WO2005034333A1

WO2005034333A1 - Method for operating an induction machine and a current inverter therefor

Info

Publication number: WO2005034333A1
Application number: PCT/EP2004/011048
Authority: WO
Inventors: Thomas Treichl; Andreas Szajek; Michael Hackner; Andreas Albrecht; Ulrich Herb; Hans-Georg Hornung
Original assignee: Sensor-Technik Wiedemann Gmbh; Agco Gmbh & Co. Ohg; Salwit Agrarenergie Gmbh; Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2003-10-04
Filing date: 2004-10-04
Publication date: 2005-04-14
Also published as: DE10346060A1

Abstract

The inventive six-phase induction machine is embodied in the form of two three-phase induction machines which are supplied by a common six-phase current inverter. The switches (S11, S12, S21, ..., S32’) of the inverter are controlled by a control circuit (CTL) in such a way that a current is obtainable from the inverter at different times.

Description

Method for operating a induction machine and inverter therefor

The present invention relates to a method for operating a three-phase machine with at least four phases using an inverter and an inverter suitable for carrying out this method. Six-phase induction machines and inverters are known, for example from US Pat. No. 6,472,845 B2, but their distribution is low in comparison to three-phase inverters and machines. Inverters for operating a three-phase machine are conventionally constructed from a number of strings corresponding to the number of phases of the three-phase machine, which are connected in parallel between a positive and a negative terminal of a DC intermediate circuit, each string having two switches arranged in series and at a point between the two Switch has an output terminal which is provided to be connected to a phase of the induction machine. 1 schematically shows such an inverter and a three-phase machine with three phases each. The three windings of the machine, each corresponding to one phase, have terminals Ul, U2, VI, V2, Wl, W2. The terminals U2, V2, W2 are interconnected to a star point; VI is with switches Sll, S12 of a first strand, Ul with switches S21, S22 of a second strand and Wl with switches S31 ,. S32 of a third string of the inverter connected. A control circuit of the inverter, not shown in the figure, opens and closes the switches of the individual strings in accordance with a predetermined time pattern in order to energize the individual phases of the induction machine in such a way that a rotating magnetic field is generated in the machine which causes a rotation of a Rotors drives. In a three-phase machine, the drive voltages applied to the individual terminals Ul, VI, Wl or phases are ideal usually sine waves offset against each other by a third period. A conventional inverter cannot easily generate these continuously variable voltage values; they are approximated by the control circuit operating cyclically with a cycle period that is many times shorter than the rotation period of the field, and in each cycle opening or closing the individual switches of the inverter with a pulse duty factor that is proportional to the voltage value currently to be generated ,

The magnetic field generated in the rotating field machine is modeled using so-called space vectors, the direction of which indicates the phase of rotation of the rotating magnetic field and the length of which corresponds to the strength of the magnetic field. A three-strand inverter as shown in FIG. 1 can assume eight different switching states, denoted uO, ul, ..., u7, to which a room pointer can be assigned in each case.

where the values 0 and 1 indicate an open and a closed switch, respectively. The switching states of the switches S12, S22, S32 are each opposite to those of the switches S11, S21, S31. In the switching states uO u7 and all terminals are Ul, VI, Wl of the induction machine at the same potential, and it flows ^'' no current from the intermediate circuit by the inverter to the machine. In the switching state ul, a current flows through the switch S11 into the terminal Ul, and two currents, each with half the current, flow out of the machine via the terminals VI, Wl and the switches S22, S32. The magnetic fields generated by the three windings overlap in the direction designated ul in FIG. 1. When the switches Sll, S21, S32 are open, equally strong currents flow into the terminals Ul, VI of the machine and the sum of these currents via the terminal Wl the, and the magnetic fields generated overlap in the direction u2.

It is easy to check that the switching states in the table labeled ul to u6 each correspond to field orientations rotated by 60 ° in the induction machine.

2 is a diagram showing the space pointers corresponding to the switching states; the space pointers uO and u7 are of vanishing length.

