METHOD FOR OPERATING AN ELECTRIC MACHINE

Abstract

The invention relates to a method for operating an electric machine (100) having a power converter (100) and multiple phases, in which method, phase currents flowing through the phases during operation of the electric machine (100) are determined and are used for continued operation of the electric machine (100), the phase currents being determined taking account of a fundamental wave and at least one harmonic of the current profile of each phase current.

Claims

1. A method for operating an electric machine (100) comprising a power converter (110) and a plurality of phases (U1, V1, W1, U2, V2, W2), the method comprising: during operation of the electric machine (100), determining phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2) flowing through the phases; and using the determined phase currents for the continued operation of the electric machine (100), wherein the phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2) are determined taking into consideration a fundamental and at least one harmonic of a current characteristic of each phase current.

2. The method as claimed in claim 1, wherein the phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2) are determined for a common time (t.sub.1).

3. The method as claimed in claim 1, wherein the phase currents are measured at different times (t.sub.1, t.sub.2) and converted into the phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2) for the common time (t.sub.1).

4. The method as claimed in claim 1, in which at least two of the phases (U1, V1, W1, U2, V2, W2) are driven so as to have a time offset in the power converter.

5. The method as claimed in claim 1, in which the plurality of phases (U1, V1, W1, U2, V2, W2) are divided into at least two three-phase current groups, and in each case two of the at least two three-phase current groups are driven so as to have a time offset in the power converter.

6. The method as claimed in claim 1, wherein the phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2) are split into phase currents of the fundamental and the at least one harmonic.

7. The method as claimed in claim 6, wherein only the phase currents of the harmonic are determined as the phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2).

8. The method as claimed in claim 1, in which closed-loop control of the electric machine (100) in respect of a preset torque takes place on the basis of the determined phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2).

9. The method as claimed in claim 1, in which a diagnosis of the electric machine (100) is performed on the basis of the determined phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2).

10. The method as claimed in claim 1, in which measured phase currents are plausibility-checked on the basis of the determined phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2).

11. An arithmetic logic unit (150), which is designed to perform all of the method steps of a method as claimed in claim 1.

12. (canceled)

13. A non-transitory, computer-readable medium containing instructions that when executed by a computer cause the computer to control an electric machine (100) that include a power converter (110) and a plurality of phases (U1, V1, W1, U2, V2, W2), by: during operation of the electric machine (100), determining phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2) flowing through the phases; and using the determined phase currents for the continued operation of the electric machine (100), wherein the phase currents (I.sub.U1, I.sub.V1, I.sub.W1, I.sub.U2, I.sub.V2, I.sub.W2) are determined taking into consideration a fundamental and at least one harmonic of a current characteristic of each phase current.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 shows, schematically, an electric machine comprising a power converter, in which a method according to the invention can be performed.

[0023] FIG. 2 shows, schematically, a sequence of a method according to the invention in a preferred embodiment.

DETAILED DESCRIPTION

[0024] FIG. 1 shows, schematically, an electric machine 100 comprising a power converter 110, in which a method according to the invention can be performed. The electric machine has (in a stator, not illustrated) six phases (phase windings), which form two three-phase current groups as subsystems and are denoted by U1, V1 and W1 and U2, V2 and W2. In this case, for example, there is an electrical phase shift of 30° between the two subsystems U1, V1, W1 and U2, V2, W2. A three-phase current group is characterized by an electrical connection of the phase windings in the stator, in this case, for example, a common neutral point, but is not electrically connected to phases of other three-phase current groups in the stator and can therefore have a dedicated drive scheme, which, in principle, can be different than drive schemes of other three-phase current groups.

[0025] The power converter 110 has two parts 111 and 112, which are each in the form of conventional bridge rectifiers, have six switching elements (not denoted), for example semiconductors such as MOSFETs, and each serve to drive one of the subsystems U1, V1, W1 or U2, V2, W2 (i.e. to connect it to the DC voltage terminals of the power converter). The power converter is interconnected with a positive and a negative terminal, for example into a vehicle power supply system of a vehicle as DC voltage terminals, via a capacitance, in this case, for example, in the form of two capacitors (not denoted). In addition, by way of example, an open-loop and/or closed-loop control unit 150 is shown which is used for driving the power converter 110, in particular for opening and closing the switching elements. It goes without saying that such a control unit can also be integrated in the power converter.

