Device for controlling a separately excited synchronous machine, separately excited synchronous machine and method

Abstract

The invention relates to a device (1) for controlling a separately excited synchronous machine (100) with slip rings (10) and brushes (12), wherein the device (1) has a full-bridge circuit (2) and a control unit (3) for controlling the full-bridge circuit (2), wherein the full-bridge circuit (2) has two half-bridges (H1, H2), each having an upper branch (ZO1, ZO2) and a lower branch (ZU1, ZU2), wherein the first half-bridge (H1) has at least one reverse-biased diode (D) in the upper branch (ZO1) and at least one transistor (T1) in the lower branch (ZU1), wherein the second half-bridge (H2) has at least one transistor (T2) in the upper branch (ZO2) and at least one reverse-biased diode (D) in the lower branch (TU2), wherein at least one forward-biased diode (D) is connected in parallel to the branch with the respective at least one reverse-biased diode (D). Diode (D1, D2) in series with at least one resistor (R1,R2), a separately excited synchronous machine (100) and a method for operating a separately excited synchronous machine (100).,

Description

[0001] The invention relates to a device for controlling a separately excited synchronous machine, a separately excited synchronous machine with such a device and a method for operating such a separately excited synchronous machine.

[0002] In separately excited synchronous machines, a rotor winding is supplied with direct current, with the current being fed in externally via slip rings mounted on a rotor shaft. Brushes then pick up the current from the slip rings and feed it to or from the rotor winding. The stationary brushes are pressed against the rotating slip rings with a certain pressure.

[0003] One area of ​​application for such separately excited synchronous machines is, for example, as an electric machine in an electric vehicle. A full bridge circuit comprising two half-bridges is used to control the separately excited synchronous machine or the rotor winding. The rotor winding, slip rings, and brushes are arranged in the bridge or bridge diagonal. The half-bridges each have an upper and a lower branch. The first half-bridge has at least one reverse-biased diode in the upper branch and at least one transistor in the lower branch. The second half-bridge has at least one transistor in the upper branch and at least one reverse-biased diode in the lower branch. Due to the different materials used for the slip rings, brushes, and rotor winding, decomposition of the slip rings can occur, particularly if the slip rings are exposed to moisture.One remedy is to arrange the slip rings in a protected space. Another measure is to temporarily apply a negative current to the slip ring, forming a protective oxide layer that counteracts decomposition. In this case, all branches of the two half-bridges must be equipped with transistors.

[0004] DE 10 2017 200 220 A1 shows a generic control method and a switching device for a separately excited synchronous machine as a drive in a hybrid or electric vehicle.In order to create a control method and a corresponding switching device by means of which the EMC problem described above is significantly reduced, a device is proposed which is provided as a switching device in a vehicle with a high-voltage network comprising high-voltage components for converting and / or distributing electrical energy within the vehicle, in particular a hybrid or electric vehicle, wherein an asymmetric full bridge is provided, in the bridge branch of which a rotor of an SSM is arranged and switches are provided in the asymmetric full bridge for providing pulse width modulation which corresponds to a desired engine speed and power of the SSM, and the device is characterized in that it comprises a short-circuit branch running parallel to the bridge branch of the asymmetric full bridge, by means of which short-circuit branch the rotor of the SSM can be short-circuited.

[0005] The generic DE 10 2019 128 721 A1 discloses a power electronics device for a separately excited synchronous machine, in particular in a motor vehicle, comprising a number of converter modules implemented as individual structural units corresponding to the number of phases of the synchronous machine and an excitation module implemented as a structural unit for generating an excitation voltage for the synchronous machine, wherein the converter modules are thermally connected to a cooling device using a coolant, in particular water, in a cooling circuit, wherein the excitation module is also thermally connected to the cooling device.

[0006] From DE 10 2010 040 520 A1 a method for determining a state of a starter motor comprising a brush-commutator arrangement is known, in which in at least one detection cycle at defined operating points on a battery which supplies the starter motor with electrical energy, a current measurement and a voltage measurement are carried out and from this a resistance of the brush-commutator arrangement is determined.

