JPH06225500A - Brush apparatus - Google Patents

Abstract

PURPOSE:To provide a brush apparatus which ensures good non-arc commutation and prevents variation of output characteristic of a DC motor. CONSTITUTION:This brush apparatus comprises a brush body 10 having a double-layer configuration consisting of a low resistance part 12 and a high resistance part 14 and a pig tail 16 as a feeding line. The brush body 10 comprises, along the rotor rotating direction, the low resistance part 12 in the upper stream side and the high resistance part 14 in the lower stream side. The end part is inclined with respect to the rotor rotating axis direction. Moreover, at this slanting part, the brush body 10 is formed so that a relative rate of the high resistance part 14 gradually increases from the end part. When the slanting part at the end part of the brush body 10 is worn, a contact area with commutator increases, but since a relative rate of the high resistance part 14 increases with progress of wearness, a contact resistance is kept almost constant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a brush device, and more particularly to an improvement of a brush device used in a DC machine.

[0002]

2. Description of the Related Art Generally, a DC motor, a DC generator or the like is constructed so that any commutator is short-circuited with a brush to perform rectification. However, due to the inductance of the armature coil or the like, the change in current during rectification is extremely delayed, and the change in current becomes large at the end of rectification, so a large reactance voltage is generated due to the inductance of the short-circuit coil. This causes a spark discharge between the brush and the commutator, roughening the commutator surface and forming an uneven surface. Therefore, the amount of wear of the brush device increases, the life of the brush device decreases significantly, and the vibration of the brush itself increases.

In particular, in order to realize downsizing of motors and the like,
When the magnet is thinned by using a high-performance magnet, the distance between the armature and the stator yoke becomes smaller. For this reason, the magnetic resistance of the armature is reduced, the inductance of the short-circuit coil is increased, and the above-mentioned problem becomes more prominent.

For this reason, various methods for performing non-spark rectification have been conventionally used. One of the representative ones is to shift the position of the brush to the magnetic center. However, in a motor with large load fluctuation or the above-mentioned motor with large inductance, the optimum shift angle of the brush fluctuates, and stable sparkless commutation cannot be performed.

There is also a method of increasing the contact resistance between the commutator and the brush to perform resistance rectification along a good rectification curve, but since the contact resistance increases, the voltage applied to the armature drops. However, there is a problem that the motor performance, especially the decrease in output torque, cannot be avoided.

For this reason, the proposal of Japanese Patent Laid-Open No. 4-46546 is known as a means for preventing the generation of sparks without a reduction in output. In this proposal, the front end of the brush is formed of a low resistance material and the rear end of the brush is formed of a high resistance material to reduce spark generation.

The tip of the brush that contacts the commutator has been devised in various ways to prevent uneven contact of the initial brush and to improve familiarity with the commutator. A typical example is a taper structure. , Bridge structure, uneven structure, etc. Here, in the taper structure, the tip of the brush is made to partially contact the commutator by inclining the tip of the brush. The bridge structure is disclosed in JP-A-55-1.
This is disclosed in Japanese Patent Publication No. 09155, and a V-shaped protrusion is provided on the tip of the brush to bring the tip of the brush and the commutator into point contact. Concavo-convex structure is based on Jitsuko Sho 60-3
It is disclosed in Japanese Patent No. 1193, and a tapered concave projection is provided at the tip of the brush. In all of these methods, the initial brush contact is partially performed at the brush tip, so that the brush tip is quickly worn and the brush and the commutator are made to fit well.

As another method, the radius of curvature of the tip of the brush is set slightly larger than the radius of the commutator, and the brush and the commutator are partially brought into contact with each other until the brush is worn. There is also.

[0009]

By the way, various methods for improving the familiarity between the brush and the commutator described above are as follows.
Since the contact area between the brush and the commutator changes with the amount of wear of the brush, the contact resistance of the brush changes, and as a result, the characteristics of the DC machine such as output torque fluctuate greatly due to fluctuations in armature current. was there.

