WO2018196925A1 - Capacitive bypass for reducing electrical shaft voltages or bearing voltages - Google Patents

Abstract

The invention relates to a device and a method for reducing a shaft voltage (UW) on a shaft or a bearing voltage (UWL) between a shaft and a housing. A rolling bearing supports the shaft against the housing and has a rolling bearing capacitance (CWL). Electrically parallel to this, another capacitance is present between the shaft and the housing (CW-G) as a result of the arrangement of the shaft and housing. According to the invention, an additional bypass capacitance (CBypass) is also arranged electrically parallel thereto.

Description

 Capacitive bypass for reducing electrical shaft voltages or bearing voltages

For the bearings of an electric motor, in addition to the mechanical wear due to shaft rotation, electrical currents flowing through the bearings of the motor shaft to ground another reason for wear. Voltages of a certain magnitude can overcome the isolation characteristics of bearing lubrication, creating sparks that can lead to pitting, corrugated surfaces, fusion craters, and eventually premature failures of the bearings and motor. A bearing voltage, d. H. the tension between the two bearing shells is usually correlated with a shaft tension, d. H. the tension between the shaft and mass.

The same applies to the bearings for other components, such as gears, where electrical discharges can cause damage.

When storing an electric motor, various effects can occur which can generate a voltage on a shaft of the motor. According to the statements from "Antriebstechnik 37" (1998) No. 7, the article "Encoder protection by installation of isolated ball bearings" describes possible causes for this:

Asymmetries in the magnetic circuit between the rotor and stator during rotation can cause unwanted magnetic fluxes that induce current in the loop formed by the shaft, bearings and housing.

There is a capacitive coupling between the stator and the rotor of an electric motor. This leads to capacitively transferred to the rotor currents that want to flow through the bearings to ground. In this case, motors that are supplied with sinusoidal AC voltage have a lower voltage between shaft or bearing and motor housing than motors that are supplied by fast-switching drives with variable frequency (frequency converter). In the latter case, these voltages can amount to a multiple in comparison. External voltages from insufficiently isolated machines can transfer a potential difference to the shaft whose resulting current wants to drain again to ground. V-belts, timing belts and lubricants that are not antistatically equipped can lead to electrostatic charging of the machine parts.

Axially interspersed magnetic fields, especially in machines with plain bearings and with asymmetries in the winding, can have a voltage difference (unipolar voltage) between the beginning and end of the bearing shells.

This listing is not considered final.

A common countermeasure is the grounding of the shaft or a constructive measures to short the bearing halves, such. As the use of conductive brushes to reduce the shaft voltage. Furthermore, conductive seals or slip rings can be used, but have the disadvantage that may deteriorate over the life of the contact properties. Conductive bearing grease or oils are based on conductive additives in which the functionality depends on their distribution in the lubricating medium and thus is not always constant.

Alternatively, as described, the bearing halves can be sufficiently isolated from each other, so that a current flow is suppressed.

An entirely different measure is provided by EP1445850A1. Here, a compensation circuit generates a compensation of the unwanted storage current. In this case, a reference signal for the compensation is provided via an artificial star point, in which the voltages of the three phases of an electric motor or converter are switched together by means of capacitors. An opposing voltage is generated from these currents via a Umpol transformer, which results in the end in a compensation current, which is fed via a coupling capacitor to the shaft. A disadvantage of this method is that only the voltages of the 3 phases serve as input parameters to provide the compensation current. This means that stray capacitances or bearing currents that do not arise directly in connection with motor or converter operation are not taken into account and therefore no compensation takes place for them.

In addition, a certain effort for the compensation circuit is necessary, which continues to consist of error-prone components

The object of the invention is to realize a technical solution that does not require a compensation circuit and error-prone components. At the same time influences of other stray capacities should be reduced. This object is achieved by a device and a method for reducing a shaft voltage (Uw) on a shaft or a bearing voltage (UWL) between a shaft and a housing. In this case, a roller bearing supports the shaft relative to the housing and has a capacity of the rolling bearing (CWL). Electrically parallel to this, there is a further capacitance between the shaft and the housing (Cw-G) due to the arrangement of shaft and housing. An additional bypass capacitance (Cypass) is also arranged electrically parallel thereto.

Advantageously, unwanted effects, which emanate from a too small total capacity, can be reduced or avoided. In particular, a shaft voltage or bearing voltage and possibly resulting destructive bearing current can thereby be reduced. The cause is advantageously irrelevant.

While conventional methods of shorting the shaft to the housing result in a mechanical resistance increase, e.g. B. by the friction of the brushes lead, so this is not the case with the proposed capacitive bypass. This is implemented by an additional capacity, which is realized constructively by a capacitive bypass. This extra capacity is the bypass capacity

(CBypass). Even a complex and error-prone compensation circuit with multiple components can be omitted.