[0007] The invention is based on the technical problem of creating a device for controlling a separately excited synchronous machine that counteracts the degradation of the slip rings with less circuit complexity. A further technical problem is the creation of such a separately excited synchronous machine and the provision of a suitable method for operating a separately excited synchronous machine.

[0008] The solution to the technical problem is provided by a device having the features of claims 1 or 7, a separately excited synchronous machine having the features of claim 8 and a method having the features of claim 9. Further advantageous embodiments of the invention emerge from the subclaims.

[0009] For this purpose, the device for controlling a separately excited synchronous machine with slip rings and brushes has a full-bridge circuit and a control unit for controlling the full-bridge circuit. The full-bridge circuit has two half-bridges, each having an upper branch and a lower branch. Furthermore, the first half-bridge has at least one reverse-biased diode in the upper branch and at least one transistor in the lower branch. Accordingly, the second half-bridge has at least one transistor in the upper branch and at least one reverse-biased diode in the lower branch. For reasons of dielectric strength, several diodes can be connected in series in the branches with reverse-biased diodes, or additional resistors can be present. In the branches with the transistor, for example, two transistors can be connected in series.At least one forward-biased diode is arranged in series with at least one resistor in parallel with each branch containing at least one reverse-biased diode. This allows a negative offset current to be passed through the slip rings with minimal circuit complexity, promoting oxide growth on the slip rings.

[0010] In one embodiment, the resistors are dimensioned such that the current through the resistors is at least 100 times smaller than the current through the conductive transistors. This ensures that the negative offset current generates no or no significant torque. Furthermore, the negative offset current is preferably 1000-3000 times smaller than the current through the conductive transistors. For example, the current through the conductive transistors is in the range of 20 A - 25 A, and the negative offset current is in the range of 10 mA - 25 mA.

[0011] In another embodiment, an additional resistor is arranged in the bridge, connected in parallel with a rotor winding. This resistor protects the transistors from overvoltage in the event of a fault in the rotor winding, as the resistor ensures current flow.

[0012] In a further embodiment, the additional resistance is at least 10 times greater than the ohmic resistance of the rotor winding. More preferably, the factor is at least 100. This ensures that a large portion of the negative offset current flows through the slip rings.

[0013] In a further embodiment, the control unit is assigned a memory in which a rotor angle of the last rest position when the separately excited synchronous machine was at a standstill is stored. The control unit is designed to detect the current rotor angle when the machine is at a standstill and compare it with the stored rotor angle. If the difference is smaller than a predetermined offset, the rotor winding is energized until the difference is at least as large as the predetermined offset. This is based on the following idea. When the vehicle is at a standstill, for example when parked or charging, the negative offset current flows with increased intensity through the areas of the slip rings on which the brushes are currently located. This leads to local oxide growth. The rotor angle when switched off is arbitrary and has no preferred position, so that all positions generally occur with equal frequency.However, if, by chance or for other reasons, the brush is again too close to the spot where the local oxide growth was previously located during a vehicle stop, the rotor is moved a little further so that at least the specified offset is maintained. The offset can be, for example, 5° - 10°. The resulting additional movement is minimal. However, this ensures that the local oxide growth is distributed more evenly across the circumference of the slip rings.

[0014] In a further embodiment, the control unit is configured to maintain the phase currents in the stator windings of the separately excited synchronous machine during coasting operation. This induces a negative current in the rotor windings, which also leads to oxide buildup.

[0015] This embodiment represents an independent, alternative solution that can also be used on its own.

[0016] The separately excited synchronous machine with slip rings and brushes has a previously described control device.

[0017] According to the method, a negative current is generated in a rotor winding of a separately excited synchronous machine under predetermined operating conditions to induce oxide growth on the slip rings. Regarding further possible embodiments, reference is made in full to the preceding explanations of the device.

[0018] The invention is explained in more detail below using preferred embodiments. The figures show: Fig. 1 a schematic representation of a device for controlling a rotor winding of a separately excited synchronous machine in a first embodiment, Fig. 2 a schematic representation of a device for controlling a separately excited synchronous machine in a second embodiment, Fig. 3 a schematic representation of slip rings on a rotor shaft and Fig. 4 a flowchart of a method for controlling a separately excited synchronous machine.