Further, at the initial stage of operation of the DC machine, the brush and the commutator are in contact with each other over a small area, and the brush resistance increases, so that the reactance voltage of the short-circuit coil also decreases and sparks can be prevented. However, as the brush wears, the contact area increases and sparks are generated between the brush and the commutator. That is, there is a problem that the rectification characteristic greatly changes with time, and it is difficult to improve the rectification from the initial operation of the DC motor to an arbitrary operation time. When this spark is generated, counter electromotive force is generated due to the inductance component, and torque is generated periodically in the opposite direction to the rotor rotation direction.
From this point as well, the output characteristics vary with the wear of the brush.

Further, in order to prevent the generation of sparks, even when the two-layer brush disclosed in the above-mentioned Japanese Patent Laid-Open No. 4-46546 is used, the brush tip is Since the commutator partially contacts, a spark is generated depending on the contact state. Therefore, there is a problem in that the above-described periodic reverse torque is generated only in the initial stage of operation, and the output characteristics fluctuate as the brush wears.

The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is to provide a brush device capable of performing good sparkless rectification and preventing fluctuations in the output characteristics of a DC machine. To do.

[0013]

In order to solve the above-mentioned problems, the present invention is one in which a low resistance portion arranged on the upstream side in the rotor rotation direction and a high resistance portion arranged on the downstream side are integrated. In a brush device including a brush main body laminated on, the tip portion of the brush main body is inclined with respect to the rotation axis direction of the commutator, and the relative ratio of the high resistance portion to the low resistance portion in the inclined portion is It is characterized in that it is gradually increased from the tip of the brush body along the radial direction of rotation of the commutator.

Further, according to the present invention, the low resistance portion is disposed on the upstream side in the rotor rotation direction and is formed so as to be gradually narrowed from the upstream side to the downstream side, and the low resistance portion is disposed on the downstream side and the upstream side to the downstream side. In a brush device including a brush main body integrally laminated with a high resistance portion formed so as to gradually widen toward a direction, a tip end portion of the brush main body is protruded by the low resistance portion in a rotation axis direction of a commutator. And the relative ratio of the high resistance portion to the low resistance portion in the inclined portion is gradually increased from the tip portion of the brush main body along the radial direction of rotation of the commutator. .

[0015]

The brush device of the present invention comprises a brush main body having a two-layer structure in which the low resistance portion is arranged on the upstream side of the rotor rotation and the high resistance portion is arranged on the downstream side. The tip of the brush main body is inclined, and the proportion of the high resistance portion in the inclined portion is formed so as to gradually increase as the distance from the tip of the brush increases. For this reason, when the brush tip is in contact with the commutator over a small area at the beginning of operation of the DC machine, most of it contacts through the low resistance part. The contact resistance of is secured. In addition, when the tip of the brush wears, the relative proportion of the high resistance part increases with the wear of the brush, so the contact resistance does not increase as the contact area increases, and the brush resistance remains almost constant. Be drunk

In the present invention, by increasing the relative proportion of the high resistance portion of the inclined portion of the brush tip in accordance with the brush wear, the contact resistance of the brush tip can be kept substantially constant, and the characteristics of the DC machine can be changed. Can be prevented.

Further, the brush main body has a two-layer structure consisting of a high resistance portion and a low resistance portion, and good spark rectification can be performed regardless of the wear state of the brush.
This good sparkless rectification ensures stable output characteristics.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings.

First Embodiment FIG. 1 is a perspective view showing the outline of the brush device of the first embodiment. 2 is a view for explaining the structure of the brush main body shown in FIG.

The brush device of this embodiment is provided on the stator side of a DC motor, and has a brush main body 10 held by a brush holder (not shown) so as to come into pressure contact with the commutator piece 22, and the brush main body 10. It is configured to include a pigtail 16 as a power supply line that connects a power source. The brush main body 10 is formed in a quadrangular prism shape having a commutator contact surface having substantially the same width as the commutator piece 22.

The feature of this embodiment is that it has a two-layer structure consisting of a low resistance portion and a high resistance portion, and has a so-called taper structure in which the tip of the brush is inclined. The point is that the target ratio gradually increases as it moves away from the tip.

Therefore, as shown in FIG. 2A, the brush main body 10 of this embodiment has a low resistance portion 12 on the upstream side and a high resistance portion 14 on the downstream side along the rotor rotation direction W. It has a two-layer structure in which these two resistance parts 12 and 14 are integrally laminated. Here, it is desirable that the low resistance portion 12 contains 30 to 70 wt% of copper, and that the resistance ratio between the low resistance portion 12 and the high resistance portion 14 is set to 5 times or more or the contact voltage drop ratio is set to 5 times or more. . The low resistance portion 12 of the embodiment contains 40 wt% of copper, and the low resistance portion 1
2 and the high resistance portion 14 are set to have a resistance ratio of 10 times.

