DE102017204106A1 - Method and device for operating a polyphase inverter - Google Patents

Abstract

The present invention relates to a method of operating a polyphase inverter (104), the method comprising a step of reading an angle and a length of a voltage vector (122) from a field-oriented controller (116) and a step of switching from a pulse width modulation a synchronous modulation for modulating the voltage vector (122) to control signals (110) for driving the phases of the inverter (104), being switched when the angle is in a switching angle range and the length is greater than a pulse width modulation length.

Description

The present invention relates to a device or a method according to the preamble of the independent claims. The subject of the present invention is also a computer program.

An electric motor can be supplied with electrical energy via an inverter. In this case, a rotational speed and a torque of the electric motor can be influenced.

Against this background, the present invention provides an improved method of operating a polyphase inverter, an improved apparatus for operating a polyphase inverter, an improved drive system and an improved computer program product according to the main claims. Advantageous embodiments will become apparent from the dependent claims and the description below.

The speed and torque of an electric motor may be influenced within limits by using pulse width modulation to generate the drive currents for the coils of the electric motor. However, due to the way in which the currents are generated by means of successive pulses, it is not possible to apply the full input voltage of the inverter to the coils, as a result of which the maximum possible torque can not be achieved.

In the approach presented here, in order to call up the maximum possible torque at the electric motor, a switch is made from a pulse width modulation to a fundamental frequency modulation.

• Einlesen eines Winkels und einer Länge eines Spannungszeigers von einer feldorientierten Regelung; und
• Umschalten von einer Pulsweitenmodulation auf eine Synchronmodulation zur Modulation des Spannungszeigers auf Steuersignale zum Ansteuern der Phasen des Wechselrichters, wenn der Winkel in einem Umschaltwinkelbereich liegt und die Länge größer als eine Pulsweitenmodulationslänge ist.

A method of operating a polyphase inverter includes the following steps:

• Reading an angle and a length of a voltage vector from a field-oriented control; and
• Switching from a pulse width modulation to a synchronous modulation for modulation of the voltage vector to control signals for driving the phases of the inverter when the angle is in a switching angle range and the length is greater than a pulse width modulation length.

In particular, the synchronous modulation may be referred to as fundamental frequency modulation. In synchronous modulation, the control signals are switched through the faster, the faster the electric motor should rotate.

From the synchronous modulation can be switched back to the pulse width modulation, if the length is smaller than a synchronous modulation length. By switching back the electric motor can be operated in each optimized modulation mode.

The pulse width modulation length may be smaller than the synchronous modulation length. The switching and the switching back can be carried out as with a two-position controller with a hysteresis. As a result, a constant change between the types of modulation can be prevented.

Between the pulse width modulation and the synchronous modulation can be switched to a transitional modulation. By means of a transition modulation, current peaks during switching and angle errors can be avoided.

As transient modulation an overmodulation can be used. Overmodulation may be a modulation of increased amplitude. Due to the short-term increased amplitude during the transition modulation current peaks can be effectively prevented.

For transition modulation, edge pulse timing can be used. Flank pulse timing can approximate a sinusoidal current profile. As a result, the transition between the pulse width modulation and the fundamental frequency clocking is less abrupt.

The field-oriented control can regulate with two degrees of freedom as a multi-variable control. Advantageously, this can be applied to existing systems. Advantageously, currents can be regulated in the field-oriented control.

In the step of switching can only be switched from the pulse width modulation to the synchronous modulation when a dependent of the angle and the length of the voltage vector condition is met. Thus, there may be a waiting time before the actual switching to the synchronous modulation, which may take until the condition is met. For example, it is only then possible to switch over from the pulse width modulation to the synchronous modulation when the rotating voltage vector is within a defined voltage range. On the other hand, can be switched immediately from the synchronous modulation to the pulse width modulation, if the length of the voltage vector falls below a predetermined value.

