DEVICE FOR OPERATING A RECTIFIER, IN PARTICULAR A SEMI-CONTROLLED RECTIFIER BRIDGE, AND METHOD FOR OPERATING A RECTIFIER

Description

[0001] The invention relates to a device for operating a rectifier, in particular a semi-controlled rectifier bridge, and a method for operating a rectifier.

[0002] It is generally known that a DC voltage can be generated from a three-phase mains voltage by means of a bridge rectifier and a capacitor connected on the output side.

[0003] From the US 6 038 155 A The closest state of the art is a three-phase SCR rectifier bridge with soft start control.

[0004] From the JP H08 214598 A A voltage control with excellent response to frequency variation is known.

[0005] From the EP 0 286 510 A1 A voltage supply circuit is known.

[0006] From the US 2011 / 044080 A1 A converter is known.

[0007] From the US 5 995 395 A Control of a bridge circuit at zero voltage is known.

[0008] From the US 6 222 749 B1 A method for limiting the current in a capacitor of a rectifier is known.

[0009] The invention is therefore based on the objective of achieving a limitation of the charging current.

[0010] According to the invention, the problem is solved in the device for operating a rectifier, in particular a semi-controlled rectifier bridge, according to the features specified in claim 1 and in the method for operating a rectifier according to the features specified in claim 6.

[0011] Key features of the invention in the rectifier bridge, in particular a converter, are that the rectifier is supplied from mains phases, in particular from a three-phase AC power network, and provides a unipolar voltage on the output side. wherein the rectifier comprises controllable switches, in particular semiconductor switches such as thyristors or the like, wherea respective current source is fed from a respective mains phase, wherein the respective generated current is used as a control signal for the controllable switch assigned to the respective mains phase depending on the state of a controllable switching unit, in particular depending on the state of a zero-voltage switch NSS.

[0012] The advantage here is that the switches are switched to the conducting state, and thus triggered, at or near the zero current crossing. This results in low losses. Furthermore, a high-voltage switch can be eliminated, and mechanical switches become unnecessary. Additionally, this prevents false tripping of the zero-voltage switch.

[0013] In an advantageous embodiment, the respective current source is formed from a respective switch S2 supplied via a half-wave rectifier from the respective mains phase, whose control input is also fed from the output, in particular from the unipolar output voltage of the half-wave rectifier, of the half-wave rectifier as long as a switch S3 of the controllable switching unit is open and whose control input can be brought to a higher potential than the respective mains phase, in particular upper intermediate circuit potential, in particular therefore upper output potential of the rectifier, when the switch S3 of the controllable switching unit is closed, in particular in such a way that no current can be supplied from one of the current sources to the controllable switches of the rectifier.

[0014] An advantage of this is that the power source is controllable and / or the forwarding of the current from the power source can be prevented, so that the controllable switches of the rectifier are not activated.

[0015] In an advantageous embodiment, switch S3 can be controlled depending on signals from a higher-level controller, in particular, wherein switch S3 can be controlled depending on a zero-voltage switch, in particular, wherein the state of the zero-voltage switch depends on the intermediate circuit voltage, in particular on the upper output potential of the rectifier and a respective mains phase voltage, as well as on signals from a higher-level controller. It is advantageous that the power source can be easily deactivated and / or switched off.

[0016] In an advantageous embodiment, a capacitor, in particular a DC link capacitor, is arranged on the output side of the rectifier, and / or a series circuit consisting of a braking resistor and a controllable switch, in particular a braking chopper, is arranged on the output side of the rectifier. An advantage of this is that excess energy, for example, energy generated by a generator, can be dissipated.

