DE3401164A1 - Multi-phase circuit for the armature winding of electrical machines and frequency converters - Google Patents

Abstract

Multi-phase windings allow relatively high force intensities and a more favourable operating behaviour (higher efficiency) to be achieved for electrical machines, particularly if they are supplied by convertor. The proposed method overcomes disadvantages of known solutions, has little reaction and manages with a small number of semiconductors. If odd numbers of phases and an alternately changed winding direction of the phases are used, reaction-free switching results for current waveforms having a gap or having linear commutation within a phase division. If other current waveforms are to be implemented, a balancing circuit is additionally used. Sub-division of the voltage sources can hence be avoided. The circuit can be used for highly demanding control tasks and for high power requirements.

Description

"Multi-phase circuit for the armature winding electrical

Machines and frequency converters " State of the art and Objective Three-phase motors for drive purposes have the conditions higher Material utilization and favorable operating properties to match. A multitude of circuit proposals for the winding and the execution of the excitation system on the machine side as well as corresponding circuits of the inverter are used this goal. The normal form of the circuit applies to synchronous and asynchronous motors the three-phase winding, with the inverter usually in a bridge circuit is applied. Due to the transition to 6-phase double star windings, in particular for greater powers, somewhat more favorable interactions between the magnetic Field and the winding currents of the polyphase winding achieved the relay of the individual phase currents takes place in accordance with the desired rotational frequency and also determines the speed of rotation of the rotor via the thrust generated. It is known in principle that a six-phase winding compared to the three-phase winding Winding design leads to greater specific forces because the force-generating Currents can be optimally assigned to the magnetic field over a larger area can.

Proposals are also known, the coils housed in the grooves the winding is to be regarded as a switching unit.

With a corresponding number of slots, this results in a multi-phase winding with an analog circuit of the inverter. The transition to a Multi-phase arrangement, the strings of which can be switched independently, has advantages for the mode of operation and material utilization of the machine.

It becomes possible to achieve a higher force or power density. For a given maximum value of the sum of all currents, a higher power can be converted will.

By using semiconductor elements that can be switched on and off or -circuits, a current control individually assigned to the coils can also be introduced will. It is thus possible to distribute the current via a pole division Operational case the machine as well as the special features of the machine type. So is For example, it is known that a rectangular current distribution similar to that of the DC machine also for inverter-fed synchronous machines at more favorable conversion conditions leads as an approximately sinusoidal current distribution.

The functional principle of the multi-phase (three-phase) machine can be counted on be that it generates a largely fluctuation-free torque. This will too in the present case aimed for and achieved. It is known that methods are proposed where only one coil of the armature winding per pole pair of the excitation field is engaged.

In order to achieve a fluctuation-free operation, this Single-phase variant, a pole pitch misalignment of the armature coils was used. However, this sets an ignition of the semiconductors that is shifted in time from pole to pole ahead and implies that some pole pitches for the force generation each whole fail. At the same time, the concentration of the armature coils is one pole pitch in only one groove to be regarded as disadvantageous. Recommended at least for larger pole pitches For thermal reasons, a structure of the winding and thus an improvement the heat dissipation.

In the case of the switching and control method at hand, a asynchronous ignition of the individual semiconductor switches under different poles avoided. The tax procedure is very simple. In addition, the force-forming interaction between the magnetic field and the armature currents in sync. The circuit arrangement of the semiconductors and the machine winding is largely suitable for all converter-fed machines, regardless of the type of their design and their size. The method can be used on synchronous machines with electrical or permanent magnetic excitation. It is equally applicable to Reluctance machines or asynchronous machines. In the latter cases come the advantages of the low-loss circuit also take full effect.

Multi-phase winding with inverter and coil-by-coil connection The coils 1 to 8 shown in Figure 1a can be used as switches supplying thinking semiconductor valves with power in two directions. Will e.g.

the valve A is ignited, the voltage source UG can supply the current in run in the direction shown through the coil from plus to minus. Becomes the semiconductor element B ignited and A blocked, the voltage source UG 'causes a coil current in the opposite direction. UG and VG 'can, for example, be achieved by using battery parts of the same size be formed, so that currents of the same size on average in symmetrical operation result in an even load on the battery halves.

In the circuit shown, the 8 coils provide 8 winding phases an armature winding in the area of 2 poles. Each of the individual strands can be supplied with power from the other strings regardless of time. The winding can also be carried out favorably as a two-layer winding.