Furthermore, an apparatus for operating a polyphase inverter is presented, the device having a device for driving a modulator, wherein the modulator is configured to modulate a voltage pointer provided by a field-oriented control to control signals for driving the phases of the inverter, wherein the device is designed to switch the modulator from one pulse width modulation to one Synchronous modulation switch when an angle of the voltage vector is in a switching angle range and a length of the voltage vector is greater than a pulse width modulation length.

A device may be an electrical device that processes electrical signals, such as sensor signals, and outputs control signals in response thereto. The device may have one or more suitable interfaces, which may be formed in hardware and / or software. For example, in a hardware configuration, the interfaces may be part of an integrated circuit in which functions of the device are implemented. The interfaces may also be their own integrated circuits or at least partially consist of discrete components. In a software training, the interfaces may be software modules that are present, for example, on a microcontroller in addition to other software modules.

The device may comprise a control device which is designed to execute the field-oriented control and to output the angle and the length of the voltage vector as a function of the reference variable. Furthermore, the device may have a modulator for modulating the voltage vector to the control signals, wherein the modulator is driven by the device for driving. The control device and the modulator can also be independent functional units.

Furthermore, a drive system is presented with a multi-phase electric motor, a polyphase inverter and a device according to the approach presented here. The electric motor may be, for example, an asynchronous or permanently excited machine. Thus, the approach described is very versatile.

• 1 ein Bockschaltbild eines Antriebssystems gemäß einem Ausführungsbeispiel der vorliegenden Erfindung;
• 2 eine Darstellung eines Wechselrichters;
• 3 eine Darstellung von Spannungsverläufen auf drei Phasen eines Wechselrichters gemäß einem Ausführungsbeispiel der vorliegenden Erfindung;
• 4 eine Darstellung eines Spannungszeigers gemäß einem Ausführungsbeispiel der vorliegenden Erfindung;
• 5 eine Darstellung eines Umschaltvorgangs gemäß einem Ausführungsbeispiel der vorliegenden Erfindung;
• 6 eine Darstellung von Spannungsverläufen auf drei Phasen eines Wechselrichters während einer Übergangsmodulation gemäß einem Ausführungsbeispiel der vorliegenden Erfindung; und
• 7 ein Ablaufdiagramm eines Verfahrens zum Betreiben eines mehrphasigen Wechselrichters gemäß einem Ausführungsbeispiel der vorliegenden Erfindung.

A computer program product with program code which can be stored on a machine-readable carrier such as a semiconductor memory, a hard disk memory or an optical memory and is used to carry out the method according to one of the embodiments described above if the program is installed on a computer or a device is also of advantage is performed.

• 1 a block diagram of a drive system according to an embodiment of the present invention;
• 2 a representation of an inverter;
• 3 a representation of voltage waveforms on three phases of an inverter according to an embodiment of the present invention;
• 4 a representation of a voltage vector according to an embodiment of the present invention;
• 5 a representation of a switching operation according to an embodiment of the present invention;
• 6 a representation of voltage waveforms on three phases of an inverter during a transition modulation according to an embodiment of the present invention; and
• 7 a flowchart of a method for operating a polyphase inverter according to an embodiment of the present invention.

In the following description of preferred embodiments of the present invention, the same or similar reference numerals are used for the elements shown in the various figures and similarly acting, wherein a repeated description of these elements is omitted.

1 shows a block diagram of a drive system 100 according to an embodiment of the present invention. The drive system 100 includes a multi-phase electric motor 102 that has a polyphase inverter 104 electrically connected. The inverter 104 draws its energy from a DC voltage source 106 such as a battery or a rectifier.

The inverter 104 is from a device 108 to operate the inverter 104 according to an embodiment of the present invention via control signals 110 driven. The device 108 has a facility 112 for providing at least one reference variable 114 for a control device 116 and a facility 118 for driving a modulator 120 on. The control device 116 depends on the reference variable 114 a voltage pointer 122 as a setpoint ready. The voltage pointer 122 has a length and an angle. The modulator 120 reads the voltage vector 122 and modulates this on the control signals 110 on. The device 118 to drive reads the voltage vector 122 one. Here are the control device 116 and the modulator 120 independent modules of the drive system 100 , The control device 116 and the modulator 120 may also be components of the device 108 be.

