Control method for electric machine

Abstract

Vehicle with an N-phase electric machine, with a first partial electrical system (BN1) and with a second partial electrical system (BN2), wherein the electric machine comprises a rotor and a stator system (1), the first partial electrical system comprises an inverter (2), the stator system (1) is assigned to the inverter, and the electric machine can be operated with an inverter controller according to the principle of field-oriented control, wherein - the electric machine is assigned a control unit (6), - the inverter (2) can be controlled by the control unit (6), and - the control unit (6) controls the inverter (2) in the form of a phase-shifted control method, characterized in that - the phase-shifted control method smooths the neutral point current, whereby the neutral point current is smoothed by a leakage inductance and by a main inductance of the machine.

Description

[0001] The invention relates to a vehicle with an N-phase electrical machine, with a first sub-vehicle network and with a second sub-vehicle network, wherein the electrical machine comprises a rotor and a stator system, the first sub-vehicle network comprises an inverter, the stator system is assigned to the inverter, and the electrical machine can be operated with an inverter controller according to the principle of field-oriented control.

[0002] Typically, components in a vehicle that consume electrical energy are supplied by an on-board power system with a nominal voltage of 14 volts. A secondary 12-volt energy storage unit, which functions as an energy source or energy sink in the on-board power system depending on the operating situation, and a 14-volt generator are designed to provide an electrical output of typically 1.5–3 kW in the vehicle.

[0003] If several consumers with increased power requirements are integrated into the vehicle's electrical system, the electrical system can comprise two or more sub-systems. A DC / DC converter then transfers electrical power between the two sub-systems. The electric motor, which can also be operated as a motor in a vehicle with an electrified powertrain, functions as an electrical energy source or sink in the vehicle, in addition to at least one energy storage device per sub-system. Such an electrical system topology is described, for example, in document DE 102 44 229 A1.

[0004] Vehicles of this type are also known from DE 10 2012 203 528 A1, DE 10 2009 007 960 A1 and US 8 253 360 B2.

[0005] It is an object of the invention to describe an improved vehicle with an N-phase electric machine, with a first sub-vehicle network and with a second sub-vehicle network, wherein the electric machine comprises a rotor and a stator system, the first sub-vehicle network comprises an inverter, the stator system is assigned to the inverter, and the electric machine can be operated with an inverter controller according to the principle of field-oriented control.

[0006] This object is achieved by a vehicle according to claim 1. Advantageous embodiments and further developments of the invention emerge from the dependent claims.

[0007] According to the invention, the phase-shifted control method smoothes the star-point current, whereby the star-point current is smoothed by a stray inductance and by a main inductance of the machine.

[0008] This means that the stator system of the electrical machine is assigned to the inverter and the electrical machine can be operated with a current controller according to the principle of field-oriented control, whereby a phase-shifted control method is used which uses both the stray inductance and the main inductance of the machine to smooth the star point current.

[0009] In other words, it is a vehicle with an N-phase electric machine, with a first sub-vehicle electrical system and at least one second sub-vehicle electrical system, wherein the electric machine comprises a rotor and a stator system, and the first sub-vehicle electrical system comprises an inverter for converting a DC voltage into an AC voltage. This can be a bridge circuit or another topology, such as a multi-level converter. At the input of this inverter, there is a DC link capacitor for smoothing the input voltage when an impressed voltage is applied, or a DC link choke for smoothing the input current when an impressed current is applied.

[0010] According to a preferred variant of the invention, it is advantageous if the stator system is designed in a star connection, the star point can be connected to the second sub-vehicle network directly or via a star point switch, the inverter controller comprises a current controller and a star point controller, the current controller controls phase currents of the stator system, and the star point controller controls a star point current.

[0011] It is expedient if the vehicle comprises measuring means for measuring a number of at least N-1 phase currents and the star point current or the vehicle comprises measuring means for measuring a number of at least N phase currents or the vehicle comprises a measuring means for measuring an intermediate circuit current, wherein the measured intermediate circuit current enables at least an indirect determination of the N phase currents of the machine based on voltage zero vectors, and the control unit comprises the inverter controller.

[0012] According to a preferred embodiment, the phase-shifted control method smoothes the star-point current, wherein the star-point current is smoothed by a stray inductance and by a main inductance of the machine, and a time offset between each phase of the electrical machine can be set.

