Method for estimating parameters of electric motor and power filter

Abstract

The invention relates to a method for estimating electrical parameters of a power filter (203) connected to a power converter (201) and of an electric motor (205). The method comprises: injecting AC signals of different frequencies within a given time window in the power filter (203) and the motor (205); measuring a response signal of the power filter (203) and the motor (205) to the injected AC signal; determining a resonant frequency of the power filter (203) based on the measured response signal; a ratio of a leakage inductance of the power filter (203) to a leakage inductance of the motor (205) is calculated.

Description

Technical Field

[0001] The invention disclosed herein relates to a method for estimating parameters of an electric motor and a power filter, a power converter for controlling an electric motor, and a computer program product according to the appended claims. Background Art

[0002] Power converters such as adjustable speed drives (ASDs) are used with motors to achieve the controllable speed and power required for industrial applications. The ASD provides a pulse width modulated voltage at its output for this controllable motion.

[0003] The disadvantage of modulation is the voltage stress on the motor windings. Therefore, an output power filter such as a sine wave filter (SWF) is installed at the output of the frequency converter, the purpose of which is to filter out high frequency components from the current flowing through the motor. In doing so, the motor isolation and bearings can be protected from high electrical transients, and the temperature of the motor is reduced, which is important for reducing fire hazards and increasing the life of the motor.

[0004] When having an output filter connected to a motor, existing products cannot estimate the parameters of the motor or the filter. Therefore, only the motor parameters can be identified, and for this the user needs to disconnect the output filter from the system. This is a laborious, time-consuming and expensive process, sometimes requiring re-certification after installation.

[0005] In this context, when having an output filter connected to a motor, the possibility to estimate parameters of the motor and / or the filter is needed. Summary of the invention

[0006] The above identified problem is solved by a first aspect of the invention disclosed herein, which relates to a method for estimating electrical parameters of an electric motor and a power filter connected to a power converter such as an adjustable speed drive (ASD).

[0007] The method includes: injecting AC signals of different frequencies into the power filter and the motor within a given time window; measuring response signals of the power filter and the motor to the injected AC signals; determining the resonant frequency of the power filter based on the measured response signals; and calculating the ratio of the inductance of the power filter to the leakage inductance of the motor.

[0008] The invention is based on a series of test signals comprising a so-called "frequency sweep", which means that AC signals of different frequencies are injected into the power filter and the motor within a given time window in order to identify the presence of the power filter and, if present, to identify the individual parameters of the motor and the power filter.

[0009] AC signals of different frequencies may be induced (ie, a "frequency sweep") by varying the duty cycle (ie, the step size of the signal output from the power converter).

[0010] According to the present invention, AC signals of different frequencies can be injected once to determine the parameters of the power filter and the parameters of the motor; or injected twice, once for detecting the presence of the power filter and the other time for determining the parameters of the power filter and the parameters of the motor.

[0011] Based on the determined parameters, the power filter and / or the motor can be precisely controlled. In particular, the determined parameters can be used to identify different time constants of the power filter and the motor. Each time constant is determined by a specific parameter or parameter combination of the system.

[0012] According to an embodiment, in case no resonant frequency of the power filter is determined, the method continues with estimating electrical parameters only for the motor.

[0013] Since in the case where it is determined that there is no resonant frequency of the power filter, there is no power filter and no further steps are required to determine the parameters of the power filter. In other words, in the case where it is determined that there is no resonant frequency of the power filter, it is detected that there is no power filter connected to the motor. Therefore, a message signaling the missing power filter can be output.

[0014] According to another embodiment, the method is performed when the filter is connected to the motor.

[0015] By using the method disclosed herein, the power filter can remain connected to the motor. Therefore, there is no need to disconnect the power filter and the motor, and the system including both the power filter and the motor can be identified, monitored and / or controlled at any time (particularly periodically and / or automatically).

[0016] According to another embodiment, the sum of the leakage inductance of the power filter and the leakage inductance of the motor is measured by using a small signal step voltage test performed at a given DC offset level.

[0017] By using a small signal step voltage test, the leakage inductance “α” of the electrical system including the power filter and the motor can be determined according to equation (1), where “L f ” is the inductance of the filter, and “L σ ” is the leakage inductance of the motor.

[0018] α=L f +L σ (1)

[0019] According to another embodiment, inductor and capacitor (LC) resonance present in the signal measured by using the small signal step voltage test is eliminated by using a digital filter that isolates the time constant interval.

[0020] The digital filter is used to specifically suppress the resonance produced by the power filter, which superimposes the signal produced by the motor. Therefore, eliminating the LC resonance results in a clean signal that provides information for both the power filter and the motor.

[0021] According to another embodiment, the ratio of the leakage inductance of the motor and the leakage inductance of the filter is measured by using the injected AC signal at a certain level of DC offset level.

