GB2063577A - Motor - Google Patents

Abstract

An electric motor has brushes made of a resilient and electrically conductive material, each brush comprising a brush base 2 and a commutator slide portion 3 integral with the brush base. A layer of a non-rubber sheet material 7 is applied on the commutator slide portion 3 to extend longitudinally along at least a portion thereof. The sheet material 7 is of paper, synthetic paper, vinyl, polyethylene, aluminium or other metal foil and is said to provide improved suppression of vibration as compared with rubber sheet material. The layer is secured adhesively, preferably by a double-sided material 15. A rubber- based adhesive is preferred. <IMAGE>

Description

SPECIFICATION Motor This invention relates to electric motors and to brush gear therefor.

A so-called "small electric motor" commonly has permanent magnet poles and is driven by a d-c power supply, which is fed to the windings of the motor via brushes and a commutator A brush in such a small electric motor suitably comprises a brush base and a commutator slide portion integral therewith, both being made of a resilient and electrically conductive material, such as a copper-berylium alloy thin strip, the commutator slide portion and the brush base make a predetermined angle relative to each other. The brush is supported by the brush base being held in an appropriate supporting means provided, for example, on a portion of the motor case. The commutator slide portion of the brush is biased into contact with the commutator by resiliency of the bent brush base.Because of the rotation of the commutator with the motor shaft, however, vibrations are caused in the commutator slide portion as it passes over the irregular surface of the commutator, particularly as it passes gaps between commutator segments. The vibrations of the commutator slide portion cause electrical sparks to generate between the commutator slide portion and the commutator, resulting in unwanted phenomena, such as electrical noise and erosion on the commutator slide portion. This erosion both reduces the life of the brush and causes damage to the commutator surface by abrasion. Abraded material tends to build up in gaps between the commutator segments, causing the commutator segments to conduct and the performance of the motor correspondingly to deteriorate. A certain degree of.

vibrations is inevitable in such electric motors.

Suitably, however, such vibrations, once generated, should be damped as soon as possible. As is well known, it has been suggested to improve vibration damping by applying a layer of rubber sheet material to the commutator slide portion of the brush.

We have now found, quite contrary to the previous belief, that greatly improved vibration damping properties can be achieved when a layer of a sheet material other than rubber (such as paper) is applied on the commutator slide portion.

Accordingly, the present invention provides, in one aspect thereof, an electric motor having brushes made of a resilient and electrically conductive material, each brush comprising a brush base and a commutator slide portion integral with the brush base, and a layer of a non-rubber sheet material being applied on the commutator slide portion to extend longitudinally along at least a portion thereof.

The invention also extends to the individual brushes as so defined.

We have found that use of an adhesive material consisting of a substrate coated with adhesive on both sides is particularly beneficial for applying the layer of sheet material on the commutator slide portion.

As we shall explain below, the nature of any adhesive employed in the application of the sheet material to the commutator slide portion has a significant effect on the vibration damping properties and the wear life of brushes and commutators.

The invention is hereafter more particularly described by way of example only with reference to the accompanying drawings, in which: Figures 1 to 4 are vibration waveform diagrams of assistance in explaining the vibration damping properties of different brush gear; Figure 5 shows, in elevation and partly in section, the commutator and brush gear of one embodiment of electric motor according to this invention; Figure 6 is a vibration waveform diagram of assistance in comparing the vibration damping properties when a commutator slide portion is caused to make contact with a revolving commutator in an electric motor in accordance with this invention and in a motor utilizing the prior art;; Figures 7A and 7B are elevational views of assistance in explaining the relationship between a brush and a commutator in an electric motor at different phases of commutator revolution; Figures 8A and 8B illustrate erosion in commutator slide portions; Figure 9 illustrates how a commutator slide portion moves during rotation of the motor; Figures lOA and 10B are diagrams of the waveforms of commutated voltages; and Figure 11 is an elevational view partly in section illustrating the commutator slide portion of another embodiment of electric motor in accordance with this invention.

