METHOD FOR OPERATING AN ELECTRICAL MACHINE AND ELECTRICAL MACHINE

Abstract

A method for operating an electric machine with a power source and with an electric motor and with an intermediary power converter, wherein an input current of the power source is converted into a multi-phase output current for the electric motor by means of a pulse width modulated control of a plurality of semiconductor switches of the converter, wherein during each period of control, for at least one phase, a pulse with a pulse duration is generated during a period duration, wherein in a period, a pulse is divided into a leading first half-pulse and a trailing second half-pulse with in each case half a pulse duration, and wherein the first half-pulse with a first displacement time and the second half-pulse with a second displacement time are mutually shifted in time within the period duration of the period.

Claims

1. A method for operating an electric machine with a power source, an electric motor, and an intermediary power converter, the method comprising: converting an input current of the power source into a multi-phase output current for the electric motor via a pulse width modulated control of a number of semiconductor switches of the converter; generating, during each period of the control, a pulse with a pulse duration for at least one phase during a period duration; dividing, in a period, a pulse into a leading, first half-pulse and a trailing, second half-pulse with half a pulse duration; and mutually shifting in time within the period duration of the period the first half-pulse with a first displacement time and the second half-pulse with a second displacement time.

2. The method according to claim 1, wherein the first half-pulse with the first displacement time is delayed in time within the period, and wherein the second half-pulse with the second displacement time is accelerated in time within the period.

3. The method according to claim 1, wherein a fraction of the period duration with an even-numbered denominator or half of the period duration is set for the pulse during the period duration as the first and/or second displacement time.

4. The method according to claim 1, wherein the first and second displacement time of the pulse are set equal in magnitude during the period.

5. The method according to claim 1, wherein the duration of the first and/or second displacement time of the pulse is modified for successive periods.

6. The method according to claim 5, wherein the duration of the first and/or second displacement time of the pulse is changed at random for each period.

7. The method according to claim 1, wherein the period duration of successive periods is varied.

8. The method according to claim 1, wherein the first and/or second displacement time for pulses of different phases are set differently.

9. The method according to claim 8, wherein the first and/or second displacement time for pulses of different phases are applied in mutually different periods.

10. An electric machine for a motor vehicle, with a power source, an electric motor and an intermediary power converter with a controller for carrying out the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

[0028] FIG. 1 illustrates an electrical machine with a power source and with an electric motor as well as an power converter connected therebetween,

[0029] FIG. 2 illustrates three phase windings of a three-phase electric motor of the machine in star connection,

[0030] FIG. 3 illustrates a bridge module of a bridge circuit of the converter for controlling a phase winding of the electric motor,

[0031] FIG. 4 illustrates an equivalent circuit diagram of the power source, and

[0032] FIG. 5 is a diagram of a PWM control of the phase windings.

DETAILED DESCRIPTION

[0033] FIG. 1 shows an electrical machine 2 for an electromotive adjustment system of a motor vehicle (not shown), for example a window lifter or a seat adjuster. For this purpose, the machine 2 comprises a three-phase electric motor 4, which is connected by means of a power converter 6 to a power source (voltage supply) 8. In this exemplary embodiment, the power source 8 comprises an energy storage device 10 inside the vehicle, for example in the form of a (motor vehicle) battery, as well as a (DC) intermediate circuit 12 which is connected to it and which at least partially extends into the converter 6.

[0034] The intermediate circuit 12 is essentially formed by a feed line 12a and a return line 12b, by means of which the converter 6 is connected to the energy store 10. The lines 12a and 12b are at least partially guided into the converter 6, in which a DC link capacitor 14 and a bridge circuit 16 are connected between the lines.

[0035] During operation of the engine 2, an input current I.sub.E supplied to the bridge circuit 16 is converted into a three-phase output current (motor current, phase current) I.sub.U, I.sub.V, I.sub.W

[0036] for the three phases U, V, W of the electric motor 4. The output currents I.sub.U, I.sub.V, I.sub.W, hereafter also known as phase currents, are guided to the respective phases (windings) U, V, W (FIG. 2) of a stator, not shown.

