Method for operating the motor

Abstract

The invention relates to a method for operating an electric machine (100) having a converter (110) and a plurality of phases, wherein the phase currents flowing through the phases are determined during operation of the electric machine (100) and used for further operation of the electric machine (100), wherein the phase currents are determined taking into account a fundamental frequency and at least one harmonic of the current profile of each phase current.

Description

Technical Field

[0001] The invention relates to a method for operating an electrical machine having a converter and a plurality of phases, as well as to a computing unit and a computer program for carrying out the method. Background Art

[0002] When used in vehicles, electric machines can be operated as motors and / or generators on a converter, particularly a converter or inverter, fed by a DC link. Clocked actuation of switching elements, particularly semiconductor switching elements such as MOSFETs or IGBTs, is common for such inverters, also known as traction inverters, for example, using pulse-width modulation (PWM). This is particularly useful for achieving a desired voltage profile, for example one that is largely sinusoidal.

[0003] For electric motors (or drives) in the automotive sector, the ISO 26262 standard is often used, which defines so-called Automotive Safety Integrity Levels (ASILs) (in this case, the safety levels in vehicles). For electric motors with an ASIL classification, a safety load is usually assigned to the generated torque, meaning that the generated torque must have a predetermined accuracy.

[0004] For cost reasons, the motor torque can be determined from the measured phase currents and the corresponding motor equations, thus eliminating the need for torque sensors. However, it is crucial to detect the phase currents as accurately as possible to meet the requirements of ISO 26262. Phase currents typically enter into current regulation, which is used to achieve the rated torque. Therefore, inaccurately detected phase currents lead to inaccurately provided torque, which in turn can lead to violations of the safety objectives of ISO 26262.

[0005] For example, a method is known from DE 10 2012 217 116 A1, in which the phase currents are determined and partially also measured during operation of such an electric machine. Summary of the Invention

[0006] According to the invention, a method for operating an electric machine, as well as a computing unit and a computer program for carrying out the method are proposed, having the features of the independent patent claims. Advantageous embodiments are the subject matter of the dependent claims and the following description.

[0007] The present invention relates to a method for operating an electric machine having a converter and a plurality of phases. For example, the converter can be an inverter (or converter), in which a DC voltage is converted into an AC voltage by means of clocked activation of switching elements (e.g., semiconductor switches such as MOSFETs or IGBTs) at a clock frequency. This clocked activation of the switching elements is preferably used to generate a sinusoidal AC voltage, for which purpose, for example, so-called sine-delta modulation can be used.

[0008] Furthermore, in this method, the phase currents flowing through the phases during operation of the electric motor are determined and used for further operation of the electric motor, for example, for regulating the torque to be provided. As already mentioned, for this purpose, the phase currents can be measured, for example, by means of a suitable current measuring device, for example, measuring the voltage drop across a measuring resistor is conceivable.

[0009] When measuring phase currents, it is now necessary to consider that there are specific times at which the measurement should be performed in order to obtain the most accurate current value possible. For example, in the case of the aforementioned constant-time control, the ideal measurement time is either in the middle of the on-time and / or in the middle of the off-time relative to the PWM cycle. This can result in the individual phase currents not being measured at the same time. This is particularly the case when the phases are controlled with a time offset, that is, when there are at least two phases that are controlled with an offset relative to each other.

[0010] The latter is the case, for example, in a motor with two times three phases, where a time offset is used or used between the activation of the first three phases and the activation of the other three phases. The method will be explained in more detail later based on this example, but it goes without saying that the proposed method cannot be used for this one example, but can be transferred to any type of motor with multiple phases.

[0011] However, when regulating the torque of a motor that uses measured or current phase currents, it should be noted that the current phase currents for all phases at a specific and, in particular, the same point in time are required, in particular as a feedback variable. However, since, as already explained above, it is usually not possible to measure all phases exactly at the same point in time, the phase currents of all phases can be converted to the same point in time.

[0012] In this conversion, it has already been shown that inaccuracies arise when simply assuming a sinusoidal curve of the phase currents. This is caused in particular by the fact that the curve of the phase currents contains not only the fundamental wave, but also different harmonics that can contribute to different strengths.