First, assume that all 8 strands are either over A or through B can be charged with the current in one direction or the other, whereby the magnitude of the current is always the same. For this it is assumed that the current after initiating the switching process quickly reaches its size, i.e. that both the switching process as well as the settling of the current take place quickly.

As the circuit according to Figure la shows, there is only one semiconductor switch at a time switched per coil, so that the losses compared to the frequently used Bridge circuits for the passage case are only half the size. In picture ib are the associated coils schematically with their two associated coil sides (e.g. 1 and 1 'shown as the coil sides of the coil 1) and their end connection.

The currents of the two coil sides are therefore to be assumed to be opposite. Effect with the direction of current drawn in the coil sides 1 to 8 and 1 'to 8' the winding currents with the field of poles N and S of an excitation system have a maximum possible thrust F if all coil currents 1 to 5 and 1 'to 5' maximum size own. This can evidently under the assumed conditions of a rapid Current rise up to the maximum value can be realized. Streams 6 to 8 and 6 'to 8 'also contribute indirectly to the generation of force, in so far as they do that Help determine the field under the poles, i.e. contribute to the excitation of the poles N and S or determine this exclusively. For some machine types it is useful to use the Size of these currents to influence the operating characteristics independent of the To adjust the currents of the pole area.

It is assumed that under the action of the thrust the excitation system (usually the rotor) moves to the left (motor operation) so is after a small distance corresponding to a coil division for a given power distribution a somewhat smaller total thrust can be expected in the coil 8, 8 '. In this position of the rotor, the previous current direction of the coil 8 is unfavorable. About the drop in thrust after a left movement around the coil division, the current is reduced the spu] e 8 reversed.

This is done by switching the semiconductor switch B on and off von A. This makes two things clear: To avoid fluctuations in the thrust force and to achieve the largest possible mean total thrust is a larger one Number of winding strands desired. A reduction in the number of phases means at the control concept described a lower and greater thrust on average Thrust fluctuations. As even as possible Winding distribution and avoid fluctuations in the magnetic conductance of the stator look cheap.

From Figure 3a it can be deduced that when the current commutates of the coil 8,8 'for a rapid increase in current, as was assumed above, favorable conditions exist. The coil 8 is located in the low-field space and includes during the process of current change the entire main flow that is in the considered Period does not change significantly. One induced from the main river In this way, voltage does not, or only slightly, counteracts the change in current. By induced the stray field of the conductors of the affected (commutating) coil Voltage is mainly decisive for commutation.

There are thus similarly favorable commutation conditions as in given to a DC machine.

Low load-dependent commutation influences on the current rise it makes sense to shift the commutation time depending on the load. This allows torque fluctuations and the commutation time to be minimized.

On the other hand, it can also be seen that in comparison to a spatial sinusoidal current and field distribution, similar to that of normal three-phase windings (as a result of the locally unequal interaction between field and current) occur, with the circuit proposed here, a greater thrust can be generated.

The circuit arrangement described in Figure 1 also means for the Semiconductor switch the highest possible utilization; there is one switch per line in engagement.

As shown in Figure 2, the semiconductor dimensioning can be component-related take place in such a way that several winding phases or coil units from one switch, e.g. a and b can be switched.

In-phase currents flow through these coils. The coils a and b can be spatially spaced by a pair of poles coils are electrically connected in series. Also a parallel connection of the two coils a and b is possible. The decision of whether to connect in series or in parallel to choose depends on the design conditions of the winding and the data of the Semiconductors. There can also be more than two in-phase coils in parallel or in Can be connected in series.

Figure 2 also indicates that, as is known, the control of the Semiconductors via their ignition currents. It is assumed here as an example that Semiconductor elements are present, which with the help of the control current or the control current pulses and a control logic St can be switched on and off.

In this case, e.g. power transistors or GTOs are to be used.

The control unit St generally closes a maximum value limitation of the current (ImaX), which prevents overloading or destruction of the semiconductor components and with a given voltage and available winding data, a rapid increase in current enabled during the switch-on process.

