Frequency converter with simplified pre-charging circuit

Abstract

The present application discloses a frequency converter, which has a rectifier (1) on the input side and at least one backup capacitor (3) arranged downstream of the rectifier (1). The rectifier (1) has a plurality of half bridges (6), and the corresponding input side phases (U, V, W) of the rectifier (1) supply power to the backup capacitor (3) through the half bridges. The input side phases (U, V, W) are connected to the network side phases (L1, L2, L3) of the multi-phase power supply network (5) through a pre-circuit (4). The network side phases (L1, L2, L3) are connected to a corresponding input side phase (U, V, W) through a corresponding phase capacitor (9) in the pre-circuit (4). In addition, the network side phase (L1, L2, L3) is directly connected to the corresponding other input side phase (U, V, W) through the corresponding switch (10) inside the pre-circuit (4), so that when the switch (10) is closed, the network side phase (L1, L2, L3) is short-circuited to the input side phase (U, V, W) and the phase capacitor (9) respectively connects two network side phases (L1, L2, L3) or two input side phases (U, V, W) to each other. The frequency converter has a control device (11), which keeps the switch (10) in an open state when the backup capacitor (3) is pre-charged, and closes the switch (10) when the backup capacitor (3) reaches a predetermined charging state (Z1).

Description

Technical Field

[0001] The present invention relates to a frequency converter.

[0002] wherein the frequency converter has on the input side a rectifier and at least one backup capacitor arranged downstream of the rectifier,

[0003] - wherein the rectifier has a plurality of half-bridges, and the corresponding input-side phases of the rectifier supply the backup capacitor via the half-bridges,

[0004] - wherein the input side phase of the rectifier is connected to the network side of the multiphase power supply network via a pre-circuit,

[0005] - wherein the network-side phase is connected to a corresponding one of the input-side phases via a corresponding phase capacitor in the pre-circuit. Background Art

[0006] Mains-powered frequency converters usually require a filter circuit and a pre-charging circuit between the supply network and the rectifier on the input side of the frequency converter. The filter circuit is used to limit network interference. The pre-charging circuit is only required when the frequency converter is connected to the supply network. The pre-charging circuit is used to limit the pre-charging current for charging the backup capacitor. As soon as the backup capacitor is charged, the current limitation can end. For this purpose, for example, the pre-charging circuit can be bridged by means of a relay.

[0007] A simple and widely used precharging circuit consists in arranging a resistor in the line from the network-side phase of the supply network to the input-side phase of the rectifier, which resistor is bridged by means of the switching contacts of a relay after the precharging of the backup capacitor. The disadvantage of this precharging circuit is that high power losses occur in the resistor during precharging and therefore a significant heating of the resistor occurs. In particular, in the case of large capacitances of the backup capacitor, the resistor must also be designed to be large. Alternatively, a larger resistance value can also be selected. However, in this case, the time required for precharging increases. In addition, repeated precharging is usually only possible to a limited extent, because the resistor needs a longer time to cool down again.

[0008] Alternatively, it is known to use special power semiconductors to precharge the backup capacitor. However, this method is complex and expensive in terms of circuit technology.

[0009] Several types of frequency converters of the type mentioned at the outset are known from EP 2 533 409 A1. In one of these frequency converters, the network-side phase is also directly connected to the corresponding identical input-side phase via a corresponding switch, so that when the switch is closed, the network-side phase is short-circuited to the identical input-side phase and thus bridges and thus deactivates the phase capacitor. In another of these frequency converters, a further capacitor is first arranged in the line path in which the switch is arranged. In addition, a further switch is arranged in the line path in which the phase capacitor is arranged. When the aforementioned switch is closed, the other switch is open and vice versa.

[0010] The frequency converter known from EP 2 533 409 A1 is advantageous compared to conventional frequency converters of the prior art because only a small power loss occurs during precharging. However, since both the precharging circuit and the filter circuit are required, the overall volume is still large. Therefore, the costs are still high.