[0013] Therefore, in the proposed method, the phase currents are determined by taking into account the fundamental and at least one harmonic of the current profile of each phase current, which is typically at least substantially sinusoidal. This preferably applies to cases in which the detected or measured phase currents are converted to another point in time, in particular to a common point in time. This prevents, for example, changes in the phase currents at harmonic-based measurement points in time from being assumed to belong to the fundamental and incorrectly included in the conversion, which would lead to erroneous values ​​for the phase currents.

[0014] For example, the number and type of harmonics considered in this case can be adapted to the individual motor or a specific motor type. For example, based on test measurements and / or simulations, it may have been determined that the fifth and seventh harmonics are particularly significant and require corresponding correction. In this case, the fifth and seventh harmonics can then also be considered in addition to the fundamental.

[0015] Phase currents determined at different times can then be converted to a common time and thus used for operation or regulation. In particular, only the fundamental phase current is determined or used as the phase current on which the operation or regulation is based.

[0016] While it is particularly preferred to regulate the electric motor to a predetermined torque based on the determined phase currents, it is also additionally or alternatively preferred to perform motor diagnostics based on the determined phase currents and / or to perform a plausibility check on the measured phase currents based on the determined phase currents. In the latter case, the determined phase currents are particularly to be understood as phase currents determined or calculated from the measured phase currents while taking into account the fundamental and harmonics. In these cases, the proposed method also enables particularly precise determination of the phase currents. As mentioned, advantageously, only the determined fundamental is used as the basis for regulation.

[0017] The proposed method is particularly suitable for electric motors used in vehicles, particularly as drive motors or traction motors, with relatively high onboard power supply voltages, for example 48 V or higher. However, it can also be used for other electric motors, particularly those relevant for ASIL, such as those used in electric steering.

[0018] A computing unit according to the present invention, for example a control unit of a motor vehicle, in particular a motor control unit or a control and / or regulating unit of an electric machine, is designed, in particular in terms of programming, to carry out the method according to the present invention.

[0019] It is also advantageous to implement the method according to the invention in the form of a computer program or computer program product having program code for executing all method steps, since this results in particularly low costs, especially when the execution control device is still used for other tasks and is therefore still available. Suitable data carriers for providing the computer program are, in particular, magnetic, optical, and electronic storage media such as hard disks, flash memories, EEPROMs, DVDs, etc. It is also possible to download the program via a computer network (Internet, intranet, etc.).

[0020] Further advantages and configurations of the invention are apparent from the description and the accompanying drawings.

[0021] The invention is schematically illustrated in the drawings based on exemplary embodiments and is described below with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 An electrical machine with a converter is schematically shown, in which the method according to the invention can be carried out.

[0023] Figure 2 In a preferred embodiment, the sequence of the method according to the invention is schematically illustrated. DETAILED DESCRIPTION

[0024] exist Figure 1 Schematically depicts an electric motor 100 with a converter 110, in which the method according to the present invention can be implemented. The electric motor (in a stator (not shown)) has six phases (windings), which form two three-phase current groups as subsystems and are denoted by U1, V1, and W1, and U2, V2, and W2. Here, there is, for example, a 30° electrical phase offset between the two subsystems U1, V1, W1, and U2, V2, W2. The three-phase current group is characterized by the electrical connection of the phase windings in the stator, here, for example, a common star point. However, it is not connected to the phases of the other three-phase current groups in the stator and can therefore have its own control scheme, which can, in principle, differ from the control schemes of the other three-phase current groups.

[0025] The converter 110 has two parts 111 and 112, each designed as a conventional bridge rectifier with six switching elements (not shown in greater detail), for example, semiconductor switches such as MOSFETs, each for controlling one of the subsystems U1, V1, W1 or U2, V2, W2 (e.g., for connection to the converter's DC terminals). The converter is connected to the vehicle's onboard power supply, for example, using a positive and a negative terminal as DC terminals, via capacitance, for example, in the form of two capacitors (not shown in greater detail). Furthermore, a control and / or regulating unit 150 is shown as an example, which is used to control the converter 110, in particular, to open and close the switching elements. Of course, such a control and / or regulating unit can also be integrated into the converter.