The multi-phase converter circuit described with a center conductor connection allows compared to the well-known multi-phase midpoint circuit and the only each e.g.

currents directed towards the center result in a doubling of the force with the same winding volume. The two-way current conduction corresponds to a double Valve effort without - as mentioned - the advantages of low passage losses (a Valve in series) are lost. The number of switch units is only half as large as when using 4-quadrant controllers. Since the current load during a Working period (ignition in both directions of all phases) for the two voltage sources fluctuates, measures to stabilize the voltages UG and ~ UG 'are desirable. The current imbalances can be reduced if some of the coils are opposite Fig. La reversed connected to the supply lines or with a different winding direction are executed. The current fluctuations in the voltage sources U, G and U UG ' can also be completely avoided by using two (or a multiple of two) groups of coils (according to Fig. 1a) with reversed current direction or changed Winding sense, but otherwise under identical conditions with separate switching arrangements work in parallel on the voltage sources. In terms of partial failure is it is advisable to combine both measures for balancing. Let through this the different voltage drops in the sub-circuits are reduced.

It should be noted that in all practical applications expected inductance of the windings to be fed for the use of reactive current diodes to force. For example, in volume 4 they are anti-parallel to the switchable semiconductor valves marked with DA, DB.

Individually switchable winding phases of a multi-phase winding arrangement were already in Proc. IEE, vol. 117, no. 10, Oct. 1970, (1) and in DT OS 16 38 238, (2) suggested. In (1) the circuit is restricted to naturally commutated ones Thyristors before, so that for this reason an adjustability of the individual currents not possible by pulsing the voltage. Such adjustability is also found in (2) not aimed at. It is not possible with different induced Voltage to realize currents of equal magnitude in the coils.

Both proposals have in common that the specified circuits the practical requirements for non-reactive operation with the greatest possible Simplicity does not match. In the following, therefore, a further development of the Described multi-phase circuit for a low-cost multi-phase arrangement. she can be expected with a low number of components, increased reliability, carries the Question of the retroactive effect and limits the advantages of an individually switchable Arrangement only a little.

A division of the voltage source into two partial voltages UG and UG provides a way of reducing the number of semiconductor G switches compared to the four-quadrant configuration, the e.g.

also in the proposal DT OS 16 38 238 was described by half to reduce. However, these circuits are not free from retroactive effects. There the multi-phase circuit anyway increased to a compared to the three-phase arrangement number of switching elements, comes the limitation of the effort in relation to the component give special importance. It is linked to the possibility of cost reduction and increasing reliability. In terms of the larger number of phases conditional larger number of power semiconductors must be mentioned, however, that this need not be a disadvantage for machines with higher capacities. The whole The number of semiconductor switches used is not higher than, for example, in a three-phase configuration.

The total switching capacity of an inverter is usually represented by a number of similar switching modules that work in parallel. The transition to a multi-phase arrangement results in the same arrangement Switching elements a greater module power; however, the inverter has one fewer units working in parallel. But also taking into account this fact appears for reasons of economy and reliability necessary to minimize the number of switching elements. The key to circuit simplification lies - as already indicated - in the application of the central conductor concept (Fig 4).

Investigations show that a subdivision of the voltage, e.g. at Powered by batteries, is not always possible.

But even if the division is possible in principle, owns they have the disadvantage that as a result of fluctuating load on the pitch circles A and B below Consideration of the internal resistance also fluctuations in the available partial voltage gen UG and UG arise. This also leads to a reduction in the average Circumferential forces of the machine and must be avoided. The measure already described the half division into coils with different winding directions is sufficient Avoidance of these phenomena does not prevent the effects of the load currents on the To avoid stress distribution. To a more universal one use the circuit with the center conductor M leads to an addition as described in Figure 5. The middle conductor is connected here at the midpoint of the two auxiliary storage tanks, given by the capacitors CA and CB.

The latter have the same size and share (without electricity in the middle conductor) the voltage U into two equal halves.

g If U = 2zug ,, then the circuit according to Fig. 5 has a similar UGl Expect operating results like the circuit shown in Figure 4. A possibly in the center conductor M current caused by unequal loading of the strands A and B can, however also cause an unequal distribution of the voltages UA and UB with this circuit. The direction and magnitude of the current flowing over the central conductor are responsible for this for the resulting voltage distribution. Especially at low frequencies larger voltage differences occur. The frequency of the center conductor current can be a multiple of the frequency of the load current or the basic frequency of the moving Be power distribution. It can be seen that an enlargement of the capacitors contributes to voltage stabilization. The way of increasing capacities via However, going beyond certain limits is not feasible for economic reasons. In particular for operation at low frequency, e.g.

