DE20007714U1 - Device for protecting the position of a dynamoelectric machine - Google Patents

Description

BONSMANN & BONSMANN"""' '··'···" '"'Kaltienkirchener Strasse 35

D-41063 Mönchengladbach PATENT ATTORNEYS _Tel| Q2161/12114 Fa

File: 00 159

Jürgen Rasch

Büdericher Str. 97, 41877 Willich

Device for protecting the bearings of a dynamoelectric machine

The invention relates to a device according to the preamble of claim 1.

In dynamoelectric machines - i.e. electric motors or generators or electric machines used in combination as electric motors or generators - it is known that so-called wave voltages are induced in the electrically conductive parts of the machine due to unavoidable asymmetries in the magnetic fields. These induced voltages can lead to a current flow in a loop formed by the rotor shaft, the rotor bearings and the housing, the so-called wave current. Other causes for the formation of wave currents can be capacitive coupling, which occurs particularly in the converter-controlled drives with high voltage change rates that are increasingly used today. The wave currents that occur are a major problem, especially in the area of the rotor bearings - usually ball bearings. As intended, a lubricating film forms in the bearings, at least during rotation, which greatly increases the transition resistance between the rotor-side and the housing-side parts of the bearing. The induced wave voltages can therefore

can no longer be compensated by a shaft current; instead, an electric field is built up. The voltages that build up are then discharged locally at certain points on the bearing, which can damage it and, in the long term, lead to bearing failure.

In order to prevent the flow of shaft currents, it is known to insulate the housing-side part of at least one bearing from the earth potential at which the other parts of the housing are usually located. This interrupts the flow of shaft current in the loop mentioned above. However, this procedure involves considerable effort, as load-bearing parts of the machine must be insulated. In addition, this type of insulation can deteriorate imperceptibly over time, which can lead to gradual bearing destruction. Furthermore, the lack of earthing of part of the motor is questionable from a safety perspective.

The invention is based on the object of counteracting the problems caused by wave currents with the simplest and most reliable means possible.

This object is achieved according to the invention according to the features of claim 1.

Appropriate embodiments of the invention emerge from the dependent claims.

The basic idea of the invention is not to prevent the flow of wave currents by means of insulation measures as in the known approaches. Rather, the invention provides a controlled flow of wave currents at the

Rotor bearings are made possible, so that damage to them is avoided. If - as is the case most often - there are two front-end machine bearings that are connected to one another via an electrically conductive shaft, then within the scope of the invention at least two devices are required for the permanent electrical connection of the rotor-side parts of the bearing with the housing-side potential. If only one device were provided on one front side, the "unprotected" bearing on the opposite front side would be subjected to even greater stress, since in this case the shaft voltage drops almost completely at the unprotected rotor bearing due to the loop formed by the rotor shafts, the rotor bearings and the housing.

Various embodiments are proposed according to the invention for permanent electrical connections. In a first embodiment, the shaft currents are diverted by at least one sliding contact resting on the rotor shaft, which can be designed as a sliding brush or carbon brush, for example. In machines with two front-side, electrically conductively connected bearings, it is important in this context that a sliding contact is provided on the front side of the machine in the area of which the bearing to be protected is located. This means that a sliding contact must always be on the side of the bearing to be protected or a corresponding extension of the shaft on this side of the machine, because otherwise the problems as explained above would tend to be exacerbated. In structural terms, it is particularly advantageous to provide the sliding contact within a tachogenerator arranged on an extension of the rotor shaft, whereby - as explained above - the tachogenerator must be on the side of the bearing to be protected.

In a second embodiment, the device for the permanent electrical connection of the rotor-side parts of the bearing with the housing-side potential is implemented by an electrically conductive sealing or sealing ring. This embodiment is particularly advantageous because no additional parts are required compared to conventional machines. The conductive design of the sealing or sealing ring can be achieved, for example, by adding graphite. The electrical resistance of the sealing or sealing ring used must be set so that damage to the bearings due to breakdowns is avoided. Of course, the sealing or sealing rings must also be designed so that they can withstand the current densities caused by the shaft currents. As already explained above, the discharge measures must be provided in the area of both bearings if the rotor shaft is continuously conductive in order to prevent damage to both bearings. The sealing or sealing ring can be designed to be flexible and thus adjustable for different shaft diameters.

In a further embodiment of the invention, the device for permanent electrical connection can have an electrically conductive sealing cord and/or an electrically conductive sealing felt, which is arranged in the front or rear bearing cover of the machine. These elements are also provided as standard in dynamoelectric machines to protect the bearings from contamination, so that no additional parts are required to divert the shaft currents.

