Arbiter Systems 931A, Руководство по эксплуатации

Страница: 45 / 161

In 1-phase 3-wire, the instrument alternates between measurements of
“Van,Ia” and “Vcn,Ic”. This is useful for making measurements on split-
phase systems. The column on the far right shows the averages for voltage,
current, phase and the totals for power quantities. The columns for “A”
and “C” show the voltage; characters in the lower right-hand corner of the
display alternate between “VanIa” and “VcnIc”, indicating the current
measurement.
In 3-phase 3-wire 2-element mode, the instrument alternates between
measurements of “Vab,Ia”,“Vcb,Ic”,“Vac,Ib”, and “Vcb,Vab”. It com-
putes “Vac” and includes it in the 3-phase average voltage. While Ib
is measured, it is not included in the 3-phase results. This procedure
yields accurate results for active power, but to estimate apparent power,
a correction of 0.866 must be arbitrarily applied since accuracy depends
on the symmetry of the phase voltages. “VcbVab” phase is measured to
determine phase rotation.
In 3-phase 3-wire 3-element mode, the instrument sequences between
measurements of “Vas,Ia”, “Vbs,Ib”; and “Vcs,Ic” (“s” denotes synthe-
sized neutral, which is the average of the signals present at voltage inputs
“a”, “b”, and “c”). Results are accurate for active power, but to estimate
apparent power synthesized neutral is used, and accuracy depends on
symmetry of the phase voltages. Reactive power, power factor, and phase
angle also are estimated. “VasVbs” phase is measured to determine phase
rotation.
In 3-phase 4-wire, 3-element mode, the instrument sequences between
measurements of “Van,Ia”, “Vbn,Ib”, and “Vcn,Ic”. All results are
accurate. This is the preferred mode for 3-phase measurements when
neutral is available. Phase rotation is determined by measuring “VanVcn”.
In 3-phase 4-wire 2.5 element mode, the instrument sequences between
measurements of “Van,Ia”, “Vb,Ib”, “Vcn,Ic”, and “VcnVan”.
NOTE: The “B” voltage input must be externally connected to
the “N” voltage input.
“Vb” is internally generated using the synthesized neutral circuit, and
the accuracy of all results depends on the symmetry of the phase voltages.
In all three-phase modes, the Model 931A also measures and displays
phase the sequence, “ABC” or “CBA”. (see Figures 3-2 and 3-3). A phase
measurement of “Vab to Vac” is made at the end of each sequence. If
the “Vab to Vac” phase is negative, the sequence is “ABC” otherwise it
is “CBA”. Phase Sequence may be checked using the waveform display.
29