Hioki 3181-01, Instruction Manual

Page: 15 / 51

7
―――――――――――――――――――――――――――――――――――――――
Chapter 3 Principle of Operation
―――――――――――――――――――――――――――――――――――
Ei （ t ）
lnx
e
x
L. P. F adder
lnx
lnx
Eo
a
＋
ー
True Rms Conversion Circuit
ln Ei （ t ）
─────
+ln E （ t ）
────
- ln Eo = ln Eo
ln Ei
2
（ t ）
──────
= ln Eo
2
∴
Eo =
Ei
2
（
t
）
─────
3.2 Current and Voltage Measurement
Thus, the active power P = EIcos
θ
[W] is the DC
component of the product of the voltage and current (the
second term in equation (
)).
When measuring AC signals using a general-purpose
measuring instrument, an average-value detection is
performed, after which conversion to the rms value is
made. Therefore, measurements on distorted or other-
wise non-sinusoidal waveforms will result in errors. In
the 3181-01, the true rms value is read by an absolute
value detection circuit and a true rms value conversion
circuit.
In the true rms conversion circuit shown in the figure
above, if we observe point a, we find the following
relationship.
Now let Ei(t) be the result of performing absolute-value
detection on the voltage and current signals from the
voltage transformer and current transformer, then the
output voltage Eo is proportional to the voltage-current
rms value.