24-04-2012, 11:08 AM
THE TWO-METER WATTMETER METHOD
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Blondel's Theorem
"The total power delivered to a load system by means of n conductors is given by the
algebraic sum of the indications of n wattmeters so inserted that each of the n wires
contains one wattmeter current-coil, its potential coil being connected between that wire
and some point of the system in common with all the other potential coils; if that
common junction of all the potential leads is on one of the n wires, the total power is
obtainable from the indications of n-1 wattmeter elements."
THE TWO WATTMETER METHOD
Many users of power monitoring equipment believe that it is necessary to measure all three pairs of
line to line voltages and all three-phase currents to properly measure power consumed by a threewire
load. This is not necessary. By measuring the current in two of the lines and the two voltages
from those two lines with respect to the third line, one can accurately measure total power
regardless of unbalance of voltage, current, or load and regardless of variations in power factor.
The method of measuring two of the currents and two of the voltages is known as the twowattmeter
method.