The control device 116 has, among other components, a voltage regulator 124 , a current regulator 126 and a conversion device 128 on. The voltage regulator 124 regulates depending on the reference variable 114 a nominal voltage. The current regulator 126 regulates depending on the reference variable 114 and / or the target voltage a desired current. The conversion facility 128 calculates the setpoints from a calculation coordinate system into a coordinate system of the voltage vector 122 and gives the voltage pointer 122 out. For example, the conversion device calculates 128 the setpoint values from a rotor-fixed, rotating coordinate system to a non-rotating, fixed coordinate system.

The modulator 120 disassembles the voltage vector 122 according to the number and position of the phases of the inverter 104 and the electric motor 102 into the respective phase components and modulates these to the control signals 110 , The control signals 110 thereby control the switches of the bridge circuit of the inverter 104 at.

The device 108 reads to operate the inverter 104 sizes 130 , such as a current speed (rpm) of the electric motor 102 , a target torque (MRef), a voltage (Udc) of the voltage source 106 and a current flow (i123) in the phases of the electric motor 102 , The device calculates this 108 the reference variable 114 , From the reference variable 114 calculates the control device 116 the voltage pointer 122 ,

The voltage pointer 122 indicates by its length a required electric power to the electric motor 102 to drive with the desired torque. The angle of the voltage vector 122 indicates a required alignment of the magnetic field generated by the current flow in the stator. The angle is constantly changing or reversing to produce a magnetic rotating field in the stator.

The modulator 120 sets the voltage pointer 122 in the control signals 110 around. In pulse width modulation PWM, the switches of the bridge circuit in the inverter 104 operated in high-frequency pulses. A fill factor between a connected section of a pulse and an off section of the pulse determines the resulting current flow in the driven phase. In pulse frequency modulation, the switches are driven with variable frequency. A distance between constant pulses determines the fill factor. The torque can thus be adjusted approximately independently of the speed.

When the voltage pointer 122 comes within the range of its maximum length, the electric motor approximately its maximum torque is retrieved. Depending on the length and angle of the voltage vector 122 controls the device 118 for driving the modulator 120 so that it changes from the pulse width modulation in a fundamental frequency modulation. The voltage is no longer pulsed generated, but the maximum voltage of the voltage source 106 is determined by the angle of the voltage vector 122 predetermined periods of time constant at the windings of the electric motor 102 created.

In other words shows 1 the device 108 as a switching logic for the type of modulation. Depending on the operating point will be on one for the system 100 Switched cheaper modulation. In field-oriented control 116 is the structure of the scheme 116 unchanged and continues to regulate the flows. The level control 124 ensures that the voltage is adjusted according to the type of modulation. The modulator 120 changes to the respective modulation mode and outputs the control signals 110 to the inverter 104 using pulse width modulation or synchronous timing.

The switching between the PWM modulation with two degrees of freedom and the fundamental frequency clocking takes place with one degree of freedom. The two degrees of freedom are angle and length of the voltage vector 122 , The one Freedom Ridge is the angle of the voltage vector 122 with a given maximum length. Switching takes place by means of the logic 108 Firstly, it calculates an optimum point in time for the switchover and, secondly, it considers efficiency optimized ranges for the different types of modulation.

The structure within the scheme 116 or the software can remain unchanged. The field-oriented control (FOR) continues to regulate with two degrees of freedom as so-called multi-variable control. Due to the fact that no structure switching takes place, the system behavior is more continuous and without major jumps in the system variables, such as current and / or voltage. The integrators in the controllers 124 . 126 can continue to operate without resetting.

