Device for measuring the impedance of a load

Abstract

The present application relates to a device for measuring the impedance of a load, in particular a device for measuring the impedance of an electric load configured to be coupled to a controlled current source. The device comprises a first coupling node configured to be coupled to a first terminal of the load and a second coupling node configured to be coupled to a second terminal of the load. The device also comprises a transformer having a primary winding and a secondary winding and a capacitor connected in series between the first terminal of the secondary winding and the first coupling node. The second terminal of the secondary winding is connected to the second coupling node. The device also comprises a processing unit, which is configured to: control an excitation signal applied to the primary winding so as to cause a change in the input current of the load (300) corresponding to the excitation signal; measure the input current and input voltage of the load (300); and determine the impedance of the load (300) based on the measured input current and input voltage.

Description

Technical Field

[0001] The present disclosure relates to an apparatus for measuring the impedance of a load. Background Art

[0002] In electrical systems, it is useful to know the impedance and / or frequency response of a load (eg, a motor) in order to identify or verify the load to allow accurate control design for the load by a control engineer.

[0003] In a motor system, a power source such as an inverter or by direct on-line power is provided to the motor to supply a controlled (e.g., constant) current to the motor. In order to accurately control the motor and predict the motor's reaction to system disturbances, an accurate model describing the motor's performance (often called a motor plant model) is required. The motor plant model depends on the motor's operating point (input voltage, speed, and load), and can therefore only be accurately obtained when the motor's rotor is magnetized and rotating.

[0004] In some systems where the power source is an inverter, the inverter may perform a frequency response analysis during or immediately after an “identification run” for control tuning. Systems where the motor is not powered by an inverter (such as where the motor is connected to the power source in direct-on-line power mode) do not typically perform a frequency response analysis. Summary of the invention

[0005] The present disclosure describes a device for measuring the impedance of an electrical load (e.g., a motor), wherein the device can be coupled in parallel with the load and can measure the impedance of the load regardless of the type of power source of the load. The frequency response of the load can be determined based on the measured impedance.

[0006] According to a first aspect, the present invention provides a device for measuring the impedance of an electrical load, the device being configured to be coupled to a controlled current source, wherein the device comprises: a first coupling node configured to be coupled to a first terminal of the load; a second coupling node configured to be coupled to a second terminal of the load; a transformer having a primary winding and a secondary winding; a capacitor connected in series between the first terminal of the secondary winding and the first coupling node, wherein the second terminal of the secondary winding is connected to the second coupling node; and a processing unit, wherein the processing unit is configured to: control an excitation signal applied to the primary winding so as to cause a change in an input current of the load corresponding to the excitation signal; measure the input current and input voltage of the load; and determine the impedance of the load based on the measured input current and input voltage.

[0007] The processing unit may be further configured to determine a frequency response of the load based on the determined impedance of the load.

[0008] The device may further include a controllable switching device configured to selectively couple the primary winding of the transformer to a voltage source to apply an excitation signal to the primary winding.

[0009] The processing unit may be configured to control the operation of the controllable switching device to generate the activation signal.

[0010] Alternatively, the device may comprise a controllable AC voltage source coupled to the primary winding of the transformer, wherein the processing unit is configured to control the controllable AC voltage source to generate the excitation signal.

[0011] For example, the excitation signal may include a sinusoidal sweep signal.

[0012] The excitation signal may include a linear frequency modulation signal.

[0013] The excitation signal may include a pseudo-random binary sequence (PRBS).

[0014] The processing unit may be configured to measure an input current and an input voltage of the load for a plurality of different excitation signal frequencies.

[0015] The processing unit may be configured to measure an input current and an input voltage of the load at a plurality of different operating points of the load.

[0016] The processing unit may include a digital signal processor, a microprocessor or a microcontroller.

[0017] According to a second aspect, the present invention provides an operating method of the device according to the first aspect, the method comprising: coupling the device in parallel with a load by coupling a first coupling node and a second coupling node of the device to a first terminal and a second terminal of the load, respectively, the load being coupled to a controlled current source; applying an excitation signal to a primary winding of a transformer, thereby causing a change in an input current of the load corresponding to the excitation signal; measuring the input current and input voltage of the load; and calculating the impedance of the load based on the measured input current and input voltage.