The tip of the brush main body 10 of this embodiment has a taper structure which is inclined with respect to the rotor (commutator) rotation axis direction. 2B is a view of the brush body 10 shown in FIG. 2A as viewed from the pigtail 16 side, the tip shape of which is a taper structure as shown in FIG.

Further, the brush main body 10 of this embodiment is formed so that the relative proportion of the low resistance portion 12 and the high resistance portion 14 in the inclined portion of the tip gradually changes. The triangular prism ABCDEF in FIG. 1 is the inclined portion (tapered portion) of the tip of the brush main body 10, but the vicinity of the tip AB is formed only by the low resistance portion 12, and it is linear as it approaches the CDEF where the inclination ends. Is formed so that the relative proportion of the high resistance portion 14 increases. Brush body 1 other than the inclined part
In 0, the relative proportion of the low resistance portion 12 and the high resistance portion 14 is constant.

That is, while the inclined portion is worn, the relative proportion of the high resistance portion 14 increases as this wear progresses, and after the brush main body 10 contacts the entire surface of the commutator 22, the low resistance portion 12 is removed. The ratio of the high resistance portion 14 becomes constant.
For example, the ratio of the low resistance portion 12 and the high resistance portion 14 is 50:
It is desirable to set it to 50 to 75:25.

The brush device of this embodiment has the above-mentioned structure, and its operation will be described below.

FIG. 3 is a diagram showing the relationship between the worn state of the brush and the contact area. 1A shows the contact state between the brush main body 10 and the commutator 20 in the initial state of the brush, and FIG. 1B shows the brush main body 10 when the inclined portion of the tip of the brush main body 10 is about half worn. The state of contact between the commutator 20 and the commutator 20 is shown in FIG. 6C, in which the inclined portion of the tip of the brush body 10 is completely worn and the brush body 10 is in full contact with the commutator 20. .

At the initial stage of operation of the DC machine, FIG.
As shown in FIG.
Only the vicinity of B contacts the commutator 20. Although the contact area S1 at this time is small, almost all of the area S1 is occupied by the low resistance portion 12, and the contact resistance R1 at this time has a large value in spite of the small contact area.

When the DC machine is operated and the inclined portion of the tip is about half worn, as shown in FIG.
The relative proportion of the high resistance portion 14 exposed gradually increases as the wear of the inclined portion progresses. Contact area S at this time
2 rapidly increases with brush wear, but this area S2
Since the relative proportion of the high resistance portion 14 in the inside gradually increases, the contact resistance R2 at this time becomes almost the same as the contact resistance R1 at the initial stage of the operation described above.

The slanted portion is completely worn and the brush body 10
After contacting the entire surface of the commutator 20, the same figure (C)
As shown in, the ratio of the low resistance portion 12 and the high resistance portion 14 becomes constant. The contact area S3 at this time becomes equal to the cross-sectional area of the brush main body 10, and becomes constant after the contact with the entire surface. Further, the contact resistance R3 after hitting the entire surface is also constant, but this contact resistance R3 is almost the same as the above-mentioned contact resistance R1 or R2.

In this way, the contact area gradually increases to S1, S2 and S3 as the brush wear of the inclined portion of the tip progresses, and becomes constant after the contact with the entire surface. However, as the contact area increases, the relative proportion of the high resistance portion 14 occupying that area also increases, so that the contact resistances R1, R2, R3 are substantially equal regardless of the wear state.

Therefore, even if the tip of the brush main body 10 having the taper structure is gradually worn and the contact area with the commutator 20 is gradually increased, the contact resistance is almost constant and a stable armature current can be supplied. You can Therefore, it is possible to prevent fluctuations in the characteristics of the DC machine such as output torque.

Next, the rectification characteristics when the brush device of this embodiment is used will be described in comparison with a conventional brush device formed of a single material.