By the presence of the voltage regulator 124 becomes the length of the voltage vector 122 regulated to the given, maximum voltage. This affects the current regulator 126 with two degrees of freedom anyway only on the angle of voltage vector 122 out. This is the current regulator 126 is still active and can respond to disturbances.

The current regulator 126 and the voltage regulator 124 are also active in the PWM modulation mode, but the voltage regulator 124 regulates to another nominal voltage. The setpoint voltage is determined by a modulation factor depending on the type of modulation.

wird erreicht. Bei der Mehrfachtaktung ist die Taktungsart synchron und ein Aussteuerungsfaktor von $\frac{1}{\sqrt{3}}\dots \frac{2}{\pi }$

wird erreicht. Bei der Grundfrequenztaktung ist die Taktungsart ebenfalls synchron und ein Aussteuerungsfaktor von $\frac{2}{\pi }$

wird erreicht. Die Modulationsart ist synchron falls die Schaltfrequenz fs gleich der Grundfrequenz der Ausgangsspannungen oder ein Vielfaches davon ist. Der Aussteuerungsfaktor ist durch $\frac{U_{sStrang}}{U_{dc}}$

definiert.In space vector modulation, the timing mode is asynchronous and a modulation factor of $0...\frac{1}{\sqrt{3}}$

is achieved. In multiple clocking, the timing mode is synchronous and a modulation factor of $\frac{1}{\sqrt{3}}...\frac{2}{\pi }$

is achieved. In the basic frequency clocking the Taktungsart is also synchronous and a modulation factor of $\frac{2}{\pi }$

is achieved. The modulation mode is synchronous if the switching frequency fs is equal to the fundamental frequency of the output voltages or a multiple thereof. The modulation factor is through $\frac{U_{sStranG}}{U_{dc}}$

Are defined.

The approach presented here is functional for all three-phase machine types, both ASM and PSM. It continues to be regulated on the streams, instead of a controlled operation. There is an operating point-dependent switching of the modulation. Better voltage utilization means more power and better efficiency. No control structure changeover is necessary. The method is not limited to three-phase machines 102 or systems 100 , The approach presented here is robust.

In the control of electric machines 102, no modulation method has prevailed as the optimum for all requirements and operating points. The space vector modulation, here called pulse width modulation or PWM, is usually the standard modulation. However, this asynchronous modulation type has two crucial weaknesses. Pulse width modulation does not fully exploit the battery voltage. From a certain ratio between the switching frequency and the drive frequency, disturbing current harmonics and possibly beats result. These two weaknesses are at the same time the strengths of the synchronous modulation methods. The variation of the modulation method presented here allows the drive system 100 operate optimally under the given boundary conditions. By the approach presented here, unwanted balancing currents can be avoided when switching, which could lead to system failure.

In order to avoid violating component-specified current limits when switching between the modulation methods, or to avoid larger compensation processes occurring in the control loop 116 can lead to instability, the approach presented here allows for a smooth transition.

In general, the modulation method is converted from a pulse width modulation via a transition modulation to a fundamental frequency timing. Back is switched from the fundamental frequency clocking via the transition modulation to the pulse width modulation. In this case, a pulse width overmodulation or PWM-OVM or a multiple clocking can be used as transition modulation.

2 shows a representation of an inverter 104 , The inverter 104 corresponds essentially to the inverter in 1 , The inverter 104 is here three-phase. He has two switches per phase 200 on that the phase either with a high voltage potential 202 or with a low voltage potential 204 the voltage source 106 connect. One phase is each with a coil of the electric motor 102 connected. The coils are in the electric motor 102 For example, interconnect in a star connection.

For the control of DC machines and induction machines 102 are predominantly voltage source inverters (WR) 104 used. 2 shows the structure of an inverter 104 for induction machines 102 , By means of different modulation methods, the desired voltages at the output of the inverter 104 be set by generating the pulse width modulation values.