[0022] The DC offset level may be the same or may be derived from a small signal step voltage test.

[0023] According to another embodiment, the AC signals injected at different frequencies start from a minimum frequency up to a maximum frequency, and are monotonically increasing or decreasing, either randomly, or in a given order, and cover the entire range with a given step size.

[0024] Based on the signals measured in response to injecting AC signals of different frequencies, the resonant frequency can then be identified and used to calculate the ratio of the filter inductance to the motor leakage inductance according to equation (2).

[0025]

[0026] According to equations (3) and (4), by combining the results from the step test and the signals measured in response to injecting AC signals of different frequencies, the inductance of the power filter and the leakage inductance of the motor can be separated.

[0027]

[0028]

[0029] Based on equation (5), the filter capacitor or filter capacitance “C f ”

[0030]

[0031] According to another embodiment, other electrical parameters are calculated based on the ratio of the leakage inductance of the power filter (203) to the leakage inductance of the motor (205), wherein the other electrical parameters are selected from the following list: stator resistance (Rs), rotor resistance (Rr), motor leakage inductance (Lσ), magnetizing inductance (Lh), resonant frequency of the power filter, capacitance (Cf) and filter inductance (Lf), wherein the capacitance (Cf) is also estimated for each phase separately to take into account the imbalance of capacitor induction.

[0032] As long as the parameters of the power filter are known, the parameters of the motor can be identified by isolating the influence of the power filter from the measured signal.

[0033] According to an embodiment, at least the ESR of the filter capacitor is determined based on the magnitude of the peak resonant frequency of the filter capacitor.

[0034] By determining the magnitude of the peak at the resonant frequency to see if the peak is significantly reduced, which means an increase in total resistance, the equivalent series resistance (ESR) of the cable, the ESR of the inductor, and the ESR of the capacitor can be determined.

[0035] According to a further embodiment, the further electrical parameter is used to adjust a control parameter for controlling the motor and / or the power filter.

[0036] As long as the electrical parameters of the power filter and / or the motor are known, the power filter and / or the motor can be precisely controlled by adapting, for example, the control parameters of the power converter. The adaptation may be performed periodically and / or in response to a user command.

[0037] According to an embodiment, the degradation levels of the component parameters, capacitance, inductance and resistance (LC or LCL) of the power filter are periodically compared at different moments in time with respect to different given thresholds.

[0038] The present invention can be used to detect parameters of motors and power filters (Lf, Cf), as well as changes in parameters over time due to degradation / aging of various components of the motor and / or filter. Detection of parameters changing over time can be performed by acquiring a baseline of the parameters, which is stored in the drive or in the cloud for future comparison. At regular intervals, new parameters can be acquired and compared to the baseline parameters plus an acceptable tolerance band in order to detect offsets / drifts in the parameters. In the event that a measured motor parameter or filter parameter (baseline, actual parameter) has exceeded an acceptable limit / threshold, an alarm / signal and / or fault condition of the drive can be triggered.

[0039] According to another embodiment, a DC test is used to measure parameters of a stator resistance (Rs) and a filter resistance (Rf), a step test is used to measure motor leakage inductance (Lσ) and filter inductance (Lf) at four DC biases, and an AC test is used to measure parameters of a magnetizing inductance (Lh) and a rotor resistance (Rr), wherein the step test is performed using a current target of 25% of a nominal magnetizing current; wherein the nominal magnetizing current is identified by the step test and the AC test; wherein the AC signals of different frequencies are injected only once at a DC bias equal to the identified nominal magnetizing current; and wherein the step test is re-run with the nominal magnetizing current using a current target of 10% of the nominal motor current, so that the result of the re-run step test is the sum of the motor leakage inductance and the filter inductance.

[0040] According to a second aspect, the invention relates to a controller for controlling an electric motor, wherein the controller comprises a processor or an FPGA configured to perform an embodiment of the method disclosed herein.

[0041] According to a third aspect, the invention relates to a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to perform an embodiment of the method disclosed herein.

[0042] The effects and other embodiments of the method, controller and computer program product according to the present invention are similar to the effects and embodiments according to the above-described attachments. Therefore, reference is made to the above description of the attachments. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Further features, details and advantages of the invention are obtained from the wording of the claims and from the following description of exemplary embodiments based on the drawings. The drawings show:

[0044] Figure 1 is a flow chart of an algorithm based on the method disclosed herein;

[0045] Figure 2 It is the circuit diagram of the electrical system;

[0046] Figure 3 is based on Figure 2 An overview of the response characteristics of electrical system components to different frequencies;

[0047] Figure 4 is an overview of tests that can be performed using the methods disclosed herein;

[0048] Figure 5 An overview of the ASD, controller, power filter (LC or LCL) and motor. DETAILED DESCRIPTION

[0049] exist Figure 1 The flow chart of the algorithm 100 is shown in FIG. The algorithm 100 starts with a detection step 101 in which a so-called frequency sweep is performed by injecting AC signals of different frequencies within a given time window in the system in order to identify the presence of a power filter. Thus, the AC signal is injected using a plurality of different duty cycles provided by the power converter.