FIG. 4 is a diagram for explaining the rectifying characteristics of a conventional brush device having a taper structure.
The contact part and the rectification curve in the initial state of the brush are
FIG. 3B shows the contact portion and the rectification curve when the brush is worn and the entire surface is contacted.

In the initial state of the conventional brush, the brush main body is only partially in contact with the commutator by the tapered portion at the tip of the brush, as shown in FIG. Therefore, the contact resistance is large, the inductance of the armature coil can be substantially ignored, and good rectification without spark generation can be obtained.

When the current flowing through the brush body is I, the current is connected to the commutator element which is in contact with the brush body.
An armature current of I / 2 flows through each armature coil of the book. Therefore, the current i flowing in the armature coil short-circuited in the brush main body is I / 2 in the initial stage of rectification and −I / 2 after the rectification period has elapsed, and within the rectification period from I / 2 to −I.
It changes linearly toward / 2.

Further, when the conventional brush is worn and sufficiently fitted to the commutator, as shown in FIG.
The brush body contacts the commutator on its entire surface. Therefore, as the contact area increases, the contact resistance also decreases, the inductance component of the armature coil cannot be ignored, and the rectification action is delayed. For this reason, a reactance voltage is generated due to a rapid current change in the latter half of rectification, and a spark is generated between the brush and the commutator.

As described above, when the conventional brush device is used, the contact area and contact resistance between the brush and the commutator change with time, and it becomes impossible to perform a constant rectifying action. Therefore, due to the counter electromotive force generated in the latter half of the rectification, the rotational torque in the direction opposite to the rotor rotation direction is applied at the same period as the rectification period, and abnormal noise or vibration is generated. In addition to the absolute value of any output torque, the periodic fluctuation state is one of the characteristics of the motor in a broad sense, and from this point as well, the characteristics of the motor change according to the amount of brush wear. .

On the other hand, when the brush device of this embodiment is used, it is possible to perform a constant rectifying action over a wide range from the initial state of the brush to the state where the brush is worn and hits the entire surface.

FIG. 5 is a diagram for explaining the rectifying characteristics of the brush device of this embodiment. In FIG. 5A, the contact portion and the rectifying curve in the initial state of the brush are shown in FIG. Shows the contact portion and the rectification curve when the brush is worn and is in full contact.

In the initial state of the brush of this embodiment,
As shown in FIG. 4A, the brush body 10 is only partially in contact with the commutator 20 at the taper portion of the brush tip. Therefore, as in the case of the conventional case described above, good rectification characteristics without spark generation are obtained.

If the inclined portion of the tip of the brush main body 10 is worn and is sufficiently fitted to the commutator 20, the brush main body 10 becomes a commutator on its entire surface as shown in FIG. Contact. However, in this brush body 10, the low resistance portion 12 is laminated on the upstream side in the rotor rotation direction, and the high resistance portion 14 is laminated on the downstream side. Therefore, the commutator piece 22 that has rushed into the rectification rises by coming into contact with the low resistance portion 12, and the rectification proceeds promptly. Actually, the rectification is delayed due to the inductance component of the armature coil, and rectification is performed along an ideal rectification curve that linearly changes as shown in FIG. 5B. It is possible to prevent changes in current and generation of sparks.

Further, even if the slanted portion of the brush body 10 is worn, the change in current in the latter half of the commutation does not increase, so that stable torque characteristics can be obtained regardless of the worn state of the brush, and the motor characteristics can be improved. Can be stabilized.

As described above, the brush main body 10 in which the relative proportion of the high resistance portion 14 in the inclined portion of the tip is gradually increased.
By using the brush device of the present embodiment including the above, since the contact resistance is almost stable, a constant armature current can be flowed regardless of the state of brush wear, and it is possible to prevent the output torque from changing over time. You can Further, since there is no abrupt current change in the latter period of commutation before and after wear, it is possible to prevent the generation of sparks, and also to prevent the generation of torque in the direction opposite to the rotor rotation direction.
It is possible to prevent periodic fluctuations in output torque. Therefore, according to the brush device of the present embodiment, it is possible to prevent the torque fluctuation and the characteristic fluctuation of the DC machine.

Other Embodiments FIG. 6 is a perspective view showing the structure of the brush device of the second embodiment. The feature of this embodiment is that the brush main body 60 is formed so that the resistance gradually increases from the upstream side to the downstream side in the rotor rotation direction, and the tip of the brush main body 60 is inclined with respect to the rotor rotation axis direction. There is a point.