The shaft voltage (Uw) usually corresponds to the voltage which the shaft has with respect to the housing or ground. The bearing voltage (UWL) is the voltage that drops between the two bearing shells or rings. When the inner ring is electrically connected to the shaft and the outer one is connected to the housing (or its potentials), the shaft voltage and the bearing voltage are identical.

The total capacity results from the sum of the capacity of the rolling bearing (CWL), the capacity between shaft and housing (CW-G) and the possibly existing bypass capacity (CBypass). These capacities are connected in parallel in the equivalent circuit diagram.

In an extended embodiment, a motor winding is connected to the shaft mechanically, but electrically isolated. Between motor winding and shaft, the capacitance between a motor winding and shaft (Cw-w) is formed.

The capacitance between a motor winding and shaft (Cw-w) forms a capacitive voltage divider with the total capacitance connected in series. The motor winding may be the electrical winding (coil) of a stator or rotor. Also, the winding may be that of a generator instead of a motor.

Instead of the rolling bearing can also be another component that can take damage by the shaft voltage. In addition to rolling bearings this applies z. As well as gears, which may be mounted on the shaft and in engagement with another housing-mounted gear similar transition properties (lubricating film with electrical resistance) and thus may have problems such as a rolling bearing. The capacity of the rolling bearing (CWL), the capacity between shaft and housing (Cw-G) and a capacity between the motor winding and shaft (Cw-w) are considered here as undesirable stray capacitances.

In a preferred embodiment, the bypass capacitance (Cßypass) is dimensioned such that the shaft voltage (Uw) or bearing voltage (UWL) does not exceed a threshold value. Advantageously, damage to the rolling bearing or components mounted on the shaft can be avoided by observing this threshold value. Furthermore, the size of the bypass capacity (Cbypass) must not be unnecessarily chosen beyond a necessary level, which would result in an unnecessary effort.

The fact that the capacity of the rolling bearing (CWL), the capacity between shaft and housing (CW-G) and the capacity between the motor winding and shaft (Cw-w) are known or can be measured beforehand once, the necessary bypass Calculate capacity, if the voltage drop between motor winding and shaft (Uw-w), as well as the maximum permissible shaft voltage (Uw) or bearing voltage (UWL) are known.

This is done according to a formula that can be derived from the capacitive voltage divider ratio:

Capacitive voltage divider: UWL / Uw-w = Cw-w / (CwL + Cw-G + Cssypass) resolved according to the bearing voltage: UWL = Cw-w / (CwL + Cw-G + Cssypass) ^* Uw-w The maximum permissible bearing voltage (and thus the threshold value) usually results in such that there are no destructive, ie abrasive, discharges. The voltage peaks are to be considered, the z. B. by the Common mode current of a frequency converter can be generated. The same applies to a shaft voltage (Uw).

In a preferred embodiment, the bypass capacity (Cßypass) is generated by a structurally independent measure.

Advantageously, this allows the bypass capacity to be provided willfully rather than randomly. This means that a constructive action is carried out without the

The desire to increase the bypass capacity (Cypass) would make no sense or have no other (primary) purpose.

Examples of such constructive measures:

The configuration of the shape of the shaft and / or the housing or the position to each other, so that the capacity between the shaft and housing (CW-G) is increased, wherein the proportion of the increase in the bypass capacity (Cßypass) corresponds. The wave can z. B. have a thickening, so that the distance is reduced to the housing and the air gap located therebetween is reduced, which leads to an increase in capacity according to the principle of a plate capacitor.

Using a wider bearing without a mechanical need for it, so that the surfaces of the bearing shells are increased against each other, which leads to an increase in capacity according to the principle of a plate capacitor.

Installation of an independent capacitive component, as described below.

In a preferred embodiment, the bypass capacitance (Cypass) is provided by an independent component or group of components. Advantageously, the provision of the capacitive bypass as an independent component simpler installation, retrofitting or maintainability can be effected. Stand-alone in this case means that the independent component serves primarily or exclusively for the purpose of providing the bypass capacity. As such component therefore z. B. the housing 3, the shaft 2 and the rolling bearing 4 itself. Depending on the use of the term may be a component or a group of components talk. A plate capacitor, for example, can be considered as one component or as a component group with both plates as components.

In a preferred embodiment, a part of the independent component or component group has an electrically conductive connection with the shaft and another part with the housing.

Advantageously, the electrically conductive connection of a separate component with a bypass capacitance (Cypass) forms a possibility for a parallel connection to the capacity of the rolling bearing (CWL) and the capacity between the shaft and the housing (CW).

G).

In a preferred embodiment, the bypass capacitance (Cypass) is provided by an array of coaxial tubes.

Advantageously, such a bypass capacity can be generated by a concrete structural measure, namely the arrangement of two tubes.

