Patent Publication Number: US-2022229100-A1

Title: Apparatus for measuring an impedance of load

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to an apparatus for measuring an impedance of a load. 
     BACKGROUND 
     In electrical systems it is useful to know the impedance and/or frequency response of a load such as an electric motor, in order identify or verify the load to permit accurate control design for the load by control engineers. 
     In an electric motor system an electric motor a source of electrical power such as an inverter or by a direct online power supply is provided to supply a controlled (e.g. a constant) current to the motor. For accurate control of the motor and prediction of the motor&#39;s reaction to system disturbances, an accurate model (often referred to as a motor plant model) describing the behaviour of the motor is required. The motor plant model is dependent upon the operating point (the input voltage, speed and load) of the motor, and so can only be obtained accurately while a rotor of the motor is magnetised and rotating. 
     In some systems in which the source of electrical power is an inverter, the inverter may perform frequency response analysis for control tuning during or immediately after an “identification run”. Systems in which an electric motor is not powered by an inverter, e.g. where the motor is connected to a power supply in a direct online power mode, typically do not perform frequency response analysis. 
     SUMMARY 
     The present disclosure describes an apparatus for measuring an impedance of an electrical load (e.g. an electric motor), in which the apparatus can be coupled in parallel with the load and can measure the impedance of the load regardless of the power source of the load. A frequency response of the load can be determined based on the measured impedance. 
     According to a first aspect, the invention provides apparatus for measuring an impedance of an electrical load that is configured to be coupled to a controlled current source, wherein the apparatus 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 capacitance connected in series between a first terminal of the secondary winding and the first coupling node, wherein a 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 that is applied to the primary winding so as to cause a variation, corresponding to the excitation signal, in an input current of the load; measure the input current and an input voltage of the load; and based on the measured input current and input voltage, determining the impedance of the load. 
     The processing unit may be further configured to determine a frequency response of the load based on the determined impedance of the load. 
     The apparatus may further comprise a controllable switching device configured to selectively couple the primary winding of the transformer to a voltage source so as to apply the excitation signal to the primary winding. 
     The processing unit may be configured to control operation of the controllable switching device so as to generate the excitation signal. 
     Alternatively, the apparatus 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. 
     The excitation signal may comprise a sinusoidal sweep signal, for example. 
     The excitation signal may comprise a chirp signal. 
     The excitation signal may comprise a pseudo-random binary sequence (PRBS). 
     The processing unit may be configured to measure the input current and the input voltage of the load for a plurality of different excitation signal frequencies. 
     The processing unit may be configured to measure the input current and the input voltage of the load at a plurality of different operating points of the load. 
     The processing unit may comprise a digital signal processor, a microprocessor or a microcontroller. 
     According to a second aspect, the invention provides a method of operation of the apparatus of the first aspect, the method comprising: coupling the apparatus in parallel a load that is coupled to a controlled current source by coupling the first and second coupling nodes of the apparatus to respective first and second terminals of the load; causing the excitation signal to be applied to the primary winding of the transformer so as so as to cause a variation, corresponding to the excitation signal, in the input current of the load; measuring the input current and the input voltage of the load; and based on the measured input current and input voltage, calculating the impedance of the load. 
     The method may further comprise determining a frequency response of the load based on the determined impedance of the load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, strictly by way of example only, with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic representation of an electrical system including a source and an electrical load, to which an apparatus for measuring an impedance of the electrical load is coupled; and 
         FIG. 2  is a schematic representation of an electrical system including a source and an electrical load, to which an alternative apparatus for measuring an impedance of the electrical load is coupled. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , an electrical system is shown generally at  100 , and includes a source  200  of electrical power coupled to an electrical load  300 . An apparatus  400  for measuring an impedance and determining a 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. 
     The source  200  is a controlled current source, and is operable to output a controlled current to the 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 the source  200  to output a constant current, the current output by the source  200  must be controlled. Thus, in this disclosure the term “controlled current” is to be understood as encompassing a constant current. However, the term “controlled current” is not to be construed as being limited to a constant current. As will be appreciated by those skilled in the art, the source  200  may control many different parameters of current can be controlled. For example, the source  200  may be configured to limit the output current that it outputs to a maximum value. 
