A brush arrangement for DC motor

Abstract: A DC motor is provided with a brush arrangement in which two parts of the brushes are arranged around the peripheral surface of a commutator so that each line across the opposite brushes with respect to the central axis of the commutator forms an angle, thus the space between the positive and negative brushes is sufficient to mount components thereon. Each width of the brushes is determined by the number of conducting segments in contact with the brushes of the same polarity, affording sufficient current to flow in the brushes.

Description: Field of the invention The present invention relates to a multipolar D.C. motor having an improved brush arrangement, and more particularly, to a brush arrangement having a relatively large brush section area for large current and sufficient space for arranging components. Prior art One type of conventional six-pole D.C. motor having an armature coil wound in the known manner of double-layer wave windings, as disclosed for instance in japanese Utility Model Applications published (first publication) under No. 58-143482 and 58-150355, comprises a pair of positive brushes and a pair of negative brushes. These four brushes are circumferentially disposed around a commutator at prescribed angular intervals, and are in sliding electrical contact with the commutator. The positive brushes are 120 degrees apart, and the negative brushes are also 120 degrees apart. A positive brush and an adjacent negative brush are 60 degrees apart, and another positive brush and another adjacent negative brush are also 60 degrees apart. In the conventional D.C. motor mentioned above, however, each space between the positive brush and the adjacent negative brush is very tight to dispose the springs and the brush holders, therefore, the width of the brush holder is restricted from the space. Also, the width of the brush is restricted from the width of the brush holder, so that the section area of the brush is restricted. Because of this, the current density in the brush is higher with respect to the rating, so that the durability of the brush is relatively low. SUMMARY OF THE INVENTION In consideration of the above described difficulties, it is an object of the present invention to provide a brush arrangement for D.C. motor in which the space between the positive brush and the negative brush can be made relatively wider. Another object of the present invention is to provide a brush arrangement for D.C. motor in which the current density in the brush can be made relatively low. In an aspect of the present invention, there is provided a DC motor as defined in claim 1. Preferably the width of each brush with respect to the circumference of the commutator is determined by the number of conducting segments in contact with the brushes having the same polarity, and the number of the conducting segments is less than 6, so that the section area of the brushes can be made larger than the one of conventional brushes. Because of this, the current density becomes lower, and the durability of the brushes becomes higher. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a section view of a general DC motor shown by. the arrows A-A in Fig. 2; Fig. 2 is a front view of a brush arrangement shown by the arrows B-B in Fig. 1; Fig. 3 and Fig. 4 are diagrammatic views showing a brush arrangement of an embodiment of the present invention; and Fig. 5 is a diagrammatic view showing in comparison a brush arrangement of the conventional type. DESCRIPTION OF THE PREFERRED EMBODIMENT Hereinafter, an embodiment of the present invention is described with reference to the drawings. Fig. 1 shows a six-pole dc motor 10 in accordance with the present invention. Magnets 11 are arranged on the inner peripheral surface of a cylindrical yoke (not shown). Armature 12 is arranged within magnets 11, and shaft 13 of armature 12 is rotatably supported by opposite end brackets 14a and 14b, both of which are secured by bolts 15. Armature 12 is provided with the core wound around in the manner of double-layer wave winding. An Annular brush stay 16 is mounted on the inner face of the rear bracket 14b so as to be arranged around the end portion of shaft 13, that is, so as to surround the end portion of shaft 13. Around the end portion of shaft 13 is provided a commutator 17 which fixedly fits the shaft 13. Brush 18a which is described later is retained by brush holder 19a, and also, is urged by the end portion of spring 20. The arrangement of the brush, brush holder, and spring is described with reference to Fig. 2. In Fig. 2, brush holders 19a and 19a are mounted on the brush stay 16. The brush holders 19b and 19b are insulated from and mounted on brush stay 16. Each brush holder 19a, 19a, 19b, and 19b is made of a metal plate formed into a gutter-like configuration having a rectangular cross section. These brush holders 19a, 19a, 19b, and 19b are circumferentially arranged at angular intervals on the annular brush stay 16 so as to surround the commutator 17. In addition, brush holders 19a and 19a receive a pair of negative brushes 18a and 18a, respectively, while the other brush holders 19b and 19b receive a pair of positive brushes 18b and 18b, respectively. These brushes 18a, 18a, 18b, and 18b are slidably positioned in brush holders 19a, 19a, 19b, and 19b in radial direction. Four spiral springs 20 are mounted on brush holders 19a, 19a, 19b, and 19b, two of which are mounted between narrower portion between brush holders 19a and 19b to urge each end surface of the positive brushes 18b and 18b, respectively, and the other two of which are mounted on the wider portion between brush holders 19a and 19a to urge each one end surface of the negative brushes 18a and 18a, respectively. Accordingly, each other end surfaces of the brushes 18a, 18a, 18b, and 18b are resiliently contacted to the outer peripheral surface of commutator 17. In addition, pigtails 21 and 21 are extended from respective negative brushes 18a and 18a, and grounded to brush stay 16. While pigtails 22 and 22 are also extended from respective positive brushes 18b and 18b, and connected each other for a power source through grommet 23. The brush arrangement is shown in Fig. 3 and Fig. 4. Fig. 4 shows that commutator 17 is slightly rotated with respect to Fig. 3. Commutator 17 is composed of a number of segments 24, and integrally formed with an insulation resin 25. Accordingly, each of the segments 24 is insulated by the insulation resin 25. The number of segments 24 is the same as the one of slots for armature 12. The number of slots is twenty six in this embodiment. One pair of positive and negative brushes 18b and 18a is opposite to each other with respect to the center of commutator 17, and the other pair of positive and negative brushes 18b and 18a is also opposite to each other. Also, when one positive brush 18b is positioned at the center of a segment 24, the other positive brush 18b is positioned between adjacent segments 24. In case of the negative brushes 18a, these are in the same relationship described above. In this case, eight and half segments 24 are present between the brushes of same polarity. In case of the six-pole dc motor, the brushes of different polarity can be arranged at every 60 degrees apart, on the other hand, the brushes of same polarity can also be arranged at every 120 degrees apart. The number of the segments 24 which is arranged between the brushes of same polarity is determined by the following equation; P = n x (120/360) = n/3 where "P" represents the number of the segments between the brushes of same polarity, and "n" represents the number of the segments. If the number of the segments "n" is substituted by "26", the following value is obtained; P = 26/3 = 8.67 It is noted that the number of the segments between the brushes of same polarity is 8.67. Thus, the number of segments 24 between the brushes of same polarity is less than the conventional arrangement of 0.17, so that the interval between the brushes of same polarity is narrower than the conventional arrangement, while the interval between the brushes of different polarity becomes wider. When the total number of segments 24 or slots is divided by "3" and the fractional portion of the result is larger than "0.5", then, one of the two brushes having the same polarity is positioned at the center of a segment 24, and the other is positioned between two adjacent segments 24. As a result, the interval angle between the brushes of different polarity becomes 62.3 degrees which is 2.3 degrees wider than the conventional interval angle which is 60 degrees. In addition, even though commutator 17 is rotated from the position shown in Fig. 3 to Fig. 4, the interval angle is determined so that the segments 24 which are not contacted with the brushes having the same polarity is not less than "6", so that the maximum width of the brushes can be obtained without reducing the number of the segments 24. In this embodiment, the number of segments 24 is twenty six, but twenty three or twenty nine segments 24 can be used. For example, assuming that "23" is substituted for "n" in the above equation, P = 23/3 = 7.67 is obtained, therefore the value in which the fractional portion, "0.67" is also larger than "0.5". In addition, "29" is substituted for "n", P = 29/3 = 9.67 is obtained. The brush arrangement of the embodiment in comparison with the one of a conventional D.C. motor is described with reference to Fig. 5. Assuming that the total number of segments 24 is twenty six as in the above embodiment. Also, each narrower opposite angle is 60 degrees apart, while each wider opposite angle is 120 degrees apart. In order that the conventional brush arrangement meets the operation principle of the dc motor, that is, when one opposite negative and positive brushes 18a and 18b are contacted with six segments 24, the other opposite negative and positive brushes 18a and 18b are contacted with four segments 24, each width of brushes 18a, 18a, 18b, and 18b must be narrower than that of the embodiment as shown by the chain double-dash line. In addition, each narrower opposite angle is 60 degrees smaller than the angle of the embodiment which is 62. 3 degrees. Accordingly, the width of the brushes in the embodiment is wider than that of the conventional brushes, so that the section area of the brushes can be made larger than that of the conventional brushes. Because of this, the current density is lower, and the durability of the brushes is higher. In addition, the narrower opposite angle is wider than that of the conventional thereof, so that it is easy to arrange components, such as springs 20 and brush holders 19a and 19b, between brush holders 19a and 19b on brush stay 16.

