Page 222 - Power Quality in Electrical Systems
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204   Chapter Fourteen


                        N : N s
                         P
                                            BNC cable
        Circuit                                              Oscilloscope
         under                                               50 Ω
         test


        Figure 14.3 A measurement system for making high current measurements using a CT
        and an oscilloscope.


        Search Coils
        The magnetic field produced by a current can be detected by a “search
        coil” as shown in Figure 14.4. The voltage output of the search coil is
        found using
                                           dB
                                   v 5 NA
                                    o
                                            dt
        where N is the number of turns in the search coil, A is the area of the
        coil, and dB/dt is the time rate of change of the magnetic flux density per-
        pendicular to the plane of the search coil. In the setup of Figure 14.4,
        the magnetic field H and the magnetic flux density B are found using
                                           i
                                    H 5
                                         2pr
                                     B 5 m H
                                          o
        where r is the mean radius from the current-carrying wire to the search
        coil, and m is the magnetic permeability of free space. Note that this
                  o
        measurement can be affected by stray time-varying fields from sources
        other than the current-carrying wire being measured.








                                       Amplifier
                                                        Analyzer

                      H


           i
        Figure 14.4 A search coil setup [14.2].
        [© 1992, IEEE, reprinted with permission]
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