Method for determining current-dependent inductances of a multiphase electrical machine and frequency converter

Abstract

Method for determining current-dependent inductances of a multiphase electrical machine, comprising the following steps: - Generating phase voltages for the multiphase electrical machine by means of pulse width modulation such that a voltage pulse (P) is generated in one of the phase voltages, - Measuring an emerging current waveform (i(t)) during the voltage pulse (P), and - Determining the current-dependent inductances as a function of the measured current waveform (i(t)).

Description

[0001] The publication DEGEL, Jan Philipp [ua]: A phase based approach for machine inductance estimation via current slope detection of an inverter fed IPMSM. In: 2021 22nd IEEE International Conference on Industrial Technology (IClT), 10-12 March 2021, Valencia, Spain. 2021, pp. 134-141. ISBN 978-1-7281-5731-3. DOI: 10.1109 / ICIT46573.2021.9453555 discloses a method for determining current-dependent inductances of a multi-phase electrical machine, in which a voltage pulse is generated within a single cycle of a pulse width modulation, the current-dependent inductances being determined as a function of an emerging current curve.

[0002] DE 10 2020 200 597 A1 discloses a method for determining current-dependent inductances of a multiphase electrical machine, in which a plurality of voltage pulses are generated at different current intensities. The resulting current changes at the different current intensities are evaluated to determine the current-dependent inductances.

[0003] The invention is based on the object of providing a method for determining current-dependent inductances of a multi-phase electrical machine and a frequency converter which can determine the current-dependent inductances quickly and reliably.

[0004] The method is used to determine current-dependent inductances of a multi-phase, in particular three-phase, electrical machine and comprises the steps of: generating phase voltages for the multi-phase electrical machine by means of pulse width modulation such that a, in particular single, voltage pulse is generated in at least one of the phase voltages, measuring an emerging current profile during the, in particular single, voltage pulse, and determining the current-dependent inductances as a function of the measured current profile.

[0005] The step of measuring the resulting current waveform during the voltage pulse comprises the following substeps: measuring or sampling a respective current value at a plurality of different discrete points in time or sampling points during the generation of the voltage pulse. The step of determining the current-dependent inductances as a function of the measured current waveform comprises the following substeps: determining the current-dependent inductances as a function of the measured respective current values.

[0006] To determine the current-dependent inductances, only a single voltage pulse is generated.

[0007] In one embodiment, the voltage pulse is generated in such a way that a current profile is caused in a torque-forming axis of the multi-phase electrical machine and / or in a field-forming axis of the multi-phase electrical machine.

[0008] The voltage pulse is generated in such a way that an instantaneous voltage value or an amplitude of the voltage pulse changes, in particular decreases, during the duration of the voltage pulse.

[0009] The instantaneous voltage value or the amplitude of the voltage pulse is changed during the duration of the voltage pulse in such a way that the difference between successive measured current values ​​is as constant as possible.

[0010] In one embodiment, a rotor of the electric machine is not blocked during the determination of the current-dependent inductances.

[0011] The voltage pulse is generated during a large number of cycles, for example between 2 and 100 cycles, of the pulse width modulation.

[0012] In one embodiment, the machine is a three-phase electrical machine, in particular a three-phase synchronous machine.

[0013] In one embodiment, the current-dependent inductances are selected from the group: differential series inductances, differential shunt inductances, and cross-saturation.

[0014] The frequency converter according to the invention serves to control a multi-phase electrical machine, wherein the frequency converter has three half-bridges, each with at least two switching means, wherein the frequency converter is designed to carry out a method described above.

[0015] The invention is described in detail below with reference to the drawings. Fig. 1 a frequency converter designed to carry out the method according to the invention, Fig. 2 a time course of currents and voltages during the determination of current-dependent inductances of a multi-phase electrical machine, and Fig. 3 determined current-dependent inductances as a function of current.

[0016] Fig. 1 shows a frequency converter 1 with a control unit 3, which controls the operation of the frequency converter 1 such that it executes the method according to the invention. The frequency converter 1 is conventionally designed to control an electrical machine in the form of a three-phase synchronous machine 2. The frequency converter 1 conventionally has three half-bridges B1, B2, B3, each fed from an intermediate circuit voltage UZK, each with two switching devices S1, S2; S3, S4; S5, S6. The frequency converter 1 is designed to execute a method described below.

[0017] The method is used to determine current-dependent inductances, for example in the form of differential series inductances Ld, differential shunt inductances Lq, and cross saturation.

[0018] To determine the current-dependent inductances, three phase voltages U, V, W are generated for the three-phase electrical machine 2 by means of pulse width modulation in such a way that a voltage pulse P is generated in one of the phase voltages U, V, W, see Fig. 2.

[0019] Then, an emerging current waveform (i(t)) is measured during the voltage pulse P and the current-dependent inductances, for example Ld, Lq, are calculated depending on the measured current waveform i(t).

[0020] The measurement of the resulting current curve i(t) during the voltage pulse P comprises the following steps, see Fig. 2: Measuring a respective current value i1 to i n at a plurality of different discrete times t1 to t nduring the generation of the voltage pulse P, where n is a natural number greater than 1. The determination of the current-dependent inductances as a function of the measured current curve i(t) comprises the following steps: Determination of the current-dependent inductances as a function of the measured respective current values ​​i1 to i n .

[0021] To determine the current-dependent inductances, only a single voltage pulse P can be generated, wherein the voltage pulse P is generated in such a way that a current curve is caused in a torque-forming axis of the electrical machine 2 and / or in a field-forming axis of the electrical machine 2.

