceivable arrangement that provides that the flag18moves by the amount the brush2a,2bis worn while remaining in electrical communication with the brush2a,2b.

In an exemplary embodiment, a piercing feature16is located on the flag18towards a second end of the flag18, distal from the first flag end. In an exemplary embodiment, the piercing features16consist of a sharpened point. In another, not shown exemplary embodiment, the piercing feature consist of a sharpened edge. The piercing feature16may take other forms that give the flag18the capability of penetrating a cover11forming the outer layer of the contact plate10.

The flag18and the contact plate10are arranged such that when the brush2a2bis worn by a predetermined amount, the piercing feature16is brought into electrically continuous contact with the contact plate10by the flag18piercing a cover11covering the contact plate10, as shown inFIG. 1. The piercing, in an exemplary embodiment, is through a sealable slot15, wherein the slot is made sealable by the cover11being made of deformable material such that when the sealable slot15is not being pierced the sealable slot15seals itself. In this way the cover11is pierced non-destructively ensuring continuous protection of the conductive core12of the contact plate10, as shown inFIG. 2.

The combination of flag18and contact plate10perform the function of a switch such that contact between a flag18with the contact plate10closes the switch by enabling current flow from the brush2ato the contact plate10. This contact may be utilised, in conjunction with known brush wear circuits (not shown), to provide a brush wear alert.

In the exemplary embodiment, shown inFIG. 1, the contact plate10extends past more than one flag18such that contact between the contact plate10and more than one flag18is possible. In this way a single contact plate10can be used to monitor more than one brush2a,2bso by simplifying the monitoring system by reducing the need for duplication of the brush wear monitoring circuit (not shown).

FIG. 2shows an expanded view of the flag18and contact plate10ofFIG. 1, showing a worn brush2amaking contact with a contact plate10. In addition, a cut through view of an end of the contact plate10shows the inner conductive core12of the contact plate10and the cover11that forms the contact plates10outer layer.

To ensure that the conductive core12remains dust free during use and the penetration of the flag18through the cover does not impart excessive stress on the flag18as it pierces the cover11, in an exemplary embodiment, the cover11has sealable slots15located on and extending through the cover1. These sealable slots15are arranged such that piercing features16of the flags18penetrate the cover only through the sealable slots15.

Sealable slots15are made sealable by the cover11being made of deformable material, for example, rubber or plastic, which have a memory. In this way sealable slots15are dust tight when not being penetrated and yet can be non-destructively penetrated by piercing features16such that they return to the original sealed state when the piercing feature16is withdrawn.

Although the invention has been herein shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures can be made within the scope of the invention, which is not to be limited to details described herein but is to be accorded the full scope of the appended claims so as to embrace any and all equivalent devices and apparatus.

REFERENCE NUMBERS

Numbers Used in the Figures