[0017] In an advantageous embodiment, the controllable switching unit of a control device, in particular a zero-voltage switch, comprises controllable switches (S3), wherein each switch is assigned to a respective network phase, and in particular wherein the control device can be controlled by a higher-level controller by means of a control signal NSS_OFF and / or the controllable switches can be controlled by means of a control signal GR_OFF from a higher-level controller. An advantage of this is that the zero-voltage switch can be easily enabled by not supplying a control voltage from the higher-level controller. Removing the enable signal, on the other hand, must then be accomplished by applying a control voltage. Thus, simple controllability is achieved.

[0018] In an advantageous embodiment, the power sources are each controllable, in particular by means of a control signal GR_OFF supplied by a higher-level control system. An advantage of this is that simple controllability can again be ensured.

[0019] In an advantageous embodiment, the controllable switches, in particular thyristors, of the rectifier are arranged in the upper branch of each half-bridge of the rectifier. The advantage here is that the rectifier is controllable and thus the DC link can only be charged according to the energy demand. Furthermore, it is possible to limit the charging current of the DC link capacitor.

[0020] In a preferred embodiment, a MOSFET switch in drain configuration is used as the current source and / or a MOSFET switch in source configuration is used as the controllable switch of the switching unit. The advantage here is that a switch with a simple and controllable circuit is used.

[0021] In an advantageous design, a surge protection device is powered from the network phases, In particular, the surge protection device comprises a capacitor and diodes, wherein current from a mains phase can be supplied to the capacitor via the respective diode, and in particular, wherein a resistor is connected in parallel to the capacitor. An advantage of this design is that a very fast response to voltage changes is possible. Especially during rapid voltage increases, the surge protection device allows the excess energy to be supplied to the capacitor, thus reducing the voltage spike.

[0022] Important features of the method for operating a rectifier, in particular one that has at least a semi-controlled rectifier bridge, especially a converter, are that the rectifier is supplied from mains phases, in particular from a three-phase AC network, and provides a unipolar voltage on the output side. wherein the rectifier has controllable switches, in particular semiconductor switches such as thyristors or the like, wherein at least one controllable switch is assigned to each mains phase, wherein the rectifier feeds a capacitor, in particular an intermediate circuit capacitor, on the output side, in particular on the DC side, whereA respective power source is supplied from a respective network phase, wherein the current generated by the respective power source is used as control current for the controllable switch assigned to the respective network phase, depending on the signal of a higher-level control system, wherein, upon release by the higher-level control system, the capacity from one of the network phases is charged with a voltage increasing from essentially zero, in particular a quarter-period of the network.

[0023] An advantage of this design is that the charging of the intermediate circuit capacitor begins at the zero crossing, and the charging current can be limited. This is because only one of the phases supplies the charging current, and the voltage rise follows the mains phase voltage rise, i.e., a sinusoidal curve over a quarter period.

[0024] In an advantageous embodiment, the current generated by the power source is fed to the controllable switch of the rectifier when the corresponding mains phase voltage is greater than the output voltage, i.e., the intermediate circuit voltage, of the rectifier and when it is less than a voltage value that is greater than the intermediate circuit voltage by a predetermined voltage difference, in particular, the value between 80 volts and 100 volts. The advantage here is that the zero-voltage switch can only be operated within this voltage window. Above this value, the power source is deactivated, and below this value, no charging current flows because the intermediate circuit voltage exceeds the mains phase voltage.

[0025] In an advantageous embodiment, the voltage applied to the capacitor, i.e., the DC link voltage, is detected, and if a threshold value is exceeded, particularly one between 200 and 300 volts, the enable signal is withdrawn, especially by the higher-level control system, and / or if the threshold value is not exceeded, the enable signal is granted. An advantage of this is the increased safety.

[0026] In an advantageous embodiment, the voltage across the capacitor, i.e., the intermediate circuit voltage, is detected, and the enable signal is issued when the peak value of the mains phase voltage is reached. In particular, the peak value is determined when the intermediate circuit voltage is within a predefined range and a further increase in the intermediate circuit voltage is not detectable for a predefined time interval. This increases safety because the zero-voltage switch is opened, thus allowing the controllable switches of the rectifier to operate in normal mode, i.e., they are supplied with control current from the power sources depending on the mains phase voltage.