The generated PWM values from the controller or controller to produce a particular voltage vector will vary with the desired voltage frequency and amplitude.

3 shows a representation of voltage curves 300 . 302 on three phases of an inverter according to an embodiment of the present invention. The three phases essentially correspond to the three phases in 2 , The voltage curves 300 . 302 . 304 are shown in three superimposed diagrams. On the abscissa a normalized angle or a time is plotted, on the ordinates a normalized voltage amplitude. The voltage curves of the individual phases are the same, but have a phase offset to one another. For each of the three Phases are voltage curves 300 . 302 . 304 represented by three different types of modulation. The first modulation type is a sine modulation 300 that closely approximates a sine wave. The second type of modulation is a space vector modulation 302 which approximately corresponds to the sinusoidal curve but has slight deviations therefrom. The third type of modulation is a fundamental frequency modulation 304 , in which the sine wave is modeled by a square wave signal. The sine modulation 300 and the space vector modulation 302 are achieved by a pulse width modulation. The gradients become 300 . 302 generated by many pulses with variable pulse width or variable filling factor. The fundamental frequency modulation 304 In contrast, has a small number of switching operations.

The approach presented here is based on space vector modulation 302 to the fundamental frequency modulation 304 changed when a certain operating condition is reached. In the fundamental frequency modulation 304 is at a phase over a half-wave to the full voltage of the voltage source.

In other words shows 3 the difference between the space vector modulation and the fundamental frequency clocking. The curve shows 300 the sine modulation, the curve 302 the space vector modulation and the curve 304 the fundamental frequency clocking.

4 shows a representation of a voltage vector 122 according to an embodiment of the present invention. The voltage pointer 122 is shown in a stator-fixed coordinate system with the axes α, β. The voltage pointer 122 is a vector from the origin of the coordinates. The voltage pointer 122 thus has a length and an angle. The length of the voltage vector 122 is variable between zero and one. The angle can be any value. Thus, the peak of the voltage vector can theoretically reach any point within a circle around the coordinate origin of radius one. The voltage pointer 122 can be called a space pointer.

dargestellt. Alle Arbeitspunkte innerhalb des Kreises 402 können mittels Pulsweitenmodulation beziehungsweise Raumzeigermodulation oder Spannungszeigermodulation erreicht werden.The length of the voltage vector 122 represents an electric power to be provided to the electric motor. Since the electric motor has a finite number of coils, not all can be represented by the voltage pointer operating points can be achieved. In fact, an area can 400 of the circle marked here by a hexagon. Here are the corners of the hexagon 400 on the circle with the radius one. One corresponds to the DC voltage U _{dc of} the voltage source. Inside the hexagon 400 is a tangent circle 402 with the radius $\frac{1}{\sqrt{3}}{U}_{dc}$

shown. All work points within the circle 402 can be achieved by means of pulse width modulation or space vector modulation or voltage vector modulation.

erreicht werden.In the approach presented here, operating points within the circuit are controlled via the pulse width modulation. If the operating points are outside of the circle 402, the system switches to fundamental frequency modulation or basic frequency modulation. By the fundamental frequency clocking, a higher power can be provided to the electric motor, as by means of the pulse width modulation. It can be effective $\frac{2}{\pi }{U}_{dc}$

be achieved.

In other words, a synchronous clocking in the field-oriented control is presented.

In the field-oriented control, both the voltage pointer length and the angle can be changed by a pulse width modulation. Due to the principle, only the angle can be changed during basic frequency clocking. The voltage length is given by the DC link voltage.

Alternatively, it is possible to switch to a control so that the currents are no longer regulated, as is the case with a standard field-oriented control. This avoids that the voltage length is changed by the current controller unintentionally. In the case of a control, a structure changeover can additionally be carried out.

In order to make maximum use of the induction machine, the maximum possible voltage is applied to the machine terminals. This is achieved by the fact that the inverter switches with the fundamental frequency control the three phases.