In a preferred embodiment, the bypass capacitance (Cypass) is provided by an array of radially aligned disks 6. Advantageously, such a bypass capacity can be generated by a concrete structural measure, namely the arrangement of two radially aligned discs. In a preferred embodiment, the component or component group is connected to the rolling bearing 4.

Advantageously, this provides a module (integrated rolling bearing) which can be sold separately as such and / or makes simplified assembly possible. Furthermore, no extra mounting step for mounting the capacitive bypass must be performed, but this happens automatically during assembly of the integrated bearing.

In an extended embodiment, the capacitive bypass or parts thereof is integrated as a seal of the rolling bearing or as a seal in the rolling bearing.

In an alternative embodiment, the capacitive bypass is mechanically separated from the rolling bearing, i. H. not connected. The component or the module of the capacitive bypass is then z. B. mechanically connected directly to the shaft and the housing and not with the rolling bearing.

Advantageously, by the separate, d. H. independent of the bearing mounting of the capacitive bypass the maintainability can be increased. Replacement of only the capacitive bypass or only the (eg worn) rolling bearing is thus made possible, without having to exchange the other component with. Also, the capacitive bypass can be offered as an additional module for retrofitting.

The term shaft is not to be understood as meaning that it is a rotating machine element which transmits only torsional forces. A shaft in the sense of the invention can be just as good an axle, rod, bolt or other component. The decisive factor is that it is an arrangement in which an undesired voltage (interference voltage), referred to here as shaft voltage, occurs. Embodiments of the invention are explained below with reference to the drawings.

Show it:

Fig. 1 a is a capacitive equivalent circuit diagram without bypass

Fig. 1b a capacitive equivalent circuit with bypass

Fig. 2a an axially integrated bypass

Fig. 2b is an axial separate bypass

Fig. 3a is a radially integrated bypass

Fig. 3b is a radial separate bypass

FIG. 1a shows a capacitive equivalent circuit diagram for a device without a bypass according to the invention. The following (stray) capacitances are present: Between the motor winding 1 and the shaft 2, a capacitance is formed between the motor winding and the shaft Cw-w. Between the shaft and the housing, a capacitance between shaft and housing CW-G is formed. Between the two bearing shells of a rolling bearing 4 (not shown here), a capacity of the rolling bearing CWL is formed. Since the bearing shells are electrically conductive and conductively connected to the shaft on one side and the housing on the other side, the capacitance between shaft and housing CW-G is parallel to the capacity of the rolling bearing

In this arrangement, the formula of the capacitive voltage divider is described as follows: UWL / Uw-w = Cw-w / (CWL + CW-G). Dissolved according to UWL UWL = (Cw-w /

By way of example, from this The following voltages result from a common mode voltage UCM of the motor or frequency converter of 400 V, a voltage drop between motor winding and shaft Uw-w of 360 V, resulting in a wave voltage Uw of 40 V by subtraction. In Figure 1 b, the capacitive equivalent circuit of Figure 1 a is extended by the inventive circuit with bypass as follows: An additional capacity, namely the bypass capacitance Cßypass is structurally introduced into the system so that they are electrically parallel to the existing capacity according to the Representation in the replacement circuit diagram acts. From the capacitive voltage divider ratio already described results in the new, lower shaft voltage Uw or bearing voltage UWL, which arises by adding the bypass capacitance Cßypass. The bypass capacitance Cssypass is effective in the representation between the shaft 2 and the housing 3 or mass. FIGS. 2 a, 2 b, 3 a and 3 b show a shaft 2, which rotates in its own rotation about the axis of symmetry SYM. To the shaft 2 is a rolling bearing 4 - representative of other rotatable components, such. As gears, shown - mounted, preferably the shaft pierces the roller bearing axially through the inner ring, with which it is mechanically and / or electrically connected. The other side of the rolling bearing 4, in particular its outer ring is connected to a housing 3, which is grounded or has a connection to ground.

The capacitive bypass is arranged in the illustration in the gap between shaft 2 and housing 3. It forms a capacitance according to the principle of a plate capacitor. The thereby effective plates or surfaces should have a substantially constant distance and area size during the rotational movement of the shaft 2, so that the rotation has no influence on the electrical, in particular the capacitive properties. The two plates are realized in the various figures either by a tube 5 and a counter tube 5b or a disc 6 and a counter-disc 6b.

For the purpose of the invention, the tube 5 or the disc 6, which faces away from the shaft 2, must be electrically connected to the housing 3 or the mass. The counter tube 5b or the counter disk 6b must be electrically connected to the shaft. As a result, the parallel connection of the bypass capacitance Cßypass is implemented.

