HT9022
EN - 7
2.1. MEASURING
AVERAGE
VALUES AND TRMS VALUES
Measuring instruments of alternating quantities are divided into two big families:
•
AVERAGE-VALUE meters: instruments measuring the value of the sole wave at
fundamental frequency from 10 to 400Hz
•
TRMS (True Root Mean Square) VALUE meters: instruments measuring the TRMS
value of the quantity being tested.
In the presence of a perfectly sinusoidal wave, the two families of instruments provide
identical results. In the presence of distorted waves, instead, the readings shall differ.
Average-value meters provide the RMS value of the sole fundamental wave, TRSM
meters, instead, provide the RMS value of the whole wave, including harmonics (within the
instrument’s bandwidth). Therefore, by measuring the same quantity with instruments from
both families, the values obtained are identical only if the wave is perfectly sinusoidal. In
case it is distorted, TRMS meters shall provide higher values than the values read by
average-value meters.
2.2.
DEFINITION OF TRUE ROOT MEAN SQUARE VALUE AND CREST FACTOR
The root mean square value of current is defined as follows: "
In a time equal to a period,
an alternating current with a root mean square value of the intensity of 1A, circulating on a
resistor, dissipates the same energy that, during the same time, would have been
dissipated by a direct current with the intensity of 1A
". This definition results in the numeric
expression:
G=
∫
+
T
t
t
dt
t
g
T
0
0
)
(
1
2
The
root mean square value
is indicated with the acronym RMS.
The Crest Factor is defined as the relationship between the Peak Value of a signal and its
RMS value: CF (G)=
RMS
p
G
G
This value changes with the signal waveform, for a purely sinusoidal wave it is
2
=1.41.
In case of distortion, the Crest Factor takes higher values as wave distortion increases.
2.3. HARMONICS
See Appendix (Par.9.4)
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