[0011] From the technical paper “Control of Active Front-End Rectifier of the Solid-State Transformer with Improved Dynamic Performance during Recharging” by HD Tafti et al., a frequency converter is known, which has a rectifier on the input side and a backup capacitor arranged downstream of the rectifier. The rectifier has a half-bridge, via which the backup capacitor is supplied by the input-side phase of the rectifier. The half-bridge has active switching elements, so that the rectifier is designed as a regenerative converter ( Umrichter). The input side phase of the rectifier is connected to the network side of the power supply network through a pre-circuit. The frequency converter has a control device, which controls the active switching element when the backup capacitor reaches a first charging state so that the backup capacitor continues to charge at a constant charging rate. Summary of the invention

[0012] The object of the invention is to design a frequency converter of the type mentioned at the outset in such a way that the filter function and the current limitation are also implemented during the precharging period and that the associated upstream circuits can be realized simply and inexpensively.

[0013] This object is achieved by a frequency converter having the features of the invention. Advantageous embodiments of the frequency converter are the subject of the individual exemplary embodiments.

[0014] According to the invention, a frequency converter of the type mentioned at the outset is designed such that

[0015] - a network-side phase is directly connected to a corresponding other input-side phase in the front circuit via a corresponding switch, so that when the switch is closed, the network-side phase is short-circuited to the input-side phase, and a phase capacitor connects two network-side phases or two input-side phases to each other, respectively, and

[0016] The frequency converter has a control device which keeps the switch open while the backup capacitor is pre-charged and closes the switch when the backup capacitor reaches a predetermined charge state.

[0017] This makes it possible for one and the same capacitor, the phase capacitor, to achieve current limiting during precharging on the one hand and to serve as a smoothing capacitor for current smoothing during normal operation on the other hand.

[0018] Interchanging the network-side and input-side phases connected to one another does not present major problems. In the case of uncontrolled rectifiers, no measures need to be taken either. In the case of controlled rectifiers (i.e. rectifiers in which the half-bridges have active switching elements), the actuation of the active switching elements can be adapted accordingly in a simple manner by the control device. In particular, only the assignment of the individual control signals needs to be adapted to the active switching elements. Such actuation can remain unchanged.

[0019] The upstream circuit preferably has an inductance in the line to the network-side phase and / or the input-side phase. This embodiment enables particularly simple and effective filtering.

[0020] The inductor is preferably arranged in the line so that the phase current fed from the supply network and / or flowing through the half-bridge flows through it both when the switch is closed and when it is switched on. As a result, the realization of the combination of the two effects of the phase capacitor (i.e. one for filtering and one for current limiting) is designed to be particularly simple and effective.

[0021] The half-bridge preferably has active switching elements, so that the rectifier is designed as a feedback-capable converter. This design is particularly advantageous in terms of energy technology. This also provides further advantages.

[0022] For example, in this case, when a predetermined first charging state is reached, the control device

[0023] - firstly, the active switching element is driven so that an input-side phase voltage is applied to the input-side phase via the active switching element, which input-side phase voltage corresponds in amplitude and phase to a network-side phase voltage applied to a network-side phase, which network-side phase is connected to the input-side phase via a phase capacitor,

[0024] - then driving the active switching element so that the input side phase voltage applied to the input side phase by the active switching element is stably converted into the network side voltage applied to the network side phase, which is connected to the input side phase through the switch, and

[0025] The switch is closed only after the input-side phase voltage has been converted into a network-side phase voltage which is applied to the network-side phase connected to the input-side phase via the switch.

[0026] As a result, current peaks and voltage jumps which may occur when the switch is closed can be significantly reduced.

