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ADE7752A Datasheet(PDF) 22 Page - Analog Devices

Part No. ADE7752A
Description  Polyphase Energy Metering IC with Pulse Output
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADE7752A Datasheet(HTML) 22 Page - Analog Devices

 
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ADE7752/ADE7752A
Rev. C | Page 22 of 24
SELECTING A FREQUENCY FOR AN ENERGY METER APPLICATION
As shown in Table 5, the user can select one of seven frequen-
cies. This frequency selection determines the maximum
frequency on F1 and F2. These outputs are intended to be
used to drive the energy register (electromechanical or other).
Since only seven different output frequencies can be selected,
the available frequency selection has been optimized for a 3-
phase 4-wire service with a meter constant of 100 imp/kWhr
and a maximum current between 10 A and 100 A. Table 8
shows the output frequency for several maximum currents
(IMAX) with a line voltage of 220 V (phase neutral). In all cases,
the meter constant is 100 imp/kWhr.
Table 8. V. F1 and F2 Frequency at 100 imp/kWhr
IMAX (A)
F1 and F2 (Hz)
10
0.18
25
0.46
40
0.73
60
1.10
80
1.47
100
1.83
The F1–7 frequencies allow complete coverage of this range of
output frequencies on F1 and F2. When designing an energy
meter, the nominal design voltage on the voltage channels
should be set to half scale to allow for calibration of the meter
constant. The current channel should also be no more than
half scale when the meter sees maximum load. This allows
overcurrent signals and signals with high crest factors to be
accommodated. Table 9 shows the output frequency on F1 and
F2 when all six analog inputs are half scale.
Table 9. F1 and F2 Frequency with Half-Scale AC Inputs
SCF
S1
S0
F1–7
Frequency on F1 and F2
(Half-Scale AC Inputs)
0
0
0
1.27
0.26
1
0
0
1.19
0.24
0
0
1
5.09
1.02
1
0
1
4.77
0.96
0
1
0
19.07
3.84
1
1
0
19.07
3.84
0
1
1
76.29
15.35
1
1
1
0.60
0.12
When selecting a suitable F1–7 frequency for a meter design, the
frequency output at IMAX (maximum load) with a 100 imp/kWhr
meter constant should be compared with column 5 of Table 9.
The frequency closest to that listed in Table 9 is the best choice
of frequency (F1–7). For example, if a 3-phase 4-wire Wye meter
with a 25 A maximum current is being designed, the output
frequency on F1 and F2 with a 100 imp/kWhr meter constant is
0.15 Hz at 25 A and 220 V (from Table 8). Looking at Table 9,
the closest frequency to 0.15 Hz in column 5 is 0.12 Hz.
Therefore, F1–7 = 0.6 Hz is selected for this design.
FREQUENCY OUTPUTS
Figure 2 shows a timing diagram for the various frequency
outputs. The outputs F1 and F2 are the low frequency outputs
that can be used to directly drive a stepper motor or electro-
mechanical impulse counter. The F1 and F2 outputs provide
two alternating high going pulses. The pulse width (t1) is set at
275 ms, and the time between the rising edges of F1 and F2 (t3)
is approximately half the period of F1 (t2). If, however, the
period of F1 and F2 falls below 550 ms (1.81 Hz), the pulse
width of F1 and F2 is set to half of their period. The maximum
output frequencies for F1 and F2 are shown in Table 6.
The high frequency CF output is intended to be used for
communications and calibration purposes. CF produces a
96 ms-wide active high pulse (t4) at a frequency proportional to
active power. The CF output frequencies are given in Table 7. As
in the case of F1 and F2, if the period of CF (t5) falls below
192 ms, the CF pulse width is set to half the period. For
example, if the CF frequency is 20 Hz, the CF pulse width is
25 ms. One exception to this is when the mode is S0 = 1,
SCF = S1 = 0. In this case, the CF pulse width is 66% of the period.
NO LOAD THRESHOLD
The ADE7752 also includes no load threshold and start-up cur-
rent features that eliminate any creep effects in the meter. The
ADE7752 is designed to issue a minimum output frequency.
Any load generating a frequency lower than this minimum fre-
quency does not cause a pulse to be issued on F1, F2, or CF. The
minimum output frequency is given as 0.005% of the full-scale
output frequency for each of the F1–7 frequency selections or
approximately 0.00204% of the F1–7 frequency (see Table 10).
For example, for an energy meter with a 100 imp/kWhr meter
constant using F1–7 (4.77 Hz), the minimum output frequency at
F1 or F2 would be 9.59 × 10–5 Hz. This would be 1. 54× 10–3 Hz
at CF (16 × F1 Hz). In this example, the no load threshold
would be equivalent to 3.45 W of load or a start-up current of
15.70 mA at 240 V.
Table 10. CF, F1, and F2 Minimum Frequency at No Load
Threshold
SCF
S1
S0
F1, F2 Min (Hz)
CF Min (Hz)
0
0
0
2.56 x 10−05
4.09 x 10−03
1
0
0
2.40 x 10−05
1.92 x 10−04
0
0
1
1.02 x 10−04
1.64 x 10−02
1
0
1
9.59 x 10−05
1.54 x 10−03
0
1
0
3.84 x 10−04
6.14 x 10−03
1
1
0
3.84 x 10−04
3.07 x 10−03
0
1
1
1.54 x 10−03
1.23 x 10−02
1
1
1
1.20 x 10−05
1.92 x 10−04


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