[0027] In an advantageous design, the release is withdrawn in the event of a fault, particularly if a brake chopper fails. This has the advantage of increasing safety.

[0028] In an advantageous embodiment, a first delay time constant is provided for the detection of the DC link voltage, and a second delay time constant is provided in the control path of the current source, in particular when the enable signal is passed to the current source, wherein the first time constant is smaller than the second delay time constant. The advantage here is that it is possible to prevent the current source from being driven before the zero-voltage switch provides a switch-off signal.

[0029] Further advantages arise from the dependent claims. The invention is not limited to the combination of features of the claims. For those skilled in the art, further meaningful combinations of claims and / or individual claim features and / or features of the description and / or the figures will become apparent, in particular from the problem statement and / or the problem arising from a comparison with the prior art.

[0030] The invention will now be explained in more detail with the help of illustrations: In the Figure 1 Figure 1 schematically shows an embodiment of the invention in which a three-phase semi-controlled mains rectifier bridge can be controlled from a control device 4, which is supplied from mains phases (L1, L2, L3) and can be operated depending on the control signals of a higher-level control system, wherein an overvoltage protection 5 is provided which reacts very quickly to voltage changes. Figure 2 A further embodiment of the invention is shown in a specific circuit configuration. In the Figure 3 The surge protection device 5 is shown as an example.

[0031] As in Figure 1As shown, the half-controlled rectifier bridge is supplied from the three-phase AC mains, which has phase lines L1, L2, and L3. Controllable power semiconductor switches, preferably thyristors, are arranged in the upper branches of the three half-bridges, and diodes are arranged in the lower branches. On the output side, the rectified voltage is fed to the intermediate circuit capacitor C1, which contributes to voltage smoothing. This output voltage is also referred to as the intermediate circuit voltage.

[0032] The DC link, i.e., the voltage applied to the DC link capacitor C1, supplies an inverter (not shown in the figures). This inverter feeds an electric motor, thus enabling its speed to be controlled. When the motor is operating as a generator, energy is transferred from the motor to the DC link via the inverter, thus causing the DC link voltage to rise.

[0033] Parallel to the intermediate circuit capacitor C1, a series circuit consisting of a controllable switch S1, i.e. the brake chopper, and a brake resistor RB is arranged, so that energy that is too high at the intermediate circuit capacitor C1 can be converted into heat by means of the brake resistor RB.

[0034] The overvoltage protection device 5 has three diodes (D3, D4, D5), each arranged between a respective mains phase line (L1, L2, L3) and a capacitor C2. They then conduct current from the mains phase lines (L1, L2, L3) to the capacitor C2 when the voltage on the mains phase lines (L1, L2, L3) is higher than the voltage applied to the capacitor C2. A discharge resistor R4 is connected in parallel to the capacitor C2, so that the capacitor is discharged with a corresponding time constant when the voltage applied to the capacitor C2 exceeds the voltages applied to the mains phase lines (L1, L2, L3).

[0035] In this way, surges can be reduced very quickly by dissipating energy to capacitor C2 when they occur. Surge protection 5 is particularly advantageous for rapidly occurring voltage changes, such as those caused by lightning or the like.

[0036] The control device 4 is supplied from the mains phase lines (L1, L2, L3) and generates the control signals for the controllable switches of the semi-controlled mains rectifier bridge. The control device 4 comprises a control element 1 functioning as a zero-voltage switch, the current sources of the current source device 3, and the controllable switching unit 2. Each mains phase is assigned one of the current sources.

[0037] The current source device 3 is provided for generating the control signals, the current sources of which can be switched off for the rectifier by means of a control signal GR_OFF generated by a higher-level control system.