[0022] Contrary to what is shown, the voltage pulse P can be generated in such a way that an instantaneous voltage value of the voltage pulse P changes during the duration of the voltage pulse P, for example in such a way that between successive measured current values ​​“i n-1' and 'i n “ a difference that is as constant as possible.

[0023] A rotor 5, see Fig. 1, the electrical machine 2 is not blocked during the determination of the current-dependent inductances.

[0024] The voltage pulse P is generated during a large number of pulse width modulation cycles.

[0025] Differential inductances in an electrical machine depend on the current operating point, specifically on the current flow. These inductances are used for various control calculations, such as current controller gain and voltage feedforward control.

[0026] The tracking of differential inductances is achieved, for example, using a predefined characteristic curve, which is stored in a motor table depending on the motor type. In the current state of the art, the characteristic curve is measured and stored offline. Errors are likely here, as automated measurement is often not available.

[0027] According to the invention, the determination of the differential inductances is achieved by outputting a fixed target voltage in the d- and q-direction, whereby a current increase is evaluated in each PWM period of a pulse width modulation or another period.

[0028] Fig. Figure 2 shows the course of current i and voltage U during the detection and identification process. The actual detection process begins at t = 0 and ends at t = 1.2 ms (12 10 -4 s in the figure).

[0029] The increased gradient of the current curve i(t) during the measurement at constant voltage is clearly visible. This indicates a decreasing inductance.

[0030] Using the voltage equation Uq=R⋅Iq+Lq⋅dIqdt+ωel⋅ΨPM can now be calculated for each measuring step n the inductance L q(q) can be calculated by applying the formula to L q is converted. For U q The specified voltage or a measurement of the resulting voltage is used here. The terms R ·I q and ω el ·ψ PM are calculated since both R and ψ PM are known from the engine data. ω el can be measured, for example, by a suitable feedback system. However, one can also assume that ω el is very small and thus the influence of ψ PM neglect.

[0031] To express it more precisely, the equation is discretized by assuming that a voltage change in a cycle leads to a current change. The equation then becomes: Δiq=LqΔuq+0.5⋅R⋅Δiq+ωel⋅ψPM. Then switched to L q and measured between two discrete points, the inductance or the current-dependent characteristic curve is obtained. The term R · Δi q can be neglected if necessary. The term ω el ·ψ PM may also be very small because the measurement is typically carried out at standstill, and can therefore also be neglected if necessary.

[0032] Due to the very short measurement time of approximately 1 ms, this method can also be carried out in the q-direction, since the accelerations during this time are negligible and even motors with very short acceleration times do not reach critical speeds.

[0033] Fig. Figure 3 shows a determined current-dependent inductance Lq as a function of the current Iq. A cross symbolizes a discrete measurement step n or a reference point of the entire measurement.

[0034] As shown, the sampling points are not evenly distributed, as the gradient of the current waveform is not constant and the sampling points are determined at identical time intervals. This effect can be counteracted, for example, by decreasing the voltage of the voltage pulse P.

[0035] To be based on the Fig. In order to generate an equidistant distribution at the support points shown in Figure 3, equidistant measured values ​​can be interpolated linearly or as a spline based on the measured values ​​at the support points, or approximated as a whole by a polynomial function using a least squares method or other fitting algorithms.

Claims

[1] Method for determining current-dependent inductances (L d , L q ) of a multi-phase electrical machine (2), comprising the steps: - generating phase voltages (U, V, W) for the multi-phase electrical machine (2) by means of pulse width modulation such that a voltage pulse (P) is generated in one of the phase voltages (U, V, W), - measuring a current waveform (i(t)) during the voltage pulse (P), and - Determine the current-dependent inductances (L d , L q ) depending on the measured current curve (i(t)), - where to determine the current-dependent inductances (L d , L q ) only a single voltage pulse (P) is generated, - wherein the voltage pulse (P) is generated during a plurality of pulse width modulation cycles, - wherein the measurement of the resulting current waveform (i(t)) during the voltage pulse (P) comprises the following steps: - Measuring a respective current value (i1 to i n ) at a plurality of different discrete points in time (t1 to t n ) during the generation of the voltage pulse (P), - where the determination of the current-dependent inductances (L d , L q ) depending on the measured current curve (i(t)) has the following steps: - Determine the current-dependent inductances (L d , L q ) depending on the measured current values ​​(i1 to i n ), - wherein the voltage pulse (P) is generated in such a way that an instantaneous voltage value of the voltage pulse (P) changes during the duration of the voltage pulse (P), and - wherein the instantaneous voltage value of the voltage pulse (P) is changed during the duration of the voltage pulse (P) in such a way that between temporally successive measured current values ​​(i1 to i n ) sets a difference that is as constant as possible. [2] Method according to claim 1, characterized by , that - the voltage pulse (P) is generated in such a way that a current curve is caused in a torque-forming axis of the multi-phase electrical machine (2) and / or in a field-forming axis of the multi-phase electrical machine (2). [3] Method according to one of the preceding claims, characterized by , that - a rotor (5) of the electrical machine (2) during the determination of the current-dependent inductances (L d , L q ) is not blocked. [4] Method according to one of the preceding claims, characterized by , that - the machine (2) is a three-phase electrical machine, in particular a three-phase synchronous machine. [5] Method according to one of the preceding claims, characterized by , that - the current-dependent inductances are selected from the set: - differential series inductances (L d ), - differential shunt inductances (L q ), and - Cross saturation. [6] Frequency converter (1) which is designed to control a multi-phase electrical machine (2), wherein the frequency converter (1) has three half-bridges (B1, B2, B3) each with at least two switching means (S1, S2; S3, S4; S5, S6), wherein the frequency converter (1) is designed to carry out a method according to one of the preceding claims.

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