EMI/RFI suppressing brush holder assembly for electric machines

#A novel commutating brush holder assembly is provided which is constructed of materials to suppress electromagnetic interference, particularly in the radio frequency and higher ranges of the frequency spectrum. The novel brush holder assembly is composed of a material such as ground ferrite-based materials suspended in a plastic carrier that suppresses the EMI generated by electric motors, generators, switch gear and like equipment which produce EMI emissions during operation. The assembly may also be used in conjunction with certain performance enhancement add-on suppression components such as radio frequency choke coils, capacitors and the like.

#This invention relates to a novel method with materials and devices for the suppression of electromagnetic interference, particularly within the radio frequency range. More specifically the invention relates to the material of construction and design of brush holder assemblies used with commutating brushes for electric motors, generators and other rotating electrical equipment such as rotary switches.

#Commutating brushes are used to maintain electrical contact between a stationary member (or stator) and a rotating member (or rotor) of rotating electrical equipment such as electric motors, generators, rotary switches, and the like. In most such equipment there is a normal current reversal and electrical connection dynamics involved in the operation of commutating devices which results in the generation of electromagnetic interference (EMI). This EMI may be conducted and/or radiated to neighboring machines and devices and result in interfering with the operation of such neighboring machines and devices thus causing serious reliability or performance problems in complex electrical systems such as those utilized in automobiles, airplanes, etc. Of particular concern are EMI signals lying within the radio frequency range referred to as radio frequency interference signals (R.F.I.), (including not only the broadcast AM and FM portions of the frequency spectrum, but extending up to and including the microwave regions. Previously, EMI/RFI has been suppressed using various add-on combinations of electrical components which are directly added to the electrical system or systems which might be affected by EMI/RFI. Capacitors and radio frequency choke coils are added in various configurations to contain EMI/RFI near its point of origin or at distant points to filter the radio signals at the point of reception. For example, U.S. Pat. No. 3,189,824 discloses the use of RFI suppression circuitry at the reception end in order to improve the performance of a radio receiver. U.S. Pat. No. 4,845,393 discloses the use of an EMI/RFI suppression circuit in the construction of an integrally built fuel pump end cap for reduction of EMI/RFI emissions from or by the fuel pump.

#The above and other objects, features and many of the attendant advantages of this invention will be appreciated more readily as the same becomes understood from a reading of the following detailed description when considered in connection with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference characters, and wherein:

#FIG. 1 in conjunction with FIG. 1A illustrates a basic form of EMI/RFI suppressing brush holder assembly constructed according to the invention, wherein FIG. 1 is a perspective end view of the lower end of the brush holder assembly, and FIG. 1A ia perspective end view from the opposite upper end of the assembly from that shown in FIG. 1;

#FIG. 2 is a perspective end view of a brush holder assembly as shown in FIGS. 1 and 1A and further including a commutating brush mounted therein together with an add-on RFI suppressing choke coil;

#FIG. 1 is a perspective, overall lower end view of a preferred form of EMI/RFI suppressing brush holder assembly comprising the invention. In FIG. 1, a brush holder assembly is shown and comprises a hollow, box-shaped body member having elongated sides 11 integrally formed into the closed box-shaped body member 10. Body member 10 has an open lower end 12 into which a commutating brush or other similar, sliding contact member is slidably fitted. The remaining upper end of the body member 10 is closed by an integrally formed cap 13 on which an elongated, cylindrical, solid tube-shaped pedestal 14 also is integrally formed with the end cap 13 and sides 11.

#The body member 10 including sides 11, end cap 13 and pedestal 14 consists essentially of ground ferrite-based materials either in powder or particulate form mixed with a polymeric plastic material such as polyphenylene sulfide, phenolic resin, nylon, or other similar plastic material. The ferrite-based particulates are magnetically susceptible and function within the brush holder assembly to suppress EMI/RFI. The polymer functions as a carrier for supporting the particulate, ferrite-like material in a suspended solid state after the two materials have been mixed with the particulate, ferrite-like material interspersed in the plastic carrier and molded into a desired physical configuration such as shown in FIG. 1. This can be achieved by injection molding or other similar process. After hardening, the polymer functions not only to hold the ferrite-like particulates in their suspended state, but provides mechanical strength and electrical and chemical resistance to the brush holder body member 10. The relative proportions of polymer to ferrite-like mater in the mixture may be varied to achieve optimum EMI/RFI suppressing characteristics in a cost effective composition having desired predetermined electrical resistivity characteristics necessary to meet performance requirements as required by particular applications in which the invention is to be used.

#A particular example of an EMI/RFI suppressing brush holder assembly according to the invention was built and tested wherein a cylindrical-shaped body member, similar to FIGS. 1 and 1A, having a hollow, open end bottom portion for receiving and supporting a commutating brush and a solid, tubular-shaped top portion was fabricated from four (4) parts of commercially available, fired #43 ferrite powder suspended in the molded, cylindrically-shaped, body member with one (1) part R-6 (PPS) commercially available, polymeric plastic binder. The brush holder thus formed was provided with a shunt wire connected through the body member between the inner end of a commutating brush supported in the hollow, open bottom end of the brush holder block and a suitable power supply terminal. Test results established that the abovenoted composition effectively suppressed EMI/RFI emissions in the 10 Kilohertz, 27 Megahertz and 100 Megahertz frequency bands.