During the start-up of the machine, measures are required to reduce the central conductor load to be kept small and / or by adding additional circuitry - such as the Balancing device SV in Figure 5 - to limit the voltage shift.

The circuit addition in Figure 5 contributes to a controlled follow-up charge of the capacitors CA and CB and balances the two partial voltages. The latter can be kept constant within narrow adjustable limits.

According to Figure 6, SV can act in such a way that with the help of the additional Switch SA and 5B charging currents via the capacitor branches (optionally in both directions) can flow. This can reduce the voltage caused by the center conductor current Voltage imbalance be reversed. When the charging current is switched on, it becomes magnetic at the same time Energy stored in the choke coil Lb. When switching off the power, you can use With the help of the free-wheeling diodes of the other switching branch, the stored energy to further balance the voltages in the capacitors.

Figure 7 shows an example of a controllable division of the partial voltages UA and UB with a slight fluctuation around the mean value Zug / 2 It is related with the selected cycle time TLV between two switch-on states.

TLV is independent of the operating frequency of the machine and the basic frequency of the inverter. The currents used for balancing are in the size range a phase current. The effort for the circuit for balancing is thus limited. In addition to the semiconductor switches 5A and SB, which can be switched on and off, are similar to the switching elements of the individual winding phases, reactive current diodes required in an anti-parallel arrangement. The use of the switches is dependent of the measured voltage distribution. A reduction in voltage imbalances requires smaller playing time TLV It is obvious that the realization of a Active balancing is not necessary on identically structured circuits made of capacitors and semiconductor switches must be limited.

Other circuit configurations are also conceivable and can be used will.

As described above, the magnitude of the potential shift depends depends significantly on the size and frequency of the central conductor current. If it succeeds, the center conductor current through a particularly favorable design and mode of operation Keeping the multi-phase winding very small in total is something special advantageous to look at. The effort for the balancing circuit SV and the size the capacitors are reduced accordingly.

If it is possible to make the center conductor current to zero, then one can Balancing circuit can be dispensed with.

Obviously, this is the most favorable case. In general is of it it can be assumed that measures have been taken to avoid a large current in the center conductor and that a low-cost dimensioning of the balancing device is thus achieved will.

As mentioned, the size of the center conductor current depends on the occurring Differences in branch streams 1A and 1B. The division of the coils into possible two halves of the same size with different winding directions is a measure which has a symmetrizing effect. However, depending on the number of phases, larger ones result Differences in the current amplitude of the central conductor current. Also the frequency of the Current proves to be dependent on the selected number of phases. She plays when using of capacitors insofar as the capacitor voltage (and also the Deviation from the symmetrical distribution) depends on the electrical-n charge is. However, for a given current amplitude, low frequencies mean a higher one Charge and thus a greater voltage shift.

As studies of the effects of the phase numbers show, they are odd Phase numbers with regard to the current amplitudes in the center conductor and the frequency (and thus for the capacitor charge) cheaper than even phase numbers. It This reduces the current amplitude to half the value and at the same time the frequency increases. For the electrical charge there is one to one Fractional reduced value.

Figure 8 shows a five-phase circuit with different winding directions of the individual coils. The winding sense of phase 1 is right-handed from phase 2 assumed left hand etc. Table 1 contains information on switching states the on and off switchable semiconductor switches A- and B for different times. Furthermore, analogous to Figures 1a and 1b, the respective coil is divided by two coil sides e.g. 1 and 1 'recorded.

The power distribution is switched on in steps that Multiples of / 5 with T of a half cycle of the fundamental frequency are given.

As a special feature, it is assumed in Table 1 that in each case one phase which has no electricity for a total of five phases. The associated current curve is shown in Fig. 9 given by phase 1 as a function of time. The currentless break is with the Change of current directions connected. It is 1/5 of the live duration a phase.

Table 1 shows the current distribution effective for the machine (the Coil sides) of all current-carrying phases by the arrows shown. It will It is clear that after each switching of the current of a phase, a further migration the current distribution is given in a kind of square wave. The electricity gap moves corresponding.

It is also clear that the current of a pole pair area like indicated by an equal number of A and B switches in the two partial circles is removed. With a total of five strands, four strands are live with two A and two B switches each. Since all currents are assumed to be equal, the phase currents close internally via their connections without causing a load is effective to the outside.