[0018] The method may further include determining a frequency response of the load based on the determined impedance of the load. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

[0020] Figure 1 is a schematic representation of an electrical system including a source and an electrical load, to which a device for measuring the impedance of the electrical load is coupled.

[0021] Figure 2is a schematic representation of an electrical system including a source and an electrical load to which an alternative device for measuring the impedance of the electrical load is coupled. DETAILED DESCRIPTION

[0022] First reference Figure 1 , an electrical system is generally shown at 100 and includes a source 200 coupled to an electrical load 300. An apparatus 400 for measuring the impedance of the electrical load 300 and determining the frequency response of the electrical load 300 is coupled to the electrical load 300 in a manner that will be explained in more detail below.

[0023] Source 200 is a controlled current source and is operable to output a controlled current to load 300. The controlled current may be, for example, a constant current. As will be appreciated by those skilled in the art, in order for source 200 to output a constant current, the current output by source 200 must be controlled. Therefore, in the present disclosure, the term "controlled current" should be understood to include a constant current. However, the term "controlled current" should not be interpreted as being limited to a constant current. As will be appreciated by those skilled in the art, source 200 may control many different parameters of the current that may be controlled. For example, source 200 may be configured to limit the output current it outputs to no more than a maximum value.

[0024] The electrical load 300 may be, for example, an electric motor. In the subsequent discussion, an electric motor will be used as an example of a load. However, those skilled in the art will appreciate that the apparatus 400 may be used to measure the impedance of other types of electrical loads (e.g., power electronic components, DC-DC converters, inverters, rectifiers, etc.), and may also determine their frequency response.

[0025] The device 400 includes a first coupling node 402 and a second coupling node 404, through which the device 400 can be coupled to the first terminal 302 and the second terminal 304 of the load 300. The first terminal 302 and the second terminal 304 of the load 300 are also coupled to the source 200. Therefore, the device 400 is coupled in parallel with the source 200 and the load 300.

[0026] The device 400 includes a transformer 406 having a primary winding 408 and a secondary winding. A capacitance 412 (e.g., a capacitor) is connected in series between a first coupling node 402 and a first terminal 414 of the secondary winding. A second terminal 416 of the secondary winding is connected to a second coupling node 404. Thus, the capacitance 412 and the secondary winding are connected in series between the first coupling node 402 and the second coupling node 404.

[0027] The primary winding 408 of the transformer 406 has a first terminal 418 and a second terminal 420. The second terminal is connected to ground, while the first terminal 418 is selectively coupled to a voltage source through a controllable switching device 428, which is connected to a voltage source at a Figure 14, the controllable switch device 428 has a control terminal coupled to the output 422 of the processing unit 424. A bypass diode 426 is connected in anti-parallel to the primary winding 408, with its anode connected to the second terminal 420 of the primary winding 408 and its cathode connected to the first terminal 418 of the primary winding 408.

[0028] Controllable switching device 428 may be, for example, a MOSFET having a gate terminal coupled to output 422 of processing unit 424 , a source terminal connected to voltage source 430 and a drain terminal connected to first terminal 418 of primary winding 408 of transformer 406 .

[0029] The processing unit 424 may be, for example, a microcontroller, a digital signal processor (DSP), a microprocessor, or some other processing unit, and the processing unit 424 is configured to control the excitation signal applied to the primary winding 408 by controlling the operation of the controllable switching device 428. When the processing unit 424 outputs a control signal (e.g., a positive voltage) to turn on the controllable switching device 428, the first terminal 418 of the primary winding 408 is coupled to the voltage source 430, thereby generating a voltage across the primary winding 408. When the processing unit 424 outputs a control signal (e.g., 0 volts) to turn off the controllable switching device 428, the first terminal 418 of the primary winding 408 is decoupled from the voltage source 430. Therefore, by controlling the operation of the controllable switching device 428, the voltage across the primary winding 408 can be controlled to generate a desired voltage waveform across the primary winding 408. This voltage waveform can be considered as an excitation signal with which the primary winding 408 is excited.

[0030] The frequency of the excitation signal can be adjusted by adjusting the rate at which the processing unit 424 outputs the control signal to the control terminal of the controllable switching device 428, so that, using the appropriate control signal, the processing unit 424 can cause the desired excitation signal to be applied to the primary winding 408.

[0031] The stimulus signal is a time-varying electrical signal, such as a swept sine wave, a linear frequency modulated signal, a pseudo-random binary sequence (PRBS), or some other time-varying electrical signal.