Claims: A DC motor (10) having four brushes (18a, 18a, 18b, 18b) and six poles, the four brushes (18a, 18a, 18b, 18b) being resiliently in contact with conducting segments (24) of a cylindrical commutator (17) having a peripheral surface and a central axis, a plurality of conducting segments (24) being formed along said peripheral surface of said commutator (17), wherein said four brushes (18a, 18a, 18b, 18b) are arranged in two pairs, said pairs being such that the constituent brushes thereof are of an opposite polarity, and two brushes of the same polarity are disposed consecutively, and each brush of a given polarity in one of said two pairs of brushes lies directly opposite the brush of opposite polarity in the other pair of said two pairs of brushes with respect to said central axis of said cylindrical commutator (17), thereby defining two lines, each line including a brush of a given polarity from one of said pairs and a brush of an opposite polarity from the other of said pairs, said two lines intersecting at the central axis of said cylindrical commutator (17), said DC motor being characterized in that said two lines form an acute angle of more than sixty degrees with respect to one another, and in that the total number of said conducting segments (24) is such that when divided by three, the fractional portion of the result of said division is greater than 0.5, as a consequence of which, when one brush of a given polarity is positioned in the center of a conducting segment (24), the other brush of the same polarity will lie between two adjacent conducting segments (24). A DC motor (10) according to claim 1, characterized in that the width of each brush (18a, 18a, 18b, 18b) with respect to the circumference of said commutator (17) is determined by the number of said conducting segments (24) in contact with said two brushes having the same polarity and in that the number of said conducting segments in contact with said two brushes of the same polarity is less than 6.

Inventors: KOBAYASHI KOUJI, KOBAYASHI, KOUJI
Applicants: MITSUBA ELECTRIC MFG CO, MITSUBA ELECTRIC MANUFACTURING CO., LTD.
Application year: 1990
Main classification: H02K13
IPC codes: H02K13, H02K23