After a series of experiments to find ways and means for more rapidly damping vibrations generated in the commutator slide portions of brushes of electric motors we have discovered that applying a layer of non-rubber sheet material, for example, a paper sheet, on the commutator slide portions of the brushes can improve vibration damping properties much better than the conventional means.

The results of our tests are shown in Figs. 1 to 4. The construction of the commutator slide portion which achieved the best results in the tests is shown in Fig. 5. The test results shown in Figs. 1 to 4 are vibration waveforms obtained under the same testing conditions, the commutator slide portion used in the particular test being illustrated at the right hand of each figure.

Fig. 1 shows the measurement results, plot ted as a graph with time as the x coodinate and the vibrational displacement converted into voltage values as the y coodinate. Vibrational displacement was obtained in the following manner. A brush 1 having the construction shown at the right hand of the figure was fixed at its brush base 2 by a fixing means (not shown), and vibrations were applied to the commutator slide portion 3 thereof.The displacement in the middle of the commutator slide portion 3 was detected with a sensor 4 and measured with an oscillograph. (The thickness of the brush 1 used was approximately 0.08 mm.) Similarly, Fig. 2 shows the measurement results obtained when a layer of rubber sheet material 5 was applied on a commutator slide portion 3 of the same size and shape as that used in the first test with a non-setting adhesive 6; Fig. 3 shows those obtained when a paper sheet 7 was applied on the commutator slide portion 3 with a setting adhesive 8; and Fig. 4 shows those obtained when a paper sheet 7 was applied on the commutator slide portion with a non-setting adhesive 6. The commutator slide portions 3 used were of the same size and shape in each case. The vibration waveform shown in Fig. 1 is that obtained in the brush 1 in the absence of any specific vibration damping means.As is apparent from the figure, it takes approx. 200 ms for vibrations to be effectively damped to zero. Fig. 2 shows the vibration waveform obtained in the brush 1 having a known vibration damping means, that is, a layer of rubber sheet material 5 applied on the commutator slide portion 3 with a non-setting adhesive 6. In this case, the time elapsed for vibrations to be effectively damped to zero is 140 ms, which is much faster than in Fig. 1.

Figs. 3 and 4 show the vibration waveforms obtained in brushes in accordance with the present invention. As shown in Figs. 3 and 4, a paper sheet 7 is applied on the commutator slide portion 3 of the brush 1. Fig. 3 shows the results of vibration tests using a brush having the paper sheet 7 applied on the commutator slide portion thereof with a quicksetting so-called "instant" adhesive 8. Fig. 4 shows the test results using a brush with the paper sheet 7 applied with a non-setting adhesive 6. As shown in Fig. 3, the elapsed time for vibrations to be effectively damped to zero in the brush with the paper sheet applied with the setting adhesive 8 is approximately 50 ms. This represents a considerable improvement in vibration damping properties, as compared with that shown in Fig. 2.Fig. 4 shows the vibration waveform obtained in a brush with the paper sheet 7 applied on the commutator slide portion 3 with a non-setting adhesive 6. As is apparent from the vibration waveform, the elapsed time for vibrations to be effectively damped to zero is as low as 20 ms. In other words, the damping time in Fig. 4 is by far the shortest and the vibration damping properties are drastically improved, compared with the results shown in Figs. 1 to 3.

As the above tests show, a small electric motor using a brush with a paper sheet 7 applied on the commutator slide portion 3 thereof with a non-setting adhesive can be expected to exhibit excellent vibration damping properties. Fig. 5 is an enlarged elevational view, partly in section, of the brush with which the results shown in Fig. 4 were obtained, and showing the brush mounted in position with its commutator slide portion 3 biased into contact with a commutator 9 having commutator segments 10 and rotated by a motor shaft 11 via an insulating cylinder 12.

