Method for operating a frequency converter and frequency converter

Abstract

Method for operating a frequency converter (100) comprising the steps: - Generating phase voltages (U, V, W) with a predetermined amplitude and frequency from an intermediate circuit voltage (UZK) using pulse width modulation, wherein the phase voltages (U, V, W) are output between associated phase terminals (1, 2, 3) of the frequency converter (100), - wherein in a first operating mode of the frequency converter (100) the procedure comprises the steps: - Measuring voltages (S1, S2, S3) between the phase terminals (1, 2, 3) and a reference potential (UZK-), - Calculating the phase voltages (U, V, W) by differentiating the measured voltages (S1, S2, S3), and - Rules of the phase voltages (U, V, W) to the specified amplitude depending on the calculated phase voltages (U, V, W).

Description

[0001] The invention is based on the objective of providing a method for operating a frequency converter and a frequency converter that enable the most accurate possible adjustment of an output voltage, particularly when using so-called slim intermediate circuits and low rotating field frequencies.

[0002] The inventive method for operating a frequency converter comprises the following steps.

[0003] Conventionally, phase voltages with a predetermined amplitude and frequency are generated from an intermediate circuit voltage using pulse-width modulation, with the phase voltages being output between the corresponding phase terminals of the frequency converter. Reference is also made to the relevant technical literature in this regard.

[0004] During the first operating mode of the frequency converter, the following steps are performed: measuring voltages between the phase terminals and a reference potential, in particular in the form of a negative DC link potential, calculating the phase voltages or phase-phase voltages by taking the difference between the measured voltages, and adjusting the phase voltages to the specified amplitude depending on the calculated phase voltages.

[0005] In one embodiment, measuring the voltages between the phase terminals and the reference potential includes sampling the voltages and a low-pass filter of the sampled voltages, wherein the cutoff frequency of the low-pass filter is smaller than the fundamental frequency of the pulse width modulation, in particular smaller than half the fundamental frequency of the pulse width modulation.

[0006] In one embodiment, the first operating mode is activated (only) as soon as the specified frequency of the phase voltages is less than or equal to a threshold frequency.

[0007] In one embodiment, a second operating mode is activated as soon as the predetermined frequency of the phase voltages is greater than the threshold frequency, wherein during the second operating mode a fundamental frequency of the pulse width modulation is higher than during the first operating mode.

[0008] In one embodiment, during the second operating mode, the phase voltages are not regulated to the specified amplitude. Instead, a duty cycle of the pulse width modulation is set depending on a level of the intermediate circuit voltage and depending on the specified amplitude; that is, the amplitude is merely set, not regulated.

[0009] In one embodiment, the reference potential is a negative intermediate circuit potential.

[0010] In one embodiment, the phase voltages are controlled by means of an I-controller, where "I" stands for integral.

[0011] In one embodiment, the manipulated variable of the control or regulator of the phase voltages is a duty cycle of the pulse width modulation.

[0012] The frequency converter according to the invention is designed to carry out the method according to the invention. The frequency converter has means for generating phase voltages with a predetermined amplitude and a predetermined frequency from a DC link voltage by means of pulse width modulation, wherein the phase voltages are output between the associated phase terminals of the frequency converter. The frequency converter further has a measuring device for measuring voltages between the phase terminals and a reference potential. The frequency converter further has a control unit for calculating the phase voltages by subtracting the measured voltages and adjusting the phase voltages to the predetermined amplitude as a function of the calculated phase voltages.

[0013] In one embodiment, the frequency converter has an intermediate circuit capacitor formed from one or more film capacitors.

[0014] The invention is described in detail below with reference to the drawings. These show: Fig. 1 a schematic block diagram of a frequency converter according to the invention and Fig. 2 a circuit diagram of a controller and a measuring device of the in Fig. 1 frequency converter shown.

[0015] Fig. Figure 1 shows a schematic block diagram of a frequency converter 100 according to the invention.