So there is no center conductor current to the outside (as the difference between the Branch streams A and B). In this case, the connection of the Center conductor to the voltage source can be dispensed with. The circuit is through their configuration and mode of operation symmetrical. It is easy to show that this symmetry is not limited to just five-phase circuits.

Starting from the smallest number three, all are odd phase numbers with m = 2g + 1; g = 1, 2, 3 (whole numbers) when using a phase as not carrying current in this sense symmetrical circuits. If there are three phases, the known bridge circuit. The multi-phase circuits can also be switched with alternating Interpret the connections as bridge circuits with two strings connected in series.

Figure 9 shows that the phase combination in the drawn half-period of the phase current 1 changes once (from 1.3 to 1.2). For the combination 1, 2, the bridge circuit is shown in Figure 10, with an A and a B valve are engaged. This form of bridge circuit with two windings in series can be used as the starting form for the mean currents to be influenced by intermediate clocks are valid. The switching arrangement is drawn without reactive current diodes.

For influencing the voltage through intermediate clocks, it is important to note that that then similar conditions as with the non-coupled four-quadrant controller exist if the prerequisites in the relationship via the midpoint connection standing subcircuits are largely the same. This means that ohmic Resistances and inductances of the strands and the voltages induced in them should be the same size. This is a corresponding design of the magnetic Circle of the machine and a symmetrical structure of its winding. Under these conditions it is possible, with the proposed circuit, to use the Make current influence with the same clock frequency as it is with a separate application of four-quadrant regulators is given.

It should be noted that it is not necessary that the to put a currentless break of the phase current in the area of the current reversal. It is possible, the current-free period at another point in the current curve to arrange. A current form changed in this way can be used for certain operating states prove to be more favorable than the current form according to Figure 9.

The prerequisite also leads to a similarly favorable result, that instead of the current break (Fig. 9) an approximately linear one in the same period of time Current change (commutation) takes place. The latter can be achieved by influencing the voltage can be achieved.

With regard to the use of all the on / off switches for setting the current amplitude should be added that operating cases are known for the Limitation of the losses on which intermediate clocks should be dispensed with. So is E.g. the use of upstream DC voltage converters for partial load operation an overall cheap solution. The semiconductor switches A and B of the polyphase winding arrangement then only take over the switching of the phase currents. The reduction in terminal voltage by pulsing is not applicable under this condition. The voltage U is determined by the Upstream controller for setting the current amplitude changed. Here you can the losses in the inverter and in the due to the increased clock frequency Machine winding can be avoided. The ones that arise in the upstream actuator Losses can be very low when using effective relief networks keep.

The variants of the multi-phase winding described above, each with two Semiconductor switches have winding phases that are operated independently of one another in four-quadrant bridge circuit a considerable simplification. For machines under general application requirements and an uninterrupted current form the brought out center conductor proves to be with stabilization according to picture 5 as useful.

Access to a symmetrically divided voltage is also for the implementation of simple relief networks is very advantageous. It is thus possible To achieve simplifications in the circuit structure of the relief circuits. So leave operate low-loss multi-phase circuits with limited component costs, which are characterized by relatively high switching frequencies. It turns out that as mentioned above, the use of an odd number of phases is also advantageous. It means practical for the energy conversion and the execution of the machine no restriction. Machines with an uneven number of phases can be made similarly inexpensive like those with an even number of phases.

For synchronous machines with effective excitation in the pole wheel, means Current shape according to Figure 9 hardly has any disadvantage.

The pole width of the machine is always worth mentioning with electrical excitation smaller than the pole pitch, so that e.g. 1/5 or 1/7 of the pole pitch be assigned to the field or pole gap and thus to the non-force-generating area can. The winding losses are reduced in this area due to the selected current form forced to zero. Also for permanent magnet synchronous machines, for larger ones Pole widths that can be achieved is a current break of 1/7 of the half cycle, for example an adequate requirement.

For machines whose excitation is not generated by the armature winding the described current pattern can be described as ideal. Compared to the conventional mode of operation with an approximately sinusoidal current distribution both greater circumferential forces and a more favorable efficiency achieved.