In order to drive an even rotation of the machine, the inverter must generate continuously rotating space pointers of constant length. That is, the tips of the space pointers have to move on a circle as denoted by K in the diagram in FIG. 2. If it is assumed that the length of the base space pointer Ui to u _e assumed to correspond to 1 when the machine is energized with the switch position assigned to the space pointer in question during a complete working cycle of the inverter with the duration T, then it is sufficient to generate a shorter space pointer , for example u'i = ru _x , 0>r> 1, a time period rT within the cycle, and during the remaining cycle time (lr) T the inverter assumes one of the switching states u ₀ or u ₇ . In order to generate a space pointer u '(t) between two base space pointers, for example u _x and u ₂ , the switching states corresponding to the neighboring base space pointers u _x , u ₂ are set within time cycles in each case, which is proportional to the length of the projection of u '(t) onto the relevant basic space vector Ux or u ₂ . An exemplary switching pattern is shown in FIG. 3. In this figure, as a function of time, the top three curves show the switching states of switches Sll to S32, the curves labeled IS11, IS21 and IS31 show the strengths of currents flowing through switches Sll, S21 and S31, and Id the strength of a resulting intermediate circuit current. In a first phase I of the working cycle, the switches S11, S21, S31 are open and the other switches are closed, corresponding to the switching state uO. The streams JS11, IS21, IS31 are 0, and as a result the intermediate circuit current Id is 0.

In a second phase II, Sll closes, the switching state ul results, and a current with a current intensity I determined by the design of the inverter and the induction machine flows through the switch Sll and the terminal Ul and is distributed to the terminal VI and the switch S22 or the terminal Wl and the switch S32. The intermediate circuit current Id is equal to IS11.

In the following phase III, the inverter changes to the switching state u2 by opening the switch S21 and closing the switch S22. The terminals U1, VI are now connected in parallel to the positive terminal of the intermediate circuit, and the sum of the currents flowing into the machine via them flows out via the terminal W1. The current flowing through Sll in phase II is now distributed to switches Sll, S21, the intermediate circuit current Id remains the same.

In the subsequent phase IV, all switches S11, S21, S31 connected to the positive terminal of the intermediate circuit are closed, and S12, S22, S32 are open, so that the intermediate circuit is effectively open. Currents through the switches Sll, S21, S31 can only be due to the induction of the induction machine and are not considered here; the intermediate circuit current Id disappears.

The process is repeated in reverse order in phases V to VIII.

As you can see, the switching pattern considered here alternates two periods in which an intermediate circuit current flows with periods without an intermediate circuit current flow in the course of each cycle. This fluctuating current requirement causes the intermediate circuit voltage to fluctuate. There are therefore phases in the operating cycle of the inverter in which less intermediate circuit voltage is available than in others, while the durations of the individual phases are calculated assuming a constant intermediate circuit voltage. There are therefore deviations between an expected and an actually generated space vector in the induction machine, whereby the synchronism of the motor is impaired. In order to limit these voltage fluctuations, it is common to connect a smoothing capacitor in parallel with the strings of the inverter. The longer the cycle period of the inverter, the greater the capacitance required for this capacitor. As a result, in order to make the capacitor small, the general aim is to make the cycle period of the inverter as short as possible. With each switchover, however, losses occur at the switches of the inverter. As a result, they have a power loss that increases with the cycle frequency of the inverter and imposes an upper limit on the cycle frequency for current IGBT or MOSFET power switches of a few kHz. Smooth capacitors with large capacitance and high dielectric strength are therefore still required for the operation of powerful machines, the costs of which are considerable.

The object of the invention is therefore to provide a method for operating an inverter and an inverter suitable for carrying out the method, which make it possible to keep the fluctuations in the intermediate circuit current or the intermediate circuit voltage low and thereby make it possible to use smaller and less expensive smoothing capacitors to use.

The object is achieved on the one hand by a method having the features of claim 1. The steps a) of defining a space vector to be generated in the induction machine and b) of establishing a switching sequence for a first group of i strings (i> 2) of the inverter Each of which complements a first i-phase machine can be carried out using any method known for i-phase machines. The specification of a switching sequence for lines that supply the other j phases of the n-phase machine (with j = 2, 3, ..., ni), which complement each other to form a second j-phase machine, can also be done in in a known manner. The same or different methods can be used to determine the switching sequences for the i-phase and the j -phase machine. A suitable selection of a phase offset between the two switching sequences ensures that the two groups of strands load the intermediate circuit at different times, so that the DC link part of the current drawn from the intermediate circuit is larger and the oscillating component can be made smaller than in a single machine with i or j phases.

In the simplest case i + j = n; i.e. the i-phase and the j -phase machine complement each other to form a complete n-phase induction machine. However, it is also possible to further divide at least one, say the j-phase machine, of i-phase and j-phase machines, so that a total of three or even more groups or machines corresponding to the groups are obtained , and to define switching sequences for each of these. Here, too, phase offsets between the groups can be determined so that the DC component of the intermediate current is large and the AC component is low.

