DE102021201858A1 - Method and control unit for switching an electric drive between an operating mode with an active short circuit and a freewheeling operating mode - Google Patents

Abstract

The invention relates to a method for switching, in particular automatic switching, of an electric drive (10) comprising a multi-phase electrical machine (18) and a power converter (12) connected upstream of the electrical machine (18) in a voltage range of a predetermined operating voltage between an operating mode with an active short circuit and a freewheel operating mode depending on at least one signal detected on the electric drive (10).
It is provided that the switchover from the freewheeling operating mode to the operating mode with an active short-circuit takes place as a function of a voltage characteristic of the collector voltage Uc of at least one transistor (36) of the converter (12) for electrical energy being fed back from the electrical machine (18). .
The invention also relates to a corresponding control unit (40).

Description

The invention relates to a method for switching over an electric drive comprising a multi-phase electric machine and a converter connected upstream of the electric machine in a voltage range of a predetermined operating voltage between an operating mode with an active short circuit and a freewheeling operating mode as a function of at least one signal detected on the electric drive.

An active short-circuit (ACS) is a measure in drive technology that is intended to prevent damage to the electrical machine or the corresponding drive electronics when excessive electrical voltages occur on the one hand and to prevent a high braking torque from occurring on the other hand would set as soon as the rectified voltage of the electrical machine exceeds the applied DC voltage. Targeted AKS is considered a safe mode for electric propulsion. For this purpose, in the case of the electric drive, the connection lines for the mostly multi-phase electric machines are short-circuited by transistors of the power converter.

The document US 2020/0067443 A1 describes an electric drive system with a control unit known as an MCU (MCU: micro control unit) and two subsystems each consisting of a three-phase electrical machine and a power converter connected upstream of this electrical machine, which is designed as a three-phase inverter. Furthermore, this document describes an operating mode of the respective subsystem with an active short circuit (called 3PS there: three-phase short circuiting) and a freewheeling operating mode of the electric drive (also referred to there as an operating mode with total phase interruption - 6SO: six switches open) as well as a switching of the respective Subsystem between these two modes of operation.

A possible variable, depending on which switching can be useful, is certainly the speed n of the electrical machine.

It is the object of the invention to provide measures that enable the electric drive to be switched over between the operating mode with an active short circuit and the freewheeling operating mode in a manner that is simple to implement and makes sense at the same time.

The present invention is achieved by a method having the features of claim 1. The object is also achieved by a computer program product having the features of claim 6 and a control unit having the features of claim 7. Preferred embodiments of the invention are in the dependent claims, described in the description and the figures, whereby further features described or shown in the subclaims and/or in the description and/or the figures can represent an object of the invention individually or in any combination, if the opposite is not clearly evident from the context results.

The subject matter of the present invention is a method for switching, in particular automatic switching, of an electric drive comprising a multi-phase electrical machine and a power converter connected upstream of the electrical machine in a voltage range of a predetermined operating voltage between an operating mode with an active short circuit and a freewheeling operating mode depending on at least one signal detected on the electric drive. It is provided that switching from the freewheeling operating mode to the operating mode with an active short circuit takes place as a function of a voltage profile of the collector voltage of at least one transistor of the power converter that is characteristic of a feedback of electrical energy from the electrical machine. The transistors of the power converter are usually bipolar transistors and in particular bipolar transistors with an insulated gate electrode (English insulated-gate bipolar transistor, IGBT for short).

The measure according to the invention switches from the freewheeling operating mode (in the document mentioned at the outset US 2020/0067443 A1 also called operating mode with total phase interruption) to the operating mode with active short circuit shortly before the onset of recuperation. The beginning of a corresponding feedback of electrical energy from the electrical machine is detected in the collector voltage. Since the recuperation current is responsible for a corresponding braking torque, this measure fulfills the required functionality. There is also a connection with the speed of the electrical machine via the collector voltage. The switching limit resulting from this (freewheeling operating mode -> operating mode with active short circuit) corresponds to a speed-dependent rectified no-load voltage.