Therefore, the brush main body 60 has a low resistance portion 62 in which the commutator contact surface has a triangular prism shape of an isosceles right triangle.
Also, the high resistance portion 64 is integrally formed. Here, the low resistance portion 62 is arranged on the upstream side in the rotor rotation direction so that the commutator contact surface becomes gradually narrower from the upstream side to the downstream side in the rotor rotation direction, and the high resistance portion 64 is the commutator contact surface. Are arranged on the downstream side in the rotational direction so as to gradually widen from the upstream side to the downstream side in the rotor rotational direction.

The tip of the brush body 60 is inclined with respect to the direction of the rotor rotation axis so that the low resistance portion 62 side projects. Therefore, when the inclined portion gradually wears, the relative proportion of the high resistance portion 64 exposed gradually increases as the wear progresses.

Therefore, even if the contact area with the commutator increases due to wear of the tip portion, the contact resistance can be kept substantially constant, and the rectifying characteristic and the output torque become constant. Is the same as in the first embodiment described above.

Further, since the brush main body 60 of the present embodiment is formed so that the brush resistance gradually increases from the upstream side to the downstream side in the rotor rotation direction, the potential of the commutator piece that has rushed into the rectification is suddenly increased. Rise and rectification progresses quickly,
The point that abrupt current change and spark generation in the latter half of rectification can be prevented is the same as in the first embodiment described above.

In particular, the brush main body 60 included in the brush device of this embodiment has the low resistance portion 62 at the inclined portion of the tip.
And the laminated state of the high resistance portion 64 is the same as the laminated state of the low resistance portion 62 and the high resistance portion 64 in other portions,
There is also an advantage that the brush main body 60 is easily manufactured.

FIG. 7 is a perspective view showing the structure of the brush device of the third embodiment. The feature of this embodiment is that the brush main body 10 of the first embodiment described above is expanded for bidirectional rotation, and the brush main body 70 is provided on the upstream side and the downstream side in the rotor rotation direction.
The two low resistance parts 72 and 76 are laminated with the high resistance part 74 in the center part, and the tip of the brush main body 70 is inclined with respect to the rotor rotation axis direction.

Therefore, the brush main body 70 has a three-layer structure in which the low resistance portion 72, the high resistance portion 74, and the low resistance portion 76 are laminated in this order along the rotor rotation direction. Further, in the inclined portion of the tip, the higher the resistance portion 74, the closer to the tip.
The relative proportion of the high resistance portion 74 in the exposed portion (the portion where the brush main body 70 and the commutator contact) when the inclined portion is gradually worn increases.

With the above construction, in a bidirectional motor, good spark rectification can be performed and stable performance can be achieved even when the inclined portion of the tip gradually wears. Can be secured.

FIG. 8 is a perspective view showing the structure of the brush device of the fourth embodiment. The feature of this embodiment is that the brush main body 60 of the above-described first embodiment is expanded for bidirectional rotation, and the brush main body 80 is provided upstream and downstream in the rotor rotation direction.
The two low resistance parts 82 and 86 are laminated with the high resistance part 84 in the central part, and the tip of the brush main body 80 is inclined with respect to the rotor rotation axis direction.

Therefore, in the brush main body 80, two low resistance portions 82 and 86 whose commutator contact surface is in the shape of a right-angled isosceles triangle triangle prism, and a high resistance portion 84 which fills the space between them are in the shape of an isosceles triangle. And are integrally combined. Here, the low resistance portion 82 is arranged on the upstream side in the rotor rotation direction such that the commutator contact surface gradually narrows from the upstream side to the downstream side in the rotor rotation direction, and the low resistance portion 86 has the commutator contact surface. It is arranged on the downstream side in the rotor rotation direction so as to become gradually narrower from the downstream side in the rotor rotation direction toward the upstream side. The high resistance portion 84 is arranged so as to fill the space between these two low resistance portions 82 and 86.

Further, the tip end of the brush main body 80 is inclined with respect to the direction of the rotor rotation axis so that the low resistance portions 82 and 86 are projected. Therefore, when the inclined portion gradually wears, the high resistance portion 8 gradually increases as the wear progresses.
4 is the same as the third embodiment described above in that the relative ratio of exposure is increased.