[0037] A star circuit 18 of the three phase windings U, V, W is shown in FIG. 2. The phase windings U, V and W are each connected with a respective (phase) end 22, 24, 26 to a respective bridge module 20 (FIG. 3) of the bridge circuit 16 and interconnected with the respective opposite end in a star point 28 as a common connection terminal. In the illustration in FIG. 2, the phase windings U, V and W are each shown by means of an equivalent circuit diagram in the form of an inductor 30 and an ohmic resistance 32 as well as a respective voltage drop 34, 36, 38. The voltage 34, 36, 38, which drops across the phase winding U, V, W, is schematically represented by arrows, and is the sum of the voltage drops across the inductor 30 and the ohmic resistance 32 as well as the induced voltage 40. The voltage 40 induced by a movement of the rotor of the electric motor 4 (electromagnetic force, EMF) is shown in FIG. 2 by means of a circle.

[0038] The star circuit 18 is triggered by means of the bridge circuit 16. The bridge circuit 16 with the bridge modules 20 is designed, in particular, as a B6 circuit. In this embodiment, during operation, a high (DC) voltage level of the feed line 12a and a low voltage level of the return line 12b are switched over at a high switching frequency in clocked fashion to each of the phase windings U, V, W. The high voltage level is in this case in particular an intermediate circuit voltage U.sub.ZK of the intermediate circuit 12, wherein the low voltage level is preferably a ground potential U.sub.G. This clocked control is implemented as a PWM control, represented in FIG. 1 by means of arrows, by a controller 42, with which control and/or regulation of the speed, the power and the direction of rotation of the electric motor 4 is possible.

[0039] The bridge modules 20 each comprise two semiconductor switches 44 and 46, which are shown schematically and exemplarily for the phase W in FIG. 2. The bridge module 20 is connected on the one hand with a potential terminal 48 to the feed line 12a and hence to the intermediate circuit voltage U.sub.ZK. On the other hand, the bridge module 20 is contacted with a second potential terminal 50 to the return line 12b and thus to the ground potential U.sub.G. Via the semiconductor switches 44, 46, the respective phase end 22, 24, 26 of phase U, V, W can be connected either to the intermediate circuit voltage U.sub.ZK or to the ground potential U.sub.G. When the semiconductor switch 44 is closed (conducting) and the semiconductor switch 46 open (non-conductive, blocking), the phase end 22, 24, 26 is connected to the potential of the intermediate circuit voltage U.sub.ZK. Accordingly, the phase U, V, W contacts the ground potential U.sub.G upon opening the semiconductor switch 44 and closing the semiconductor switch 46. As a result, it is possible by means of the PWM control to apply two different voltage levels to each phase winding U, V, W.

[0040] In FIG. 3, a single bridge module 20 is shown in simplified form. In this exemplary embodiment, the semiconductor switches 44 and 46 are implemented as MOSFETs (metal-oxide semiconductor field-effect transistors), each of which are switched over in clocked fashion by means of the PWM control between a switched-on state and a blocking state. For this purpose, the respective gate connections are routed to corresponding control voltage inputs 52, 54, by means of which the signals of the PWM control of the controller 42 are transmitted.

[0041] FIG. 4 shows an equivalent circuit diagram for the power source 8. During operation, the energy storage 10 generates a battery voltage U.sub.Bat and a corresponding battery current I.sub.bat for the operation of the power converter 6. In FIG. 4, the internal resistance of the energy storage 10 is shown as an ohmic resistor 56, and a self-inductance of the energy storage device 10 as an inductor 58. A shunt resistor 60 is connected in the return line 12b, at which the intermediate circuit voltage U.sub.ZK drops.

[0042] FIG. 5 subsequently shows and describes the waveform on the individual phase terminals 22, 24, 26, and how the voltage or PWM signals can advantageously be controlled or regulated at the individual phase windings U, V, W, as well as which consequences result therefrom with respect to the currents I.sub.U, I.sub.V, I.sub.W in the phase windings U, V, W and the input current I.sub.E of the external power source 8. In the embodiment in FIG. 5, the phase winding U is applied to a constant, low voltage potential, i.e. in particular, to ground potential U.sub.G. The phases V and W are supplied with the pulse width modulated control signals.

