Page 220 - Power Quality in Electrical Systems
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202   Chapter Fourteen





        V s                           Average          V rms



                                (a)


                        AC-rms
        V s             converter          V rms


                          (b)

                       Heater
        V s
                                        Temp. to
                            Temp
                            sensor        DC         V rms
                                        converter


                                (c)
        Figure 14.1  Multimeter types for making AC voltage measure-
        ments. (a) Rectify and average. (b) Analog computation. (c) Thermal.

        True-rms multimeters are more accurate in the measurement of the
        rms value of periodic waveforms.


        Oscilloscopes
        Today’s oscilloscopes have more than enough bandwidth to display the
        harmonics present in all power-quality events. For instance, the 20th har-
        monic of the line frequency in the U.S. is 1200 Hz, which is much lower
        than the bandwidth of even an inexpensive oscilloscope. Impulsive tran-
        sients (like that caused by lightning strikes or power switching) typically
        last a few microseconds. It is well within the capability of oscilloscopes
        to measure and display these waveforms with good accuracy.
                                                              2
          Scopes are often used in conjunction with current probes to measure
                                                      3
        AC line currents. Often, differential voltage probe modules are used to

          2
           See, for example, the Tektronix P6042 current probe, which has a response from DC
        to 50MHz.
          3
           The differential probe overcomes the practical problem of using a single-ended scope
        probe, which has a signal input and a ground clip.
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