Fig. 6 shows the vibration test results obtained with brushes actually incorporated in a small electric motor. The following description will show that an electric motor according to this invention exhibits excellent damping properties in response to vibrations generated during rotation of motor and in comparison with a similar motor according to the prior art. The waveform shown by arrow A in Fig. 6 is the vibration waveform obtained using a brush with a layer of rubber sheet material 5 applied on the commutator slide portion 3 thereof with a non-setting adhesive 6, as illustrated diagrammatically at A' at the left-hand side of the figure.The waveform shown by an arrow B in the figure is the similar vibration waveform obtained using a brush with a paper sheet 7 applied on the commutator slide portion 3 thereof with a non-setting adhesive 6, as illustrated diagramatically at B' at the lefthand side of the figure.

The vibration waveforms A and B were obtained by measuring displacements in the middle of the commutator slide portion 3 with a sensor 4 during the rotation of the commutator 9 at 2,400 rpm.

In general, a commutator such as 9 comprises a plurality of segments 10 (three segments in the example shown in Fig. 6) disposed at predetermined equal spacings. Vibrations are generated as the commutator slide portion 3 passes over the edges of each segment 10 in turn. The waveforms at times t, and t2 represent those vibrations. An observation of the gradual damping of the vibrations generated at times t, reveals that, whereas the vibration in the waveform A remains undamped even immediately before time t2 when the next vibration is generated, the vibration in the waveform B generated at time t, has been almost completely damped 2 ms after its generation. It is apparent from the test results shown in Fig. 6, therefore, that the brush used in test B which is in accordance with this invention has excellent vibration damping properties as compared with the brush in accordance with the prior art.

In the examples cited above, we have referred specifically to a brush in which a paper sheet was used as the layer of sheet material applied on the commutator slide portion. We have found that the same effect can also be achieved by using layers of other non-rubber sheet materials, for example, a synthetic paper, vinyl, polyethylene, or aluminium and other metallic foil sheet. Why a brush with a layer of non-rubber sheeting applied on the commutator slide portion thereof with a nonsetting adhesive should have such excellent vibration damping properties is not completely clear to us at present. We believe, however, that the smaller coefficient of elasticity of nonrubber sheet materials as compared with that of conventional vibration-damping materials such as rubber may be significant.

As is apparent from the above description, a small electric motor having good vibration damping properties can be achieved by applying sheet materials on the commutator slide portions of the brushes. We have found, however, that among small electric motors of the above-mentioned construction which we produced for test purposes, there were some motors in which commutator segments were caused to conduct after approximately 100 hours of service. Investigation of the cause of such conduction between commutator segments has shown that this conduction may be attributed to the difference of adhesive type used for applying the sheet 7 on the commutator slide portion 3.

That is, those small electric motors in which the above-mentioned conduction occurred used a Si-based adhesive (Trade Name: Densil 2078) as the adhesive 6 while those motors which did not suffer this effect used a rubberbased adhesive (Trade Name: Sony Chemical T-4120). Plan views of the commutator slide portions 3 of both such types of motors taken out after a predetermined period of service are shown in Fig. 8A and 8B. In the figures, reference numerals 1 3 and 13' refer to eroded portions on the commutator slide portions 3, which were produced by electric sparks between the commutator slide portions 3 and the commutator segments 10 during the rotation of the motors. Arrow a represents the position at which the commutator slide portion 3 makes contact with only one commutator segment 10 (i.e. positions corresponding to that shown by arrow a in Fig.

7A). Arrows b and c represent the positions at which the commutator slide portion 3 makes contact with two adjoining commutator segments 10 (i.e. positions corresponding to those shown by arrows b and C in Fig. 7B). In Figs. 8A and 8B, the eroded portions 1 3 at the positions shdwn by arrows b and C in the figures are inevitably produced as the commutator slide portion 3 is separated from one commutator segment 10 and makes contact with the next, as shown in Fig. 7B.Inspection shows that the commutator slide portion 3 shown in Fig. 8A has no eroded portion at the position where the commutator slide portion 3 makes contact with a single commutator segment alone during rotation (the position corresponding to arrow a in Fig. 7A) and so causes no corresponding damage to the surface of the commutator segment 10 in the state shown in Fig. 7A. On the other hand, the commutator slide portion 3 has eroded portion 13' at the position shown by arrow a in Fig. 8B and so correspondingly cuts the surfaces of the revolving commutator segments 10 with the consequence that abraded material resulting from the cutting builds up in the gaps between the commutator segments 10, causing unwanted conduction between the commutator segments 10.