[0013] The phase-shifted control of the individual phases is carried out by a respective pulse pattern and the pulse patterns are advantageously shifted by an angle of 360° / N.

[0014] According to a further variant of the invention, the phase shift of the control method serves to smooth the star point current by minimizing current ripples of the star point current and the phase shift acts torque-free in the rotor when controlling the star point current averaged over one revolution of the machine.

[0015] This means that, averaged over one revolution of the machine, no additional torque is generated in the rotor.

[0016] Particular advantages arise when the phase offset of the control method can be used for an increased adjustment frequency in the pulse pattern generation in order to reduce a switching frequency of the inverter in order to improve the efficiency or, at a given switching frequency, to extend the operating range of a pulse width modulated (PWM) control, which is limited by the speed of the machine, towards a higher speed of the machine, and the star point controller provides a control value for the control method.

[0017] Thus, for a given switching frequency, the PWM operating range limited by the pole change frequency of the machine and thus by the speed of the machine can be extended to higher speeds.

[0018] A further advantageous embodiment of the invention is obtained when the measuring means N currents (I u , I v , I w ) of the electrical machine, and the control unit measures the measured currents (I u , I v , I w ) via an extended Clarke-Park transformation into a field-oriented current vector (I d , I q , I0) with a flux-forming component (I d ), with a moment-forming component (I q ) and transformed with a zero current component (I0), whereby the star point current (I Stern ) is three times the zero current component.

[0019] In addition, it is advantageous if the inverter controller has the flux-forming component as a controlled variable, the inverter controller has the torque-forming component as a controlled variable, the inverter controller has the zero current component as a controlled variable, the inverter controller has a first setpoint current value (I*d ) for the flux-forming component, the inverter controller has a second setpoint current value (I* q ) for the torque-generating component, the inverter controller has a starpoint setpoint current as a setpoint value (I* Stern ), the inverter controller uses as a control variable a first stator control voltage (U* d ), the inverter controller outputs a second stator control voltage (U* q ) and the inverter controller outputs a third stator control voltage (U* o ) outputs.

[0020] This is expediently supplemented by the fact that the N-phase electrical machine is designed as a 3-phase electrical machine, the inverter comprises six inverter switches (HS1, HS2, HS3, LS1, LS2, LS3), the six inverter switches are arranged in three half-bridges for the three phases of the stator system, the first stator control voltage, the second stator control voltage and the third stator control voltage are converted into phase voltages (U a , U b , U c ) of the stator system, and the inverter switches the phase voltage for each of the phases in a switching cycle according to the principle of pulse width modulation.

[0021] According to a further variant of the invention, when the star point switch is closed, the electrical machine transfers a star point current (I Stern) with a current direction from the star point to the second sub-system electrical power from the first sub-system to the second sub-system and acts as a step-down converter.

[0022] With a direct connection or closed star point switch, the electrical machine transfers a star point current (I Stern ) with a current direction from the second sub-system to the star point, electrical power is transferred from the second sub-system to the first sub-system and acts as a boost converter.

[0023] Furthermore, by specifying the starpoint setpoint current and setting the starpoint control voltage, a starpoint current (I Stern ) and the electrical machine acts as a bidirectional power controller when the star point switch is closed.

[0024] In the following, a preferred embodiment of the invention is described with reference to the accompanying drawings. Further details, preferred embodiments and developments of the invention emerge from these. In detail, schematically Fig. 1: Schematic topology of an electrical machine with loaded star point Fig. 2: Optimization of pulse pattern generation to reduce current ripple of the star point current Fig. 3: Principle of symmetrical pulse pattern generation with a triangle modulator Fig. 4: Phase-shifted pulse pattern generation by multiple adjustment of the duty cycle during a triangle modulation Fig. 5: Phase-shifted pulse pattern generation by phase shift between modulators of the same type and a constant duty cycle Fig. 6: Phase-shifted pulse pattern generation by different modulator types with the same duty cycle for the different phases

[0025] The embodiment according to Fig. Figure 1 shows an example of an N-phase machine as a three-phase electrical machine with a rotor and a stator system (1). The three phases are designated (u, v, w). An inverter (2) and an intermediate circuit capacitor (3) are assigned to the electrical machine. The electrical machine, the inverter, and the intermediate circuit capacitor are components of a first sub-vehicle electrical system (BN1) of a vehicle. This sub-vehicle electrical system also has at least one first electrical energy storage device (3'), which can be embodied as an electrochemical energy storage device.