[0026] The clocked activation of the two subsystems U1, V1, W1 and U2, V2, W2 is carried out here via two separate activation circuits 115 and 116 and with a time offset of, for example, 25 μs. U1 , I V1 and l W1 or I U2 , I V2 and I W2 These phase currents can be measured or detected, for example, by means of current sensors or current measuring devices—such a current measuring device is indicated schematically and exemplarily by 120 .

[0027] As already mentioned, the ideal measuring time, for example, with respect to the aforementioned PWM period of clocked activation, lies in the middle of the on-time and / or the middle of the off-time. Therefore, it is impossible to measure the respective phase current at the same time for all phases.

[0028] exist Figure 2 In a preferred embodiment, the process of the method according to the invention is schematically shown. For this purpose, the following can be used: Figure 1 The motor 100 shown in FIG. 1 has a converter 110 . In particular, the motor 100 for adjusting the current I to a specific value is shown here. dq,soll In a control scheme (in so-called dq coordinates), the specific current describes the torque of the electric machine.

[0029] The setpoint value I is given in the controller 210 dq,soll and the corresponding actual value I dq,ist The difference between the two determines the corresponding set value U of the voltage dq,soll After conversion into actual voltage values ​​for phases U1 , V1 , W1 or U2 , V2 , W2 , these setpoint values ​​can be adjusted at converter 110 , which results in corresponding phase currents and thus a torque of electric machine 100 .

[0030] Here, the actual phase current I ist,t1 and I ist,t2 The measured value, such as the reference Figure 1 As mentioned above, the phase current I ist,t1 Represents the phase current I U1 , I V1 and l W1 The actual value of the phase current I ist,t2 Represents the phase current or I U2 , I V2 and I W2 Here, it should be noted that, as already mentioned Figure 1 As mentioned, the two subsystems U1, V1, W1 and U2, V2, W2 are controlled in a time-shifted manner, which also occurs with a time-shifted measurement of the phase currents in the two subsystems (relative to the respective ideal measurement time), namely at times t1 and t2.

[0031] Taking the fundamental and harmonics into account, the actual phase currents I ist,t1 and I ist,t2 Within the scope of transformation 220, it is converted into the current actual value I dq,ist This will be explained in more detail below on the basis of the corresponding equations and the exemplary system described so far.

[0032] A symmetrical design and symmetrical control of the motor should be assumed. Considering only the fundamental wave, the six phase currents result as follows:

[0033]

[0034] and

[0035] .

[0036] Here, use Description of phase current The corresponding contents apply to the other phase currents. The phase angle is shown. The phase current of the second subsystem at time t2 can now be converted to time t1 (in this example, Applicable):

[0037]

[0038] and

[0039] .

[0040] During this conversion, the phase currents are assumed to be purely sinusoidal, while any harmonics are ignored. Because of the incorrect assumption of the fundamental, information about these harmonics is not available for phase current control, as already mentioned. This results in increased torque ripple and a possible violation of safety targets (based on ASIL requirements). The method proposed within the scope of the present invention now allows (significant) harmonics to be taken into account in addition to the fundamental of the phase currents, thereby reducing torque ripple.

[0041] By using all measured phase currents at the time points t1 and t2, significant harmonics can be inferred from the number of measured currents. Take the example of a 2×3-phase motor with a 30° electrical phase offset between the subsystems (as described in relation to Figure 1 As explained above), the following mathematical relationship can be derived for two further exemplary harmonics in the phase current (the fifth and seventh harmonics are selected here as prominent harmonics):

[0042] .

[0043] Here, 、 and The amplitudes of the phase currents in the fundamental, fifth harmonic, and seventh harmonic are shown respectively. The corresponding phase angles are expressed as 、 and In the same way, the equations can be formulated for the other five phases using the corresponding times t1 and t2 and the corresponding amplitudes and phase angles, which equations take into account the fifth and seventh harmonics.

[0044] Due to the symmetrical design and symmetrical operation of the motor, the following can also be derived or assumed: the fundamental and harmonics have the same amplitude, i.e. for the fundamental, for example:

[0045] .