[0038] When the current source device 3 is enabled by means of the control signal GR_OFF, the current generated by the current sources assigned to the respective mains phases (L1, L2, L3) is passed through a controllable switching unit 2 and used as a control signal for the switch, in particular semiconductor switch, of the rectifier that can be controlled for the respective corresponding mains phase.

[0039] The controllable switches of the controllable switching unit 2, which are provided for the conduction of the current from the respective current source of the current source device 3, are controlled by the control device 1, which can be enabled or disabled by means of the control signal NSS_OFF generated by the higher-level control system. The control device 1, which functions as a zero-voltage switch, is connected to the mains phases (L1, L2, L3) and has a comparator that compares the DC link voltage with the respective phase voltage, in particular taking into account a threshold value.

[0040] In this way, according to Figure 1When enabled by the control signal NSS_OFF, for example when switching on the system comprising the rectifier or the electrical device comprising the rectifier, such as a converter or the like, the power source can be disconnected with the associated control input of the controllable switch if the difference between the intermediate circuit voltage and the voltage value of a respective mains phase falls below a positive threshold value or zero.

[0041] However, if the difference between the DC link voltage and the voltage value of a respective network phase exceeds a positive threshold or zero, the current source is connected to the control input of the controllable switch, provided that the enable signal from the higher-level control system is present.

[0042] In addition, the control device 1 compares the intermediate circuit voltage, i.e., the voltage across the intermediate circuit capacitor C1, with a threshold value, for example, with a further threshold value between 200 volts and 300 volts. If the intermediate circuit voltage Uz exceeds this further threshold value, the supply from the power sources is also interrupted.

[0043] Therefore, if no signal voltages NSS_OFF and GR_OFF are present initially, i.e., when the electrical device is switched on, the current sources are enabled. In this case, they are supplied from the mains phases (L1, L2, L3) and, as the mains phase voltage rises, each delivers a control current to the inputs of the controllable switches of the semi-controlled mains rectifier bridge. Thus, the controllable switches, in particular thyristors, are triggered depending on the mains phase voltage. After triggering, the DC link capacitor C1 is charged by the corresponding mains phase voltage as it rises from zero. This limits excessive charging current and / or charging current rise. In particular, it allows charging from only one of the mains phases to be carried out within a single mains quarter period.

[0044] According to the invention, the current sources assigned to the respective network phases can be switched by means of the low-voltage control signals GR_OFF, and the current can be supplied to the control inputs of the controllable switches via the switches of switching unit 2, which are also controllable by the low-voltage control signal NSS_OFF. A medium-voltage switch or high-voltage switch is therefore not necessary.

[0045] Therefore, when the enable signals NSS_OFF and GR_OFF are issued by the higher-level controller, the controllable switches are triggered after the respective phase voltage crosses zero. This is because the respective power source is supplied from the respective mains phase, for example, using half-wave rectification, preferably through a single diode. In the event of a fault or overload, the enable signal GR_OFF can be withdrawn, preventing charging of the intermediate circuit capacitor C1. Charging of the intermediate circuit capacitor C1 can be performed within a quarter-cycle of the mains voltage.

[0046] A MOSFET in drain configuration can be used as the current source.

[0047] MOSFETs in source configuration can be used as switches in switching unit 2.

[0048] In the further embodiment according to the invention, the control device for each network phase (L1, L2, L3) is arranged according to Figure 2 The remaining circuitry corresponds to the aforementioned embodiment.

[0049] As in Figure 2As shown, a half-wave rectifier, i.e., diode D1, is supplied from the exemplary mains phase L1. A series circuit consisting of the controllable switch S2, in particular a MOSFET switch, and the resistor R3 is fed from the output voltage of the half-wave rectifier. The current released by switch S2 from the mains phase L1 is supplied via diode D1 and the aforementioned series circuit to the control input, in particular the gate, of the controllable switch, in particular a thyristor, of the rectifier associated with the mains phase L1 when the corresponding switch S3 is open.

