ADE7752A datasheet(22/24 Pages) AD | Polyphase Energy Metering IC with Pulse Output

22 / 24 page

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)

0.18

0.46

0.73

1.10

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

F1–7

Frequency on F1 and F2

(Half-Scale AC Inputs)

1.27

0.26

1.19

0.24

5.09

1.02

4.77

0.96

19.07

3.84

19.07

3.84

76.29

15.35

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

F1, F2 Min (Hz)

CF Min (Hz)

2.56 x 10−05

4.09 x 10−03

2.40 x 10−05

1.92 x 10−04

1.02 x 10−04

1.64 x 10−02

9.59 x 10−05

1.54 x 10−03

3.84 x 10−04

6.14 x 10−03

3.84 x 10−04

3.07 x 10−03

1.54 x 10−03

1.23 x 10−02

1.20 x 10−05

1.92 x 10−04

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