In FIGS. 2a and 2b, the capacitive bypass is formed by coaxial tubes. This has two such tubes or tubular pieces with a conductive surface, which assume the functionality of both plates of the plate capacitor, in particular a tube 5 and a counter tube 5b. The plates are not flat, but shaped according to the tube round. Tube 5 and counter tube 5b engage each other here, to one of the two consequently has a smaller diameter, so that between the opposite surfaces (plates), push through one another, an air gap is formed, which separates the two plates from each other. The two tubes or plates are aligned in the illustration parallel to the shaft.

Alternatively, the functionality of one of the plates can also be formed by the shaft 2 itself, so that the counter tube 5b can be saved, since the shaft, if it is conductive and has a corresponding shape, can take over this function. The air gap is then formed between the tube 5 and the shaft 2.

In FIGS. 3 a and 3 b, the capacitive bypass is formed by disks which extend in the radial direction to the shaft 2. This has two such discs or annular pieces with a conductive surface, which take over the functionality of both plates of the plate capacitor, in particular a disc 6 and a counter-disc 6b. The surfaces should be flat in this case, since they would otherwise have a variable distance when rotated against each other, which affects the capacity. The disk 6 and the counter disk 6b rotate with each other at an axial distance by the rotation of the shaft. The axial distance creates an air gap, which separates the two plates (surfaces) from each other. The two disks or surfaces are oriented orthogonally or radially to the shaft 2 in the illustration.

FIGS. 2 a and 3 a show an integrated roller bearing 4 b, i. H. the rolling bearing forms a structural unit or module with the capacitive bypass. In this case, the parts of the capacitive bypass are mounted directly on the rolling bearing 4, so that an integrated rolling bearing 4b can be provided and delivered as a module.

In contrast, FIGS. 2b and 3b show a separate (or stand-alone) capacitive bypass which is not mechanically connected directly to the roller bearing 4. In this case, the tubes 5 or discs 6 have no direct mechanical connection with the roller bearing 4. Instead, they are electrically connected to the housing 3. prevented. The optional counter tube 5b or counter disk 6b is electrically conductively connected to the shaft 2.

LIST OF REFERENCE NUMBERS

1 motor winding

 2 wave

 3 housing

4 rolling bearings

 4b integrated rolling bearing

 5 coaxial tube

 5b counter tube

 6 radial disc

6b counter-disc

 Cw-w Capacity between motor winding and shaft CWL Capacity of the rolling bearing

CW-G Capacity between shaft and housing CBypass bypass capacity

SYM symmetry axis

Claims

claims

1 . Device for reducing a shaft voltage (Uw) on a shaft (2) or a bearing voltage (UWL) between a shaft (2) and a housing (3), wherein a roller bearing (4) the shaft (2) relative to the housing (3 ) and thereby has a capacity of the rolling bearing (CWL),

while electrically in parallel there is a further capacitance between shaft and housing (CW-G) through the arrangement of shaft (2) and housing (3),

characterized in that

an additional bypass capacitance (Cypass) is also arranged electrically parallel thereto.

2. Apparatus for reducing a shaft voltage (Uw), or a bearing voltage (UWL), according to one of the preceding claims, characterized in that the bypass capacitance (Cßypass) is dimensioned so that the shaft voltage (Uw), or bearing voltage (UWL) does not exceed a threshold.

3. A device for reducing a shaft voltage (Uw), or a bearing voltage (UWL), according to one of the preceding claims, characterized in that the bypass capacity (Cßypass) is generated by a structurally independent measure.

4. A device for reducing a shaft voltage (Uw), or a bearing voltage (UWL), according to one of the preceding claims, characterized in that the bypass capacitance (Cßypass) is provided by an independent component or component group.

5. A device for reducing a shaft voltage (Uw), or a bearing voltage (UWL), according to claim 4, characterized in that

a part of the independent component or component group has an electrically conductive connection with the shaft (2) and another part with the housing (3).

6. A device for reducing a shaft voltage (Uw), or a bearing voltage (UWL), according to one of claims 4 to 5, characterized in that the bypass capacitance (Cßypass) is provided by an array of coaxial tubes (5).

7. Device for reducing a shaft voltage (Uw), or a bearing voltage (UWL), according to one of claims 4 to 5, characterized in that the bypass capacitance (Cßypass) by an arrangement of radially aligned discs (6) is provided.

8. A device for reducing a shaft voltage (Uw), or a bearing voltage (UWL), according to one of claims 4 to 7, characterized in that the component or the component group with the rolling bearing (4) is connected.

9. A method for reducing a shaft voltage (Uw) on a shaft (2), or a bearing voltage (UWL) between a shaft (2) and a housing (3), wherein a roller bearing (4) the shaft (2) relative to the Housing (3) and thereby has a capacity of the rolling bearing (CWL),

while electrically in parallel there is a further capacitance between shaft and housing (CW-G) through the arrangement of shaft (2) and housing (3),

characterized in that

an additional bypass capacitance (Cypass) is also generated electrically in parallel therewith.