[0050] A Zener diode D2 is provided to generate a defined control voltage for switch S2. This series circuit is followed by another series circuit consisting of Zener diode D2 and a resistor R2, connected in parallel. The control input of switch S2 is connected to the center tap of this second series circuit.

[0051] Thus, a power source is formed by means of switch S2, which is supplied from the mains phase L1.

[0052] Furthermore, another controllable switch S3 is provided, by means of which the upper potential of the intermediate circuit voltage, i.e. the output voltage of the controlled rectifier, can be connected to the center tap, i.e.

[0053] Functionally, switch S3 corresponds to a switch of the controllable switching unit 4 according to Figure 1 , wherein when switch S3 is closed, the ignition of the controllable switches of the rectifier is prevented, and when switch S3 is open, the controllable switch becomes conductive via the half-wave rectifier D1 depending on the mains phase voltage.

[0054] Switch S3 is controlled depending on the NS_OFF and GR_OFF signals from the higher-level control system.

[0055] The half-wave rectifier is implemented only as diode D1 and does not have a smoothing capacitor.

[0056] As in Figure 1 The NSS is used as a control device 1 for switch S3 of the controllable switching unit. Switch S3, which is assigned to the mains phase L1, is only actuated by the NSS, and thus the triggering of the controllable switches, such as thyristors, of the rectifier is only enabled, if the voltage present on the mains phase (L1) is greater than the DC link voltage, i.e., the voltage across the capacitor (C1), but less than an upper voltage value that is greater than the DC link voltage by a voltage difference value.

[0057] Therefore, switch S3 remains open as long as the mains phase voltage is above the DC link voltage within this voltage range. At even higher voltages, switch S3 is closed by the zero-voltage switch NSS.

[0058] If the mains phase voltage is lower than the intermediate circuit voltage, a current flow through the voltage source, i.e., a current flow through switch S2, is automatically prevented.

[0059] The zero-voltage switch (NSS) also incorporates a voltage sensing device for detecting the intermediate circuit voltage. A delay time constant τ_1 is implemented in this process.

[0060] Likewise, a time delay means is preferably provided in the control path of switch S2, which delays the activation of switch S2. The delay time constant tau_2 is provided here, which is greater than the delay time constant tau_1. This prevents the gate of switch S2 from being activated before a switch-off signal generated by the zero-voltage switch NSS takes effect. The time delay can be implemented using RC elements, so that a respective precisely defined delay time constant is generated. Figure 2 The delay time constants are caused by parasitic properties that are not explicitly shown. Figure 4 is different from Figure 2 For clarification, a concrete implementation of the delay time constant tau_2 using component C3, i.e., capacitance C3, is shown. The delay time constant tau_1 is only symbolic. In addition, Figure 4A Zener diode D6 is shown to stabilize the supply voltage for the zero-voltage switch (NSS). The Zener diode D7 in the control path ensures a minimum voltage difference in the control path of the respective controllable switch of the rectifier.

[0061] The exemplary embodiment according to Figure 2 and after Figure 4 is otherwise carried out in the same way.

[0062] The higher-level controller revokes the NSS_OFF enable of the zero-voltage switch NSS when it detects that the intermediate circuit voltage has reached the peak value of the mains voltage. To do this, it is monitored whether the recorded intermediate circuit voltage values ​​lie within a value range that is characteristic of a peak value and, to increase reliability, it is observed whether the intermediate circuit voltage no longer increases. If an increase is no longer observable for a reasonable period of time and the reached value lies within a specified value range, it is assumed that the intermediate circuit voltage has reached the peak value of the AC voltage network. The higher-level controller then issues the NS_OFF signal, thus deactivating the zero-voltage switch NSS and, in particular, also causing switch S3 to open or remain open.Thus, the controllable switches of the rectifier, in particular thyristors, are triggered depending on the mains phase voltage, since the current source, especially switch S2, is supplied and controlled by the mains phase voltage. In this case, the rectifier then operates in the known manner of a bridge rectifier.

