WEAR-MONITORING DEVICE OF A BRUSH OF A CURRENT-TRANSFERRING DEVICE IN AN ELECTRIC MACHINE

Abstract

The invention relates to a wear-monitoring device of a brush of a current-transferring device in an electric machine comprising a measuring element, which is arranged at a distance from the brush and forms an electrical capacitor together with the brush, the capacitance of which electrical capacitor depends on the position of the brush relative to the measuring element.

Claims

1. A wear-monitoring device of a brush (1) of a current-transferring device in an electric machine, in which wear of the brush (1) is determinable depending on a position of the brush (1) in a brushholder (2), the wear-monitoring device comprising a current-conducting measuring element (6) assigned to the brush (1), said measuring element having an electrical voltage which is dependent on the position of the brush (1) in the brushholder (2), characterized in that the measuring element (6) is arranged at a distance from the brush (1) and, together with the brush (1), forms an electrical capacitor having an electrical capacitance which is dependent on the relative position of the brush (1) with respect to the measuring element (6).

2. The wear-monitoring device as claimed in claim 1, characterized in that the measuring element (6) is arranged fixedly in or on a brushholder (2), in which the brush (1) is guided displaceably.

3. The wear-monitoring device as claimed in claim 2, characterized in that the measuring element (6) is integrated in a wall of the brushholder (2).

4. The wear-monitoring device as claimed in claim 1, characterized in that the measuring element (6) partially or completely envelops the brush (1).

5. The wear-monitoring device as claimed in claim 1, characterized in that the measuring element (6) is in the form of an electrical plate which is arranged opposite, the brush (1).

6. The wear-monitoring device as claimed in claim 1, characterized in that, in an unused state, the brush (1) has at least 50% of an area of the measuring element (6).

7. The wear-monitoring device as claimed in claim 1, characterized in that the current-transferring device is in the form of a commutation device.

8. The wear-monitoring device as claimed in claim 1, characterized in that the current-transferring device is in the form of a slipring system in a slipring rotor machine.

9. The wear-monitoring device as claimed in claim 1, comprising a drive circuit (7) for generating a field voltage in the brush (1).

10. A method for operating a wear-monitoring device as claimed in claim 9, the method comprising generating a PWM-type field voltage in the drive circuit (7).

11. A current-transferring device comprising a wear-monitoring device as claimed in claim 1.

12. An electric machine comprising a current-transferring device as claimed in claim 11.

13. The wear-monitoring device as claimed in claim 1, characterized in that the measuring element (6) is in the form of an electrical plate which is arranged parallel to and opposite the brush (1).

14. The wear-monitoring device as claimed in claim 1, characterized in that, in an unused state, the brush (1) has at least 90% of an area of the measuring element (6).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Further advantages and expedient embodiments are set forth in the further claims, the description of the figures and the drawings, in which:

[0022] FIG. 1 shows a schematic illustration of a brush in a brushholder of a current-transferring device in an electric machine, illustrated in the unused initial state and in a state with wear, in each case with an assigned measuring element,

[0023] FIG. 2 shows the characteristic of a field voltage and the characteristic of the voltage in the measuring element in response to the field voltage, illustrated in the initial state and in the state with wear of the brush,

[0024] FIG. 3 shows illustrations corresponding to FIG. 1 of the brush in the initial state and with wear, but with three assigned measuring elements on the brushholder,

[0025] FIG. 4 shows illustrations corresponding to FIG. 2 of the field voltage and the response voltage in the first measuring element,

[0026] FIGS. 5 to 8 show, in cross section, a brush in a brushholder with various variant embodiments of a measuring element,

[0027] FIG. 9 shows a drive circuit for generating a field voltage.

DETAILED DESCRIPTION

[0028] Identical parts have been provided with the same reference symbols in the figures.

[0029] FIG. 1 shows, in two different states of wear, a brush 1 in a brushholder 2 of a current-transferring device in an electric machine. Armature windings are energized with the aid of the current-transferring device. The current-transferring device is, for example, a commutation device comprising a collector 3 rotating with the armature, with the brush 1 resting against the lateral surface of said collector. The brush 1 is mounted displaceably in the brushholder 2 and is pressed against the lateral surface of the collector 3 by a spring element 4, which is supported on the base of the brushholder 2. Owing to the frictional contact between the end side of the brush 1 and the lateral surface of the collector 3, the brush is subjected to wear. In FIG. 1, the top image shows the brush in the unused initial state, and the bottom image shows the brush in the state of wear in which the brush length is reduced in comparison with the unused state. The brush 1 is energized by means of an electrical conductor 5.

[0030] In order to be able to detect the present state of wear and possibly to generate a warning signal when a wear limit is reached, the current-transferring device is provided with a wear-monitoring device, which comprises a current-conducting measuring element 6 which is assigned to the brush 1. The measuring element 6 is in the form of, for example, an electrical conductor or a current-conducting plate and is arranged at a distance from the brush 1, but parallel thereto. The measuring element 6 is positioned, for example, in the wall of the brushholder 2. In any case, direct contact between the brush 1 and the measuring element 6 is ruled out.

[0031] The brush 1 and the current-conducting measuring element 6 each form capacitor halves and together form an electrical capacitor having a capacitance which is dependent on the relative position between the brush 1 and the measuring element 6. The measuring element 6 is arranged fixed to the housing, in particular is fixedly connected to the brushholder 2, in particular is integrated in the wall of the brushholder 2. As the degree of wear increases, the length of the brush 1 is shortened, as a result of which the relative position between the brush 1 and the measuring element 6 changes. As a result, a changing capacitance of the capacitor comprising the capacitor halves comprising the brush 1 and the measuring element 6 is also set.

