SMY133-SMY134-SMP133 Operating Manual
4.3 Description of Operation
4.3.1 Method of Measurement
The measurement consists of three processes being performed continuously and simultaneously:
frequency measuring, sampling of voltage and current signals and evaluation of the quantities from
the sampled signals.
4.3.1.1 Voltage Fundamental Frequency Measurement Method
The voltage fundamental frequency is measured at the U1 voltage signal. It is measured continuously
and evaluated every 10 seconds.
The fundamental frequency output is the ratio of the number of integral mains cycles counted during
the 10 second time clock interval, divided by the cumulative duration of the integer cycles.
If value of frequency is out of measuring range, such state is indicated with flashing indicator at
upper right corner of the actual data window.
4.3.1.2 Voltage and Current Measurement Method
Both voltage and current signals are evaluated continuously as required by IEC 61000-4-30, ed. 2
standard. The unitary evaluation interval,
a measurement cycle
, is a ten / twelve ( value behind slash
is valid for f
NOM
= 60 Hz )
mains cycles
long period ( i.e. 200 ms at frequency equal to preset f
NOM
),
which is used as a base for all other calculations.
The sampling of all voltage and current signals is executed together with the frequency of 128 / 96
samples per mains cycle. The sampling rate is adjusted according to the frequency measured on any
of the voltage inputs
U1
,
U2
,
U3
.
If the measured frequency is in measurable range at least on one of
these inputs, then this value is used for subsequent signal sampling.
If the measured frequency is out
of this range, the preset frequency value ( f
NOM
) is used and measured values may be incorrect.
When exceeding the measuring range of any voltage or current, the instrument indicates overload by
indicator at upper right corner of the actual data window.
Effective values of voltages and currents are calculated from sampled signals over the measurement
cycle using formulas (examples for phase No. 1) :
Phase voltage (effective value) :
U
1
=
√
1
n
∑
i
=
1
n
U
1
i
2
Line voltage (effective value) :
U
12
=
√
1
n
∑
i
=
1
n
(
U
1
i
−
U
2
i
)
2
Current (effective value) :
I
1
=
√
1
n
∑
i
=
1
n
I
1
i
2
where : i …........ sample index
n ........... number of samples per measurement cycle ( 1280 / 1152 )
Ui
1
, Ii
1
… sampled values of voltage and current
Phase Current Sum :
∑
I
=
I
1
+
I
2
+
I
3
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