Cartridge Brush With Integral Filter Inductor

This invention relates generally to devices for suppressing electromagnetic interference (EMI) in direct current machines, and more particularly, to the integration of a highly reliable, inexpensive suppression device into the structure of a brush holder. Such a brush holder is used to hold a brush of the type which makes a sliding electrical contact with a moving part.

Electromagnetic interference is commonly generated by dynamic electrical connections, such as the changing connections produced by the relative movement between the brushes and commutator of an electric motor or generator. The rapid changes in the electric and associated magnetic fields caused by the electrical switching EMI can interfere with the operation of other electrical systems, particularly digital systems operating at low voltage and low current levels.

Commonly, EMI problems are solved by adding noise suppression circuitry to each system which may be affected by the noise, rather than by reducing the noise at the source of the EMI. Such solutions are costly and result in circuit complexity since each system requires individual protection.

A more cost efficient and reliable means of solving the problem is to suppress EMI emissions at their source. Prior arrangements for suppressing EMI at the source have been made. For example, U.S. Pat. No. 2,453,114, issued May 16, 1945 to Brandt, teaches the integration of a capacitor into the structure of a brush holder. The integral capacitor/brush holder is comprised of a hollow casing made of insulative material and an outer electrically conductive layer acting as one capacitor plate. The capacitor shunts the high frequency EMI to ground through a low impedance path. While the Brandt patent teaches a means to suppress EMI at the source, it is neither a simple nor low cost solution. First, a good electrical connection to ground near the EMI source is necessary for effective capacitive filtering. Therefore, when a good ground connection is not readily available, or it is undesirable for some reason to inject EMI into ground, capacitive filtering is not necessarily the most effective EMI suppression technique. Second, the materials and manufacturing processes involved in constructing a capacitor of the type shown in Brandt are expensive, complex and time-consuming. For example, the physical dimensions of a capacitor are usually proportional to its capacitive value. Therefore, separate tooling and material processing may be required to construct Brandt casings of various capacitive values. As a result of the limited flexibility in constructing Brandt type casings, the flexibility to design the EMI filter to operate over specific frequency ranges is also limited.

Accordingly, there is a need for a highly reliable, inexpensive, improved suppression device integral to a small electric machine brush holder, to suppress the EMI commonly generated by such machines.

This need is met by the present invention, whereby a highly reliable, inexpensive, improved suppression device integral to a small electric machine brush holder is provided to suppress the EMI commonly generated by such machines. The brush device comprises a brush holder and a brush assembly.

The brush holder includes a housing, which may be substantially tubular, and noise suppression means, mounted on the housing and electrically connected to a brush assembly. The noise suppression means suppresses electrical noise generated during the interaction of the brush assembly and the moving part by providing an inductive impedance in the electrical path of the brush. Such noise suppression means may comprise an electrically conductive coil wound about the housing. The housing can be formed from low reluctance material and, further, act as a core for the coil.

The brush holder may further include a connector cap, electrically connected to the noise suppression means and adapted to provide a mating connection between the noise suppression means and an external connector. Such a connector cap may also be mounted on the housing.

The brush assembly, positioned within the housing, may include a brush of the type used to make sliding electrical contact with a moving part, and terminal means for connecting the brush to the noise suppression means. A spring means, interposed between the terminal means and the brush, may also be included for biasing the brush toward the moving part to make electrical contact therewith. Such a brush assembly may further include engaging means for engaging the housing. The engaging means may comprise an opening defined in the housing through which the noise suppression means is electrically connected to the terminal means. The terminal means comprising a substantially L-shaped terminal having longer and shorter portions, and a shunt wire connecting the brush to the shorter portion of the terminal. A locking tab on the longer portion of the terminal engages the opening in the housing when the brush assembly is inserted into the housing.

FIG. 1 is a side view of a brush device of the present invention with the lower portion of the brush device in section to illustrate more clearly the construction of the device;

FIG. 2 is an exploded, perspective view of the brush device of FIG. 1, illustrating the interrelationship of the brush holder and brush assembly.