[0026] The electrical machine, which can be operated both as a generator and electrically, is controlled via the inverter according to the field-oriented control (FOR) principle, which is familiar to a person skilled in the art. For this purpose, the inverter is preferably designed as a bridge converter, i.e., each phase of the electrical machine is assigned an electrical half-bridge of the inverter. A half-bridge consists of a series connection of two switches, with the switch assigned to the higher electrical potential being referred to as the high-side switch (HS1, HS2, HS3) and the switch assigned to the lower electrical potential being referred to as the low-side switch (LS1, LS2, LS3).

[0027] As an alternative to a bridge converter, a multilevel converter can also be used. Without limiting this generality, we will continue to assume a bridge converter.

[0028] The vehicle also has a second sub-vehicle electrical system (BN2), which includes, for example, a second electrical energy storage device (4') with an optional intermediate circuit capacitor (4) and electrical loads (12). The second sub-vehicle electrical system can have various embodiments. These have in common that the lower potential of the second sub-vehicle electrical system corresponds to the lower potential of the first sub-vehicle electrical system. In particular, there is no electrical connection between the higher potential of the second sub-vehicle electrical system and the higher potential of the first sub-vehicle electrical system.

[0029] However, the star point (1a) of the electrical machine is permanently electrically connected to the higher potential of the second sub-vehicle network or can be electrically connected via a switch referred to as a star point switch (5).

[0030] This topology eliminates the need for a separate DC / DC converter, as its function is taken over by the combination of an N-phase electrical machine with at least one winding in star connection, an inverter, optionally a bidirectional blocking switch between the star point of the machine and the higher potential of the second partial power supply system, whose voltage is advantageously lower than the voltage of the first partial power supply system, as well as an output capacitor for voltage smoothing. The prerequisite for determining the star point current is that either all currents in the N phases or N-1 phases + star point current of the machine or the total current through the intermediate circuit capacitor can be measured. The latter option, however, has the limitation that a measurement is only possible if no voltage zero vectors are set, i.e.Using the example of a bridge circuit, never close either all upper switches or all lower switches at the same time.

[0031] A suitable method for controlling the neutral current is, for example, field-oriented control using the zero-sequence component of the extended Clarke-Park transformation, which requires the determination of the d / q and zero currents in the field-oriented coordinate system. The transformation equation can be written as follows: [IdIqI0]=23⋅[cos(βel)cos(βel−120°)cos(βel+120°)−sin(βel)−sin(βel−120°)−sin(βel+120°)121212]⋅[IUIVIW]

[0032] In contrast to the normal Clarke-Park transformation, it can be seen that the starpoint current does not cancel out to 0, but represents an additional degree of freedom in the system. For the electrical angle β el , which is required for the rotation of the rotor-fixed coordinate system, generally applies: βel=βmech⋅Zp+βS

[0033] β S Represents the slip-dependent additional component of an asynchronous machine, which can be positive or negative depending on whether the machine is operating in motor or generator mode. In the case of a synchronous machine, this parameter is set to 0.

[0034] The actual value of the current I0 determined in equation 1 can now be controlled to a defined setpoint using a controller. Common controller types include continuous controllers with optional proportional and / or integral and / or derivative components, as well as state controllers or discontinuous controllers with, for example, two- or three-point behavior or fuzzy controllers. Due to its simplicity and common practice, the following assumes a PI controller, which in the current state of the art is usually implemented digitally in a control unit.

[0035] The star point controller outputs a stator control voltage as the control value U0* which is then subjected to an extended, inverse Clarke-Park transformation. This can be expressed as follows: [UU*UV*UW*]=[cos(βel)sin(βel)1cos(βel−120°)−sin(βel−120°)1cos(βel+120°)−sin(βel+120°)1]⋅[Ud*Uq*U0*]

[0036] The resulting control voltages UU*,UV* and UW* can now be fed to a PWM modulator within the control unit, which generates pulse pattern signals to control the inverter. Without further measures in the control process, as shown in Fig. 3, a symmetrical pulse pattern is generated, thus operating the inverter in such a way that the starpoint current is distributed synchronously across each of the N phases of the machine. While this prevents a main flux from developing in the machine, which could lead to a torque in the rotor, it also prevents the main inductance of the machine from acting, so that only the stray inductance can be used to smooth the starpoint current. The following derivation illustrates this:

[0037] Derived from the general voltage equations of an electric machine in a stator-fixed UVW system [UUUVUW]=ddt[ΨuΨvΨw]︸Ψ¯uvw+[Rs,uvw000Rs,uvw000Rs,uvw]⋅[IuIvIw]︸I¯uvw The linked magnetic flux can generally be described by a leakage inductance L σ,uvw and one with the main inductance L m describe the effective portion Ψ¯uvw=Lσ,uvw⋅E¯⋅I¯uvw+Lm⋅M¯⋅I¯uvw using the unit vector E and the matrix M, which represents the geometric arrangement of the phases relative to each other. E¯=[100010001],M¯=[cos(0°)cos(120°)cos(240°)cos(240°)cos(0°)cos(120°)cos(120°)cos(240°)cos(0°)]

[0038] Using the extended Clarke-Park transformation, the flux equation in the rotor-fixed d / q / 0 coordinate system can be given in general form as follows Ψ¯dq0=[ΨdΨqΨ0]=Lσ,uvw⋅E¯⋅[IdIqI0]+[32⋅Lm00032⋅Lm0000]⋅[IdIqI0]

[0039] For machines that have a pronounced saliency in the d- and q-axis, the corresponding inductances L d and L q be used. Ψ¯dq0=[ΨdΨqΨ0]=Lσ,uvw⋅E¯⋅[IdIqI0]+[Ld000Lq0000]⋅[IdIqI0]

[0040] It can be seen that the components of the main inductance L mnot affect the zero current I0, but only the leakage inductance L σ,uvw is effective.

[0041] Since the general state of the art in an electrical machine aims to keep the stray inductance as small as possible in favor of a good magnetic coupling between stator and rotor, this can result in a very high current ripple in the star point current. The following example illustrates this:

[0042] The zero current I0, which results from the extended Clarke-Park transformation, is distributed essentially evenly to 1 / N each on the N phases of the electrical machine I0=13⋅IU+13⋅IV+13⋅IW+…

[0043] Using the example of a 3-phase machine, the zero current is I0=13⋅[I^u⋅sin(ωt+0°)+I^v⋅sin(ωt+120°)+I^w⋅sin(ωt+240°)]

[0044] Since the sum of the three sinusoidal currents, each offset by 120°, is zero and the goal is to have the same amplitudes in all phases, the star point current can be simplified as IStern=3⋅Io be described.

[0045] If, for example, an energy transfer is to take place from the partial energy network 1 to the partial energy network 2, the current ripple for a buck converter known to the expert can be specified in general form as follows: ΔI0=Ue−UaL⋅D⋅ts ΔIStern=3⋅Δl0=3⋅Ue−UaLσ,uvw⋅D⋅ts

[0046] The duty cycle D is defined for the buck converter as UaUe, the period t s corresponds to the inverse of the switching frequency f s For example, if the partial energy system 1 has a current voltage of U e = 48V, the partial energy network 2 has a current voltage of U a= 12V, the leakage inductance of the electrical machine is of the order of 20µH and the inverter is operated with a switching period of t s = 62.5µs, the star point current results in a current ripple of ΔIStern=3⋅48V−12V20μVs / A⋅12​V48V⋅62.5μs ≈84.4A

[0047] In the event that an energy transfer from partial on-board power system 2 to partial on-board power system 1 is to take place in the opposite direction, the system is operated as a boost converter, as is familiar to those skilled in the art. The current ripple in the boost converter can generally be specified as follows: Δl0=UeL⋅D⋅ts

[0048] The duty cycle of the boost converter is defined as D=Ua−UeUa

[0049] Since the same inductance is effective as in buck converter operation, the star point current also results in a current ripple of ΔlStar=3⋅Δl0=3⋅UeLσ,uvw⋅Ua−UeUa⋅ts ΔlStar=3⋅12V20μVs / A⋅48V−12V48V⋅62.5μs ≈ 84.4 A

[0050] A current ripple of this magnitude means that without an additional external smoothing inductance, the output capacitor (12) must be dimensioned very large.