The multi-phase electrical machine is in particular a three-phase electrical machine. Then the operating mode with active short-circuit is a so-called 3PS operating mode (3PS: three-phase short-circuiting) and the free-wheeling Operating mode (all-phase-break operating mode) a so-called 6SO operating mode (6SO: Six Switches Open). Switching from the freewheeling operating mode to the operating mode with an active short circuit is a 6SO -> 3PS switch.

According to a preferred embodiment of the method according to the invention, it is provided that an operating point suitable for switching over is determined by means of an evaluation circuit for evaluating the voltage profile of the collector voltage Uc of the transistor, which corresponds to the actual collector voltage Uc at least in a currentless phase of a respective half-bridge of the converter, in which this transistor is connected, a low-energy measuring current is superimposed. Such a collector voltage evaluation circuit is already provided in many prior art drivers.

The superimposed measurement current makes it possible to switch reliably from the freewheeling operating mode to the operating mode with an active short circuit before the onset of feedback from the EM to the DC circuit. Without this measuring current, it would only be possible to reliably detect the onset of feedback. The onset of feedback is linked to the occurrence of an unwanted braking torque.

In principle, switching from the operating mode with an active short circuit to the freewheeling operating mode is possible in various ways, but according to a preferred embodiment of the invention, the switching from the operating mode with an active short circuit to the freewheeling operating mode takes place as a function of a fixed speed value/a fixed one Speed threshold n1 of the electrical machine. Here, the required functionality is implemented in a particularly simple manner. The speed of the electric machine can be determined from different signals from the electric drive.

For three-phase systems, switching between the active short-circuit mode of operation and the free-wheeling mode of operation is a 3PS -> 6SO switch, respectively.

It is advantageously provided that the switchover from the operating mode with an active short circuit to the freewheeling operating mode also takes place as a function of the collector voltage of the at least one transistor. The collector voltage can be used to determine whether the fixed speed value/fixed speed threshold n1 has been reached.

According to a further preferred embodiment of the invention, when the electric drive is switched over in the voltage range of the predetermined operating voltage between the operating mode with an active short circuit and the freewheeling operating mode, a hysteresis occurs with regard to the rotational speed. This hysteresis prevents unwanted switching back and forth between the two operating modes and is clearly visible at least in the voltage range of the specified operating voltage.

According to yet another preferred embodiment of the invention, switching over takes place automatically by means of a control unit that monitors the collector voltage of the at least one transistor of the power converter.

The subject matter of the present invention is also a control unit for automatically switching over an electric drive comprising a multi-phase electric machine and a power converter connected upstream of the electric machine in a voltage range of a predetermined operating voltage between an operating mode with an active short circuit and a freewheeling operating mode depending on at least one detected on the electric drive Signal. It is provided that the control unit is set up to switch the electric drive from the freewheeling operating mode to the operating mode with an active short-circuit as a function of a voltage curve in the collector voltage of at least one transistor of the power converter that is characteristic of a feedback of electrical energy from the electrical machine. It is preferably provided that this control unit is set up to carry out the above-mentioned method.

According to a preferred embodiment of the control unit according to the invention, it is provided that the control unit is set up to determine an operating point that is suitable for switching using an evaluation circuit for evaluating the voltage profile of the collector voltage Uc of the transistor, which in turn is set up to determine the actual collector voltage Uc at least in a currentless Phase of a respective half-bridge of the converter, in which the transistor is connected, to superimpose a low-energy measuring current, this evaluation circuit being formed in particular by a corresponding driver for this transistor.

Furthermore, it is advantageously provided that the control unit is set up to switch the electric drive from the operating mode with an active short circuit to the freewheeling operating mode as a function of a fixed Switch speed value / a fixed speed threshold n1 of the electric machine.

Another preferred embodiment of the control unit according to the invention provides that the control unit is set up to switch the electric drive from the operating mode with an active short circuit to the freewheeling operating mode, also depending on the collector voltage of the at least one transistor.

Furthermore, it is provided in particular that when the electric drive is switched over in the voltage range of the predetermined operating voltage between the operating mode with an active short circuit and the freewheeling operating mode, a hysteresis results with regard to the rotational speed.

