阅读说明：本技术 用于对电机的转子进行电励磁的设备和运行方法 (Device for electrically exciting a rotor of an electric machine and method for operating the device ) 是由 托马斯·希尔丁格 于 2018-10-26 设计创作，主要内容包括：用于对电机的转子进行电励磁的设备,其中,在滑动触头中具有较小的电流密度的运行条件下,滑动触头耗损通过额外地馈入与直流电流叠加的交流电流而减少,其中,由于随同旋转的电容器使转子极相对于交流电表现为开路。(Device for electrically exciting a rotor of an electric machine, wherein, in operating conditions with a low current density in the sliding contacts, the sliding contact losses are reduced by additionally feeding an alternating current superimposed on a direct current, wherein the rotor poles appear open circuit to the alternating current due to the co-rotating capacitor.)

1. Device for electrically exciting an electrical machine, comprising a rotor having a rotor body (7) comprising at least one rotor pole and a shaft (3) comprising at least two slip rings (4), and wherein the device further comprises a voltage source (2) which can provide a direct voltage and a control section (1) which is connected to the voltage source and is designed such that the control section can control the direct voltage of the voltage source (2), and wherein the device further comprises at least two contact elements (5) for contacting the slip rings (4) and an electrical line (6) for connecting the slip rings (4) with the at least one rotor pole, and wherein the voltage source (2) is connected to the contact elements (5) and the electrical conductor (6) is configured such that a direct current can flow through the formed excitation circuit, in order to excite said at least one rotor pole, characterized in that a voltage source (2) is able to provide an alternating voltage which is superimposed on the direct voltage of the voltage source (2), and in that the control section (1) is connected to the voltage source (2) and is configured in such a way that it is able to control the alternating voltage of the same voltage source independently of the direct voltage, and in that the device further comprises a capacitor (8) which is arranged in the part of the excitation circuit which is located on the rotor and is designed in such a way that it is able to make said at least one rotor pole appear as an open circuit with respect to the alternating voltage component of the voltage source (2).

2. Method for operating the device according to claim 1, the method comprising the method steps performed in the given order:

v1: confirming the running state of the motor;

v2: selecting a required direct current voltage component;

v3: selecting a desired alternating voltage component;

v4: controlling the voltage source (2) according to the voltage component selected in V2 and V3;

wherein the steps V2 to V3 are performed by the control part (1), and

wherein in step V2 the selection of the required direct voltage component is made on the basis of the operating state confirmation made in step V1, and in step V3 the required alternating voltage component is selected according to the following criteria: when the dc voltage component selected in V2 is not sufficient to increase the dc current density in the contact element (5) above a predefined value that ensures a sufficient lubricating effect of the contact element (5) on the slip ring (4), then the following ac voltage components are selected: so that the effective value of the alternating current density flowing through the contact element, caused by the selected alternating voltage component, is at least as large as the deviation of the direct current density from the mentioned predefined value.

Technical Field

The present invention relates to a device for electrically exciting a rotor of an electric machine, and to a method for operating such an electric machine. The present invention relates to a motor excited by a direct current.

Background

In many electrical machines, the rotor is electrically excited via sliding contacts. The sliding contact may be a carbon brush placed on the slip ring. The current required for operating the motor in the rotor is fed via the formed sliding contacts.

During operation of the electric machine, the sliding contact elements are subjected to more or less severe wear. If one or more of the sliding contact elements are worn, maintenance measures need to be taken, which leads to costly downtime. In order to prolong the motor failure caused by this reason as long as possible, various measures have been proposed in the prior art. For this purpose, reference is made exemplarily to DE 102006040262B 3 and EP 1885034 a 1.

One reason for the increased wear of the contact elements is that the current density through the contact elements exceeds a certain value (see for example DE 102006040262B 3). However, the inventors have realized that an increased wear on the contact element also occurs when the current density through the contact element is below a certain value. From a certain current density, the passing current through the known and widely used carbon brushes will lead to a lubricating effect, which reduces the wear. For example, the contact elements of a synchronous generator are usually designed in such a way that the wear under rated load is as low as possible, i.e. the current density through the contact elements moves in a range that is beneficial for wear under rated load. However, certain operating conditions exist in which the dc-excited electrical machine is operated for a longer time with a lower current density in the contact elements. This relates, for example, to synchronous machines which are operated in phase-shift mode (see, for example, DE 102015111394 a 1). Depending on which reactive power consumption is currently required, such synchronous machines are either under-excited or over-excited.

Disclosure of Invention

The inventors set themselves the task of reducing the wear of the contact elements of a dc-excited electrical machine when the current density through the contact elements leaves a useful range in the direction of a low current density. According to the solution of the invention, in operating conditions with a low current density in the sliding contact piece, the wear of the sliding contact piece is reduced by additionally feeding an alternating current superimposed on the direct current, wherein the rotor poles appear open to the alternating current due to the capacitor rotating therewith, i.e. the alternating current does not flow through the rotor poles. It can also be said that the rotor poles are short-circuited with respect to the alternating current by a capacitor.

This object is achieved by a device having the features of claim 1. This object is also achieved by a method for operating such a device having the features of claim 2.

Drawings

The solution according to the invention is elucidated below with reference to the drawing. Wherein in detail:

fig. 1 shows a device for electrically exciting a rotor of an electric machine according to the prior art;

fig. 2 shows an apparatus for electrically exciting a rotor of an electrical machine according to the invention;

fig. 3 shows a flow chart of a method according to the invention.

