GB1574229A - Spark quencher attachment for an electric motor - Google Patents

Description

(54) A SPARK QUENCHER ATTACHMENT FOR AN ELECTRIC MOTOR (71) We, MATSUSHITA ELECTRIC IN DUSTRIAL CO. LTD., a Corporation organised and existing under the laws of Japan of 1006, Oaza-Kadoma, Kadoma City, Osaka Prefecture, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a spark quencher attachment for a small electric motor. An example of a small motor is shown in the United States Patent No. 3,662,240. The invention is applicable to a small-sized motor having a commutator.

Conventionally, according to the prior art, a plurality of non-ohmic resistors whose resistance increases with applied voltage (hereinafter called varistors) are generally used together as a spark quenching device to be connected across contacts. Among varistors, there are two o kinds, namely "bulk and lead wires- type" and "sheet-type". The use in a small sized motor of the former type, the bulk and lead wires-type, necessitates troublesome con nections of numerous lead-in and lead-out wires, to the commutator terminals and the governor terminals. One needs, in fact, two lead wires per varistor. Furthermore, since the connections are made by soldering by hand or spot-welding by hand, the connections are not necessarily reliable.

In case of the conventional use of the "sheet type" varistor, its connecting electrodes are made of silver or the like conductive metal films and the electrodes are soldered to the contacting metals of the commutator con nected to the commutator segment metals and a governor switch. Such soldering is made by first adjusting the angular position of the varistor electrodes by observing and manually adjusting the position, so as to face the metallic contacts of the commutator, and second, soldering the varistor electrodes and the met allic contacts. The adjustment of positions of the electrodes and the soldering are tasks which tend to make the manufacturing procedure inefficient. Furthermore, when areas of the sheet-type varistor are made equal for the different current capacities, then the area connected across the contacts having the larger current capacity is less effective in spark quenching and is likely to be damaged sooner than the other areas. Besides, ordinary high melting point solder which is used to connect the varistor electrodes and the metallic contacts adversely affects the life and characteristic of the varistors.

The present invention seeks to provide a spark quencher attachment for a small electric motor which simplifies connection to the motor. In accordance with this invention there is provided a spark quencher attachment for small electric motors comprising a disc adapted to be mounted on the commutator of said motor in a plane normal to its axis and in a predetermined angular position with respect to the commutator, at least two electrodes located in different angular positions on one face of said disc and available for electrical connection to respective segments of the commutator when said disc is mounted thereon, and at least two varistors angularly positioned in or on said disc between said electrodes, with each varistor being electrically connected between said electrodes. or between adjacent pairs of electrodes, if more than two.

Preferably the attachment is adapted to be mounted on the commutator and secured in its predetermined angular position thereon by means of an interfitting key and keyway means on said disc and on said commutator. In order to simplify still further the assembly of the disc on the commutator each electrode on the disc comprises a layer of low melting-point solder on the face of each electrode for electrically and mechanically connecting the varistor disc to the commutator by simple mounting of the disc on the commutator with the electrodes on the disc in physical contact with leads to the commutator segments, and then heating the disc to fuse the solder and make the desired electrical and mechanical connection.

The spark quencher attachment of this invention is compact, and quickly installed; firmly secured to the commutator and has good revolving balance due to its rigid construction.

The invention will be better understood from the following non-limiting description of examples thereof given with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of the rotor of a small-sized motor embodying the present invention.

Figure 2 is a fragmental elevation view of a commutator and a varistor of the rotor of Figure 1.

Figure 3 is a plan view of the varistor of Figure 2.

Figure 4 is a plan view of a modified ex ample of the varistor of Figure 2.

As shown in the circuit diagram of Figure 1, varistors 4, 5, 6 and 7 are electrically connected across the commutator segment pairs 2d-2a, 2a-2b, 2b-2c, and 2c-2d, respectively. The varistor 7 is also connected between the contact points of the governor switch 3 so as to serve both to quench sparks between the commutator segments 2c and 2d and those between the contacts of the governor switch 3.

The varistors 4, 5, 6 and 7 are formed on a substrate disc 11 of, for example, alumina of about 1 mm thickness, as shown in Figure 2.

This Figure also shows metallic commutator segments 2a-2d embedded in a molded and hardened plastics resin core 21. An end part of each metallic commutator segment 2a-2d is buried in a base part 10 of the plastics resin which is integral with the core 21. The governor switch 3 is mounted on the base part 10.

Metallic contacts 22c, 22d, are connected in any convenient way to the metallic commutator segments 2c, 2d, respectively.

Figure 3 is a plan view of an example of a varistor disc. The alumina disc 11 has a through.

bore 111 and a keying indent 112 for engaging with a detent 10a of a bottom protrusion 101 of the base 10 to ensure a precise angular positioning of the disc 11 with respect to the base 10. Varistors 4, 5, 6 and 7 are formed as a ring-shape film on one face of the alumina disc 11 by a known method. The varistor electrodes 8a, 8b, 8c and 8d are formed by means of coating known silver paint on the ring-shaped varistor film, and subsequently a coating of a low-melting-point solder is formed by a known screen printing method on each of the electrodes.