[0043] The diagram in FIG. 5 comprises five horizontal, superimposed sections. The time is plotted horizontally, i.e., on the x-axis or abscissa axis. By way of example, three periods 63, 64, 66 of the PWM control are shown in FIG. 5, a period 62, 64, 66 in each case having a period duration 68, 70 and 72, which here, for example, is between 20 s (microseconds) and 50 s.

[0044] FIG. 5 shows a PWM control in which the phase terminals 22, 24, 26 of the electric motor 4 are each actuated with a PWM (pulse) signal P.sub.V, P.sub.W of a different duty cycle. The current desired voltages U.sub.U, U.sub.V U.sub.W of the three phases U, V and W are shown in FIG. 5 with an instantaneous value 74, 76 and 78 respectively shown as a horizontal line. In this case, the desired voltage values vary over the time as a function of the rotational speed of the electric motor 4 in each case in the manner of a sinusoidal function. This causes the lines of the instantaneous values 74, 76 and 78 to move up and down periodically in the vertical direction, i.e., along the Y-axis or ordinate axis.

[0045] The saw tooth-shaped line in the upper section of the diagram represents a periodically linearly increasing and linearly decreasing counter reading 80 of a counter integrated in the controller 42. The points of intersection between the thresholds of the individual phases U, V, W which are fixed for a specific point in time, that is to say, the instantaneous values 74, 76, 78 with the saw tooth-like counter reading 80, represent the point in time for generating and terminating the (PWM) pulses P.sub.V, P.sub.W, with which the phase windings U, V, W are applied. This means that in the case of a high voltage threshold, the instantaneous value 74, 76, 78 is low, so that the sample time of the pulse-shaped pulse P.sub.V, P.sub.W is long, that is to say, that the respective phase V, W is supplied with the phase current I.sub.V, I.sub.W or applied with a voltage for a prolonged time. A counter reading 82, which is phase shifted by 180 relative to the counter reading 80, is shown by dashed lines in FIG. 5.

[0046] In the second section 84 and third section 86 of the diagram of FIG. 5, the voltage profiles at the phase terminals 22, 24 and 26 are shown in a time-resolved manner.

[0047] In the second section 84, a PWM control is shown in which in each period 62, 64 and 66, always the same pulses P.sub.V and P.sub.W are generated; in the following, therefore, only the first period 62 is described by way of example. In this exemplary embodiment, the phase W is switched on at the beginning of the period 62 and is switched off at a point in time 88. Delayed in time, a voltage is then applied to the phase winding W at a point in time 90. After a pulse duration T.sub.V, at a point in time 92, the pulse PV is terminated. Subsequently, the phase W is switched on at a point in time 94 up to the end of the period 62. The pulse P.sub.W thus essentially extends over in each case two adjacent periods 62, 64, 66 during a pulse duration T.sub.W. This voltage profile is periodically repeated for the PWM control in the second section 84 with a clock frequency (base frequency) f.sub.periode.

[0048] In the fourth section 96 and the fifth section 98 of FIG. 5, a respective time profile of the alternating current I.sub.res, I.sub.res resulting from the PWM control is shown in the power source, for example, in the intermediate circuit 12. Section 96 hereby shows the alternating current I.sub.res for the PWM control according to section 84, and section 98 shows the alternating current I.sub.res for a PWM control according to section 86.

[0049] In section 96, the alternating current I.sub.res is shown for an operating situation in which the flow direction of the phase currents I.sub.V and I.sub.W of the phases V and W correspond in terms of the directions from and to the star point 28. The amperage in phase V, for example, is 1 A, and in phase W, the amperage is 3 A. The amperage of phase U, for example, has 4 A and has a flow direction opposite phases V and W. At the beginning of period 62, thus, an alternating current I.sub.res with the amperage 3 A is generated up to the point in time 88. Accordingly, during the pulse duration TV, an alternating current I.sub.res of 1 A is generated.