[0032] The change in capacitance of the capacitor comprising the brush 1 and the measuring element 6 can be detected via the electrical voltage potential U.sub.1 of the measuring element 6. An electrical field E is produced between the brush 1 and the measuring element 6, and this electrical field generates the voltage potential U.sub.1 in the measuring element 6. The voltage U.sub.1 of the measuring element 6 can be determined with the aid of an electrical measuring device. In the event of a change in the voltage U.sub.1, triggered by a change in the capacitance owing to a wear-related shortening and position change of the brush 1, a warning signal can be generated as soon as the voltage U.sub.1 of the measuring element 6 reaches a threshold value.

[0033] The measuring element 6 is arranged axially at a distance from the open end side of the brushholder or the collector 3. In the unused initial state of the brush 1, said brush has a greater length than the measuring element 6 and is arranged opposite the measuring element 6 in such a way that the brush 1 extends completely to the height of the measuring element 6. In the used state of wear shown in the image at the bottom in FIG. 1, on the other hand, the brush 1 has been displaced so far in the direction of the collector 3 owing to wear that there is only a partial overlap between the brush 1 and the measuring element 6, as a result of which there is a lower electrical capacitance.

[0034] In the exemplary embodiment shown in FIG. 1, the electrical measuring device comprises precisely one measuring element 6, which is arranged so as to be fixed to the housing or on the brushholder 2.

[0035] FIG. 2 shows voltage characteristics U for a field voltage U.sub.err (top image) and a measuring element voltage U.sub.1 (bottom image) over time. The field voltage U.sub.err is applied to the brush 1, for example with the aid of a drive circuit, as illustrated in FIG. 9. The field voltage U.sub.err is in the form of a rectangular PWM (pulse width modulation) signal, which results in the field current characteristic I.sub.err illustrated in the top graph. Owing to the capacitive coupling between the brush 1, to which the field voltage U.sub.err is applied, and the measuring element 6, the measuring element voltage U.sub.1 is set in accordance with the bottom graph. The graph at the bottom shows the measuring element voltage U.sub.LA for the unused initial state of the brush 1 and the measuring element voltage U.sub.1,B for a brush which, as shown in the image at the bottom in FIG. 1, has been reduced owing to wear. In the unused initial state, the measuring element voltage U.sub.1,A is greater than in the used state in accordance with the measuring element voltage U.sub.1,B. This difference can be detected via the measuring device, wherein the warning signal is generated as soon as the measuring element voltage falls below a threshold value.

[0036] FIG. 3 shows a variant embodiment having a plurality of, in particular three, electrically conductive measuring elements 6 arranged one above the other which each form, with the brush 1, a capacitor. Each capacitor, consisting of the brush 1 and one of the measuring elements 6, has a specific capacitance, which is, however, dependent on the relative position of the brush 1 in the brushholder 2 and with respect to each measuring element 6.

[0037] The top graph in FIG. 4 shows the field voltage U.sub.err and the field current I.sub.err, which are identical to the field voltage and the field current in FIG. 2. The field voltage U.sub.err is in the form of a rectangular PWM signal.

[0038] The bottom graph in FIG. 4 shows the measuring element voltage U.sub.1 for the first measuring element. The voltages of the further measuring elements are denoted by U.sub.2 and U.sub.3. As can be seen from the graph, the measuring element voltage U.sub.1,A in the unworn initial state has been provided with a high amplitude, whereas in the used state of the brush 1 in which said brush has a shortened length, the measuring element voltage U.sub.1,B drops to zero. As shown in the image at the bottom in FIG. 3, the brush 1 has been shortened to such an extent that there is no longer any overlap with the measuring element 6 above, with the result that, correspondingly, the electrical capacitance drops to zero and the measuring element voltage U.sub.1,B is likewise zero. This complete drop to zero can be detected, as is likewise true for the second measuring element with the measuring element voltage U.sub.2 and the third measuring element with the measuring element voltage U.sub.3. A warning signal is generated, for example, as soon as the measuring element voltage U.sub.2 or the measuring element voltage U.sub.3 drops to zero.

[0039] FIGS. 5 to 8 show various variant embodiments for the measuring element 6. All of the variant embodiments have the common feature that the measuring element 6 is completely integrated in the wall of the brushholder 2 and there is thus no touching contact with the brush 1.

[0040] As shown in FIG. 5, the measuring element 6 extends in the form of a plate only on one side of the brushholder 2 and is opposite the brush 1. In FIG. 6, the measuring element 6 is angular and is opposite the brush 1 on two sides. In FIG. 7, the measuring element 6 is U-shaped and is opposite the brush 1 on three sides. In FIG. 8, the measuring element 6 is formed all round in rectangular form and completely surrounds the brush 1 so that the measuring element 6 is opposite the brush 1 on all sides.

[0041] FIG. 9 shows a drive circuit 7 for generating a field voltage U.sub.err in the brush. The drive circuit 7 comprises a transistor 8, for example a MOSFET, with the drain 8a thereof being connected to the voltage B+ of a voltage source 9, whereas the source 8b of the transistor is connected to the positive terminal F+ of a rotor or armature winding via a brush. The negative terminal F of the armature winding is connected to ground GND via a further brush. A freewheeling diode 10 in the reverse direction is connected in parallel with the armature winding. The transistor is driven by a clocked signal, wherein the level of the field current or the field voltage can be set via the duty cycle of this signal.

[0042] The drive circuit 7 can also, if appropriate, be provided with an H bridge, which opens up the possibility of further functions in the field circuit.