[0027] Furthermore, in the case of an active switching element, starting from reaching a second charging state of the backup capacitor, the control device drives the active switching element so that an input-side phase voltage is applied to the input-side phase via the active switching element, the input-side phase voltage being opposite to the network-side phase voltage applied to the network-side phase connected to the input-side phase via the phase capacitor, wherein the second charging state is assumed to be before the first charging state during precharging of the backup capacitor. With this design, the precharging time can be significantly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] According to the following description of the embodiments of the present invention in combination with the embodiments, the above characteristics, features and advantages of the present invention and the ways and methods of achieving them are explained in more detail. Herein, it is shown in a schematic diagram:

[0029] Figure 1 shows a frequency converter,

[0030] Figure 2 Shows network connections, front-end circuitry and input-side rectifier

[0031] Figure 3 shows a flow chart,

[0032] Figure 4 shows a time chart, and

[0033] Figure 5 Another time diagram is shown. DETAILED DESCRIPTION

[0034] according to Figure 1 , the frequency converter has a rectifier 1 on the input side. A further device 2 is arranged downstream of the rectifier 1 on the output side. The frequency converter also has at least one backup capacitor 3, which is arranged between the rectifier 1 and the further device 2. The further device is usually designed as an inverter. In this case, we are talking about an intermediate circuit converter. However, the further device can also be designed differently, for example as a DC voltage consumer, a DC voltage network, a photovoltaic system or an energy storage device.

[0035] The rectifier 1 has phases U, V, W on the input side, which are connected to phases L1, L2, L3 of the power supply network 5 via a pre-circuit 4. Therefore, both the rectifier 1 and the power supply network 5 are multi-phase, i.e., have multiple phases U, V, W or L1, L2, L3, respectively. The phases U, V, W of the rectifier 1 are referred to as input-side phases U, V, W below to distinguish them from the phases L1, L2, L3 of the power supply network 5. Similarly, in order to distinguish them from the input-side phases U, V, W of the rectifier 1, the phases L1, L2, L3 of the power supply network 5 are referred to as network-side phases L1, L2, L3.

[0036] The number of input side phases U, V, W is usually three. However, regardless of the number of input side phases U, V, W, the number of input side phases U, V, W is equal to the number of network side phases L1, L2, L3.

[0037] It is possible that the network-side phase L1 is electrically ahead of the network-side phase L2 by 120°, and likewise, the network-side phase L2 is electrically ahead of the network-side phase L3 by 120°, and the network-side phase L3 is electrically ahead of the network-side phase L1 by 120°. However, it is preferred that, on the contrary, the network-side phase L1 is electrically behind the line-side phase L2 by 120°, and likewise, the network-side phase L2 is electrically behind the network-side phase L3 by 120°, and the network-side phase L3 is electrically behind the network-side phase L1 by 120°.

[0038] like Figure 2 As shown, the rectifier 1 has a plurality of half bridges 6. A corresponding input side phase U, V, W of the rectifier 1 supplies power to the backup capacitor 3 via each half bridge 6. The half bridge 6 has Figure 2 The diode 7 shown. The half bridge preferably also has an active switching element 8 connected in parallel with the diode 7. The switching element 8 (if present) is an electronic semiconductor switch, such as a MOSFET. Due to the active switching element 8, the rectifier 1 is feedback-capable. If the active switching element 8 is present, the source switching element 8 is driven by corresponding control signals C1 to C6. If the active switching element 8 is present, the diode 7 can be a separate component. Alternatively, the diode 7 can be an integrated component of the active switching element 8.

[0039] according to Figure 2 , the network side phases L1, L2, L3 are connected to a corresponding one of the input side phases U, V, W via a corresponding capacitor 9 inside the front circuit 4. Specifically, the network side phase L1 is connected to the input side phase U, the network side phase L2 is connected to the input side phase V, and the network side phase L3 is connected to the input side phase W via a corresponding capacitor 9. In order to distinguish it from the backup capacitor 3 in language, the capacitor 9 is referred to as a phase capacitor 9 below.