We explain below how, in our view, the eroded portion 13' is produced.

Fig. 9 shows the vertical displacement of the commutator slide portion 3 at the point shown by arrow a in Fig. 7 during one revolution of the motor shaft 11. Thus the commutator slide portion 3 at the point shown by arrow a in Fig. 7 remains on a high level, as shown by arrow H in Fig. 9 so long as the commutator slide portion 3 keeps in contact with the surface of a single commutator segment 10 alone (as shown in Fig. 7A).

For example, when the motor shaft 11 revolves from the state shown in Fig. 7A in the direction shown by an arrow in the figure, the position of the commutator slide portion 3 at the point shown by arrow a remains on the level H in Fig. 9 until the side edge b' of the segment 10 reaches the point a. As the motor shaft 11 further revolves, the level of the commutator slide portion 3 lowers to the state shown in Fig. 7B, that is, to a lower level, as shown at L in Fig. 9 when the commutator slide portion 3 is in contact with both adjoining segments 10. And then, the commutator slide portion 3 is gradually pushed up by the side edge c' of the segment 10 until the side edge d reaches the point a in the figure. After the side edge d passes over the point a the position of the commutator slide portion 3 is restored to the original high level as shown by arrow H in Fig. 9.In this way, the position of the commutator slide portion 3 at the point a drops to a lower level, as shown by arrow L in Fig. 9 every time the commutator slide portion 3 passes a gap between the commutator segments 10. The side edge c' of the commutator segment 10 revolves while pushing up the commutator slide portion 3 during the period in which the position of the commutator slide portion 3 at the point a is shifted from the level L to the level H in Fig. 9; in other words, during the period in which the side edge d of the next commutator segment 10 reaches the point a as the motor revolves from the state shown in Fig. 7B. This causes the commutator slide portion 3 both to be bent, during this period, and to be moved in the direction shown by arrow F due to a force exerted in the direction shown by arrow F.

This causes electric sparks, and as a result, the eroded portion 13' shown in Fig. 8B. In other words, as the movement of the commutator slide portion 3 is caused in the direction F every time the commutator slide portion 3 passes a gap between the commutator segments 10 is increased, so unwanted sparks are generated, and as a result, the eroded portion 13' is produced. That this is the case is further supported by the results of measurement we have conducted on commutated voltage waveforms, as shown in Figs. 1 0A and lOB. That is, the commutated voltage waveform of a small electric motor having only the eroded portions 1 3 as shown in Fig. 8A has only one voltage fluctuation as shown in Fig.

1 OA. On the other hand, the commutated voltage waveform of a small electric motor having the eroded portion 13' in addition to the eroded portions 1 3 has two voltage fluctuations as a result of the generation of unwanted sparks, as shown in Fig. lOB.

As is apparent from the above discussion, the cause of the eroded portion 13' shown in Fig. 8B is the longitudinal movement (movement in the direction F) of the commutator slide portion 3. One possibility for preventing this movement, therefore, which comes to mind is increasing the strength of brushes. In the embodiment in Fig. 5, the brushes made of a resilient electrically conductive material, for example, copper-berylium alloy strip, each brush comprising a brush base 2 and a commutator slide portion 3 integrally formed therewith by bending the strip. This construction necessarily limits the thickness of the brushes since the vibration coefficient of the commutator slide portions must be maintained at a desired level. It is not desirable, therefore, to increase the thickness of the brushes to improve their strength.