If the performance of the n-phase induction machine is low and the period of time within each inverter cycle in which one of the groups of strings draw current is shorter than one mth of the cycle period (if m is the number of

Groups), it is generally possible to choose the phase shift between the groups so that the times in which they draw current do not overlap, that is to say that during the course of a working cycle of the inverter, either only one of the groups at any time or no group draws electricity.

Conversely, if the required power of the induction machine is large, so that each of the m groups draws current for more than one mth of the cycle period, the phase shift can generally be set such that whenever one of the groups has no current draws, at least one of the other groups draws current, so that the intermediate circuit current never disappears completely. In both cases, the oscillating component of the intermediate circuit current is considerably reduced in comparison to a conventional i- or j-phase machine designed for the same output, especially if i and j have small values such as 2 or 3.

If the switching sequences of each group have only one time interval in each cycle in which the group draws current from the intermediate circuit, then an optimal phase shift between the groups is given by a phase difference between the two switching sequences of 360 °, divided by the number of groups, guaranteed. If the shift sequences of each group have two time intervals per cycle, for which the group draws a current from the intermediate circuit, the phase difference between the switching sequences of the groups can expediently be set to 180 ° divided by the number of groups.

On the other hand, the object is achieved by an inverter with n strings fed from a DC intermediate circuit (n> 4), a smoothing capacitor parallel to the strings and a control circuit for actuating switches of the n strings, in which the control circuit is set up, a method as above to be defined.

Further features and advantages of the invention result from the following description of exemplary embodiments with reference to the attached figures. Show it:

1, already discussed, a block diagram of a three-phase induction machine and an inverter with three strands;

2, already treated, space pointers to be generated in a rotating field machine;

3, already dealt with, a switching sequence of the three-phase inverter from FIG. 1;

4A shows a three-phase machine and an inverter, each with six phases;

Figs. 4 B, C alternative six-phase induction machines;

5A is a timing diagram of a non-out-of-phase switching sequence for a six-phase inverter;

5B, C are time diagrams of phase-shifted switching sequences according to the invention; FIG. 6A shows a phase-shifted switching sequence for the six-phase inverter at low power;

6B shows a phase-shifted switching sequence for the six-phase inverter at high power; 7 shows a cutting sequence which is determined according to different methods for the two three-phase machines which complement one another for the six-phase machine;

8 shows an induction machine and an inverter, each with four phases;

FIG. 9 space pointers to be generated in the four-phase machine of FIG. 8, analogous to FIG. 2;

FIG. 10 shows a time diagram of a non-phase-shifted switching sequence for the four-phase inverter from FIG. 8; 11 shows a time diagram of a phase-shifted switching sequence according to the invention; and

12 is an illustration of the mode of operation of the invention when a three-phase machine is divided into three sub-machines which are driven out of phase.

4A shows a block diagram of a six-phase inverter and a six-phase induction machine connected to it. The six-phase induction machine can be understood as a combination of two three-phase machines acting on a common shaft, whereby the connecting terminals of the first three-phase machine with Ul, U2, VI, V2, Wl, W2 and those of the second machine with the same symbols, each with an additional apostrophe. The terminals U2, V2, W2, U2 ', V2', W2 'of both machines are connected to a common star point; however, this is not important for the functioning of the machine. The terminals U2, V2, W2 of the first three-phase machine and those of the second three-phase machine could also each be interconnected to a star point. A combination of two three-phase machines in a delta connection as shown in FIG. 4B can also be considered. A structure as shown in FIG. 4C is also possible, in which six windings are connected in series to form a hexagon and each winding has both a phase of the first three-phase machine with terminals U1, VI, Wl and the second machine with the Terminals Ul ', VI', Wl 'is to be attributed. The inverter is fed via an intermediate circuit with a positive rail (+) and a negative rail (-) by a rectifier (not shown). A smoothing capacitor C and six strings, each with two switches, are connected in parallel between the two rails +, -. As in Fig. 1, the switches connected to the terminals Ul, VI, Wl are designated Sll, S12, S21, S22, S31, S32, and the corresponding switches assigned to the second three-phase machine have the same symbols, each with an additional one Apostrophe. Since the windings of the second sub-machine are oriented opposite to those of the first, in order to generate a given space vector, they must each be supplied with a current whose sign is opposite to that which flows through the corresponding winding of the first sub-machine. That is, the switching states for generating a given basic space vector are in each case opposite for the switches Sll ', ..., S32' assigned to the second sub-machine to those of the switches Sll, ..., S32 assigned to the first sub-machine, as in the table below specified.

