These instruments provide the product
of the two input signals (both voltages) through an analog multiplier
that evaluates a voltage output proportional to the power of the
input signals. This output can be followed by a filtering block.
The output signal is proportional to
the instantaneous electric power flowing through the line. To
calculate the energy, it is now necessary to complete the process by
integrating over the observation time.
This last procedure can be performed in
two different ways.
1st Procedure
. The power signal at the output of the
analog multiplier is applied to the input of a voltage-frequency
converter. Thus, the power information is converted from a voltage
level to the frequency of pulse sequence, for which the counting
process performs the integration of the power in the observation
interval, i.e., the measurement of energy.
The final measurement can be performed
by means of an electronic counter with digital display or using a dc
step motor incrementing the rotor angular position every pulse by a
fixed angular increment. The rotor position is shown by a mechanical
counter (similar to the system mounted on the induction energy
meters) indicating the total number of complete rotations performed
by the system, proportional to the energy of the system under
measurement.
This second arrangement is normally
adopted because it allows a permanent record of the energy
information, which is not subject to possible lack of electric energy
as in the first case.
2nd Procedure
. This arrangement is based on an
analog-to-digital converter (ADC) connected to the output of the
analog multiplier. The sampling process is driven by an internal
clock and performed by a sample and hold circuit.
Thus, the ADC provides uniform sampling
over the signal period and, under the condition imposed by the
sampling theorem, the sum of the samples is proportional to the
integral of the power signal, i.e., to the energy during the
observation interval.
The calculation is performed by means
of a dedicated CPU and then the results are sent to the digital
memory to be stored and displayed. They can also be used to manage
any other automatic process based on the energy measurement.
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