[0046] The corresponding content then applies to and The fifth and seventh harmonics of . Likewise, the following can be derived or assumed: the phase angles are respectively identical, i.e., for the fundamental, for example:

[0047] .

[0048] The corresponding content applies to phase offset or phase angle and The fifth and seventh harmonics of

[0049] Using these assumptions, we have a solution with six unknowns (amplitude: 、 、 and phase angle 、 、 ), the unknowns can be determined by simple mathematics. If the phase currents are not all measured at the same point in time, as is the case, for example, when the phases of two subsystems are controlled with a time offset (as explained above), all six phase currents, including their dominant harmonics, can now be determined for one point in time.

[0050] In the following example, the phase current of phase U2 is calculated for time t1 together with the fifth and seventh harmonics:

[0051] .

[0052] Accordingly, for example, the phase currents of phases V2 and W2 can be calculated for time t1 together with the fifth and seventh harmonics. Thus, in this way, the phase currents of phases actuated with a time offset can be converted very accurately to the corresponding phase currents at the time when the phases were actuated without a time offset. From this, the actual value Idq,ist can then be determined, allowing regulation to be performed.

[0053] Furthermore, for a 2 × 3 phase system, Eq.

[0054] and

[0055] This must be applied, so that in addition to the six current equations, two more equations exist, allowing the third harmonic to be determined. This results in eight equations with eight unknowns, as described above, under the aforementioned assumptions. In general, for an n×3-phase system, in addition to the fundamental, 2n-1 harmonics can be considered in this way.

[0056] Within the scope of the present invention, the electric machine is now preferably operated, for example its torque is regulated, taking into account only the determined fundamental frequency or the current value of the fundamental frequency.

Claims

1. A method for operating an electric machine (100) having a converter (110) and a plurality of phases (U1, V1, W1, U2, V2, W2), wherein at least two of the phases (U1, V1, W1, U2, V2, W2) are controlled in a time-shifted manner in the converter, wherein the phase currents (I U1 , I V1 、l W1 , I U2 , I V2 , I W2 ) is determined and used for further operation of the motor (100), The phase current (I U1 , I V1 、l W1 , I U2 , I V2 , I W2 ) is determined taking into account the fundamental frequency and at least one harmonic of the current profile of each phase current, The phase currents (I U1 , I V1 、l W1 , I U2 , I V2 , I W2 ), in which the phase currents at different time points (t1, t2) are measured and converted into the phase current (I U1 , I V1 、l W1 , I U2 , I V2 , I W2 ).

2. The method according to claim 1, wherein a plurality of phases (U1, V1, W1, U2, V2, W2) are subdivided into at least two three-phase current groups and two of the at least two three-phase current groups are controlled in the converter in a time-shifted manner.

3. The method according to claim 1, wherein the phase current (I U1 , I V1 、l W1 , I U2 , I V2 , I W2 ) is divided into a phase current of a fundamental wave and at least one harmonic.

4. The method according to claim 3, wherein as the phase current (I U1 , I V1 、l W1 , I U2 , I V2 , I W2 ) only determines the phase current of the fundamental wave.

5. The method according to any one of the preceding claims 1 to 4, wherein based on the determined phase current (I U1 , I V1 、l W1 , I U2 , I V2 , I W2 ) adjusts the motor (100) to a predetermined torque.

6. The method according to any one of the preceding claims 1 to 4, wherein based on the determined phase current (I U1 , I V1 、l W1 , I U2 , I V2 , I W2 ) performs diagnosis of the motor (100).

7. The method according to any one of the preceding claims 1 to 4, wherein based on the determined phase current (I U1 , I V1 、l W1 , I U2 , I V2 , I W2 ) performs a plausibility check on the measured phase currents.

8. A computing unit (150) which is set up to carry out all method steps of the method according to any one of the preceding claims 1 to 7 when it executes a computer program.

9. A computer program product comprising a computer program which, when executed on a computer unit (150), causes the computer unit (150) to carry out all method steps of the method according to any one of claims 1 to 7.

10. A machine-readable storage medium having a computer program stored thereon, said computer program being set up to perform all method steps of the method according to any one of the preceding claims 1 to 7 when it is executed on a computing unit (150).

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