[0040] In addition, the network side phases L1, L2, L3 are connected to the corresponding other one of the input side phases U, V, W via the corresponding switch 10 inside the front circuit 4. Specifically, through the corresponding one of the switches 10, the network side phase L1 is connected to the input side phase V, the network side phase L2 is connected to the input side phase W, and the network side phase L3 is connected to the input side phase U. The connection between the network side phase L1 and the input side phase V, the network side phase L2 and the input side phase W, and the network side phase L3 and the input side phase U is direct. When the switch 10 is closed, the network side phases L1, L2, L3 and the input side phases U, V, W are short-circuited. In this case, the phase capacitor 9 connects the two power side phases L1, L2, L3 or the two input side phases U, V, W to each other, respectively. In particular, each phase capacitor is connected to

[0041] - the network-side phase L1 and the input-side phase V (which are short-circuited to one another in this case by one of the switches 10 ) are connected to the network-side phase L2 and the input-side phase W (which are short-circuited to one another in this case by another of the switches 10 ),

[0042] the network-side phase L1 and the input-side phase V are connected to the network-side phase L3 and the input-side phase U (which are also short-circuited to one another in this case via another of the switches 10 ), and

[0043] The network side phase L2 and the input side phase W are connected to the network side phase L3 and the input side phase U.

[0044] according to Figure 1 The frequency converter further comprises a control device 11. If necessary, the control device 11 generates control signals C1 to C6. When the backup capacitor 3 is precharged, the control device 11 opens the switch 10. As a result, the backup capacitor 3 is slowly charged via the phase capacitor 9. Keeping the switch 10 in the open state can be achieved, for example, by the relay 12 that actuates the switch 10 is not loaded with current by the control device 11. When a predetermined charging state Z1 (hereinafter referred to as the first charging state Z1) is reached, the control device 11 closes the switch 10. For example, in Figure 2 The corresponding control signal for the relay 12 is shown as C0 in FIG. In this state (ie, when the switch 10 is closed), the normal operation of the frequency converter is realized. In normal operation, the control device 11 also drives another device 2. For example, according to Figure 1 , electrical energy can be supplied to the load 13 via the inverter. The first charging state Z1 can be defined, for example, by the support voltage UZK dropped across the backup capacitor 3 reaching a predetermined percentage of its rated value, for example at least 90%.

[0045] exist Figure 1 and 2 In particular Figure 2 In the figure, not only the basic design of the present invention is shown, but also a series of advantageous designs are shown. In particular, the pre-circuit 4 can have an inductance 15 to the network side phases L1, L2, L3 in the line 14. If the inductance 15 is present, they are preferably arranged in the line 14 so that the phase currents I1, 12, 13 (network side phase currents I1, 12, 13) fed from the power supply network 5 flow through them when the switch 10 is closed and opened. Alternatively or additionally, the pre-circuit 4 can have an inductance 16 in the line 14 to the input side phases U, V, W. If the inductance 16 is present, they are preferably arranged in the line 14 so that the phase currents IU, IV, IW (input side phase currents IU, IV, IW) flowing through the half bridge 6 flow through them when the switch 10 is closed and opened. The nodes 17, 18 at which the switch 10 is connected to the line 14 are therefore arranged between the phase capacitors 9 and the inductors 15, 16.

[0046] Combine the following Figure 3 The operation of the frequency converter is described again. In this context, two further advantageous embodiments are described. On the one hand, these embodiments can be implemented independently of one another. On the other hand, these embodiments can also be implemented independently of whether inductors 15, 16 are arranged in line 14. However, the prerequisite is the presence of active switching elements 8.

[0047] Before explaining the basic design according to the invention, the first of the two advantageous designs is also explained below. This is done because it is executed first in the chronological order of taking measures.

[0048] In the context of this advantageous embodiment, when the rectifier 1 is connected to the power supply network 5 in step S1, the control device 11 controls the power supply network 5 according to the control device 11. Figure 3 Check whether the backup capacitor 3 has reached the charging state Z2, hereinafter referred to as the second charging state Z2. In particular, the control device 11 can check in step S1 whether the support voltage UZK has reached or exceeded a suitable predetermined value, for example, a value between 30% and 70% of the rated value, in particular between 40% and 60% of the rated value.