[0063] If the DC link voltage Uz is lower than a further threshold value, for example a value between 200 volts and 300 volts, the zero-voltage switch NSS is enabled, i.e., switch S3 for the zero-voltage switch NSS is enabled. Thus, the zero-voltage switch NSS operates as described above and causes switch S3 to open when the mains phase voltage is within the window defined by the voltage difference above the DC link voltage. List of reference symbols

[0064] 1 Control device Zero-voltage switch 2 Controllable switch unit 3 Power source device 4 Control device 5 Overvoltage protection NSS Zero-voltage switch C1 DC link capacitor C2 Capacitor C2 Capacitor D1 Diode D2 Zener diode D3 Diode D4 Diode D5 Diode D6 Zener diode D7 Zener diode R1 Resistor R2 Resistor R3 Resistor R4 Resistor RB Braking resistor S1 Controllable switch, in particular braking chopper S2 Controllable switch L1 First mains phase L2 Second mains phase L3 Third mains phase Tau_1 Delay time constant Tau_2 Delay time constant NSS_OFF Control signal for zero-voltage switch GR_OFF Control signal for rectifier +Uz Positive DC link potential -Uz Negative DC link potential

Claims

1. System comprising a rectifier and a device for operating the rectifier, in particular a half-controlled rectifier bridge, wherein the rectifier can be connected to network phases, in particular from a three-phase AC voltage network, and makes a unipolar voltage available at the output, wherein the rectifier has first controllable switches, in particular semiconductor switches such as thyristors or the like, wherein the system comprises current sources and the associated actuation paths, wherein each current source can be fed by a respective network phase, wherein the system further comprises controllable switching units, wherein the controllable switching units each comprise a third switch (S3), wherein the system further comprises half-wave rectifiers, wherein the current of each current source is used as an actuation signal for the first controllable switch assigned to the relevant network phase depending on the status of the relevant controllable switching unit, wherein each controllable switching unit further has a respective zero-voltage switch (NSS), wherein the system further comprises a capacitor, in particular a DC link capacitor (C1), wherein the capacitor is arranged at the output of the rectifier, wherein the system further comprises a means for detecting the voltage applied to the capacitor, i.e. the DC link voltage, wherein the means for detecting the DC link voltage comprises a first time constant of time delay, wherein the current sources are each controllable, in particular by means of a release signal delivered by a superordinate controller, and the actuation path of the current source comprises a second time constant of time delay, i.e. in particular for relaying the release signal to the current source, wherein the first time constant of time delay is less than the second time constant of time delay, wherein each current source is formed from a respective second switch (S2) to which power is supplied from the relevant network phase via the relevant half-wave rectifier (D1), - the control input of which second switch is likewise fed from the output of the half-wave rectifier as long as the third switch (S3) of the controllable switching unit is open, and - the control input of which second switch can be brought to a higher potential, in particular an upper DC link potential, i.e. in particular an upper output potential of the rectifier, compared with the relevant network phase when the third switch (S3) of the controllable switching unit is closed, such that current cannot be supplied to the controllable switches of the rectifier from any of the current sources, wherein furthermore each third switch (S3) can be actuated depending on signals of a superordinate controller, wherein furthermore each third switch (S3) can be actuated depending on the relevant zero-voltage switch (NSS) of the relevant switching unit, wherein each zero-voltage switch (NSS) is configured such that the third switch (S3) assigned to the relevant network phase (L1) is actuated by the relevant zero-voltage switch (NSS), and thus the triggering of the controllable switches of the rectifier is released for triggering, only when the voltage at the relevant network phase (L1) is greater than the DC link voltage, i.e. the voltage applied to the capacitor (C1), but less than an upper voltage value that is greater than the DC link voltage by a voltage difference value.