As can be seen in comparison to a three-phase inverter is, it can be assumed with a higher number of phases that the proportion of simultaneously current-carrying switching elements increases with the number of phases. With the three-phase inverter Two out of a total of six carry the current, four out of a five-phase inverter 10 and with seven phases six out of fourteen semiconductors etc .. So while with three of the phases only a third of the valves are inserted, increases for infinity many phases the proportion to 50%.

In addition to the advantages for the electrical machine, this also results also advantages with regard to the improved component utilization of the inverter.

As already mentioned, the current curve is used for reluctance machines not according to Figure 9, but e.g. according to a shape according to Figure 11b. Part of the stream (e.g.

Section 1,1 ') is used to generate the excitation field. He should be in general case be separately adjustable in size. A stream that over the If the entire half-cycle is to flow, a center conductor current is generated even with an odd number of phases in finite size.

For the example of the five-strand winding, Fig. 8a is in one i1 corresponding scale shown. It is an alternating current whose Half period corresponds to a phase division. Table 2 indicates for two points in time that either switch A or B an additional Current flows in the size of a phase current. This current also occurs in the middle conductor on. Without a break in the phase currents, there is a center conductor load that is not can be suppressed.

As shown in Figure 12, however, a six-phase Circuit to double the center conductor current in amplitude. He would also be have increased period duration, from which one is increased by a multiple Charge amount calculated. It can be seen that even with a current carrying time of 180 the choice of an odd number of phases helps to reduce the asymmetry. From this this results in a more favorable dimensioning of the balancing circuit. The usability the circuit with a reduced number of switching elements can be described by the Significantly improve measures. It is hereby an economically favorable and Reliable switching configuration to a greater extent due to the reduced number of components to disposal. The advantages of the multi-phase machine can be demonstrated by the specified Introducing the circuit into practice much more cheaply.

In the special case of the multi-phase reluctance machine can be Use of switching elements that can be switched on and off independently, one of the DC machine achieve appropriate control and control behavior. By pulsing the course of a phase current can be influenced so that it is in the, the excitation current corresponding section in comparison to the remaining electricity share (the actual Armature current) represents an independent variable, see Figure 13.

An excitation current that is independent of the armature current corresponds to the actuating behavior a separately excited DC machine. A multi-phase inverter with a appropriately multi-phase armature winding allows both in terms of Quality of the energy conversion as well as with a view to adjustability and controllability a very expect favorable behavior.

Claims (7)

Protection claims multi-phase circuit for electrical machines in Connection to a converter, in motor operation based on a DC voltage input and a number of parallel windings to be operated with alternating Winding direction as shown in Figure 4 are connected, each strand with two Semiconductor switches and two diodes and one strand end to a center point is performed, d a d u r c h characterized that the number of phases is odd and greater than is 3. 2. Multi-phase circuit for electrical machines in connection with a converter according to claim 1, d a d u r c h characterized in that in each case at least a phase assigned to a certain rotor position is de-energized and the others Phases can be operated with a current of largely the same size. 3. Multi-phase circuit for electrical machines in connection with a converter according to claim 1, d a d u r c h characterized in that the commutation period adapted to the duration of a phase segment and possibly by influencing the voltage an approximately linear change in current is achieved. 4. Multi-phase circuit for electrical machines in connection with a converter according to claim 1, characterized in that during operation a circuit is used with a current form deviating from claim 2, which allows the midpoint of the without subdivision of the input DC voltage Connect strings in such a way that there is a symmetrical voltage distribution even under load for the two branches (A and B according to Figure 5) with respect to the center point. 5. Multi-phase circuit for electrical machines in connection with a converter according to claims 1 and 3, d u r c h g e k e n n n z e i c h n e t that the largely symmetrical voltage distribution through the use of 2 capacitors of the same size in connection with 2 DC voltage converters (e.g. 5A and SB according to Figure 6) and a choke coil (Ld in Figure 6) is achieved. 6. Multi-phase circuit for electrical machines in connection with a converter according to the above claims, characterized in that by Influencing the current form, e.g. according to Figure 13 for machines that are operated after Reluctance principle work or by the excitation from the pole gap area in their Field can be influenced (mixer excitation), both a field and an (anchor) current-dependent influencing of the operating behavior is brought about. 7. Multi-phase circuit for electrical machines in connection with a converter according to the above claims, characterized in that the Influencing the magnitude of the currents by changing the mean voltage value, for example by pulsing, at least for a limited operating range, over the as Switching elements associated with the strands of semiconductor switches takes place.