[0049] If this is not the case, the control device 4 returns to step S1 via step S2. In step S2, the control device 11 keeps both the switch 10 and the active switching element 8 open. Figure 3 It is indicated in the figure that the control signals C0 to C6 are not output (indicated by the symbol “. / .”).

[0050] When the backup capacitor 3 reaches the second charging state Z2, the control device 11 drives the active switching element 8 in step S3. The driving causes the input side phase voltage UU to be fed to the corresponding phase capacitor 9 via the active switching element 8, wherein the input side phase U is switched by the switching element, and the change of the input side phase voltage is opposite (anti-phase) to the network side phase voltage UI. Figure 4 In the left region thereof, the correlation curves of the network-side phase voltage UI and the input-side phase voltage UU as a function of time t are shown.

[0051] In a similar manner, the control device 11 controls the active switching element 8, by which the input side phase V is switched, so that the corresponding phase capacitor 9 is provided with an input side phase voltage UV, which changes in the opposite (inverse phase) to the network side phase voltage U2. Similarly, the control device 11 controls the active switching element 8, by which the input side phase W is switched, so that the corresponding phase capacitor 9 is provided with an input side phase voltage UW, which changes in the opposite (inverse phase) to the network side phase voltage U3. As a result, a relatively large potential difference is applied to the phase capacitor 9, so that a relatively large phase current I1, 12, 13, IU, IV, IW flows in the line 14. As a result, the backup capacitor 3 is quickly charged. Since the network side phase voltages U1, U2, U3 change over time, the input side phase voltages UU, UV, UW also change. The control of the active switching element 8 must meet these conditions. However, the corresponding determination of the control of the active switching element 8 is generally known to those skilled in the art and does not need to be explained in detail.

[0052] The basic embodiment of the invention and the second of the two advantageous embodiments associated therewith will now be described. This is done in conjunction with the second of the two advantageous embodiments, since it relates to measures taken when the first charging state Z1 is reached immediately before the switch 10 is closed.

[0053] During the precharging of the backup capacitor 3, it is assumed that the second charge state Z2 precedes the first charge state Z1. Therefore, the control device 11 checks in step S4 whether the backup capacitor 3 has reached the first charge state Z1. In particular, the control device 11 can check in step S4 whether the support voltage UZK reaches or exceeds a suitable predetermined value. This value must be greater than the value for the second charge state Z2.

[0054] As long as this is not the case, the control device 4 returns to step S3. Otherwise, it proceeds to step S5. In step S5, the control device 11 drives the active switching element 8 for the U phase so that the input-side phase voltage UU is applied to the input-side phase U via the relevant active switching element 8, which input-side phase voltage is consistent with the network-side phase voltage U1 in terms of amplitude and phase. Figure 4In the right area thereof, the relative curves of the phase voltage U1 and the phase voltage UU as a function of time t are shown. Figure 5 The left-hand region of is likewise shown the correlation curves of the phase voltage U1 and the phase voltage UU as a function of the time t.

[0055] Figure 4 It also shows how and in what way the transition from the opposite curve (left area) to the same-phase curve (right area) is achieved. Figure 4 As shown, this changeover preferably takes place at a zero crossing of the phase voltages U1 , UU.

[0056] In a similar manner, in step S5, the phase voltage UV of the input side phase V is also converted from a curve opposite to the curve of the phase voltage U2 to a curve of the same phase as the curve of the phase voltage U2. The same applies to converting the phase voltage UW of the input side phase W from a curve opposite to the curve of the phase voltage U3 to a curve of the same phase as the curve of the phase voltage U3.