The other way round: the switching sequence with which the switches of the second sub-machine have to be actuated in order to generate a given space pointer like the pointer u '(t) in FIG. 2 is the same as that used for the first sub-machine by one to generate 180 ° phase-shifted space pointer -u '(t). 5A specifically shows an example of such a switching sequence. For the switches S11 to S32 of the first sub-machine, this switching sequence is identical to that of FIG. 3. In phases I to VIII of the switching sequence, they generate the basic space pointers uO, ul, u2, u7, u7, u2, ul, uO one after the other , The first sub-machine loads the intermediate circuit with the current Id. The states of the switches Sll 'to S32', which are assigned to the second sub-machine, correspond to those of the switches Sll to S32. opposite; in phases I to VIII, they generate the basic spatial pointer u7, u2, ul, uO, uO, u7, u2, ul. The second sub-machine draws the intermediate circuit current Id 'in the phases II, III, VI, VII. In general, the transition time between the phases II and III or V and VI is not for both sub-machines - in contrast to the figure simplifying in this regard the same. The currents Id, Id 'overlap in phase with Id, dead. A comparison of the power consumption in comparison to the three-phase machine of FIG. 1 is not yet associated with this.

However, if, as shown in FIG. 5B, a phase offset between the switching sequences of the two sub-machines is introduced according to the invention, this also leads to a phase shift in the intermediate circuit currents Id, Id 'drawn by the sub-machines. As shown in FIG. 5B, in which a phase shift Δφ of approximately 45 ° is assumed, any phase shift between the switching sequences of the two sub-machines leads to an equalization of the intermediate current and thus to a reduction in the voltage fluctuations in the intermediate circuit.

If, as in the switching sequence considered in FIGS. 3 and 5A, one of the neutral switching states uO, u7 forms ends and middle of a working cycle of the inverter and both neutral phases are each of the same length, the optimum value of the phase shift is 90 °. In the ideal case shown in FIG. 5A, in which the phases I, IV, V, VIII of the neutral states make up half of the cycle duration, such a phase offset leads to the fact that, as shown in FIG. 5C, the one pulled by the first sub-machine The intermediate circuit current Id does not disappear exactly when the intermediate circuit current Id 'disappears, and vice versa. The total intermediate circuit current Id, tot is then constant. In most practical cases, the duration of the neutral switching states uO, u7 will be less than or greater than half the cycle time. Here, too, however, there is an equalization of the intermediate circuit current, as shown in Figs. 6A, 6B. In FIG. 6A, which relates to the case of operating the six-phase induction machine at low power, the duration of the neutral states is greater than half a cycle duration, and phases alternate, in each of which one of the sub-machines has current pulls and phases in which no current is drawn draw each other. In the case of operation at high power shown in FIG. 6B, where the duration of the neutral switching states is less than half a cycle duration, phases in which one of the sub-machines draw current alternate with phases in which both draw current.

In the examples considered so far, the intermediate circuit currents Id or Id 'caused by the sub-machine have a period that is half as long as the duty cycle of the inverter. Therefore, a phase shift of the switching sequences of the sub-machines against each other of 90 ° ensures a good equalization of the total intermediate circuit current Id, tot. Of course, the period of the intermediate circuit currents can also match the cycle time of the inverter, for. B. if a switching sequence is used for each sub-machine in which the neutral phases IV and V with the space vector u7 in the middle of the switching sequence are omitted and the neutral phases I and VIII with the space vector uO compared to the switching sequence of FIG are extended accordingly; then a good equalization results from a phase shift of 180 °. 7 shows switching cycles according to a second embodiment of the method. In a departure from the approach followed in FIGS. 5B, 5C and 6A, 6B, the switching sequences for the switches Sll to S32 on the one hand and Sll 'to S32' on the other hand are to be determined according to the same rule and by a phase shift between the switching cycles

To make the load on the DC link as even as possible are shown in Figs. 7A, 7B different regulations are used for the determination of the switching sequences. This makes use of the fact that it is irrelevant for the behavior of the motor whether the state uO or u7 is used as the neutral switching state or what proportion these switching states have in the duration of the cycle. While the duration of both switching states was the same for the switching sequences considered so far, in the case of FIG. 7, phases IV, V, which correspond to switching state u7, are extended for switches Sll to S32 that supply the first sub-machine and for switches Sll ' shortened to S32 'of the second sub-machine, and vice versa, phases I, VIII, which correspond to the switching state uO, are shortened for switches Sll to S32 and extended for Sll' to S32 '. This shifts the phases of the loading tion of the intermediate circuit through the first sub-machine to the ends of the cycle and for the second sub-machine to the middle of the cycle, as can be seen from the intermediate circuit currents Id, Id 'in the sub-machines in FIG. 7. There is an equalization of the whole

DC link current Id ', the clearer the more the switching state u7 is extended for switches Sll to S32 and the switching state uO is shortened and for switching switches Sll' to S32 'the switching state uO is extended and u7 is shortened.