[0051] The control voltages generated by the controllers in field-oriented form Ud∗,Uq∗ and U0∗ are either directly converted into stator-fixed control voltages using an inverse Clark-Parke transformation according to equation 2 UU∗,UV∗ and UW* transformed and fed to a PWM modulator or by means of an inverse Clarke transformation [Uα∗Uβ∗U0∗]=[101−12321−112−321]⋅[Ud∗Uq∗U0∗] initially in stator flux-oriented components Ua∗,Uβ∗ and U0∗ transformed, from which corresponding duty cycles for the PWM modulator can be generated for each phase of the electrical machine using a space vector modulation known to the person skilled in the art.

[0052] The pulse pattern for controlling the inverter is now modified in such a way that not only the stray inductance but also the main inductance of the electrical machine is used to smooth the star point current.

[0053] In the associated Fig. 2 to 6, a pulse pattern is shown on the vertical axis against time t on the transverse axis. The pulse pattern results from the value of the duty cycle of the respective phase (also on the vertical axis) in relation to a respective modulator signal (dashed on the vertical axis). If the modulator signal assumes the value of the duty cycle or a local maximum, the pulse essentially switches in the respective phase, resulting in a phase shift of the pulse pattern, for example, with different modulator types for each phase (see Fig. 6). In the Fig. 3 to 6 is the pulse pattern over a period of two periods t s shown.

[0054] The pulse pattern can be modified either by using the center-aligned pulse pattern generation that is common in electrical machines and known to those skilled in the art, but by adjusting the duty cycle generated by the inverter controller multiple times during a modulation period. The advantage here is that the modulator can remain the same for all phases. The adjustment frequency can be increased by a factor of 2, for example, during a modulation period by modifying the duty cycle for each phase output by the inverter controller in such a way that two different values ​​result, but the average value over a modulation period is the same as if the duty cycle were only adjusted once per modulation period. This can be done, for example, by carrying out the current control at twice the frequency, which means that all Clarke-Park transformations must also be carried out twice as fast.Furthermore, the zero vectors of the space vector modulation can be distributed differently within a modulation period.

[0055] Another possibility is to store a previously calculated pattern in the control unit's memory and call it up when needed. Fig. 4 shows the corresponding embodiment.

[0056] In addition, the duty cycle can be kept constant across all phases and different modulator types such as triangle or sawtooth, each in rising or falling form, can be used in combination (see Fig. 5) The last two embodiments are known to the person skilled in the art as edge aligned methods.

[0057] A final and most advantageous possibility for generating a phase-shifted pulse pattern is to use the same modulator types, such as the triangle modulator and setting a defined time offset between the individual phases (see Fig. 6). This represents the simplest type of implementation and offers the most degrees of freedom, since on the one hand the phase offset between the individual modulators can be adjusted as required during operation, and on the other hand a simple adaptation to the number of phases of the electrical machine is possible by setting the phase offset, for example, to Phase shift = 360°N phases is set. Fig. Figure 2 shows, using the example of a three-phase machine, how the current ripple in the star point current is significantly reduced by more than 50% by shifting the pulse pattern of phases U and W while leaving the pulse pattern of phase V unchanged. List of reference symbols 1 N-phase machine in star connection 1a star point 2 inverters, e.g. bridge circuit or multi-level inverter 3 Intermediate circuit capacitor for partial power supply system 1, assigned to the inverter 3' Energy storage in the partial energy network 1 4 Energy storage in the partial energy network 2 4' DC link capacitor for partial power supply system 1 5 Switch for connecting the star point with the higher potential of the partial energy network 2 (optional) 6 Control device (SV) 11 Consumers in the partial energy network 1 12 consumers in the partial energy network 2

