Patent Application
20170149189
Electric machines often use brushes to contact a rotating commutator and conduct electric power to a winding. Brushes are wear items in electric machines which employ them and generally function well until the brushes are worn beyond their service life. At the end of the service life of one or more brushes in a particular machine may fail to function. Because a machine that does not function impacts other operations, it is desirable to avoid a failure of the machine. Maintenance of wear items can certainly avoid failures but often the employment of a maintenance schedule entails the replacement of parts of the electric machine before its actual service life is over. In part this is because each electric machine and its unique environment of operation will have a distinct service life. Early replacement of parts or of the electric machine increases costs. Unfortunately heretofore there has been no mechanism or method to monitor the brushes actual life such that reliable replacement at an actual needed interval could be accomplished leaving the industry with scheduled maintenance (and inherent added cost) as the only option. The industry then would be receptive to reliable alternatives.
An electric machine configured to increase resistance between a brush and a commutator of the electric machine at a wear condition of the brush consistent with end of service life.
A brush for an electric machine includes a brush body, an interrupter operable to cause a rise in resistance between the brush body and a commutator of an electric machine in which the brush is installed, when the brush is near an end of service life.
The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike:
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To ensure a proper understanding of the functionality of the arrangement, it is noted that the wear of the brush body 32 at the time the interrupter 30 is brought into contact with the commutator 12 will be of the electrical rather than frictional variety. As will be appreciated by one of skill in the art, the degree of electrical wear of a brush in an electric machine is reduced with the force applied to the commutator through the brush. Because of the interrupter in the present disclosure, the force on the material of the brush body 32 material about the interrupter 30 becomes less due to the load being assumed by the interrupter 30. Resultantly, the lesser force on the brush body 32 creates an environment where electrical wear of the brush body material increases eroding the material around the interrupter 30 and leaving the interrupter proud of the surface of the brush body 32. This causes a gap to form between the commutator and the brush body thereby exacerbating both the rise in resistance and the electrical erosion of the brush body 32. With the increase in resistance, the voltage drop (from brush to commutator) will consequently increase. The resistance is not so great as to prevent the electric machine functioning at this point but the voltage drop is sufficient to be detected by a controller and can therefore be monitored. Also, in machines with 4 or 6 brushes, such as an electric starter motor, parallel circuits will still carry most of the motor current, even if one brush has worn enough to cause the interrupter to contact the commutator. As more of the interrupter 30 is exposed, it will also begin to cause the brush to bounce on the commutator each time the interrupter crosses an undercut 36 in the commutator (or more aptly each time an undercut 36 of the commutator passes beneath the brush 34 and contacts the interrupter 30. The bouncing serves to increase the gap between the brush and the commutator thus further increasing the resistance and voltage drop
The positioning of the interrupter 30 in the brush body 32 will be such that the interrupter 30 does not become exposed to the commutator until near the end of service life for the brush 34.
Accordingly, as noted above the resistance change is a signal that is readable by a controller, which then may alert a user by selected means that repair is needed. It is noted that in some embodiments the interrupter will have an electrical conductivity lower than the material of the brush to further punctuate the resistance change.
The signal discussed above may take the form of battery terminal voltage variation; current draw by the motor (starter in one embodiment) requiring a current sensor not shown but well known as a sensor itself, a pattern of variation in either voltage or current during cranking or overrun or start to start trends in voltage or current peaks. Each of the voltage changes can be measured directly by the ECU or other controller and each of the current based indications would use a current sensor that then supplies a signal to the ECU or other controller to detect patterns.
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