[0050] During a period 62, 64, 66, the alternating current I.sub.res thus has a three-section current block or alternating current component I.sub.block, which periodically repeats. By way of example, only the middle current block I.sub.block is described below which corresponds to the pulse P.sub.V, wherein the two lateral current blocks, generated by the pulse P.sub.W, can be similarly described because of the linearity.

[0051] By Fourier transform, the alternating current component I.sub.block is mapped on a frequency spectrum F.sub.block(), wherein w is the angular frequency. By application of the displacement law, the following is obtained for the total spectrum F() of alternating-current components I.sub.block of several (n) periods of the period duration T.sub.periode

F()=.sub.ne.sup.jT.sup.periodeF.sub.block(),

[0052] wherein j is the imaginary unit. It follows for the clock frequency f.sub.period or the respective multiple nf.sub.periode that period or

e.sup.jT.sup.periode=e.sup.j2f.sup.periode.sup.nT.sup.periode=e.sup.j2n=1.

[0053] This results in that the frequency or alternating current components of the AC current I.sub.res add up for nf.sub.periode. This results in so-called EMC needles, which adversely affect the EMC behavior of the machine 2.

[0054] A method for reducing the EMV needles is described below with reference to sections 86 and 98 of FIG. 5. In the embodiment of FIG. 5, the pulses P.sub.V and P.sub.W in period 64 are divided in each case into two half-pulses P.sub.V1 and P.sub.V2 and P.sub.W1 and P.sub.W2 The half-pulses P.sub.V1 and P.sub.V2 and P.sub.W1 and P.sub.W2 in this case each have a pulse duration T.sub.V or T.sub.W, which correspond to the respective half, original pulse duration T.sub.V or T.sub.W. In principle, the method is applicable to all three phases U, V, W. In the embodiment of FIG. 5, phase U is, however, by way of example, permanently at low ground potential U.sub.G so that no displacement takes place.

[0055] The half-pulses P.sub.V1, P.sub.W1, P.sub.V2, P.sub.W2 are then displaced by a respective displacement time within the period 64. The half-pulses P.sub.V1, and P.sub.W1 are in this case delayed in time by means of a displacement time .sub.T1 as compared to the unshifted pulses P.sub.V and P.sub.W of section 84 in this embodiment. The half-pulses P.sub.V2 and P.sub.W2 are temporally accelerated in time with a displacement time .sub.T2 so that they lead the respectively associated half-pulse P.sub.V1 and P.sub.W1 during the period duration 70.

[0056] The displacement times .sub.T1 and .sub.T2 are equal in magnitude in the illustrated embodiment. In particular, the displacement times .sub.T1 and .sub.T2 are equal in magnitude to half the period duration 70. The following applies:

e.sup.j2f.sup.periode.sup.nT.sup.periode.sup./2=e.sup.jn=1.

[0057] which means that due to the time shift, a phase shift of 180 is produced by means of the displacement times .sub.T1 and .sub.T2. In other words, the counter reading 80 is converted into the counter reading 82.

[0058] The resulting alternating current I.sub.res thus has a phase sequence, which is shifted by 180 during the period 64. As can be seen comparatively clearly in section 98, the periodicity of the alternating current I.sub.res or its alternating-current components is hereby disturbed. Thus, the alternating-current components no longer add up for nf.sub.periode, but instead are distributed over a plurality of frequency components. In the modulation scheme of section 86, preferably such a pulse displacement means is performed each second period by means of the displacement times .sub.T1 and .sub.T2.

[0059] The invention is not limited to the embodiment described above. Rather, other variants of the invention can also be derived from those skilled in the art without departing from the scope of the invention. In particular, all the individual features described in connection with the exemplary embodiment can also be combined with one another in another manner without departing from the subject matter of the invention.

[0060] For example, it is equally conceivable to vary the period durations 68, 70, 72 such that the periods 62, 64 and 72 have different period durations. It is also conceivable, for example, that a plurality of pulses P.sub.V, P.sub.W of consecutive periods 68, 70, 72 are shifted in time. It is essential that the active time per phase U, V, W, i.e., the pulse duration, remain substantially constant during a period 62, 64, 66. This results only in a disruption of the periodicity of the alternating current I.sub.res, but not of the operation of the electric motor 4.

[0061] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.