According to yet another preferred embodiment of the control unit according to the invention, it is provided that the control unit is set up to monitor the collector voltage of the at least one transistor of the power converter. This monitoring takes place in particular via a corresponding driver.

• 1 einen Elektroantrieb für ein Kraftfahrzeug mit einem Stromrichter, einer elektrischen Maschine und einer Steuereinheit zum automatischen Umschalten zwischen Betriebsmodi des Elektroantriebs,
• 2 die Ansteuerung eines Transistors des Stromrichters durch eine Steuereinrichtung in schematischer Darstellung,
• 3 ein Spannungs-Drehzahl-Kennfeld des Elektroantriebs mit seinen diversen Bereichen,
• 4 ein Blockdiagramm einer ersten erfindungsgemäßen Realisierung des Umschaltens,
• 5 ein Diagramm, welches ausgewählte Parameter bei dieser ersten Realisierung des Umschaltens zeigt,
• 6 eine schematische Darstellung einer zweiten erfindungsgemäßen Realisierung des Umschaltens und
• 7 ein Diagramm, welches ausgewählte Parameter bei dieser zweiten Realisierung des Umschaltens zeigt.

In the following, the invention is shown in drawings using a preferred exemplary embodiment and described in more detail below. show:

• 1 an electric drive for a motor vehicle with a power converter, an electric machine and a control unit for automatically switching between operating modes of the electric drive,
• 2 the activation of a transistor of the power converter by a control device in a schematic representation,
• 3 a voltage-speed map of the electric drive with its various areas,
• 4 a block diagram of a first implementation of switching according to the invention,
• 5 a diagram showing selected parameters in this first implementation of switching,
• 6 a schematic representation of a second inventive realization of switching and
• 7 Figure 12 is a diagram showing selected parameters in this second implementation of switching.

the 1 shows an electric drive 10 for a motor vehicle with an electric or hybrid drive system. The electric drive 10 comprises a converter 12 designed as an inverter (inverter). This has a circuit arrangement 14, 16 with a DC part 14 with two DC voltage connections and an AC part 16 with phase connections for three phases to which one in the drive as a traction motor serving electric machine 18 can be connected, or - is connected as shown here. As shown here, this electrical machine 18 can be designed purely as an electric motor or else as a motor generator. A rechargeable battery (an accumulator) or some other type of electrical energy store is usually connected to the DC voltage connections of the DC part 14 .

The DC part of the power converter 12 has a voltage side 20 (high side) and a zero potential side 22 (low side) between which a capacitor 24 and three half-bridges 26, 28, 30 (one for each of the phases) each with two switching devices 32, 34 per half-bridge 26, 28, 30 is arranged. The circuit arrangement generally contains current and voltage measuring units (not shown here) for measuring various currents and voltages in the circuit arrangement. Each of the switching devices 32, 34 of the example shown here consists of a transistor 36 and a diode 38 connected in parallel with the transistor 36. The transistors 36 are in the example in the form of IGBTs (IGBT: insulated-gate bipolar transistor—bipolar transistor with insulated gate electrode). .

the 2 shows in a greatly simplified schematic representation the control of the transistor 36 or the transistors 36 by a control device 40. The control device 40 receives data (from current and voltage measuring units only indicated by arrows), from which it calculates the operating point or the operating points of the transistor 36 or the transistors 36 are determined. The control device 40 controls the transistor(s) 36 via a driver 42 (in the sense of a device driver for the transistor(s) 36). For this purpose, the control device 40 outputs corresponding signals to the driver 42, which then converts them into corresponding hardware-related control signals for the corresponding transistor 36. In addition, signals from the transistor 36 also go back to the corresponding driver 42 . Among other things, a signal that corresponds to the collector voltage Uc of at least one transistor 36 of the power converter 12. Controller 40 and driver 42 are implemented within a computer-based controller.

