US3509398A - Commutating device - Google Patents

Description

A ril 28, 1970 E. s. TREIBLE, JR 3,509,398

COMMUTATING DEVICE Filed July 9, 1968 INVENTOR EDWIN S. TRE/BLE, JR.

AGENT United States Patent US. Cl. 310-220 9 Claims ABSTRACT OF THE DISCLOSURE A brush holder for commutating .a dynamo-electric machine having a plasma gap electrode for controlling arcing plasma. The electrode is electrically tied to, and carried at the trailing edge of, the brush and is spaced from the commutator; the electrode receives and maintains incipient arcing plasma at the end thereof.

This invention pertains to a commutation device for use with dynamo-electric machines of the commutatortype, and in particular to such a device having means for controlling and maintaining the plasma occurring between the brush and commutator.

In dynamo-electric machines there occurs incipient arcing between the brush and commutator which is destructive of the brush holder and brush. Additionally, the arcing can give rise to the more serious condition of flashover.

Flashover is a destructive short circuit occurring through the conductance of the plasma developed in the electric arcing. It usually starts when commutation is interfered with to such an extent that an arc is formed from the trailing edge of a commutator bar to the trailing edge of the commutating brush. In fiashover, the energy source is no longer just that which is in the coil undergoing commutation, but is a condition in which a major part of the power of the dynamo-electric machine becomes available.

The arcing plasma can be moved by air circulation, boundary layer wiping, or other means to adjacent structures. The arcing-produced plasma can be carried around the commutator to a brush of opposite polarity, completing a direct, ionized path therebetween. This allows for a direct discharge therebetween resulting in severe damage to the machine. Flashover originates under the brushes. It can occur when there is a current pulse which is greater than the brush can handle.

Commutating devices are known which dissipate the arcing plasma, or neutralize it, or damp its effects, yet there are dynamo-electric applications in which it is not desired tosquelch the plasma. Only by way of example, a universal motor equipped with a motor-reaction, dynamic brake is one of such. In this exampled application, arcing plasma forms during periods of high current density in the brush-commutator sliding contacts. This will normally result in accelerated brush wear and brush holder damage. Even so, the plasma is required for strong braking activity because of its current shunting action.

In this example, the braking activity is initiated by interrupting the line current; this is followed by a reversal of the field and a shunting of it across the armature.

Mechanical energy stored in the armature causes the conductors thereof to cut the few lines of flux remaining in the field due to field residual induction, and thus generates a voltage.

This generated voltage, and resulting current, produces a stronger field, and thus more flux density. More lines of flux are cut generating a higher voltage, and the cycle repeats constantly. This is similar to the building up manifested in self-excited DC generators. In order for the 3,509,398 Patented Apr. 28, 1970 build-up to occur, the residual induction must be ample so that about four or five volts can be generated initially. This is to overcome the voltage drop across the brushcommutator contacts which is approximately a constant.

As build up occurs, commutation problems begin. The current density in the sliding contacts becomes excessive; also, due to armature reaction, strong cross-magnetization forces start to exist which tend to distort the magnetic field in the direction of armature rotation. Now lines of flux are cut in the neutral plane where brushes are shorting out armature coils and current reversal takes place.

The are forms when a cathode begins to emit electrons thermally. If the cathode happens to be a carbon brush, the arc will appear when there is a voltage drop of 25 volts across the brush. If the cathode is a copper armature bar, the arc will appear sooner, or when there is a voltage drop of 15 volts. The are is a plasma which only requires a voltage drop of 10-15 volts to maintain an arc of 1 cm.

The arc which forms between the brush and commutator bars ionizes the air in the immediate vicinity. This produces a lower resistance path between the commutator bars and the brass brush holder. The are thus assumes the lower resistance path and produces a shunt across the brush-commutator sliding contacts. A shunting action is desirable, as it lowers the resistance of the circuit causing a strong braking action by motor reaction until the given foot-lbs. of rotary mechanical energy in the armature are consumed. However, the plasma melts the brush holder and vaporizes the lagging edge of the brush.

Accordingly, for such applications as that of the example just given, and those similar thereto, it is an object of this invention to provide a commutating device with means for controlling plasma which occurs in an area between the brush and the commutator, and for maintaining said plasma in a zone removed from said :area.

A feature of this invention comprises the use of a plasma gap electrode mounted adjacent the brush, an end of which electrode is spaced from the commutator. The electrode is electrically tied to the brush and accepts the arcing plasma in the gap formed between the end of the electrode and the commutator surface.

Further objects and features of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying figures in which:

FIGURE 1 is a sectioned, side elevation of the commutating device according to the invention, in conjunction with a commutator of a dynamo-electric machine;

FIGURE 2 is an end view of the commutating device of FIG. 1 taken along line 22 of FIGURE 1; and

FIGURE 3 is a sectional view of the commutating device taken along line 3-3 of FIGURE 1.

The commutating device 10, according to the invention, as shown in FIGURE 1, comprises a casing 12 of electrically insulating material which receives a cartridge 14, which cartridge is formed of electrically conducting material, at one end thereof. A brush retaining cap 16 screws into the end of the cartridge 14, compressing a brush spring 18. A set screw 20, threaded into the machine housing 22, retains the casing 12 in the housing.