The varistor disc produced in the abovementioned way is then installed at the bottom face of the base 10 of the commutator as shown in Figure 2.

The varistor electrodes 8a, 8b, 8c and 8d are correctly disposed to face corresponding metallic contacts 22a, 22b, 22c and 22d, respectively, by mounting the bore 111 of the disc on the bottom protrusion 101 and the engagement of key indent 112 with the key detent 10a.

Then the assembly is put in a chamber at an elevated temperature of about 200 OC in order that the low-temperature solder on each electrode melts and firmly connects the electrodes 8a, 8b, 8c and 8d with the metallic contacts 22a, 22b, 22c and 22d, respectively. As the metallic contacts 22a, 22b 22c and 22d are connected to the metallic commutator segments 2a, 2b, 2c and 2d, respectively, and moreover, the metallic contacts 22c and 22d are also connected across the contacts of the governor switch 3, there is formed the circuit of Figure 1.

The projecting end parts of the metallic contacts 22a, 22b, 22c and 22d are used to make connections to the ends of the rotor coils of the motor.

The keying indent and keying detent can be reversed if desired; e.g. a keying indent can be formed on the bottom protrusion of the base of the commutator and a keying detent can be formed on the varistor disc.

Figure 4 shows a modified example of the varistor disc. In this disc, one varistor, 7, is formed to have a larger area than the other three varistors, to take account of the extra requirement for a quenching of sparks between the commutator segments pair 2c-2d and contacts of the governor switch 3.

As elucidated in the above-mentioned detailed description, the spark quencher arrange ment disclosed is characterized in that the varistor disc has a keying indent or detent for engaging with keying detent or indent formed on the bottom protrusion of the base part of of the resin mold of the commutator and the silver-painted electrode on the varistor disc has a further coating of the low-melting point solder.

As a result of the above-mentioned con struction, the spark quencher of the present invention is easily and precisely fixed to the commutator base part by means of the engagement of the keying indent and keying detent with each other, the varistors are rigidly and compactly fixed to the commutator base part, thereby ensuring stable and smooth revolution as designed without vibration due to eccentricity of center of weight by dislocation of the varistors, and there is no possibility of damaging the varistors by excessive heating thereof when soldering to the contact metals of the commutator. Furthermore, if one wishes to employ the varistor disc with different sizes of varistors, a satisfactory and suostantially uniform spark quenching effect for various pairs of metallic commutator segments, including the pair which is in parallel with the governor switch, is obtainable.

It should now be apparent that the spark quencher of a small-sized motor as described hereinabove, possesses useful advantages.

Claims (9)

WHAT WE CLAIM IS:

1. A spark quencher attachment for small electric motors comprising a disc adapted to be mounted on the commutator of said motor in a plane normal to its axis and in a predetermined angular position with respect to the commutator, at least two electrodes located in different angular positions on one face of said disc and available for electrical connection to respective segments of the commutator when said disc is mounted thereon, and at least two varistors positioned on said disc between said electrodes, with each resistor being electrically connected between said electrodes, or between adjacent pairs of electrodes, if more than two.

2. A spark quencher according to claim 1, wherein said disc is provided with a central aperture shaped to receive an axial boss formed on one end of the commutator, and with a key or keyway by means of which the disc may be keyed onto said boss in said predetermined angular position.

3. A spark quencher according to claim 1 or 2, for use with a small electric motor fitted with a governor switch, wherein one of said varistors, located on said disc in a position electrically to bridge said switch, is larger than the other varistors.

4. A spark quencher according to claim 1, 2 or 3, wherein said disc is a disc of alumina and wherein said resistors are formed by deposition of a film of non-ohmic resistor material on one surface of the disc.

5. A spark quencher according to any one of the preceding claims, wherein said electrodes are formed from a low melting point solder thereby to permit the electrical connections to the commutator segments to be made by mounting the disc in said predetermined angular position on said commutator and with said electrodes in physical contact with leads to the commutator segments and heating the disc to fuse the electrodes to said leads.

6. A spark quencher according to claim 1, substantially as hereinbefore described with reference to Figure 3 or Figure 4 of the accompanying drawings.

7. An electric motor when fitted with a spark quencher attachment as claimed in any one of the preceding claims.

8. An electric motor according to claim 7, wherein the spark quencher attachment is as required by claim 2, or any one of claims 3-5, as dependent thereon, wherein the commutator of said motor has an axially extending boss on an end face thereof and onto which is keyed said disc, the said commutator having electrical leads extending therethrough from the commutator segments to said end face there to make electrical contact with the said electrodes on the face of the disc.

9. An electric motor according to claim 8, wherein said spark quencher attachment is substantially as hereinbefore described in Figure 3 or 4 of the accompanying drawings and is mounted on the commutator of said motor in a manner substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.