2. System according to claim 1, characterised in that the status of each zero-voltage switch (NSS) is dependent on the DC link voltage, in particular on the upper output potential of the rectifier and a relevant network phase voltage, as well as on signals of a superordinate controller.

3. System according to at least one of the preceding claims, characterised in that the first controllable switches, in particular thyristors, of the rectifier are each arranged in the upper branch of each half-bridge of the rectifier.

4. System according to at least one of the preceding claims, characterised in that a MOSFET switch is used as the current source in each drain circuit and / or in that a MOSFET switch is used as the second switch (S2) in each source circuit.

5. System according to at least one of the preceding claims, characterised in that the system further comprises an overvoltage protector, and this overvoltage protector (5) can be fed from the network phases, the overvoltage protector (5) in particular having a capacitor and diodes (D2, D3, D4), it being possible to feed current from a network phase to the capacitor via the relevant diode (D2, D3, D4), a resistor (R1) in particular being wired in parallel with the capacitor.

6. Method for operating a rectifier which in particular has an at least half-controlled rectifier bridge, power being supplied to the rectifier from network phases, in particular from a three-phase AC voltage network, and the rectifier making a unipolar voltage available at the output, the rectifier having first controllable switches, in particular semiconductor switches such as thyristors or the like, each network phase being assigned at least one of the first controllable switches, the rectifier feeding, at its output, i.e. in particular on the DC voltage side, a capacitor, in particular a DC link capacitor (C1), a particular current source being fed from a particular network phase, the current generated by each current source being used as a control current for the first controllable switch assigned to the relevant network phase depending on the signal of a superordinate controller, the capacitor, upon release by the superordinate controller, being charged from one of the network phases by a voltage, in particular a network quarter period, that increases from substantially zero, the voltage applied to the capacitor, i.e. the DC link voltage, being detected, characterised in that a first time constant of time delay is provided during detection of the DC link voltage, and a second time constant of time delay is provided in the actuation path of the current source, i.e. in particular when relaying the release signal to the current source, the first time constant of time delay being less than the second time constant of time delay, each current source being formed from a respective second switch (S2) supplied with power from the relevant network phase via a half-wave rectifier, each generated current being used as an actuation signal for the controllable switch (S1, S2) assigned to the relevant network phase depending on the status of a relevant controllable switching unit, the controllable switching unit furthermore having a third switch (S3), the third switch (S3) furthermore being actuatable depending on signals of a superordinate controller, the third switch (S3) assigned to the network phase (L1) being actuated by a zero-voltage switch (NSS), and thus the triggering of the first controllable switches of the rectifier being released for triggering, only when the voltage at the network phase (L1) is greater than the DC link voltage, i.e. the voltage applied to the capacitor (C1), but less than an upper voltage value that is greater than the DC link voltage by a voltage difference value.

7. Method according to claim 6, characterised in that the current generated by the current source is fed to the relevant first controllable switch of the rectifier when the corresponding network phase voltage is greater than the output voltage, i.e. the DC link voltage, of the rectifier and less than a voltage value that is greater than the DC link voltage by a predetermined voltage difference value, the magnitude of which is in particular between 80 V and 100 V.

8. Method according to claim 7 or claim 6, characterised in that the voltage applied to the capacitor, i.e. the DC link voltage, is detected, and the release is cancelled, in particular by the superordinate controller, when a threshold value, which is in particular between 200 V and 300 V, is exceeded, and / or the release is issued when the threshold value is not exceeded.

9. Method according to claims 6 to 8, characterised in that the voltage applied to the capacitor, i.e. the DC link voltage, is detected, and the release is issued when the peak value of the network phase voltage is reached, the reaching of the peak value being established in particular when the DC link voltage is in a predetermined value range and no further rise in the DC link voltage can be established for a predetermined time period.

10. Method according to any of claims 6 to 9, characterised in that the release is cancelled in the event of a fault, in particular a breakdown of a brake chopper.