[0016] The frequency converter 100 features conventional components 6 and 7 for generating phase voltages U, V, and W with a predetermined amplitude and frequency from an intermediate circuit voltage UZK using pulse width modulation. Component 6 is a rectifier for generating the intermediate circuit voltage UZK from a mains input voltage UN. Component 7 is a conventional inverter. Reference is made to the relevant technical literature for further information.

[0017] The phase voltages U, V and W are output between the corresponding phase terminals 1, 2 and 3 of the frequency converter 100.

[0018] The frequency converter 100 also has a measuring device 5, see also Fig. 2. For measuring voltages S1, S2, and S3 between phase terminals 1, 2, and 3, respectively, and a reference potential or negative DC link potential UZK-. The DC link voltage UZK corresponds to the potential difference between a positive DC link potential UZK+ and the negative DC link potential UZK-.

[0019] The measuring device 5 has a low-pass filter whose cutoff frequency is lower than the fundamental frequency of the pulse-width modulation, in particular lower than half the fundamental frequency of the pulse-width modulation. The sampled voltage, in this example voltage S1, is applied to an input of the measuring device 5. The low-pass filtered voltage S1' is present at the output of the low-pass filter. In the circuit shown, the measuring device 5 has a voltage divider with resistors 10 and 11 and a downstream Pi filter with capacitors 12 and 14 and a resistor 13.

[0020] The frequency converter 100 further comprises a control unit 8, which, in a first operating mode, serves to calculate the phase voltages U, V, and W by subtracting the measured voltages S1, S2, and S3 or the filtered voltages S1', S2', and S3'. The control unit further serves to regulate the phase voltages U, V, and W to the specified amplitude as a function of the calculated phase voltages U, V, and W.

[0021] The phase voltage U corresponds, for example, to S1-S2, the phase voltage V corresponds, for example, to S2-S3, and the phase voltage W corresponds, for example, to S3-S1.

[0022] The frequency converter 100 further includes an intermediate circuit capacitor 9, which is composed of one or more film capacitors.

[0023] The first operating mode is activated as soon as the predefined frequency of the phase voltages U, V, and W is less than or equal to a threshold frequency, for example, 10 Hz. Similarly, a second operating mode is activated as soon as the predefined frequency of the phase voltages U, V, and W is greater than the threshold frequency. During the second operating mode, the fundamental frequency of the pulse-width modulation is higher than during the first operating mode. The fundamental frequency during the second operating mode can be, for example, 8 kHz or 16 kHz, and the fundamental frequency of the pulse-width modulation during the first operating mode can be, for example, 2 kHz.

[0024] During the second operating mode, the phase voltages U, V and W are not regulated, but set or controlled by adjusting the duty cycle of the pulse width modulation depending on the level of the intermediate circuit voltage UZK and depending on the specified amplitude.

[0025] The phase voltages U, V and W are controlled by means of an I-controller 4, see Fig. 2, which can be part of the control unit 8. The quantity S represents, by way of example, the setpoint of the control or the specified amplitude, and the quantity I represents, by way of example, the actual value of the control.

[0026] The control variable for regulating the phase voltages U, V and W is a duty cycle of the pulse width modulation.

[0027] According to the invention, in the first operating mode, output voltage errors of the voltages U, V and W are reduced by means of control, particularly in frequency converters with a slim intermediate circuit and low switching frequency.

[0028] Conventionally, the amplitude of the phase voltages U, V, and W is set by adjusting a duty cycle based on the measured DC link voltage UZK, but not regulated in a closed control loop. This is sufficiently accurate for frequency converters with an electrolytic DC link.

[0029] At low switching frequencies in conjunction with a slim DC link or small DC link capacitances, very high voltage fluctuations occur in the DC link, which can cause the amplitude of the phase voltages U, V and W to deviate considerably from the specified amplitude if the amplitude is set depending on the measured DC link voltage UZK.

[0030] According to the invention, the phase voltages U, V, and W, or their amplitudes, are calculated from the differences between the three voltages S1, S2, and S3. If the calculated amplitude of the phase voltages U, V, and W deviates from the specified amplitude, this is corrected by means of the I-controller 4.