Of course, the drainage effect according to the invention can also be achieved by a combination of the embodiments described above

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The invention is explained in more detail below using the drawings as examples. They show:

Fig. 1 is a schematic representation of a motor with protective devices according to certain embodiments of the invention;

Fig. 2 is a schematic representation of a motor with protective devices according to further embodiments of the invention;

Fig. 3 Detailed representations of electrically conductive sealing rings for use in the present invention.

According to Fig. 1, a schematically illustrated electric motor with a rotor arrangement 10 and a stator arrangement 12 in a housing 14 has a motor shaft 16 which is mounted on the front sides of the motor via ball bearings 18 and 20. As indicated schematically at 22, when the motor is operating, inductively and/or capacitively coupled shaft voltages occur which are induced or coupled into the electrically conductive motor shaft, among other things. Without the measures according to the invention, shaft currents arise between the rotor and the housing-side parts of the bearings 18, 20. Since the bearings have an increased contact resistance during rotation due to the lubricating film, discharge-like processes occur which, over time, lead to damage to the bearings.

Within the scope of the present invention, however, these wave currents are bypassed by the bearings. In the embodiment shown in Fig. 1, the wave currents endangering the bearing 20 are diverted by a carbon contact 19, which is

electrical connection with the housing. The opposite bearing 18 has a sealing ring 24 which is made of a conductive material and is in permanent electrical contact with both the motor shaft and the housing. The sealing ring is made of conventional materials which are provided with conductive additives (e.g. graphite additives or conductive fiber additives) with a sufficiently low electrical resistance. As a further sealing element, a sealing felt 26 in a rear bearing cover of the motor is also designed to be conductive.

In Fig. 2, further connecting devices are shown schematically, with identical parts being provided with identical reference numerals. In the bearing 18', bearing sealing disks 28, which are used for the purpose of splash protection, are made of conductive material. The bearing sealing disks also lie permanently on the housing and shaft 16, whereby any shaft currents that arise can be diverted. In the other bearing 20', the diversion does not take place directly in the area of the bearing, but rather through a carbon brush 32, which is integrated into a tachogenerator 30 arranged in the extension of the shaft 16, with the shaft of the tachogenerator 30 and the motor shaft 16 being conductively connected to one another.

Fig. 3 shows various sealing rings (gamma rings, shaft seals), with the numbers 1 to 6 designating those parts of the rings which can be made of conductive material according to the invention.

Claims (8)

1. Device for protecting the bearings of a dynamoelectric machine from shaft currents, the machine having a housing ( 14 ) and a movable rotor ( 10 ) as well as at least one rotor bearing ( 18 , 20 ) which has a fixed housing-side part and a rotor-side part, shaft voltages being induced or coupled into the rotor-side part during rotation and the rotor and housing-side parts of the bearing ( 18 , 20 ) having an increased contact resistance to one another at least during rotation, characterized by at least one device for permanently electrically connecting the rotor-side parts of the bearing to the housing-side potential in such a way that potential equalization is established between the rotor and housing-side parts of the bearing without damaging the at least one bearing ( 18 , 20 ). 2. Device according to claim 1, characterized in that the device for permanent electrical connection has a sliding contact ( 19 , 32 ) which is electrically and mechanically connected to the housing ( 14 ) and bears against a rotor shaft ( 16 ), the rotor shaft ( 16 ) being electrically connected to the rotor-side part of the bearing and the sliding contact being provided in the region of the front side of the machine in the region of which the bearing to be protected is located. 3. Device according to claim 2, characterized in that the sliding contact ( 32 ) is provided in a tachogenerator ( 30 ) connected to the rotor shaft ( 16 ) outside the machine housing ( 14 ). 4. Device according to claim 2 or 3, characterized in that the grinding contact is designed as a grinding brush. 5. Device according to claim 2 or 3, characterized in that the sliding contact is designed as a carbon brush ( 19 , 32 ). 6. Device according to one of claims 1 to 5, characterized in that the device for permanent electrical connection has an electrically conductive sealing ring ( 24 ) which rests against the rotor shaft ( 16 ) and is connected to the housing ( 14 ). 7. Device according to claim 6, characterized in that the sealing ring ( 24 ) is made of rubber or plastic which is made conductive by an additive. 8. Device according to one of claims 1 to 7, characterized in that the device for permanent electrical connection has an electrically conductive sealing cord and/or an electrically conductive sealing felt ( 26 ) which is arranged in the front or rear bearing cover of the machine.