[0057] Then, in step S6, the control device 11 drives the active switching element 8 for the phase U so that an input-side phase voltage UU is applied to the input-side phase U via the relevant active switching element 8, which is consistent with the network-side phase voltage U3 in terms of amplitude and phase. Figure 5 As shown, continuous conversion is achieved. For example, this can be achieved in the following way, that is, according to Figure 5 As shown, the input side phase voltage UU is constantly maintained at a certain voltage value starting from time t1 until the network side phase voltage U3 reaches the certain voltage value at time t2. After reaching the certain voltage value, the phase voltage UU follows the network side phase voltage U3.

[0058] In a similar manner, in step S6, the phase voltage UV of the input-side phase V is also switched from a curve in phase with the curve of the phase voltage U2 to a curve in phase with the curve of the phase voltage U1. The same applies to switching the phase voltage UW of the input-side phase W from a curve in phase with the curve of the phase voltage U3 to a curve in phase with the curve of the phase voltage U2.

[0059] The conversion of the phase voltages UU, UV, UW as described above can be performed simultaneously. In particular, it is advantageous if the network-side phase L1 lags the network-side phase L2 electrically by 120°, and if the network-side phase L2 also lags the network-side phase L3 electrically by 120°, and if the network-side phase L3 lags the network-side phase L1 electrically by 120°. Because the corresponding input-side phase voltages UU, UV, UW thus only have to maintain a constant value for one-third of the period of the network-side phase voltages U1, U2, U3. Otherwise, the input-side phase voltages UU, UV, UW have to maintain a constant value for two-thirds of the period of the network-side phase voltages U1, U2, U3.

[0060] The individual input-side phases U, V, W can be controlled independently of one another by the corresponding half-bridge 6. As an alternative, the conversion of the phase voltages UU, UV, UW as described above can therefore be performed sequentially. In this case, it is even possible to perform a direct, immediate (sudden) switchover of the input-side phase voltages UU, UV, UW from a curve in phase with one of the network-side phase voltages U1, U2, U3 to a curve in phase with another of the network-side phase voltages U1, U2, U3 without causing a voltage jump, thereby performing the switchover continuously as before. However, in this case, for example, for phase U, the switchover must be performed at a point in time when the network-side phase voltages U1 and U3 have the same value. In a similar manner, in this case, the switchover for phase V must be performed at a point in time when the network-side phase voltages U1 and U2 have the same value. In this case, the switchover for phase W must also be performed at a point in time when the network-side phase voltages U2 and U3 have the same value.

[0061] After the switching of the phase voltages UU, UV, UW as described above, the control device 11 closes the switch 10 in step S7. The closing of the switch 10 should be carried out as soon as possible after the time t2. With the closing of the switch 10, the precharging of the backup capacitor 3 and thus the frequency converter is completed as a whole. This is followed by normal operation, in which the other device 2 is operated in the usual way. In order to close the switch 10, the control device 11 can, for example, drive the relay 12 so that the switch 10 is closed.

[0062] In summary, the present invention relates to the following facts:

[0063] The frequency converter has a rectifier 1 and at least one backup capacitor 3 arranged downstream of the rectifier 1 on the input side. The rectifier 1 has a plurality of half bridges 6, through which the respective input-side phases U, V, W of the rectifier 1 supply power to the backup capacitor 3. The input-side phases U, V, W are connected to the network-side phases L1, L2, L3 of the multiphase power supply network 5 via a pre-circuit 4. The network-side phases L1, L2, L3 are connected to a respective one of the input-side phases U, V, W in the pre-circuit 4 via a respective phase capacitor 9. In addition, the network-side phases L1, L2, L3 are directly connected to the respective other input-side phases U, V, W in the pre-circuit 4 via a respective switch 10, so that when the switch 10 is closed, the network-side phases L1, L2, L3 are short-circuited to the input-side phases U, V, W, and the phase capacitors 9 respectively connect two network-side phases L1, L2, L3 or two input-side phases U, V, W to each other. The frequency converter has a control device 11 which keeps the switch 10 in an open state when the backup capacitor 3 is precharged, and closes the switch 10 when the backup capacitor 3 reaches a predetermined charging state Z1.