Claims

[1] Vehicle with an N-phase electrical machine, with a first partial on-board network (BN1) and with a second partial on-board network (BN2), wherein the electrical machine comprises a rotor and a stator system (1), the first partial on-board network comprises an inverter (2), the stator system (1) is assigned to the inverter, and the electrical machine can be operated with an inverter controller according to the principle of field-oriented control, wherein - a control unit (6) is assigned to the electrical machine, - the inverter (2) can be controlled by the control unit (6), and - the control unit (6) controls the inverter (2) in the form of a phase-shifted control method, characterized by , that - the phase-shifted control method smoothes the star point current, whereby the star point current is smoothed by a stray inductance and by a main inductance of the machine. [2] Vehicle according to claim 1, characterized by , that - the stator system is designed in a star connection, - the star point can be connected to the second sub-system directly or via a star point switch (5), - the inverter controller comprises a current controller and a star point controller, - the current controller regulates phase currents of the stator system, and - the starpoint controller regulates a starpoint current. [3] Vehicle according to claim 1 or 2, characterized by , that - the vehicle comprises measuring means for measuring a number of at least N-1 phase currents and the star point current or the vehicle comprises measuring means for measuring a number of at least N phase currents or the vehicle comprises a measuring means for measuring an intermediate circuit current, wherein the measured intermediate circuit current enables at least an indirect determination of the N phase currents of the machine based on voltage zero vectors, - and the control unit includes the inverter controller. [4] Vehicle according to one of the preceding claims, characterized by , that - a time offset between each phase of the electrical machine can be set. [5] Vehicle according to one of the preceding claims, characterized by , that - the phase-shifted control of the individual phases is carried out by a respective pulse pattern, and - the pulse patterns are shifted by an angle of 360° / N. [6] Vehicle according to one of the preceding claims, characterized by , that - the phase shift of the control method serves to smooth the star-point current by minimizing current ripples of the star-point current, and - the phase shift when controlling the star point current acts torque-free in the rotor, averaged over one revolution of the machine. [7] Vehicle according to one of claims 2 to 6, characterized by , that - the phase shift of the control method is used for an increased adjustment frequency in the pulse pattern generation in order to reduce a switching frequency of the inverter in order to improve efficiency or, at a given switching frequency, to extend the operating range of a pulse-width modulated control, which is limited by the speed of the machine, towards a higher speed of the machine, and - the starpoint controller provides a control value for the control process. [8] Vehicle according to one of claims 3 to 7, characterized by , that - the measuring instruments measure N currents (lu, Iv, Iw) of the electrical machine and - the control unit transforms the measured currents (lu, Iv, Iw) via an extended Clarke-Park transformation into a field-oriented current vector (Id, Iq, I0) with a flux-forming component (Id), with a moment-forming component (Iq) and with a zero current component (I0), whereby the star point current (IStar) is three times the zero current component. [9] Vehicle according to one of the preceding claims, characterized by , that - the inverter controller has the flux-forming component as a controlled variable, - the inverter controller has the torque-generating component as a controlled variable, - the inverter controller has the zero current component as a controlled variable, - the inverter controller has as a setpoint a first setpoint current value (I*d) for the flux-forming component, - the inverter controller has a second setpoint current value (I*q) for the torque-generating component as a setpoint value, - the inverter controller has a star point current as a setpoint (I*Star), - the inverter controller outputs a first stator control voltage (U*d) as a control variable, - the inverter controller outputs a second stator control voltage (U*q) as a control variable, and - the inverter controller outputs a third stator control voltage (U*0) as a control variable. [10] Vehicle according to claim 9, characterized by , that - the N-phase electrical machine is designed as a 3-phase electrical machine, - the inverter comprises six inverter switches (HS1, HS2, HS3, LS1, LS2, LS3), - the six inverter switches are arranged in three half-bridges for the three phases of the stator system, - the first stator control voltage, the second stator control voltage and the third stator control voltage can be transformed into phase voltages (Ua, Ub, Uc) of the stator system by an extended inverse Clarke-Park transformation, and - the inverter switches the phase voltage for each of the phases in a switching cycle according to the principle of pulse width modulation. [11] Vehicle according to one of claims 2 to 10, characterized by that when the star point switch is closed, the electrical machine - with a star point current (IStar) with a current direction from the star point to the second sub-system, electrical power is transferred from the first sub-system to the second sub-system and acts as a step-down converter. [12] Vehicle according to one of claims 2 to 11, characterized by that with direct connection or closed star point switch the electrical machine - with a star point current (IStar) with a current direction from the second sub-system to the star point, electrical power is transferred from the second sub-system to the first sub-system and acts as a boost converter. [13] Vehicle according to one of claims 9 to 12, characterized by , that - a starpoint current (IStar) can be set by specifying the starpoint target current and setting a starpoint control voltage, and - the electrical machine acts as a bidirectional power controller when the star point switch is closed.

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