[0032] When the excitation signal is applied to the primary winding 408 of the transformer 406, a corresponding varying (time-varying) voltage is generated across the secondary winding, which causes a varying current I2 to flow through the capacitor 412. The current I2 is combined with the controlled (e.g., constant) current I1 flowing from the source 200 to the load 300 (when the load is operating) to form a current I3. Thus, the current I3 includes a time-varying component having a frequency corresponding to the frequency of the excitation signal. As the frequency of the excitation signal changes (e.g., in the case where the excitation signal is a swept sine wave or a linear frequency modulation signal), the frequency of the time-varying component I2 of the current I3 changes accordingly.

[0033] The processing unit 424 is configured to measure the voltage across the load 300 and the current I3 through the load, and based on the measured voltage and current, determine or calculate the impedance of the load 300 at a specific operating point of the load 300 by dividing the measured voltage by the measured current. The processing unit 424 can make voltage and current measurements for a plurality of different frequencies of the excitation signal, and the processing unit 424 can determine or calculate the impedance of the load 300 at each excitation frequency. The determined or calculated impedance can then be used by the processing unit 424 to determine the frequency response of the load 300 by using known techniques or models that relate the frequency response of the load 300 to its impedance.

[0034] For example, where the load 300 is a motor, the motor can be arranged to operate at a selected operating point by setting the source 200 to output an appropriate current I1 for the selected motor load to achieve the desired speed and input voltage. Once the motor is operating at the desired operating point, an excitation signal (e.g., a sine wave sweep, a linear frequency modulation, or a PRBS) output by the processing unit 424 is applied to the primary winding 408 of the transformer 406 (by controlling the operation of the controllable switching device 428 as described above) to inject the current I2 into the system 100. The processing unit 424 measures the current I3 through the motor and the voltage across the motor terminals at a plurality of different excitation frequencies, and the processing unit 424 determines the impedance of the motor at these different excitation frequencies based on the measured current and voltage. The processing unit 424 can then determine the frequency response of the motor at the selected operating point based on the determined impedance. This process can be repeated for a plurality of different selected operating points of the motor in order to more completely characterize or verify the motor.

[0035] Reference now Figure 2 , generally shown at 500, is an alternative apparatus for measuring the frequency response of an electrical load electrical system, which is Figure 1 The alternative device 500 is coupled to the controlled current source 200 and the electric load 300 in the same manner as the device 400. The alternative device 500 includes a number of elements that are the same as the elements of the device 400, so Figure 1 and Figure 2 The same elements in the drawings are denoted by the same reference numerals.

[0036] Alternative device 500 and Figure 1 The apparatus 400 differs in that the bypass diode 426 and the controllable switching device 428 are replaced by a controllable AC voltage source 502 coupled to the output 422 of the processing device 424 .

[0037] In operation of the alternative device 500, the processing unit 424 controls the controllable AC voltage source 502 to output an excitation signal in the form of an AC voltage to the first winding 408 of the transformer 406, which induces an alternating voltage across the secondary winding, thereby causing a varying current I2 to flow through the capacitor 412. Figure 1 As in the device 400 of FIG. 4 , the current I2 is combined with the current I1 flowing from the source 200 to the load 300 (when the load is operating) to form the current I3. Therefore, the current I3 includes a component whose frequency corresponds to the frequency of the excitation signal. The processing unit 424 can control the controllable voltage source to generate an excitation signal whose frequency varies with time, for example, a swept sine wave or a linear frequency modulation signal. As the frequency of the excitation signal changes, the frequency of the I2 component of the current I3 changes accordingly.

[0038] As in Figure 1 As in the apparatus 400, the processing unit 424 is configured to measure the voltage across the load 300 and the current I3 through the load, and based on the measured voltage and current, determine or calculate the impedance of the load 300 at a specific operating point of the load 300. The processing unit 424 can make voltage and current measurements for a plurality of different frequencies of the excitation signal, and the processing unit 424 can determine or calculate the impedance of the load 300 at each excitation frequency. The determined or calculated impedance can then be used by the processing unit 424 to determine the frequency response of the load 300 using known techniques.