8 shows a three-phase machine and an inverter, each with four phases. The windings of the induction machine corresponding to the individual phases are each arranged at 90 ° to each other and have input terminals Ul, VI, Ul ', VI', each of which is connected to a switch Sll, S12 or S21, S22 or Sll ', S12' or S21 ', S22' are connected to the string of the inverter, and output terminals U2, V2, U2 ', V2' connected to a star point.

By energizing one of the four windings of the machine, the space pointers denoted by ul to u4 in the diagram of FIG. 9 can be generated. Analogous to the six-phase induction machine described above, two

Switching states of the inverter, in which the switches Sll, S21, Sll ', S21' are all open or all closed, and the corresponding, disappearing space pointers are denoted by uO. In order to drive a uniform rotation of the four-phase induction machine, a space vector u '(t) which rotates evenly on a circle K must be generated. For this purpose, the respective base space pointer is generated during a time period corresponding to the projection of u '(t) onto the two adjacent base space pointers, here ul and u2, or when the time periods in which the base space pointers are generated , overlap, a space pointer is generated parallel to the bisector of the angle spanned by the base space pointer ul, u2.

A switching sequence, not according to the invention, with which the control circuit CTL of the inverter from FIG. 8 can control its switch in order to generate the desired space vector u '(t) during a working cycle with phases I to X of the inverter, is shown in FIG. 10. In each work cycle, the switches of each string switched twice, the two switching times of each string being equidistant from a center point of the working cycle between phases V and VI. In addition, in the case of two strands which supply mutually opposite connections, for example Ul, Ul ', the machine

Switching times of the switches Sll, S12 of one strand are just as far from the beginning or end of the switching cycle as the switching times of the switches Sll ', S12' of the other strand from the middle of the switching cycle. There are therefore two time intervals per switching cycle in which the

The intermediate circuit current IdU of the inverter, which is due to the current flow between the connections Ul and Ul ', is different from zero, and these two time periods are each centered in the first and second half of the switching cycle. The same applies to an intermediate circuit current IdV based on the current flow between the connections VI, VI ', so that a highly uneven overall intermediate circuit current Id, dead results with the curve shown in the bottom line of FIG. 10.

If one considers the four-phase induction machine as a superimposition of two two-phase machines, to which the phases Ul, Ul 'and VI, VI' belong, respectively, it is easy to see that the total intermediate circuit current can be equalized by introducing a phase offset of 90 ° between the switching sequences of the two two-phase machines. The corresponding switching sequences and the resulting intermediate circuit currents are shown in FIG. 11 over time. Periods in which the machine formed by phases VI, VI 'loads the intermediate circuit coincide with time intervals in which the machine formed by phases Ul, Ul' does not draw any intermediate circuit current. The DC link is loaded evenly, with a low AC component, and a small smoothing capacitor C is sufficient to effectively suppress disturbing voltage fluctuations.

Of course, the six-phase induction machine already described above can also be understood as a combination of three two-phase machines, each of which corresponds to the terminal pairs Ul, Ul ', VI, VI' and Wl, Wl '. In the switching sequence not shown according to the invention shown in FIG. 5A, it is easy to understand that there is a share in each of the two-phase machine formed by U1, U1 'and the machine formed by U1, U1' IdU or IdW on the intermediate circuit current with the same profile as Id in FIG. 5A would be omitted, while the two two-phase machine VI, VI 'only draws an intermediate circuit current if the space vector u' (t) to be generated is a non-vanishing one Projection onto the base space vector u2, u5 that can be produced by this machine, ie if, in a departure from the illustration in FIG. 5A, the switching times of the switches S21, S22 on the one hand and S21 'and S22' on the other are different.