Detailed Description

Fig. 1 shows a highly schematic illustration of a device for electrically exciting a rotor of an electric machine with direct current according to the prior art. Here, the rotor comprises a rotor body, which is denoted by 7 and which comprises electric rotor poles. The rotor further comprises a shaft, which is indicated with 3 and which carries slip rings, of which one slip ring is indicated with 4. For feeding the rotor poles, the device comprises a voltage source, which is denoted by 2 and which supplies a direct voltage. The device further comprises contact elements for contacting the slip ring, one of which is indicated with 5. On the shaft 3 there are electrical conductors which connect the slip ring 4 with the rotor poles, and one of which is indicated with 6. The voltage source 2 is connected to the contact element 5. The voltage source 2, the electrical connection to the contact element 5 and the conductor 6 on the shaft are arranged such that a direct current can flow through the formed excitation circuit in order to excite the rotor pole. The electric machine further comprises a control unit, which is denoted by 1 and is designed and connected to the voltage source 2 in such a way that it can control the voltage source. Obviously, at least two slip rings (4) and two contact elements (5) are required.

Fig. 2 shows a highly schematic illustration of a device according to the invention for electrically exciting a rotor of an electrical machine. The reference numerals of fig. 2 correspond to those of fig. 1. In contrast to fig. 1, the voltage source 2 of fig. 2 may provide a voltage comprising both a direct voltage component and an alternating voltage component. The two components are superimposed. This is indicated by the parallel connection of the voltage source symbols for the direct voltage and the alternating voltage. Fig. 2 does not mention a practical implementation of the voltage source 2. Various implementation possibilities suitable for this are sufficiently known to the person skilled in the art. The control unit 1 is designed and connected to the voltage source 2 in such a way that it can set the dc voltage component and the ac voltage component of the same voltage source independently of one another. Of course, equivalently, two separately controllable voltage sources can also be used, which each provide a dc voltage component and an ac voltage component and are connected to one another accordingly. Within the scope of the invention, this arrangement will be understood as a voltage source 2. Furthermore, the rotor comprises a capacitor, which is denoted by 8 and which is arranged and designed in the circuit located on the rotor for feeding the rotor poles, so that the capacitor causes the rotor poles to appear as open circuits with respect to the alternating voltage component of the voltage source 2. It is thereby ensured that a current comprising a direct current component and an alternating current component can flow through the contact element 5, but the rotor poles are only flown through by the direct current, or that the current flowing through the rotor poles has a negligible alternating current component compared to the direct current component. How the skilled person designs the capacitor 8 for this purpose will be explained below.

Fig. 3 shows a flow chart of a method according to the invention for operating the device according to the invention according to fig. 2. The individual method steps are denoted by V1 to V4. The method steps comprise the following items performed in the given order:

v1: confirming the running state of the motor;

v2: selecting a required direct current voltage component;

v3: selecting a desired alternating voltage component;

v4: the voltage source 2 is driven according to the selected voltage component of V2 and V3.

In step V1, the operating state of the motor is confirmed. This may be done by an external request to the control unit 1 or may be done by the control unit 1 itself. In step V2, control unit 1 selects the dc voltage component required for the operating state of voltage source 2 confirmed at V1. In step V3, control unit 1 selects the ac voltage component of voltage source 2. The selection of V3 is based on the following criteria. The dc voltage component selected from V2 results in a dc current component of the current density flowing through the contact element 5. If the direct current density in the contact element 5 exceeds a predefined value in this case, the alternating voltage component is not required, since the direct current component alone is sufficient to increase the current density in the contact element 5 above the lower threshold of the range of acceptable contact element wear, that is to say the lubricating effect in the contact element is sufficient. However, if the direct current density is lower than the above-mentioned predefined value, an alternating voltage component is required to achieve a sufficient lubricating effect. The lubricating effect is generated here by the contact element 5 being heated by the passing current through it. If the DC current density is lower than a predefined value by Δ _ i _ DC, it is necessary to have an AC current density with an effective value of at least i _ AC _ effective ≧ Δ _ i _ DC in order to obtain an acceptable lubricating effect. It is clear that the mentioned predefined values depend on the material and geometry of the contact element 5 used. However, it is clear enough for a person skilled in the art how the predefined value can be confirmed. The person skilled in the art can also determine this value by experiment if necessary, in that: the wear of the contact element 5 is measured as a function of the dc current density. The sought value will then be derived from the measured wear of the contact element and the desired minimum service life. In step V4, the control section 1 controls the voltage source 2 according to the dc voltage component and the ac voltage component selected in steps V2 and V3.

The design of the capacitor 8 depends on the frequency f of the alternating voltage component of the voltage source 2 and the inductance l of the rotor poles in order to make the undesired alternating current component through the rotor poles negligibly small, the capacitance C of the capacitor 8 must satisfy the condition (4 п)²*f²*C*L)>>1. This condition can therefore be achieved by a sufficiently large frequency f and/or by a sufficiently large capacitance C of the capacitor 8. Since the capacitor 8 also rotates with the rotor, it is endeavoured to design the capacitor as small as possible not too large. On the other hand, the supply frequency is selected most simply for f, since this makes it possible to design the voltage source 2 in a simplified manner. The skilled person can easily select from said advantageous combinations of parameters f and C, depending on the current boundary conditions.

Finally, it should also be mentioned that the selection of the voltage components in V2 and V3 can be performed by the control unit 1 on the basis of a pre-prepared table and/or on the basis of calculations.