The control unit 40 is provided, among other things, for automatically switching the electric drive 10 between an operating mode with an active short circuit (AKS) and a freewheeling operating mode as a function of at least one signal detected on the electric drive 10 . The switching can be divided into two sub-areas: (a) a switch from the active short-circuit mode of operation to the free-wheel mode of operation; and (b) a switch from the free-wheel mode of operation to the active short-circuit mode of operation. In the case of the three-phase AC part 16 shown here or the three-phase electrical machine shown here, the operating mode with active short-circuit is also referred to as 3PS (3PS: three-phase short-circuiting) and the freewheeling operating mode is also referred to as 6SO operating mode (6SO: six switch open) referred to.

the 3 shows a voltage-speed characteristic diagram of the electric drive 10 with its areas. The switchover of the electric drive 10 between the operating mode with active short circuit (AKS) and the freewheeling operating mode is to be described below on the basis of this voltage/speed characteristic diagram. Only the normal operating range is considered here, ie the map ranges in the voltage range U _{HVmin ≦} U _B ≦U _HVmax of a specified operating voltage U _B . These map areas are: a freewheeling area (SSO area) 44, an active short circuit area (AKS area or 3PS area) 46 and a hysteresis area 48 lying between these two areas 44, 46. The freewheeling area 44 extends from the speed n=0 to a first limit 50 at a constant speed n=n1 of the electrical machine 18. The active short-circuit area 16 is located above a second limit 52, which is given by a rectified open-circuit voltage.

Switching from the operating mode with an active short circuit to the freewheeling operating mode (3PS -> 6SO) takes place at the fixed speed value n1, i.e. at the first limit 50. The speed n and the achievement of the fixed speed value n1 become at least one by monitoring the collector voltage Uc Transistor 36 of the converter 12 (in particular on the low-side) determined. Switching from the freewheeling operating mode to the operating mode with an active short circuit (6SO -> 3PS) takes place in the no-load range of the second limit 52 depending on a feedback for electrical energy from the electrical machine 18 (via the AC part 16 into the DC part 14) characteristic voltage course in the collector voltage of at least one transistor 36 of the power converter 12. This results in a (switching) hysteresis with the resulting hysteresis range 48.

Two possible implementations of such a speed-dependent switchover are now to be described below using corresponding block diagrams. The two operating modes are referred to as 3PS and 6So for short, without this being to be understood as limiting.

4 Figure 12 shows a block diagram of the first implementation of switching. The input signal is the collector voltage Uc. The output signal is a 3PS/6SO switching signal.

The block 54 (Frequ Mon) resets the output if the determined speed n falls below n1. As soon as the output is reset (/6SO), the Frequ Mon is set to a high frequency value independently of the Uc input and maintained at this value as long as /6SO remains set. This ensures that, although no frequency can be measured in the /6SO state, /6SO is not immediately requested again immediately after the return to the 3PS state, which would result in the circuit chattering.

The block 56 (Rekup Mon) sets the output of the circuit as soon as current flow through the monitored transistor 36 or its parallel diode 38 is detected, or as soon as the voltage across the transistor 36 under consideration drops to such an extent that a current flow occurs if the speed is to be expected soon.

The block 58 (Lock 6SO) together with the two downstream logic gates (inverter 60 and AND element 62) is intended to prevent a downstream flip-flop 64 from being set in the 3PS state. That wouldn't be too bad, but the switch-off delay of block 58 (Lock 6SO) causes flip-flop 64 to be set again immediately after switching to state 6SO, Rekup Mon. This prevents bouncing and, above all, prevents the decaying AKS current (lu, Iv, Iw - builds up in the 3PS state) from triggering the recuperation detection.

• - Im obersten Plot sind jeweils die drei Phasenströme 66 dargestellt.
• - Im zweiten Plot sind die Spannungen 68 den drei Lowside-Transistoren 36 dargestellt.
• - Im dritten Plot sind der Ausgang der Drehzahlauswertung im 3PS 70 mit der zugehörigen, einstallbaren Vergleichsschwelle 72 dargestellt. Außerdem sind die Anforderung 3PS-> 6SO 74 und der Ausgang der Schaltung 76 dargestellt.
• - Im untersten Plot sind das vom Treiber kommende Signal 78 der Spannung Uc, die im Zustand 6SO aktive Schwelle für die Rekuperationserkennung 80, die Anforderung 6SO -> 3PS 82 sowie das invertierte Ausgangssignal 84 des Blocks 58 (Lock 6SO) dargestellt.