[0050] If a different resonant frequency is found in the signal output by the system in response to the injected AC signal (i.e., a frequency within a range calculated based on the injected AC signal, or a frequency specified in a lookup table), a flag or message is set or generated that signals that a power filter has been found. In response to the flag or message being issued that signals that a power filter has been found, all settings corresponding to the system including the motor and the power filter are enabled.

[0051] In a removal step 103, a moving average filter or moving average filtering is applied to the signal output by the system in response to the injected AC signal to remove the resonant component of the power filter from the current being measured.

[0052] In the changing step 105 , the frequency of the injected AC signal is changed, in particular, the frequency of the injected AC signal is increased.

[0053] In a determination step 107 , the resistance and inductance of the power filter are determined and a nominal magnetizing flux is calculated based on the resistance and inductance of the power filter, wherein a given parameter set is used at the beginning.

[0054] In a test step 109 , a test for determining the leakage inductance and the sum of the filter inductance and the leakage inductance with an increased pulse length and a current target lower than the estimated nominal magnetizing current is calculated.

[0055] The sum of the leakage inductance of the motor and the leakage inductance of the filter is measured using a small-signal step voltage test, for example performed with a certain DC offset level.

[0056] In an injection step 111, an AC signal of different frequencies for a given time window is injected into the system again for a second frequency sweep at a DC bias equal to the estimated nominal magnetizing current. The resonant frequency is extracted and the ratio of the leakage inductance of the filter to the leakage inductance of the motor is calculated by applying Equation 1.

[0057] α=L f +L σ(1)

[0058] The frequency sweep starts from the minimum frequency up to the maximum frequency and can be performed in any order, monotonically increasing or decreasing, or randomly to cover the entire range with a given step size.

[0059] In the separation step 113 , the leakage inductance of the filter and the leakage inductance of the motor are separated by applying Equation 3 and Equation 4.

[0060]

[0061]

[0062] In calculation step 115 , the filter capacitor or filter capacitance is calculated by applying Equation 5.

[0063]

[0064] exist Figure 2 In FIG. 2 , system 200 includes a power converter 201 forming an adjustable speed drive, a sine wave power filter 203 and a motor 205 , where Lf is the filter inductance, Cf is the filter capacitance, Rr is the rotor resistance of the motor, Lh is the magnetizing inductance Lh, and Lσ is the leakage inductance.

[0065] exist Figure 3 In the figure, a first graph 301 and a second graph 303 are shown, wherein the first graph 301 shows the frequency in [Hz] on its x-axis and the amplitude in [dB] on its y-axis, and the second graph 303 shows the frequency in [Hz] on its x-axis and the phase in [deg] on its y-axis.

[0066] As shown by curves 305, 307, 309 and 311 Figure 2 The frequency characteristics of the system are shown in Figure 1. By using tests for each frequency region, the parameters of the system can be fully identified.

[0067] In particular, frequencies in a low region such as, for example, from 0.1 Hz to 10 Hz result in a difference between response signals output from the power filters 307 , 309 and response signals output from the motors 305 , 311 , as indicated by arrows 313 and 315 .

[0068] exist Figure 4 4 shows a graph 400 which shows the frequency in [Hz] on its x-axis and the DC voltage on its y-axis.

[0069] A DC test 401 is performed at a very low frequency to measure the stator resistance Rs and the filter resistance Rf.

[0070] An AC test 403 is performed at a small frequency (eg, 1 / τm) to measure the magnetizing inductance Lh and the rotor resistance Rr.

[0071] A step test 405 is performed at an intermediate frequency (eg, 1 / τσ) to measure the motor leakage inductance Lσ and the filter inductance Lf at four DC biases.

[0072] A frequency sweep test 407 is performed at a higher frequency (eg, from 1 / τs to 1 / τp) to determine the ratio of the filter inductance Lf and the motor leakage inductance Lσ.

[0073] exist Figure 5 The system 500 is shown in . The system 500 comprises a power converter 501 in the form of an adjustable speed drive, a controller 503 , a power filter 505 (LC or LCL) and a motor 507 .

[0074] Here, the power converter 501 includes a mounting cable 509 , which may be non-insulated or insulated and may have different lengths depending on the specific arrangement of the system 500 .

[0075] The present invention is not limited to one of the above-described embodiments. The present invention can be modified in various ways.

[0076] All features and advantages resulting from the claims, the description and the drawings, including construction details, spatial arrangements and procedural steps, are essential to the invention per se and in any combination.