Der gestrichelte Kreis zeigt die bei der Grundfrequenztaktung effektiv erreichbare Strangspannung von $\frac{2}{\pi }{U}_{DC.}$

In order to avoid structure switching that is difficult to control in the closed-loop control, the synchronous clocking is implemented as an extension in the field-oriented control by continuing to regulate the currents without switching over the control structure. This shows the hexagon 400 the maximum achievable fundamental wave of the strand voltage (peak value). The registered circle 402 shows the maximum achievable in the space vector modulation strand voltage of $\frac{1}{\sqrt{3}}{U}_{DC},$

The dashed circle shows the effectively achievable in the basic frequency clocking strand voltage of $\frac{2}{\pi }{U}_{DC,}$

5 shows an illustration of a switching operation according to an embodiment of the present invention. The representation essentially corresponds to the illustration in FIG 4 , Here is a switching angle range 500 shown. The angle range 500 is a circular sector of the by a pulse width modulation length 502 defined circle 402 and from the origin to a corner of the hexagon 400 directed. Similar switching angle ranges not shown here are for further vertices of the hexagon 400 Are defined. Furthermore, several operating points A, B, C are marked. The points A and B lie on the inscribed circle 402 , The point C lies on the corner of the hexagon 400 to which the switching angle range 500 is aligned. The point C is outside the range coverable with pulse width modulation and can be reached via the synchronous modulation.

If the endpoint of the voltage vector is at point A or point B, the length of the voltage vector is equal to the pulse width modulation length 502 and thus fulfills one of the requirements for switching to the synchronous modulation. However, point A is not within the switching angle range 500 , Point B is within the switching angle range 500 and meets both requirements. If the end point of the voltage vector is at point B, the system switches to synchronous modulation and thus to point C.

Once a modulation scheme is released, it does not become active immediately. For the switching time from the pulse width modulation to the synchronous clocking is waited until conditions are met. One of the most important conditions is the voltage pointer length and the angle.

In a synchronous clocking is only switched if the rotating voltage vector is in a defined voltage range 500 located. An area 500 is shown here by way of example. The voltage pointer length must be a certain length 502 have exceeded. The control voltage to be controlled is guided when switching to the respective modulationsartabhängige Austeuerung.

It is obvious that a jump between point B and point C is less problematic than a jump between point A and point C. This is due to the angle errors and amplitude errors caused by the jump, which in turn cause equalizing currents.

If the voltage pointer length falls below a predetermined value, it is immediately switched to the pulse width modulation. Switching over the voltage pointer length is done with a hysteresis. Other constraints such as currents, torques and speeds may be considered.

The approach presented here is functional for all three-phase machine types, both asynchronous and permanent-magnet machines (ASM, PSM). This results in smooth transitions between the types of modulation and no major voltage jumps or equalizing currents through the modulation type switching. The overmodulation results in a continuous transition between the pulse width modulation to the fundamental frequency timing. By a multiple clocking, for example, a triple, quintuple or seven-fold timing and / or a center pulse timing or Flankenpulstaktung results in a high robustness.

6 shows a representation of voltage waveforms on three phases of an inverter during a transition modulation according to an embodiment of the present invention. Here is a transitional modulation as a triple-timing is shown as an example of a multiple clocking or a Flankenpulstaktung. The voltage curves have a rectangular shape. This is the actual switching pulse 600 in each case a short switching pulse 602 upstream or downstream.

Furthermore, two voltage curves resulting from the voltage curves shown above are shown in the stator-related α, β coordinate system. The fundamental oscillation output amounts to α ₀ = 2 * cos α - 1. The basic oscillation output can be changed via the angle α. For the case α = 0 results in the fundamental frequency clocking.