[0026] The clocked driving of the two subsystems U1, V1, W1 and U2, V2, W2 in this case takes place via two separate drive circuits 115 and 116, and with a time offset of, for example, 25 μs. In this case in each case one phase current I.sub.U1, I.sub.V1 and I.sub.W1 or I.sub.U2, I.sub.V2 and I.sub.W2 flows through the phases. These phase currents can be measured or detected, for example, by means of a current sensor or a current-measuring device—one such device is denoted schematically and by way of example by 120.

[0027] As already mentioned, an ideal measurement time is, for example, in the middle of the On time and/or in the middle of the Off time in relation to a PWM period of the mentioned clocked driving. Thus, a measurement of the respective phase current is in any case not possible for all phases at the same time.

[0028] FIG. 2 shows schematically a sequence of a method according to the invention in a preferred embodiment. In this regard, the electric machine 100 comprising the power converter 110 can be used as shown in FIG. 1. Shown in particular here is a closed-loop control scheme for the closed-loop control with respect to a specific current I.sub.dq,set (in so-called d-q coordinates), which specifies a torque of the electric machine.

[0029] A difference between the setpoint value I.sub.dq,set and the associated actual value I.sub.dq,act is given in a controller 210, in which corresponding manipulated values for the voltage, U.sub.dq,set, are determined. These can be set, if appropriate after conversion into actual voltage values for the phases U1, V1, W1 or U2, V2, W2, at the power converter 110, which results in corresponding phase currents and therefore a torque of the electric machine 100.

[0030] In this case, the actual phase currents I.sub.act,t1 and I.sub.act,t2 are measured, as mentioned with reference to FIG. 1, to be precise at different times t.sub.1 and t.sub.2. The phase currents I.sub.act,t1 in this case represent the actual values for the phase currents I.sub.U1, I.sub.V1 and I.sub.W1, and the phase currents I.sub.act,t2 represent the actual values for the phase currents I.sub.U2, I.sub.V2 and I.sub.W2. It should be noted here that, as already mentioned with reference to FIG. 1, the driving of the two subsystems U1, V1, W1 and U2, V2, W2 takes place with a time offset, which also goes along with a measurement of the phase currents in the two subsystems which is offset in time (in relation to the in each case ideal measurement time), i.e. at times t.sub.h1 and t.sub.2, respectively.

[0031] These actual phase currents I.sub.act,t1 and I.sub.act,t2 are converted into the present actual value I.sub.dq,act as part of a transformation 220, taking into consideration fundamental and harmonics. This will be explained in more detail below using corresponding equations and the exemplary system described up to now.

[0032] In this case, a symmetrical design of the electric machine and symmetrical driving should be assumed. The six phase currents are given as follows, purely under consideration of the fundamental:

[00001]$I_{U1}\left(t_1\right)={\hat{I}}_{U1}.Math.\sin \left(\omega t_1+\phi \right),$$I_{V1}\left(t_1\right)={\hat{I}}_{V1}.Math.\sin \left(\omega t_1-\frac{2\pi }{3}+\phi \right),$$I_{W1}\left(t_1\right)={\hat{I}}_{W1}.Math.\sin \left(\omega t_1-\frac{4\pi }{3}+\phi \right),$$I_{U2}\left(t_2\right)={\hat{I}}_{U2}.Math.\sin \left(\omega t_2-\frac{\pi }{6}+\phi \right),$$I_{V2}\left(t_2\right)={\hat{I}}_{V2}.Math.\sin \left(\omega t_2-\frac{2\pi }{3}-\frac{\pi }{6}+\phi \right)$$\mathrm{and}$$I_{W2}\left(t_2\right)={\hat{I}}_{W2}.Math.\sin \left(\omega t_2-\frac{4\pi }{3}-\frac{\pi }{6}+\phi \right).$

[0033] In this case, Î.sub.U1 specifies the amplitude of the phase current I.sub.U1; corresponding designations apply for the other phase currents. φ specifies a phase angle. The phase currents of the second subsystem at time t.sub.2 can now be converted to time t.sub.1 (in the example in this case t.sub.2−t.sub.1=25 μs):

[00002]$I_{U2}\left(t_1\right)={\hat{I}}_{U2}.Math.\sin \left(\omega t_1-\frac{\pi }{6}+\phi \right),$$I_{V2}\left(t_1\right)={\hat{I}}_{V2}.Math.\sin \left(\omega t_1-\frac{2\pi }{3}-\frac{\pi }{6}+\phi \right)$$\mathrm{and}$$I_{W2}\left(t_1\right)={\hat{I}}_{W2}.Math.\sin \left(\omega t_1-\frac{4\pi }{3}-\frac{\pi }{6}+\phi \right).$

[0034] During this conversion, the phase current is assumed to be purely sinusoidal, whereas possible harmonics are ignored and the information thereon is not available to the phase current closed-loop control owing to the assumption of a false fundamental, as already mentioned above. An increased torque ripple and a possible infringement of the safety target (on the basis of an ASIL specification) would be the consequence. With the method proposed here within the scope of the invention, now also (incident) harmonics can be taken into consideration as well as the fundamental of the phase current, and therefore the torque ripple can be reduced.

