Table of Contents
Advertisement
Fig 1.5: Phasor Diagram showing Three-phase, Four-wire Delta Connected System
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 poly phase
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, as follows:
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 to-
gether to obtain the total three phase value. In older analog meters, this measurement was accomplished using up
to three separate elements. 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 be-
tween the three phases and the common neutral conductor. A current coil is required in each of the three phases.
e
Electro Industries/Gauge Tech
120 V
Vnc
Vbc
Vbn
120 V
Doc# E149701
Vca
Vcn
Vab
200 V
1-4
Advertisement
Table of Contents