[0019] In the Fig. 1 schematically shows a device 1 for controlling a separately excited synchronous machine. The device 1 has a full-bridge circuit 2 and a control unit 3, wherein the control unit 3 is assigned a memory 4. The full-bridge circuit 2 has a first half-bridge H1 and a second half-bridge H2. The first half-bridge H1 has an upper branch ZO1 and a lower branch ZU1. In the upper branch ZO1, at least one reverse-biased diode D is arranged, to which a series circuit comprising a forward-biased diode D1 and at least one first resistor R1 is connected in parallel. At least one first transistor T1 is arranged in the lower branch ZU1. Accordingly, a second transistor T2 is arranged in the upper branch ZO2 of the second half-bridge H2.In the lower branch ZU2 there is a reverse-biased diode D, in parallel with which there is a series circuit consisting of a forward-biased diode D2 and at least a second resistor R2. The rotor winding LR is arranged in the bridge or the bridge diagonal, with another resistor R3 connected in parallel with it. Not shown are the slip rings and brushes through which the current is fed into the rotor winding LR. The positive current direction is symbolized by an arrow. A positive current flows when the two transistors T1, T2 are switched on. A negative offset current constantly flows through the resistor R1, the diode D1, the parallel circuit consisting of the further resistor R3 and the rotor winding LR, the diode D2 and the second resistor R2.Resistors R1 and R2 are dimensioned such that the negative offset current is at least 100 times smaller than the positive current flowing when transistors T1 and T2 are switched on. At standstill, i.e., when both transistors T1 and T2 are switched off, only the negative offset current flows, leading to oxide growth on the slip rings. Transistors T1 and T2 are controlled by control unit 3. At least the rotor angle φ is stored in memory 4. R0 from the last vehicle standstill. If the separately excited synchronous machine is to be switched off (brought to a standstill), the current rotor angle φ R1 read by the control unit 3. The rotor angle φ R1 For example, a rotor position sensor provides this current rotor angle φ R1 is then calculated with the stored rotor angle φ R0 compared. If the difference is smaller than a specified offset Δφ R, the rotor winding LR continues to be energized via the transistors T1, T2 until the difference is greater than Δ φ R The current rotor angle is then used as the new φ R0 stored in memory 4 and, in the case of an electric vehicle, a parking brake is activated.

[0020] In the Fig. 2 shows an alternative embodiment which can also be used in combination with the embodiment according to Fig. 1 can be applied. Identical parts are given the same reference numerals as in Fig. 1. Additionally shown are freewheeling diodes D3 of the transistors T1, T2, which are either separate components or intrinsic diodes of the transistors T1, T2 if they are embodied as MOSFETs. A stator winding LS is also shown schematically. In terms of hardware, the device 1 is constructed as in the prior art. According to the invention, in a coasting mode of the separately excited synchronous machine 100, the current supply to the rotor winding LR is stopped (T1, T2 are blocked), whereas the stator windings LS continue to be supplied with current. The stator winding LS induces a voltage at the rotor winding LR, so that a negative current flows. The directions of the currents are symbolized by arrows. The negative current then flows through the freewheeling diode D3 into the battery and from there back via the other freewheeling diode D3.Since the rotor shaft rotates during sailing operation, the slip rings are subjected to rotationally symmetrical negative current, resulting in a symmetrical oxide buildup.

[0021] In the Fig. Figure 3 schematically shows how two slip rings 10 are arranged on a rotor shaft 11, with stationary brushes 12 collecting the current and connected to the rotor winding LR. The slip rings 10 are electrically connected to the center taps of the full-bridge circuit 2.

[0022] In the Fig. 4 shows a flowchart of a method. In a first step S1, the device 1 detects that the separately excited synchronous machine 100 (see also Fig. 2) should be parked (e.g. an electric vehicle should be parked). In a second step S2, the current rotor angle φ R1 determined.