[0077] Reference numerals

[0078] 100 Algorithms

[0079] 101 Detection Steps

[0080] 103 Removal Steps

[0081] 105 Change Steps

[0082] 107 Determine the steps

[0083] 109 Test Steps

[0084] 111 Injection Steps

[0085] 113 Separation Steps

[0086] 115 Calculation steps

[0087] 200 Systems

[0088] 201 Power Converter

[0089] 203 Sine wave power filter

[0090] 205 Motor

[0091] 301 First Graphic

[0092] 303 Second Graphic

[0093] 305 Curve

[0094] 307 Curve

[0095] 309 Curve

[0096] 311 Curve

[0097] 313 Arrow

[0098] 315 Arrow

[0099] 400 graphics

[0100] 401 DC Test

[0101] 403 AC Test

[0102] 405 Step Test

[0103] 407 Frequency Sweep Test

[0104] 500 System

[0105] 501 Power Converter

[0106] 503 Controller

[0107] 505 Power Filter

[0108] 507 Motor

[0109] 509 Installing Cables

Claims

1. A method for estimating electrical parameters of a power filter (203, 505) and an electric motor (205, 507) connected to a power converter (201, 501), The method comprises: - injecting AC signals of different frequencies in the power filter (203, 505) and the motor (205, 507) within a given time window; - measuring the response signals of the power filter (203, 505) and the motor (205, 507) to the injected AC signal; - determining the resonant frequency of the power filter (203, 505) based on the measured response signal; - Calculating the ratio of the leakage inductance of the power filter (203, 505) to the leakage inductance of the motor (205, 507).

2. The method according to claim 1, characterized in that In the event that no resonant frequency of the power filter (203, 505) is determined, the method proceeds with estimating electrical parameters only for the motor (205, 507).

3. The method according to claim 1, characterized in that The method is performed when the power filter (203, 505) is connected to the motor (205, 507).

4. The method according to any one of the preceding claims, characterized in that The sum of the leakage inductance of the power filter (203, 505) and the leakage inductance of the motor (205, 507) is measured by using a small signal step voltage test performed at a given DC offset level.

5. The method according to claim 4, characterized in that Inductor and capacitor (LC) resonances present in the signal measured by using the small signal step voltage test are eliminated by using a digital filter that isolates the time constant interval.

6. The method according to claim 4 or 5, characterized in that: The ratio of the leakage inductance of the motor and the leakage inductance of the filter is measured by using the injected AC signal at a certain level of the DC offset level.

7. The method according to any one of the preceding claims, characterized in that The AC signals injected with different frequencies cover the frequency range from the minimum frequency to the maximum frequency, and are monotonically increasing or decreasing, or random, or in a given order, and cover the entire range with a given step size.

8. The method according to any one of the preceding claims, characterized in that Other electrical parameters are calculated based on the ratio of the leakage inductance of the power filter (203, 505) to the leakage inductance of the motor (205, 507), wherein the other electrical parameters are selected from the following list: stator resistance (Rs), rotor resistance (Rr), motor leakage inductance (Lσ), magnetizing inductance (Lh), capacitance (Cf) and filter inductance (Lf), wherein the capacitance (Cf) is also estimated for each phase separately to take into account the imbalance of capacitance.

9. The method according to claim 8, characterized in that The other electrical parameters are used to adjust control parameters for controlling the motor (205, 507) and / or the power filter (203, 505).

10. The method according to claim 9, characterized in that Adjustments to the control parameters are made periodically and / or in response to user commands.

11. The method according to claim 10, characterized in that The degradation levels of the component parameters, capacitance, inductance and resistance (LC or LCL) of the power filter are periodically compared at different moments in time relative to different given thresholds.

12. The method according to any one of the preceding claims, characterized in that The DC test is used to measure the parameters of the stator resistance (Rs) and the filter resistance (Rf). The step test is used to measure the parameters of the motor leakage inductance (Lσ) and the filter inductance (Lf) under four DC biases. The AC test is used to measure the parameters of the magnetizing inductance (Lh) and the rotor resistance (Rr). wherein the step test is performed using a current target of a predefined percentage of a nominal magnetizing current; wherein the nominal magnetizing current is identified by the step test and the AC test; wherein the AC signals of different frequencies are injected only once at a DC bias equal to the identified nominal magnetizing current; and Therein, the step test is re-run at the nominal magnetizing current using a current target that is a predefined percentage of the nominal motor current, such that the result of the re-run step test is the sum of the motor leakage inductance and the filter inductance.

13. The method according to any one of the preceding claims, characterized in that At least the ESR of the filter capacitor is determined based on the magnitude of the peak resonant frequency of the filter capacitor.

14. A controller for controlling an electric motor (205), in, The controller comprises a processor or an FPGA configured to perform the method according to any one of claims 1 to 13.

15. A computer program product comprising instructions which, when executed by a computer, cause the computer to perform the method according to any one of claims 1 to 13.

Images