As described earlier, the commutator slide portion 3 is subject to a force in the direction shown by arrow F in Fig. 5 or Fig. 7 due to the rotation of the motor. This force causes the degree of bend of the commutator slide portion 3 to change, amplifying the movement of the commutator slide portion 3 in the direction F. To prevent or mitigate such amplification of the movement of the commutator slide portion 3 changes in the degree of bend of the commutator slide portion 3 should be minimised.

Applying a layer of sheet material 7 on the commutator slide portion, as in the embodiment shown in Fig. 5, has an effect in reducing changes in the degree of bend.

From our tests, we found that the degree to which changes in the degree of bend are prevented or mitigated greatly varies with the types of adhesive used for applying the sheet 7 on the commutator slide portion 3. That is, the use of a Si-based adhesive (for example, Densil 2078) has a certain degree of effect, compared with the commutator slide portion without the layer of sheet material, but the effect is not so remarkable since the test results showed that unwanted abnornal erosion 13', as shown in Fig. 8B, still occurs after approximately 100 hours of service.

When a rubber-based adhesive (for example, Sony Chemical T-4120) is used, a considerable degree of prevention in changes in the degree of bend can be expected since no erosion occurs at the point shown by arrow a in Fig. 8A. The difference in effect between the two adhesive types is, we believe, attributable to the difference in viscosity of adhesive.

The silicon-based adhesive having higher viscosity seems to permit relative displacement between the commutator slide portion 3 and the sheet material 7.

The embodiment of Fig. 11, which is based on these observations, uses an adhesive material 1 5 consisting of a substrate 14 on both sides of which an adhesive is coated, for applying a layer of non-rubber sheet mterial of paper, etc. on the surface of the commutator slide portion 3. The reason why the brush shown in Fig. 11 is more effective than that having the sheet material 7 applied by means of adhesive 6 only, as shown in Fig. 5, seems to be the use of the double coated adhesive material 1 5 which forms a multilayer construction consisting of the adhesive, the substrate 14, more adhesive and the sheet material 7, resulting in reduced relative displacement between the sheet material 7 and the commutator slide portion 3 and in an enhanced effect in mitigating or preventing changes in the degree of bend. In order to ensure positive degree of prevention of relative movement of the sheet material 7, the thickness of the adhesive material 1 5 itself should be as thin as practicable, preferably under 0.5 mm, for example, as based on our test results.

Claims (1)

1. A small electric motor having brushes made of a resilient and electrically conductive material, each brush comprising a brush base and a commutator slide portion integral with the brush base, and a layer of non-rubber sheet material being applied on the commutator slide portion to extend longitudinally along at least a portion thereof.

2. An electric motor as claimed in Claim 1, wherein said layer is applied on said commutator slide portion by adhesive.

3. An electric motor as claimed in Claim 1, wherein said layer is applied on the commutator slide portion by using an adhesive material consisting of a substrate on both sides of which an adhesive is coated.

4. An electric motor as claimed in Claim 2 or Claim 3, wherein said adhesive is rubber based.

5. An electric motor as claimed in any preceding Claim, wherein said sheet material comprises paper or synthetic paper.

6. An electric motor as claimed in any of Claims 1 to 4, wherein said sheet material comprises a vinyl sheet.

7. An electric motor as claimed in any of Claims 1 to 4, wherein said sheet material comprises a polyethylene sheet.

8. An electric motor as claimed in any of Claims 1 to 4, wherein said sheet material comprises a metallic foil sheet.

9. For an electric motor brush gear, a brush as defined in any preceding claim.

10. An electric motor according to Claim 1 and substantially as hereinbefore described.

CLAIMS (23 Sept 1980)

1. An electric motor having brushes made of a resilient and electrically conductive material, each brush comprising a brush base and a commutator slide portion integral with the brush'base, and a layer of non-rubber sheet material being applied on the commutator slide portion to extend longitudinally along at least a portion thereof.