     The electrical load  300  may be, for example, an electric motor. In the discussion that follows an electric motor will be used as an example of a load. However, it will be appreciated by those skilled in the art that the apparatus  400  can be used to measure the impedance, and also to determine the frequency response, of other types of electrical load such as power electronics components, DC-DC converters, inverters, rectifiers and the like. 
     The apparatus  400  includes a first coupling node  402  and a second coupling node  404 , by means of which the apparatus  400  can be coupled to the first and second terminals  302 ,  304  of the load  300 . The first and second terminals  302 ,  304  of the load  300  are also coupled to the source  200 . Thus, the apparatus  400  is coupled in parallel with the source  200  and the load  300 . 
     The apparatus  400  includes a transformer  406  having a primary winding  408  and a secondary winding  410 . A capacitance  412  (e.g. a capacitor) is connected in series between the first coupling node  402  and a first terminal  414  of the secondary winding  410 . A second terminal  416  of the secondary winding  410  is connected to the second coupling node  404 . Thus, the capacitance  412  and the secondary winding  410  are connected in series between the first and second coupling nodes  402 ,  404 . 
     The primary winding  408  of the transformer  406  has first and second terminals  418 ,  420 . The second terminal is connected to a ground, whilst the first terminal  418  is selectively coupled to voltage source, represented by node  430  in  FIG. 1 , by means of a controllable switching device  428 , which has a control terminal that is coupled to an output  422  of a processing unit  424 . A bypass diode  426  is connected in anti-parallel with 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 . 
     The controllable switching device  428  may be, for example, a MOSFET having a gate terminal coupled to the output  422  of the processing unit  424 , a source terminal connected to the voltage source  430  and a drain terminal connected to the first terminal  418  of the primary winding  408  of the transformer  406 . 
     The processing unit  424 , which may be, for example, a microcontroller, a digital signal processor (DSP), a microprocessor or some other processing unit, is configured to control an excitation signal that is 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 cause the controllable switching device  428  to switch on, the first terminal  418  of the primary winding  408  is coupled to the voltage source  430 , and thus a voltage develops across the primary winding  408 . When the processing unit  424  outputs a control signal (e.g.  0  volts) to cause the controllable switching device  428  to switch off, the first terminal  418  of the primary winding  408  is decoupled from the voltage source  430 . Thus, 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 regarded as an excitation signal with which the primary winding  408  is excited. 
     By adjusting the rate at which control signals are output by the processing unit  424  to the control terminal of the controllable switching device  428  the frequency of the excitation signal can be adjusted, and thus, with appropriate control signals, the processing unit  424  can cause a desired excitation signal to be applied to the primary winding  408 . 
     The excitation signal is a time varying electrical signal such as, for example, a swept sinusoid, a chirp signal, a pseudo random binary sequence (PRBS) or some other time varying electrical signal. 
     When the excitation signal is applied to the primary winding  408  of the transformer  406 , a corresponding changing (time varying) voltage develops across the secondary winding  410 , which causes a varying current I 2  to flow through the capacitor  412 . The current I 2  combines with a controlled (e.g. constant) current I 1  that flows from the source  200  to the load  300  (when the load is operational) to form a current I 3 . Thus the current I 3  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. where the excitation signal is a swept sinusoid or a chirp signal) the frequency of the time varying component  12  of the current I 3  changes accordingly. 
     The processing unit  424  is configured to measure a voltage across the load  300  and the current I 3  through the load and, based on the measured voltage and current, determine or calculate an impedance of the load  300  at the particular operating point of the load  300 , by dividing the measured voltage by the measured current. Voltage and current measurements may be taken by the processing unit  424  for a number of different frequencies of the excitation signal, and the impedance of the load  300  at each excitation frequency can be determined or calculated by the processing unit  424 . The determined or calculated impedance(s) can then be used by the processing unit  424  to determine the frequency response of the load  300  using known techniques or models that relate the frequency response of the load  300  to its impedance. 