[0035] Owing to the use of all of the measured phase currents at times t.sub.1 and t.sub.2, it is possible to draw a conclusion on incident harmonics depending on the number of measured currents. Using the example of a 2×3-phase electric machine having an electrical phase shift of 30° between the subsystems (as explained in relation to FIG. 1), the following mathematical relationship can be derived for, for example, two further harmonics (in this case the fifth and the seventh were selected as incident harmonics) in phase currents:

I.sub.U1(t.sub.1)=Î.sub.U1,F.Math.sin(ωt.sub.1+φ.sub.U1,F)+Î.sub.U1,5.Math.sin(5.Math.ωt.sub.1+φ.sub.U1,5)+Î.sub.U1,7.Math.sin(7.Math.ωt.sub.1+φ.sub.U1,7).

[0036] In this case, Î.sub.U1,f, Î.sub.U1,5 and Î.sub.U1,7 specify the amplitudes of the phase currents in the fundamental, the fifth harmonic and the seventh harmonic, respectively. The corresponding phase angles are denoted by φU1,F, φU1,5 and φU1,7. In the same way, equations which consider the fifth and seventh harmonics can be specified correspondingly for the further five phases with corresponding times t.sub.1 and t.sub.2 and corresponding amplitudes and phase angles.

[0037] Using the symmetrical design of the electric machine and symmetrical driving, the assumptions can still be derived and met that the fundamental and the harmonics have the same amplitude, i.e. that the following applies for the fundamental, for example:

Î.sub.F=Î.sub.U1,F=Î.sub.V1,F=Î.sub.W1,F=Î.sub.U2,F=Î.sub.V2,F=Î.sub.W2,F.

[0038] The corresponding then applies to the fifth and seventh harmonics with the amplitudes Î.sub.5 and Î.sub.7. Likewise, the assumption can be derived or met that the phase angles are in each case identical, i.e. the following applies for the fundamental, for example:

φ.sub.F=φ.sub.U1,F=φ.sub.V1,F=φ.sub.W1,F=φ.sub.U2,F=φ.sub.V2,F=φ.sub.W2,F.

[0039] The corresponding then applies to the fifth and seventh harmonics with the phase shifts or phase angles φ.sub.5 and φ.sub.7.

[0040] With these assumptions, six equations result with six unknowns (the amplitudes: Î.sub.F, Î.sub.5, Î.sub.7 and the phase angles φ.sub.F, φ.sub.5, φ.sub.7), which can be determined by simple mathematics. If the phase currents are not all measured at the same time, as takes place, for example, during time-offset driving of the phases of the two subsystems (as explained above), now all six phase currents can be determined for one time together with their dominant harmonics.

[0041] In the following example, the phase current of phase U2 is calculated for time t.sub.1 together with the fifth and seventh harmonics:

[00003]$I_{U2}\left(t_1\right)={\hat{I}}_F.Math.\sin \left(\omega t_1-\frac{\pi }{6}+{\phi }_F\right)+{\hat{I}}_5.Math.\sin \left(5.Math.\omega t_1+{\phi }_5\right)+\widehat{I_7}.Math.\sin \left(7.Math.\omega t_1+{\phi }_7\right).$

[0042] Correspondingly, for example, the phase currents of the phases V2 and W2 can be calculated for the time t.sub.1 together with the fifth and seventh harmonics. In this way, therefore, the phase currents of the phases with time-offset driving can be converted very accurately into corresponding phase currents at the time of the driving of the phases without the time offset. In turn, it is then possible to determine from this the actual value Idq, with the result that the closed-loop control can run.

[0043] Furthermore, in the case of a 2×3-phase system, the equations

I.sub.U1+I.sub.V1+I.sub.W1=0 and I.sub.U2+I.sub.V2+I.sub.W2=0

[0044] need to apply, as a result of which, in addition to the six current equations, two further equations are present, with the result that it would also be possible for a third harmonic to be determined as well. In this way, eight equations with eight unknowns are obtained under the assumptions as described above. In general, in this way 2n−1 harmonics can be taken into consideration in addition to the fundamental for an n×3-phase system.

[0045] Within the scope of the invention, the electric machine is now preferably operated taking into consideration only the fundamental determined or current values of the fundamental, for example is subjected to closed-loop control in respect of its torque.