[0039] As can be appreciated from the foregoing, the apparatus 400, 500 disclosed herein and shown in the accompanying drawings provides a cost-effective mechanism for determining the impedance and frequency response of a load coupled to a controlled current (e.g., constant current) source during operation of the load, thereby facilitating accurate identification and verification of the load, and allowing control engineers to accurately design controls for the load. It should be noted that the apparatus 400, 500 described above and shown in the accompanying drawings can be used regardless of the source 200. For example, the apparatus 400, 500 can be used in electrical systems that use rectifiers as current sources, and in electrical systems that operate in direct-on-line mode.

[0040] Although electric motors are discussed as examples of loads in the above disclosure, those skilled in the art will appreciate that the apparatus 400, 500 of the present disclosure may be used to measure the frequency response of other types of electrical loads (e.g., power electronic components, DC-DC converters, inverters, rectifiers, etc.). In addition, although single-phase motors are given as examples of loads in the above discussion, the apparatus (400, 500) of the present disclosure is also applicable to three-phase motors, in which case the impedance of each phase may be characterized as described above using one apparatus per phase. Additionally or alternatively, a three-phase analysis method such as a DQ transform may be used to calculate the impedance in a rotating synchronous coordinate system.

[0041] It should be noted that the above embodiments are intended to illustrate rather than limit the present invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, and "a" or "an" does not exclude a plurality. Any reference signs in a claim should not be construed as limiting its scope.

Claims

1. A device (400, 500) for measuring the impedance of an electric motor (300) configured to be coupled to a controlled current source (200), wherein: The device (400, 500) comprises: A first coupling node (402) configured to be coupled to a first terminal (302) of the motor (300); a second coupling node (404) configured to be coupled to a second terminal (304) of the motor (300); A transformer (406) having a primary winding (408) and a secondary winding; a capacitor (412) connected in series between a first terminal (414) of the secondary winding and the first coupling node (402), wherein a second terminal (416) of the secondary winding is connected to the second coupling node (404); and A processing unit (424), wherein the processing unit (424) is configured to: controlling an excitation signal applied to the primary winding (408) so as to cause a change in input current of the motor (300) corresponding to the excitation signal; measuring the input current and input voltage of the motor (300) at a plurality of different operating points of the motor, wherein the motor is arranged to be able to output a suitable current for a selected load to obtain a desired speed and input voltage, so that the motor operates at different operating points, the operating points being defined by input voltage, speed and load; and Based on the measured input current and input voltage, the impedance of the motor (300) is determined.

2. The device according to claim 1, wherein: The processing unit is further configured to determine a frequency response of the electric motor based on the determined impedance of the electric motor.

3. The apparatus according to claim 1 or 2 further comprises a controllable switching device configured to selectively couple the primary winding of the transformer to a voltage source to apply the excitation signal to the primary winding.

4. The device according to claim 3, wherein: The processing unit is configured to control the operation of the controllable switching device to generate the activation signal.

5. The device according to claim 1 or 2, wherein: The apparatus comprises a controllable AC voltage source coupled to the primary winding of the transformer, wherein the processing unit is configured to control the controllable AC voltage source to generate the excitation signal.

6. The device according to claim 1 or 2, wherein: The excitation signal includes a sinusoidal scanning signal.

7. The device according to claim 1 or 2, wherein: The excitation signal includes a linear frequency modulation signal.

8. The device according to claim 1 or 2, wherein: The excitation signal comprises a pseudo-random binary sequence PRBS.

9. The device according to claim 1 or 2, wherein: The processing unit is configured to measure an input current and an input voltage of the motor for a plurality of different excitation signal frequencies.

10. The device according to claim 1 or 2, wherein: The processing unit includes a digital signal processor, a microprocessor or a microcontroller.

11. A method of operating the device (400, 500) according to any one of the preceding claims, the method comprising: The device (400, 500) is coupled in parallel with the motor (300) by coupling a first coupling node and a second coupling node (402, 404) of the device (400, 500) to a first terminal and a second terminal (302, 304) of the motor (300), respectively, wherein the motor (300) is coupled to a controlled current source (200), Applying an excitation signal to a primary winding (408) of a transformer (406) to cause a change in input current of the motor (300) corresponding to the excitation signal; measuring the input current and input voltage of the motor (300); as well as Based on the measured input current and input voltage, the impedance of the motor (300) is calculated.

12. The method according to claim 11, further comprising: Based on the determined impedance of the motor, a frequency response of the motor is determined.

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