[0064] The invention has many advantages. In particular, not only can a low-loss precharging of the backup capacitor 3 be achieved, but also the pre-circuit 4 can be greatly simplified. In particular, compared with the prior art, the pre-charging resistors and pre-charging capacitors required therein can be saved. Therefore, compared with the prior art frequency converter, the frequency converter according to the invention can achieve a great cost saving. In particular, compared with the prior art, the cost of the pre-circuit 4 can be reduced by more than 50%. The required capacitance value of the phase capacitor 9 can be adjusted so as to provide fast precharging and a good filtering effect.

[0065] Although the present invention has been illustrated and described in more detail by means of preferred embodiments, the present invention is not limited to the disclosed examples and a person skilled in the art can derive other variants therefrom without departing from the scope of protection of the present invention.

Claims

1. A frequency converter, - wherein the frequency converter has, on the input side, a rectifier (1) and at least one backup capacitor (3) arranged downstream of the rectifier (1), - in, The rectifier (1) has a plurality of half-bridges (6), and a corresponding input-side phase of the rectifier (1) supplies power to the backup capacitor (3) via the half-bridges. - wherein the input side phase of the rectifier (1) is connected to the network side of the multi-phase power supply network (5) via a pre-circuit (4), - wherein the network side phase is connected to a corresponding input side phase via a corresponding phase capacitor (9) in the front circuit (4), It is characterized in that - the network side phase is also directly connected to the corresponding other input side phase via a corresponding switch (10) in the front circuit (4), so that when the switch (10) is closed, the network side phase is short-circuited to the input side phase, and the phase capacitor (9) connects the two network side phases and the two input side phases to each other, respectively, and The frequency converter has a control device (11) which keeps the switch (10) open when the backup capacitor (3) is pre-charged and closes the switch (10) when the backup capacitor (3) reaches a predetermined charging state (Z1).

2. The frequency converter according to claim 1, characterized in that: The upstream circuit (4) has an inductance (15, 16) in a line (14) to the network-side phase and / or the input-side phase.

3. The frequency converter according to claim 2, characterized in that: The inductor (15, 16) is arranged in the line (14) so ​​that when the switch (10) is opened and closed, the phase current fed from the power supply network (5) and / or flowing through the half bridge (6) flows through the inductor.

4. The frequency converter according to any one of claims 1 to 3, characterized in that: The half bridge (6) has active switching elements (8), so that the rectifier (1) is designed as a feedback-capable converter.

5. The frequency converter according to claim 4, characterized in that: When a predetermined first charging state is reached, the control device (11) - firstly, the active switching element (8) is driven so that an input-side phase voltage (UU, UV, UW) is applied to the input-side phase via the active switching element (8), the input-side phase voltage being consistent in amplitude and phase with the network-side phase voltage (U1, U2, U3) applied to the network-side phase, the network-side phase being connected to the input-side phase via the phase capacitor (9), - then driving the active switching element (8) so that the input side phase voltage (UU, UV, UW) applied to the input side phase through the active switching element (8) is stably converted into the network side phase voltage (U1, U2, U3) applied to the network side phase connected to the input side phase through the switch (10), and The switch (10) is closed only after the input-side phase voltage (UU, UV, UW) has been converted into the network-side phase voltage (U1, U2, U3) applied to the network-side phase, wherein the network-side phase is connected to the input-side phase via the switch (10).

6. The frequency converter according to claim 5, characterized in that: Starting from a second charging state (Z2) of the backup capacitor (3) obtained during precharging of the backup capacitor (3) before the first charging state, the control device (11) drives the active switching element (8) so that the input side phase voltage (UU, UV, UW) is applied to the input side phase via the active switching element (8), and the input side phase voltage is opposite to the network side phase voltage (U1, U2, U3) applied to the network side phase, wherein the network side phase is connected to the input side phase via the phase capacitor (9).

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