Device for reducing harmful bearing voltages

Abstract

The invention relates to a device for reducing harmful bearing voltages in an electrical machine (M) fed by a DC link voltage of a DC link, said electrical machine comprising a stator (3), which has windings (7) and is insulated from ground (GND), and a rotor (2) and a motor shaft, wherein furthermore a rotor-side bearing (LA.sub.R) and a stator-side bearing (LA.sub.S) are each insulated from the ground (GND) and the rotor (2) and the stator (3) are electrically connected to each other by means of a bypass capacitance (C.sub.Bypass) having a predefined capacitance.

Claims

1. A device for reducing harmful bearing voltages in an electrical machine (M) fed by a DC link voltage of a DC link, said electrical machine comprising a stator (3), which has windings (7) and is insulated from ground (GND), and a rotor (2) and a motor shaft, wherein furthermore a rotor-side bearing (LA.sub.R) and a stator-side bearing (LA.sub.S) are each insulated from the ground (GND) and the rotor (2) and the stator (3) are electrically connected to each other by means of a bypass capacitance (C.sub.Bypass) having a predefined capacitance, wherein the bypass capacitance (C.sub.Bypass) is implemented by introducing a bypass capacitor between the rotor (2) and the stator (3).

2. The device according to claim 1, characterized in that a level of the bypass capacitance (C.sub.Bypass) corresponds to a multiple of the bearing capacitance.

3. The device according to claim 1, characterized in that the bypass capacitance (C.sub.Bypass) is implemented by introducing a dielectric between the rotor (2) and the stator (3).

4. The device according to claim 1, characterized in that the implementation of the bypass capacitance (C.sub.Bypass) is accomplished by means of discretely constructed capacitors.

5. The device according to claim 1, characterized in that the bypass capacitor is electrically connected to the motor shaft via slip rings or brushes.

6. The device according to claim 1, characterized in that a Y capacitor with sufficient insulation strength is connected to one side of a stator core of the stator (3) and a shaft contact ring or shaft capacitor is connected to the other side of the stator core of the stator (3).

7. The device according to claim 6, characterized in that the configuration of the shaft contact ring or shaft capacitor is such that it also has a capacitance relative to the motor shaft.

8. A method of configuring the capacitance of a bypass capacitor in a device according to claim 1, in which the common-mode voltage U.sub.CM is applied, having the following steps: a) varying the bypass capacitance (C.sub.Bypass) starting from a lower capacitance to a capacitance increased relative thereto; b) identifying the BVR ratio of the common-mode voltage U.sub.CM applied to the voltage at the bearing both for the rotor-side bearing LA.sub.R and for the stator-side bearing LA.sub.S; c) determining that value of the bypass capacitance (C.sub.Bypass) at which the BVR ratio exceeds a predetermined setpoint, the capacitance corresponding thereto being selected and used as a bypass capacitance (C.sub.Bypass) for the device.

9. The device according to claim 1, wherein the electrical machine is characterized by a system-related capacitance C.sub.RS between the rotor and the stator, and the rotor bypass capacitance (C.sub.Bypass) is in addition to and parallel with the system-related capacitance C.sub.RS.

Description

(1) In the drawings:

(2) FIG. 1 shows an equivalent circuit diagram of the capacitance network for an exemplary embodiment of the invention;

(3) FIG. 2 shows an illustration of the BVR for a floating stator, a stator at common potential and a grounded stator for the rotor-side and the stator-side bearings; and

(4) FIG. 3 shows the BVR ratio with a varying rotor-ground capacitance without the bypass capacitor on the left side of the illustration and with a bypass capacitor on the right side of the illustration next to it.

(5) In the following, the invention will be described in more detail based on preferred exemplary embodiments with reference to FIGS. 1 to 3, wherein like reference numerals refer to the same functional and/or structural features.

(6) FIG. 1 shows an equivalent circuit diagram for an electric motor of an exemplary embodiment of the invention. The equivalent circuit diagram shown represents the capacitance network for the corresponding implementations with the system-related capacitances C.sub.ws, C.sub.W-LAs, C.sub.W-LAr, C.sub.WR, C.sub.BS, C.sub.BR, C.sub.RS, C.sub.RE, C.sub.SE, C.sub.Y, C.sub.S-LAS, C.sub.s-LAr, in each case representing, by way of example, the capacitances between W=winding, LA=outer bearing race (r=rotor side, s=stator side), R=rotor and S=stator as well as further capacitances between the respective components of the motor, which, however, will not be discussed in greater detail. Furthermore, the potential PE of the protective earth in the network is shown, as well as the ground reference potential GND and the common-mode voltage U.sub.CM. The motor winding is denoted by reference numeral 7.

(7) According to the invention, rotor 2 and stator 3 are electrically connected to one another via a bypass capacitor C.sub.Bypass, whereby the potentials between rotor 2 and stator 3 are adapted to one another, and consequently, the voltage applied to rotor-side bearing LA.sub.R and stator-side bearing LA.sub.S respectively decreases.

(8) For an evaluation of the effect of a bypass capacitor, as shown in FIG. 2, the capacitance of the bypass capacitor is first varied between 1 pF and 10 nF and the BVR (bearing voltage ratio), i.e., the ratio of the applied common-mode voltage U.sub.CM to the voltage at the bearing for both rotor-side bearing LA.sub.R and the stator-side one LA.sub.S, is determined. The results are shown as a solid line for rotor-side bearing LA.sub.R and as a dashed line for stator-side bearing LA.sub.S. Thus, if bypass capacitor C.sub.Bypass is increased between the respective bearing LA.sub.R, LA.sub.S and the motor shaft or rotor 2 conductively connected thereto, the BVR of the stator 3 connected to common potential or ground significantly increases, that of rotor 2 with the insulated bearing seat significantly decreasing in case of a sufficiently large bypass capacitor C.sub.Bypass, achieving a reduction in bearing voltage.

(9) FIG. 3 shows the relationship of the BVR ratio with a varying rotor-ground capacitance C.sub.RE, namely without a bypass capacitor C.sub.Bypass on the left side of the illustration and with a bypass capacitor C.sub.Bypass on the right side of the illustration next to it.