[0023] In a third step S3, the rotor angle φ R0of the last standstill is read from a memory 4. Steps S2 and S3 can also be interchanged or performed simultaneously. In a fourth step S4, the two rotor angles φ R0 , φ R1 compared, whereby the difference is calculated. If the difference is greater than a specified offset Δφ, the separately excited synchronous machine 100 is switched off and the rotor angle φ R1 stored (step S5). Otherwise, the rotor winding LR continues to be energized by device 1 until the difference of the current rotor angle φ R1 and the stored rotor angle φ R0 is greater than Δφ (step S6). The current rotor angle φ R1 as new stored rotor angle φ R0 saved and the separately excited synchronous machine 100 is switched off (step S7). List of reference symbols 1 device 2 full bridge circuit 3 Control unit 4 storage 10 slip ring 11 Rotor shaft 12 brushes 100 synchronous machine T1, T2 transistors D-diode D1, D2 diodes D3 freewheeling diode R1 first resistance R2 second resistor R3 further resistance LR rotor winding LS stator winding

Claims

[1] Device (1) for controlling a separately excited synchronous machine (100) with slip rings (10) and brushes (12), wherein the device (1) has a full-bridge circuit (2) and a control unit (3) for controlling the full-bridge circuit (2), wherein the full-bridge circuit (2) has two half-bridges (H1, H2), each having an upper branch (ZO1, ZO2) and a lower branch (ZU1, ZU2), wherein the first half-bridge (H1) has at least one reverse-biased diode (D) in the upper branch (ZO1) and at least one transistor (T1) in the lower branch (ZU1), wherein the second half-bridge (H2) has at least one transistor (T2) in the upper branch (ZO2) and at least one reverse-biased diode (D) in the lower branch (TU2), characterized by that at least one forward-biased diode (D1, D2) is arranged in series with at least one resistor (R1, R2) in parallel to the branch with the at least one reverse-biased diode (D). [2] Device according to claim 1, characterized by that the resistors (R1, R2) are dimensioned such that the current through the resistors (R1, R2) is at least a factor of 100 smaller than the current through the switched transistors (T1, T2). [3] Device according to claim 1 or 2, characterized by that a further resistor (R3) is arranged in the bridge, which is connected in parallel to a rotor winding (LR). [4] Device according to claim 3, characterized by that the additional resistor (R3) is at least 10 times greater than the ohmic resistance of the rotor winding (LR). [5] Device according to one of the preceding claims, characterized by that the control unit (3) is assigned a memory (4) in which a rotor angle (φ R0) of the last rest position when the separately excited synchronous machine (100) is at a standstill, wherein the control unit (3) is designed to determine the current rotor angle (φ R1 ) and compare it with the stored rotor angle (φ R0 ), whereby, if the difference is smaller than a predetermined offset (Δφ), the rotor winding (LR) is energized until the difference is at least as large as the predetermined offset (Δφ). [6] Device according to one of the preceding claims, characterized by in that the control unit (3) is designed such that in a sailing operation in which the rotor shaft rotates and the current supply to the rotor winding (LR) is stopped, whereas the stator windings (LS) continue to be supplied with current, the phase currents in the stator windings (LS) of the separately excited synchronous machine (100) are maintained. [7] Device (1) for controlling a separately excited synchronous machine (100) with slip rings (10) and brushes (12), wherein the device (1) has a full-bridge circuit (2) and a control unit (3) for controlling the full-bridge circuit (2), wherein the full-bridge circuit (2) has two half-bridges (H1, H2), each having an upper branch (ZO1, ZO2) and a lower branch (ZU1, ZU2), wherein the first half-bridge (H1) has at least one reverse-biased diode (D) in the upper branch (ZO1) and at least one transistor (T1) in the lower branch (ZU1), wherein the second half-bridge (H2) has at least one transistor (T2) in the upper branch (ZO2) and at least one reverse-biased diode (D) in the lower branch (TU2), characterized by , that the control unit (3) is designed to maintain the phase currents in the stator windings (LS) of the separately excited synchronous machine (100) in a sailing operation in which the rotor shaft rotates and the current supply to the rotor winding (LR) is stopped, whereas the stator windings (LS) continue to be supplied with current. [8] Separately excited synchronous machine (100) with slip rings (10) and brushes (12), wherein the separately excited synchronous machine (100) has a device (1) for controlling with the features of one of the preceding claims. [9] Method for operating a separately excited synchronous machine (100) having the features of claim 8, characterized by that under given operating conditions a negative current is generated in a rotor winding (LR).

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