A commutating brush 24 of carbon, or like material, is slidably mounted within cartridge 14 and urged outwardly from the cartridge by spring 18, into abutting contact with a commutator 26. Casing 12 also receives a plasma gap electrode 28. Electrode 28 is supported by cartridge 14, in casing 12, and is spaced from the outer surface of the commutator 26. Brush 24 is disposed point of As the arrow in FIGURE 1 indicates, commutator 26,

in this preferred embodiment, rotates in a given direction. The direction indicated establishes that electrode 28 is carried at the trailing edge of brush 24 and forms a plasma gap zone 32 between its projecting extremity and the surface of commutator 26.

As shown in FIGURES 2 and 3, the electrode 28 is bonded or otherwise fixed to cartridge 14. Accordingly, there is electricalconductivity between brush 24, which, as noted, may be carbon, or the like, and which is mounted in the electrically-conductive cartridge 14, and the carbon or other high-melting-point material electrode 28 In operation, arcing plasma occurring in area 30 is carried to the trailing edge of brush 24 by means of boundary layer wiping, and moves from there to the plasma gap zone, for the reason priorly noted, where it is retained by virtue of the fact that it will always follow the path of lowest resistance.

It is taught by my disclosure to carry electrode 28 on cartridge 14 within casing 12. But the teaching of my invention is broader than that manifested by the structure of my preferred embodiment. Clearly it is my teaching to use an electrode, which is electrically conductive with the brush, to form a plasma gap zone adjacent the brush and spaced from the bars of the commutator. Thus, carriage of such an electrode on structures other than those such as cartridge 14, or within components such as casing 12 or, very simply, the maintenance of a plasma gap zone by an electrode adjacent, and electrically tied, to the brush, are embodiments within the spirit of my invention.

The provisioning, of the gap in zone 32 avoids -the possibility of boundary layer wiping carrying the plasma around the commutator 26 to a brush of opposite polarity, and the low resistance path thus formed further inhibits any incidence of flashover. Equally important, the life of the brushes and brush holder are greatly extended. The gap must be provided in zone 32, of course, for further reasons: if electrode 28 closed upon and contacted the commutator 26, this would upset the voltage distribution across the commutator. Also, if electrode 28 contacted the commutator it would simply be a redundant brush; this would teach no novelty or improvement. Accordingly, the gap in zone 32 is necessarily provided for all of the foregoing reasons.

According to my invention, then, there is provided a commutating device, for use with a dyamo-electric machine of the commutator type, having means for controlling plasma occurring at the brush-commutator area and for maintaining the plasma in a zone removed from said area. Further, the plasma-controlling means of my invention, in cooperation with the arcing plasma, provides a low resistance, path for electrical current which parallels the brush. 1

While I have described my invention in connection with a specific embodiment thereof, it is to be clearly understood that this is done only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and the accompanying claims.-

v 1. A commutating device, for use with a dynamo-electric machine of the commutator type, comprising:

a casing;

a brush, for commutation of said machine, carried by said casing;

means carried by said casing for supporting said brush for abutting-contactwiththe machine commutator; and r means carried adjacent said brush cooperative with arcing plasma to provide a low-resistance path for elecv trical current, which path parallels said brush, for controlling such arcing plasma which occurs in an area between said brush and said commutator, and for maintaining said plasma in a zone removed from said area. i '2. The device, according to claim 1, wherein: said plasma-controlling means are carried by said casing.

3. The device, according to claim 1, wherein: said plasma-controlling means comprise an electrode, fixed to said supporting means, having a face opposed to and spaced from the commutator of said machine so as to define said zone between said face and said commutator. 4. The device, according to claim 1, wherein: said plasma-controlling means are electrically connected to said brush.

5. The device, according to claim 1, wherein: said plasma contr'olling means are supported immediately adjacent the trailing edge of said brush.

6. The device, according to claim 1, wherein: said supporting means comprise abrush cartridge mounted in said casing; and 1 said plasma-controlling means is fixed to said cartridge, one end of said plasma-controlling means being mounted, in common with said cartridge, in said casing.

7. The device, according to claim 6, wherein: said plasma-controlling means comprise an electrode.

8. The device, according to claim 7, wherein: said cartridge and said electrode are both formed of electrically-conductive material; and said cartridge and electrode both manifest electrical conductivity. individually and therebetween, conductivity therebetween proceeding from the fixing of said electrode to said cartridge.

9. The device, according to claim 1,' wherein: said casing is formed of electrically-insulative material.

7 References Cited UNITED STATES PATENTS 758,151 "4/1904 Thomson 310221 1,331,897 2/1920 Burnham 310-221 1,331,936 2/1920 'Linebaugh 310-221 1,423,996 7/1922 Burnham 310 221 1,519,270 12/1924 Shand 310-221 MILTON O. HIRSHFIELD, Primary Examiner L. L. SMITH, Assistant Examiner US. Cl. X.R.

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