7 FIG. 12 shows a flowchart of a method of operating a polyphase inverter according to an embodiment of the present invention. The method can be used, for example, on a device such as that described in US Pat 1 is shown executed. The method has one step 700 of reading in and one step 702 switching over. In step 700 When read in, an angle and a length of a voltage vector are read in by a field-oriented control. In step 702 the switching is switched from a pulse width modulation to a synchronous modulation for the modulation of the voltage vector to control signals for driving the phases of the inverter when the angle is in a switching angle range and the length is greater than a pulse width modulation length.

If an exemplary embodiment comprises a "and / or" link between a first feature and a second feature, this can be read so that the embodiment according to one embodiment, both the first feature and the second feature and according to another embodiment, either only the first Feature or only the second feature.

LIST OF REFERENCE NUMBERS

100

drive system

102

electric motor

104

inverter

106

power supply

108

Device for operating

110

control signals

112

Device for providing

114

command variable

116

control device

118

Device for switching

120

modulator

122

voltage vector

124

voltage regulators

126

current regulator

128

conversion unit

130

input variables

200

switch

202

high potential

204

low potential

300

sinusoidal modulation

302

Space vector modulation, pulse width modulation

304

Fundamental frequency modulation

400

first area

402

second area

500

Umschaltwinkelbereich

502

Pulse width modulation length

600

switching pulse

602

switching pulse

700

Step of the rules

702

Step of switching

Claims (14)

A method of operating a polyphase inverter (104), the method comprising the steps of: Reading (700) an angle and a length of a voltage vector (122) from a field-oriented controller (116); and Switching (702) from a pulse width modulation (302) to a synchronous modulation (304) for modulating the voltage vector (122) to control signals (110) for driving the phases of the inverter (104) when the angle lies in a switching angle range (500) and the Length is greater than a pulse width modulation length (502). Method according to Claim 1 in which, in the step (702) of switching from the synchronous modulation (304) to the pulse width modulation (302) is switched, if the length is smaller than a synchronous modulation length. Method according to one of the preceding claims, wherein in step (702) the switching between the pulse width modulation (302) and the synchronous modulation (304) is switched to a transition modulation. Method according to Claim 3 in which an overmodulation is used in step (702) of the switching as transition modulation. Method according to Claim 3 in which, in step (702) of switching as transition modulation, edge pulse timing is used. Method according to one of the preceding claims, in which the field-oriented control (116) regulates with two degrees of freedom as a multi-variable control. Method according to one of the preceding claims, wherein in the field-oriented control (116) is regulated to currents. Method according to one of the preceding claims, in which, in the step of switching (702), switching is effected from the pulse width modulation (302) to the synchronous modulation (304) only when a condition dependent on the angle and the length of the voltage vector (122) is satisfied , Method according to one of the preceding claims, wherein in the step of switching (702), switching from the pulse width modulation (302) to the synchronous modulation (304) only occurs when the rotating voltage pointer (122) is in a defined voltage range (500). Method according to one of the preceding claims, wherein in the step of switching (702) immediately from the synchronous modulation (304) to the pulse width modulation (302) is switched when the length of the voltage vector (122) falls below a predetermined value. Apparatus (108) for operating a polyphase inverter (104), the apparatus (108) having means (118) for driving a modulator (120), the modulator (120) being adapted to receive a field oriented control (116 ) to modulate control signals (110) for driving the phases of the inverter (104), the device (118) being adapted to switch the modulator (120) from a pulse width modulation (302) to a synchronous modulation (304). when an angle of the voltage vector (122) is within a switching angle range (500) and a length of the voltage vector (122) is greater than a pulse width modulation length (502). Device (108) according to Claim 11 control device (116) adapted to perform field oriented control and output the angle and length of the voltage vector (122) in response to a command variable (114), and a modulator (120) for modulating the voltage vector (122 ) to the control signals (110), wherein the modulator (120) is driven by the means (118) for driving. A drive system (100) comprising a polyphase electric motor (102), a polyphase inverter (104) and a device (108) according to any one of Claims 11 to 12 , Drive system (100) according to Claim 13 in which the electric motor (102) is an asynchronous or a permanently excited machine.

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