The waveforms of the circuit 4 is in 5 shown. The upper part (a) shows the signal curves with decreasing engine speed (3HP -> 6SO) and the lower part (b) with increasing engine speed (6SO -> 3HP).

• - The three phase currents 66 are shown in the top plot.
• - The voltages 68 for the three low-side transistors 36 are shown in the second plot.
• - The third plot shows the output of the speed evaluation in the 3PS 70 with the associated adjustable comparison threshold 72. Also shown is the 3PS->6SO request 74 and the output of the circuit 76.
• - In the bottom plot are the signal 78 of the voltage Uc coming from the driver, the active threshold for the recuperation in the state 6SO tion detection 80, the requirement 6SO -> 3PS 82 and the inverted output signal 84 of the block 58 (Lock 6SO).

the 6 shows an alternative implementation of the speed-dependent 3PS / 6SO switchover 3 in a particularly simple embodiment.

The same comparator 86 is used for both states (3PS and 6SO) to evaluate the Uc signal. However, this has a variable threshold, which is just above 0V in the 3PS state and just below the voltage present at Uc in the 6SO state when no recuperation current is flowing.

A pulse is subsequently generated by block 88 in an edge-controlled manner. This starts as soon as Uc < Uth.

The pulse resets the subsequent counter 90. In an even simpler embodiment, the edge-controlled pulse generator 88 can be dispensed with and the inverted output of the Uc comparator 86 can be connected directly to the OR gate 92. The counter 90 is then reset in state 3PS throughout the negative current split wave. However, since the positive and negative partial current waves are different during the transient process of the AKS current, there is an error in the speed measurement. The measured speed can range from at least half the value of the actual frequency to very high.

The reset can be suppressed via an edge-controlled pulse (block 94). This pulse is started when the output signal changes from 3PS to /6SO and is present at least until the phase currents have decayed.

The counter 90 counts the period duration in the state 3PS. The recurring reset pulses 94 reset the counter 90 before it reaches the value T>1/n1 (block 96). If the speed becomes so slow that the period exceeds the value 1/n1, the status /6SO is activated immediately. Two logic gates 98 designed as inverters ensure the necessary signs for the logic links.

In the /6SO state, the reset signal remains off and the counter 90 reaches the stop. As soon as recuperation begins, the Uc < Uth monitor sends a pulse via comparator 86 which resets counter 90. It is switched to the 3PS state.

• - Im obersten Plot sind jeweils die drei Phasenströme 66 dargestellt.
• - Im zweiten Plot sind die Spannungen 68 den drei Lowside-Transistoren 36 dargestellt.
• - Im dritten Plot sind das vom Treiber kommende Signal 70 der Kollektorspannung Uc, die Schwelle für die Uc Auswertung 72, sowie die beiden Eingänge des OR Gates 92 aus 6 (Signale 100) dargestellt.
• - Im untersten Plot ist der Ausgang des Periodendauerzählers 90 (Signal 102) mit der zugehörigen Vergleichsschwelle (Signal 104) dargestellt. Außerdem ist das Ausgangssignal der Schaltung (3PS /6SO in 6 - Signal 106) dargestellt.

The waveforms of the circuit 6 are in 7 shown. Here, too, the upper part (a) shows the signal curves with decreasing engine speed (3HP -> 6SO) and the lower part (b) with increasing engine speed (6SO -> 3HP).

• - The three phase currents 66 are shown in the top plot.
• - The voltages 68 for the three low-side transistors 36 are shown in the second plot.
• - In the third plot, the signal 70 coming from the driver for the collector voltage Uc, the threshold for the Uc evaluation 72, and the two inputs of the OR gate 92 are off 6 (Signals 100) shown.
• - The bottom plot shows the output of the period counter 90 (signal 102) with the associated comparison threshold (signal 104). In addition, the output signal of the circuit (3PS /6SO in 6 - Signal 106) shown.