Particularly, the brush main body 80 included in the brush device of this embodiment has the low resistance portion 8 at the inclined portion of the tip.
2, 86 and the high resistance portion 84 are in the same laminated state and the other portions are laminated in the same state.
There is also an advantage that 0 can be easily manufactured.

FIG. 9 is a perspective view showing the structure of the brush device of the fifth embodiment. The feature of the present embodiment is that the change of the contact resistance due to the change of the spring pressure of the brush is corrected in the brush device of the first embodiment.

For this reason, the brush main body 90 has the same structure as the brush main body 10 of the first embodiment in the inclined portion of the tip, and the relative proportion of the low resistance portion 92 in the other portions becomes farther from the brush tip. It has a two-layer structure including a low resistance portion 92 and a high resistance portion 94 so as to increase gradually.

With such a structure, when the wear of the inclined portion of the tip progresses, stable output characteristics can be obtained and good spark-free rectification can be performed as in the first embodiment. it can.

After all the inclined parts are worn and the brush main body 90 contacts the entire surface of the commutator, even if the spring length becomes long according to the wear amount of the brush and the spring pressure becomes weak, The relative proportion of the low resistance portion 92 is increased in proportion to the amount of wear, and a substantially constant contact resistance can be secured.

The present invention is not limited to the above embodiments, and various modifications can be made within the scope of the gist of the present invention.

For example, in each of the above-mentioned embodiments, the DC motor has been described as an example, but the present invention is not limited to this and can be applied to a DC generator. Further, in each of the embodiments, the brush main body having a square or rectangular cross section is described, but the cross sectional shape is not particularly limited to a parallelogram or a trapezoid.

[0064]

As described above, in the present invention, the contact resistance of the brush tip can be kept substantially constant by increasing the relative proportion of the high resistance portion of the inclined portion of the brush tip as the brush wears. It is possible to prevent changes in the characteristics of the DC machine.

Further, the brush main body has a two-layer structure consisting of a high resistance portion and a low resistance portion, and it is possible to perform good spark rectification regardless of the wear state of the brush.
This good sparkless rectification ensures stable output characteristics.

[Brief description of drawings]

FIG. 1 is a perspective view showing an outline of a brush device according to a first embodiment.

FIG. 2 is a view for explaining the structure of the brush main body shown in FIG.

FIG. 3 is a diagram showing a relationship between a worn state of a brush and a contact area.

FIG. 4 is an explanatory diagram of rectifying characteristics of a conventional brush device having a taper structure.

FIG. 5 is an explanatory diagram of rectifying characteristics of the brush device according to the present embodiment.

FIG. 6 is a perspective view showing an outline of a brush device according to a second embodiment.

FIG. 7 is a perspective view showing an outline of a brush device according to a third embodiment.

FIG. 8 is a perspective view showing an outline of a brush device according to a fourth embodiment.

FIG. 9 is a perspective view showing an outline of a brush device according to a fifth embodiment.

[Explanation of symbols]

 10 Brush main body 12 Low resistance part 14 High resistance part 16 Pigtail

Claims (2)

[Claims] 1. A brush device including a brush main body in which a low resistance portion arranged on an upstream side in a rotor rotation direction and a high resistance portion arranged on a downstream side are integrally laminated, wherein a tip of the brush main body is provided. The portion is inclined with respect to the rotation axis direction of the commutator, and the relative ratio of the high resistance portion to the low resistance portion in this inclined portion is gradually increased from the tip of the brush body along the radial direction of rotation of the commutator. Brush device characterized by increasing. 2. A low resistance portion which is arranged on the upstream side of the rotor rotating direction and is formed so as to become gradually narrower from the upstream side to the downstream side, and a low resistance portion which is arranged on the downstream side and gradually increases from the upstream side to the downstream side. In a brush device including a brush body integrally formed with a high resistance portion formed to be wide, a tip end portion of the brush body is inclined with respect to a rotation axis direction of a commutator so that the low resistance portion projects. At the same time, the relative proportion of the high resistance portion to the low resistance portion in the inclined portion is gradually increased from the tip end portion of the brush main body along the radial direction of rotation of the commutator.