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1.1.3: Blondell's Theorem and Three Phase Measurement
In 1893 an engineer and mathematician named Andre E. Blondell set forth the first
scientific basis for polyphase metering. His theorem states:
If energy is supplied to any system of conductors through N wires, the total power in
the system is given by the algebraic sum of the readings of N wattmeters so arranged
that each of the N wires contains one current coil, the corresponding potential coil
being connected between that wire and some common point. If this common point is
on one of the N wires, the measurement may be made by the use of N-1 Wattmeters.
The theorem may be stated more simply, in modern language:
In a system of N conductors, N-1 meter elements will measure the power or energy
taken provided that all the potential coils have a common tie to the conductor in
which there is no current coil.
Three-phase power measurement is accomplished by measuring the three individual
phases and adding them together to obtain the total three phase value. In older
analog meters, this measurement was accomplished using up to three separate ele-
ments. Each element combined the single-phase voltage and current to produce a
torque on the meter disk. All three elements were arranged around the disk so that
the disk was subjected to the combined torque of the three elements. As a result the
disk would turn at a higher speed and register power supplied by each of the three
wires.
According to Blondell's Theorem, it was possible to reduce the number of elements
under certain conditions. For example, a three-phase, three-wire delta system could
be correctly measured with two elements (two potential coils and two current coils) if
the potential coils were connected between the three phases with one phase in
common.
In a three-phase, four-wire wye system it is necessary to use three elements. Three
voltage coils are connected between the three phases and the common neutral
conductor. A current coil is required in each of the three phases.
In modern digital meters, Blondell's Theorem is still applied to obtain proper
metering. The difference in modern meters is that the digital meter measures each
phase voltage and current and calculates the single-phase power for each phase. The
meter then sums the three phase powers to a single three-phase reading.
Electro Industries/GaugeTech
Electro Industries/GaugeTech
The Leader In Power Monitoring and Smart Grid Solutions
The Leader In Power Monitoring and Smart Grid Solutions
1: Three Phase Power Measurement
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