Reference List

10

electric drive

12

power converter

14

DC part

16

AC part

18

electric machine

20

voltage side

22

zero potential side

24

capacitor

26, 28, 30

half bridges

32

switching device (voltage side)

34

Switching device (zero potential side)

36

transistor

38

diode

40

control device

42

driver

44

freewheel area

46

AKS area

48

hysteresis range

50

first border

52

second border

54 - 64

blocks

66 - 84

signals

86 - 98

blocks

100 - 106

signals

UHV

tension

Uc

collector voltage

t

time

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US 2020/0067443 A1 [0003, 0008]

Claims (10)

Method for switching, in particular automatic switching, of an electric drive (10) comprising a multi-phase electrical machine (18) and a power converter (12) connected upstream of the electrical machine (18) in a voltage range of a predetermined operating voltage between an operating mode with an active short circuit and a freewheeling Operating mode as a function of at least one signal detected on the electric drive (10), characterized in that the switchover from the freewheeling operating mode to the operating mode with an active short circuit is dependent on a voltage curve of the collector voltage that is characteristic of electrical energy being fed back from the electrical machine (18). Uc of at least one transistor (36) of the converter (12). procedure after claim 1 , characterized in that an operating point suitable for switching over is determined by means of an evaluation circuit for evaluating the voltage profile of the collector voltage Uc of the transistor (36), which corresponds to the actual collector voltage Uc at least in a currentless phase of a respective half-bridge (26, 28, 30) of the Converter (12), in which this transistor (36) is connected, superimposed a low-energy measuring current. procedure after claim 1 or 2 , characterized in that switching from the operating mode with an active short circuit to the freewheeling operating mode takes place as a function of a fixed speed value/a fixed speed threshold n1 of the electrical machine (18). Procedure according to one of Claims 1 until 3 , characterized in that switching from the operating mode with an active short circuit to the freewheeling operating mode also takes place as a function of the collector voltage Uc of the at least one transistor (36). Procedure according to one of Claims 1 until 4 , characterized in that when switching the electric drive (18) in the voltage range of the predetermined operating voltage between the operating mode with active short circuit and the freewheeling operating mode with respect to the speed n results in a hysteresis. Procedure according to one of Claims 1 until 5 , characterized in that the switching takes place automatically by means of a control unit (40) which monitors the collector voltage Uc of the at least one transistor (36) of the converter (12). Control unit (40) for automatically switching over an electric drive (10) comprising a multi-phase electrical machine (18) and a power converter (12) connected upstream of the electrical machine (18) in a voltage range of a predetermined operating voltage between an operating mode with active short circuit and a freewheeling operating mode depending on at least one signal detected on the electric drive (10), characterized in that the control unit (40) is set up to switch the electric drive (10) from the freewheeling operating mode to the operating mode with an active short-circuit depending on an electrical energy feedback of the electrical machine (18) characteristic voltage profile of the collector voltage Uc of at least one transistor (36) of the power converter (12). control unit after claim 7 , characterized in that the control unit (40) is set up to determine an operating point suitable for switching by means of an evaluation circuit for evaluating the voltage profile of the collector voltage Uc of the transistor (36), which in turn is set up to determine the actual collector voltage Uc at least in a currentless phase of a respective half-bridge (26, 28, 30) of the power converter (12), in which the transistor (36) is connected, to superimpose a low-energy measuring current, this evaluation circuit being controlled in particular by a corresponding driver (42) for this transistor (36) is formed. control unit after claim 7 or 8th , characterized in that the control unit (40) is set up to switch the electric drive (10) from the operating mode with an active short circuit to the freewheeling operating mode as a function of a fixed speed value/a fixed speed threshold n1 of the electric machine (18). Control unit according to one of Claims 7 until 9 , characterized in that the control unit (40) is set up to switch the electric drive (10) from the operating mode with an active short circuit to the freewheeling operating mode, also depending on the collector voltage Uc of the at least one transistor (36).

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