FIG. 12 schematically shows the time course of the portions of the intermediate circuit current attributed to the individual two-phase machines on the assumption that, as in the switching sequence of FIG. 5A, each two-phase machine in each working cycle from tO to tl ¹ the control circuit draws current twice, and that the times in which the individual machines draw current are each phase-shifted by 60 °. The time intervals with a non-vanishing portion IdU, IdV, IdW of the intermediate circuit current are each shown as a vertical line with a dashed horizontal double arrow, which symbolizes the variable width of the current impulses u '(t) depending on the required machine power and the orientation of the space vector to be generated. When the machine is low, represented by the curve Id, tot (l), the current pulses attributed to the individual two-phase machines do not overlap, and the intermediate circuit current fluctuates between 0 and the maximum current consumed by a single two-phase machine Im. If the required power increases so that adjacent intervals of non-vanishing DC link current of the two-phase machines overlap, the current oscillates between Im and 21m, as shown for curve Id 'tot ^' (2). If the required power increases again, so that neighboring intervals begin to overlap the next, the total intermediate circuit current Id, tot (3) fluctuates between 21m and 31m. In all cases, the alternating component of the total intermediate circuit current is effectively minimized. As you can easily see, this concept can easily be transferred to induction machines with even higher phase numbers; the DC link current can be evened out more effectively the larger the number of phases of the considered induction machine or the larger the number of sub-machines can be broken down into this induction machine.

Claims

claims

1. A method for operating an n-phase induction machine (n> 4), where n is a natural number> 4, with the aid of an inverter, which feeds n strings (Sll, S12; S21, S22; S31, S32; Sll ', S12'; S21 ', S22'; S31 ', S32') for supplying one phase each (Ul, VI, Wl, Ul ', VI', Wl ') of the n-phase induction machine, in which in successive work cycles, the following steps are carried out: a) determining a space vector (u '(t)) to be generated in the n-phase induction machine; b) defining a first switching sequence for the switches (Sll, S12, S21, S22, S31, S32) of a first group of i strands (i = 2, 3, ..., n-2) (Sll, S12; S21, S22; S31, S32), wherein 2 <i≤n-2, each supplying phases (U, V, W) of the n-phase induction machine, which complement each other to form a first i-phase machine, in such a way that these i phases provide a first contribution to the room pointer; c) Definition of a second switching sequence for the switches (Sll ', S12', S21 ', S22', S31 ', S32') of a second group which contains the j strands not belonging to the first group (j = 2 + i, 3+ i, ..., n- 2-i) (Sll ', S12'; S21 ', S22'; S31 ', S32'), 2 + i≤j <n-2-i, comprises, in such a way that the phases of the n-phase induction machine fed by these j strands, which complement each other to form a j-phase machine, make the remaining contribution to the space vector and that the times in which the strands of the second group draw a current from the intermediate circuit , with the times in which the strands of the first group draw a current from the intermediate circuit do not or only incompletely match, d) controlling the groups of strands on the basis of the defined switching sequences.

2. The method of claim 1, wherein i + j = n.

3. The method according to claim 1, in which the j phases fed by the second group of the n-phase induction machine are treated as a j -phase induction machine and steps b) to d) are also applied to the j - phase induction machine.

4. The method according to any one of the preceding claims, in which the contributions of all groups to the space vector (u '(t)) are defined in parallel.

5. The method according to any one of the preceding claims, wherein the number of strands of all groups is the same.

6. The method according to any one of the preceding claims, in which the switching times of the switching sequence relating to each working cycle are determined for each group according to a uniform rule and the working cycles of the groups have a phase shift.

7. The method as claimed in claim 6, in which the phase offset is selected such that, at any time when one of the groups draws current from the intermediate circuit, none of the other groups draws current.

8. The method according to claim 6, in which the phase offset is selected such that at any time when one of the groups does not draw any current from the intermediate circuit, at least one of the other groups draws a current from the intermediate circuit.

9. The method according to any one of the preceding claims, wherein the switching sequences of each group per cycle have a time interval in which the group draws a current from the intermediate circuit, and that the phase difference between the switching sequences is 360 ° divided by the number of groups ,

10. The method according to any one of claims 1 to 7, wherein the switching sequences of each group have two time intervals per work cycle in which the group NEN draws current from the DC link, and that the phase difference between the switching sequences is 180 ° divided by the number of groups.

11. Inverters with n (n> 4) strings fed from a DC intermediate circuit (Sll, S12; S21, S22; S31, S32; Sll ', S12'; S21 ', S22'; S31 ', S32'), one to the Strands of parallel smoothing capacitor (C) and a control circuit (CTL) for driving switches (Sll, S12, ..., S32 ') of the n strands, characterized in that the control circuit (CTL) is set up, a method according to a of the preceding claims.