The brush device of the present invention, shown in FIGS. 1 and 2, and generally designated by the reference numeral 2, provides electrical contact with a moving part, such as a commutator, while suppressing electrical noise generated by the interaction of the brush device with the moving part. The brush device comprises a brush holder, generally designated by the reference numeral 4 in FIG. 2, and a brush assembly, generally designated by the reference numeral 6 in FIG. 2.

The brush holder 4 includes a housing 8, a portion of which is shown in section (FIG. 1), preferably generally tubular, adapted to receive a brush assembly 6 therein; and noise suppression means, mounted on the housing 8 and electrically connected to a brush assembly 6, for suppressing electrical noise generated during the interaction of the brush assembly 6 and the moving part. The noise suppression means, comprising an electrically conductive coil 10 wound about the housing 8, provides an inductive impedance in the electrical path between the brush assembly 6 and an external connector (not shown). The inductance of the coil 10 depends on several factors, e.g., the number of windings, the conductivity of the conductor, the cross-sectional area of the conductor, and the material composition of the housing.

The inductance of the coil 10 may be selected so that the electrical noise suppressed by the brush device 2 is within specific frequency ranges. The electrical noise suppression characteristics are further enhanced if the housing 8 is formed from low reluctance material, such as ferrite impregnated plastic, so that the housing acts as a core for the coil 10.

Preferably, the brush holder 4 further includes a connector cap 12, electrically connected to the noise suppression means, adapted to provide a mating connection between the noise suppression means and the external connector (not shown). As shown in FIG. 2, the connector cap 12 is made of conductive metal and is substantially cylindrical with the closed end 14 forming a truncated cone. After the brush assembly 6 has been inserted into the housing 8, the connector cap 12 is mounted on the housing 8 such that it seals the first end 16 of the housing 8. The electrical connection between the coil 10 and the connector cap 12 is completed by feeding the first end 18 of the coil 10 through a channel 20 in the connector cap 12 and then mechanically securing it to the closed end 14 of the connector cap 12.

The brush assembly 6 includes a brush 22 of the type used to make sliding electrical connection with a moving part; and terminal means for connecting the brush 22 to the coil 10. The terminal means includes a substantially L-shaped terminal 24 having longer and shorter portions, and a shunt wire 26 connecting the brush 22 to the shorter portion 28 of the terminal 24. The brush assembly 6 further includes spring means, interposed between the terminal 24 and the brush 22, for biasing the brush 22 toward the moving part to make electrical contact therewith. The spring means, as shown in FIG. 2, comprises a spring 30 attached to a ferrite bead 32. As is well known in the art, a ferrite bead adds series inductance to the electrical path of a conductor passing through its center, and provides very high frequency noise suppression. The shunt wire 26 passes through the centers of both the spring 30 and the ferrite bead 32.

The brush assembly 6 further includes engaging means, comprising a locking tab 34 on the longer portion 36 of the terminal 24, for engaging the housing 8. In the preferred embodiment, the housing 8 defines an opening 38 through which coil 10 is electrically connected to the longer portion 36 of the terminal 24. The locking tab 34 engages the opening 38 in the housing 8 once the brush assembly 6 is fully inserted into the first end 16 of the housing 8. The second end 40 of the coil 10 is then electrically and mechanically connected to the locking tab 34 through the opening 38. This completes the electrical path between the brush assembly 6 and the connector cap 12 through the coil 10. Once the locking tab 34 has engaged the opening 38 in the housing 8, the brush assembly 6 is positioned within the housing 8. Sliding electrical contact may be made with the moving part by the brush 22 which protrudes from the second end 42 of the housing 8.

Having described the invention in detail and by reference to the preferred embodiment thereof, it will be apparent that other modifications and variations are possible without departing from the scope of the invention defined in the appended claims. For example, it is contemplated by this invention that the construction of the brush assembly may be varied to accommodate differing engaging means, such as a threaded coupling. Accordingly, it is the appended claims, including all equivalents, which are meant to define the scope of the present invention.