     For example, where the load  300  is an electric motor, the electric motor may be arranged to operate at a selected operating point by setting the source  200  to output an appropriate current I 1  for a selected motor load to achieve a required speed and input voltage. Once the motor is operating at the desired operating point, an excitation signal such as a sinusoidal sweep, a chirp 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), so as to inject a current I 2  into the system  100 . The current I 3  through the motor and the voltage across the motor terminals are measured by the processing unit  424  at a plurality of different excitation frequencies, and the impedance of the motor at those different excitation frequencies is determined by the processing unit  424 , based on the measured currents and voltages. The processing unit  424  can then determine the frequency response of the motor at the selected operating point based on the determined impedances. In order to characterise or verify the motor more completely, this process may be repeated for a plurality of different selected operating points of the motor. 
     Referring now to  FIG. 2 , an alternative apparatus for measuring a frequency response of an electrical load electrical system is shown generally at  500 , coupled to a controlled current source  200  and an electrical load  300  in the same manner as the apparatus  400  of  FIG. 1 . The alternative apparatus  500  includes a number of elements that are the same as elements of the apparatus  400 , and so like elements in  FIGS. 1 and 2  are denoted by like reference numerals. 
     The alternative apparatus  500  differs from the apparatus  400  of  FIG. 1  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 . 
     In operation of the alternative apparatus  500 , the processing unit  424  controls the controllable  502  AC voltage source 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  410 , causing a varying current I 2  to flow through the capacitor  412 . As in the apparatus  400  of  FIG. 1 , the current I 2  combines with a current I 1  that flows from the source  200  to the load  300  (when the load is operational) to form a current I 3 . Thus the current I 3  includes a component having a frequency corresponding to the frequency of the excitation signal. The processing unit  424  may control the controllable voltage source  520  so as to produce an excitation signal whose frequency varies over time, e.g. a swept sinusoid or a chirp signal. As the frequency of the excitation signal changes the frequency of the  12  component of the current I 3  changes accordingly. 
     As in the apparatus  400  of  FIG. 1 , the processing unit  424  is configured to measure a voltage across the load  300  and the current I 3  through the load and, based on the measured voltage and current, determine or calculate an impedance of the load  300  at the particular operating point of the load  300 . Voltage and current measurements may be taken by the processing unit  424  for a number of different frequencies of the excitation signal, and the impedance of the load  300  at each excitation frequency can be determined or calculated by the processing unit  424 . The determined or calculated impedance(s) can then be used by the processing unit  424  to determine the frequency response of the load  300  using known techniques. 
     As will be appreciated from the foregoing, the apparatus  400 ,  500  disclosed herein and illustrated in the accompanying Figures provides a cost effective mechanism for determining the impedance and frequency response of a load coupled to a controlled current (e.g. a constant current) source during operation of the load, thus facilitating accurate identification and verification of the load and permitting accurate control design for the load by control engineers. It will be noted that the apparatus  400 ,  500  described above and illustrated in the accompanying Figures can be used regardless of the source  200 . For example, the apparatus  400 ,  500  can be used in electrical systems that use a rectifier as a current source, and in electrical systems that operate in a direct online mode. 
     Whilst in the disclosure above an electric motor is discussed as an example of a load, but it will be appreciated by those skilled in the art that the apparatus  400 ,  500  of the present disclosure can be used to measure the frequency response of other types of electrical load such as power electronics components. DC-DC converters, inverters, rectifiers and the like. 
     Moreover, whilst in the discussion above a single phase motor is given as an example of a load, the apparatus ( 400 ,  500 ) of the present disclosure is equally suitable for use with a three-phase electric motor, in which case the impedance per phase can be characterised as described above by using one apparatus per phase. Additionally or alternatively, three-phase analysis methodologies such as the DQ-transformation can be used to calculate impedance in a rotational-synchronous coordinate system. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the 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, “a” or “an” does not exclude a plurality. Any reference signs in the claims shall not be construed so as to limit their scope.