ADE7752B_15 datasheet(22/24 Pages) AD

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ADE7752B

Rev. 0 | Page 22 of 24

SELECTING A FREQUENCY FOR AN ENERGY METER APPLICATION

As shown in Table 5, the user can select one of seven frequencies.

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). Because 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 maxi-

mum current of 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. 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 to 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 to 7 (Hz)

Frequency on F1 and F2

(Half-Scale AC Inputs) (Hz)

2.24

0.23

4.49

0.46

1.12

0.12

4.49

0.46

5.09

0.52

1.12

0.12

0.56

0.06

0.56

0.06

When selecting a suitable f1 to 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 that is closest in Table 9 determines the best

choice of frequency (f1 to 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 0.46 Hz at 25 A and 220 V (see Table 8). Looking at

Table 9, the closest frequency to 0.46 Hz in Column 5 is 0.46 Hz.

Therefore, f1 to 7 = 4.49 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

120 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

90 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

190 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.

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