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]