[0031] Since the phase voltages U, V, and W can only be a few volts at low rotational speeds, but the measurement is derived from the differences of the high voltages S1, S2, and S3, even a small measurement tolerance leads to a distortion of the measured U, V, and W. Therefore, the cutoff frequency of the voltage regulation is designed to be below the rotating field frequency (approx. 1 Hz) and only regulates an integral component.

[0032] By using the invention, the motor torque does not collapse at low PWM frequencies, with a narrow DC link, and at low rotating field frequencies, but is maintained independently of the PWM frequency due to the control mechanism. In other words, the phase voltages are regulated in U / F operation (asynchronous motor).

Claims

[1] Method for operating a frequency converter (100) comprising the steps: - Generating phase voltages (U, V, W) with a predetermined amplitude and frequency from an intermediate circuit voltage (UZK) using pulse width modulation, wherein the phase voltages (U, V, W) are output between associated phase terminals (1, 2, 3) of the frequency converter (100), characterized by , that - in a first operating mode of the frequency converter (100) the procedure comprises the following steps: - Measuring voltages (S1, S2, S3) between the phase terminals (1, 2, 3) and a reference potential (UZK-), - Calculating the phase voltages (U, V, W) by differentiating the measured voltages (S1, S2, S3), and - Rules of the phase voltages (U, V, W) to the specified amplitude depending on the calculated phase voltages (U, V, W). [2] Method for operating a frequency converter (100) according to claim 1, characterized by , that - measuring the voltages (S1, S2, S3) includes sampling the voltages (S1, S2, S3) and low-pass filtering the sampled voltages (S1, S2, S3), wherein the cutoff frequency of the low-pass filter is smaller than the fundamental frequency of the pulse width modulation, in particular smaller than half the fundamental frequency of the pulse width modulation. [3] Method for operating a frequency converter (100) according to any one of the preceding claims, characterized by , that - the first operating mode is activated as soon as the specified frequency of the phase voltages (U, V, W) is less than or equal to a threshold frequency. [4] Method for operating a frequency converter (100) according to claim 3, characterized by , that - a second operating mode is activated as soon as the specified frequency of the phase voltages (U, V, W) is greater than the threshold frequency, whereby during the second operating mode a fundamental frequency of the pulse width modulation is higher than during the first operating mode. [5] Method for operating a frequency converter (100) according to claim 4, characterized by , that - during the second operating mode, the phase voltages (U, V, W) are not regulated to the specified amplitude, and a duty cycle of the pulse width modulation is set depending on a level of the intermediate circuit voltage (UZK) and depending on the specified amplitude. [6] Method for operating a frequency converter (100) according to any one of the preceding claims, characterized by , that - the reference potential is a negative intermediate circuit potential (ICP-). [7] Method for operating a frequency converter (100) according to any one of the preceding claims, characterized by , that - the regulation of the phase voltages (U, V, W) is carried out by means of an I-controller (4). [8] Method for operating a frequency converter (100) according to any one of the preceding claims, characterized by , that - the control variable for regulating the phase voltages (U, V, W) is a duty cycle of the pulse width modulation. [9] Frequency converter (100) configured to carry out the method according to one of the preceding claims, comprising: - Means (6, 7) for generating phase voltages (U, V, W) with a predetermined amplitude and a predetermined frequency from an intermediate circuit voltage (UZK) by means of pulse width modulation, wherein the phase voltages (U, V, W) are output between associated phase terminals (1, 2, 3) of the frequency converter (100), - a measuring device (5) for measuring voltages (S1, S2, S3) between the phase terminals (1, 2, 3) and a reference potential (UZK-), and - a control unit (8) for calculating the phase voltages (U, V, W) by means of difference formation from the measured voltages (S1, S2, S3) and controlling the phase voltages (U, V, W) to the specified amplitude as a function of the calculated phase voltages (U, V, W). [10] Frequency converter (100) according to claim 9, comprising: - an intermediate circuit capacitor (9) formed from one or more film capacitors.

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