SA9904A 데이터 시트보기 (PDF) - South African Micro Electronic Systems

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SA9904A

Three Phase Power / Energy IC with SPI Interface

South African Micro Electronic Systems 

SA9904A

sames

TYPICAL APPLICATION

In figure 8, the components required for the three phase

power/energy metering section of a meter, is shown. The

application uses current transformers for current sensing. The

4-wire meter section is capable of measuring 3x230V/80A with

precision better than Class 1.

The most important external components for the SA9904A

integrated circuit are the current sense resistors, the voltage

sense resistors as well as the bias setting resistor.

BIAS RESISTOR

R7 defines all on-chip and reference currents. With R7=47kW,

optimum conditions are set.

CT TERMINATION RESISTOR

The voltage drop across the CT termination resistor at rated

current should be at least 16mV. The CT’s used have low

phase shift and a ratio of 1:2500.The CT is terminated with a

2.7W resistor giving a voltage drop across the termination

resistor 86.4mV at rated conditions (Imax for the meter).

CURRENT SENSE RESISTORS

The resistors R1 and R2 define the current level into the

current sense inputs of phase one of the device. The resistor

values are selected for an input current of 16µA on the current

inputs at rated conditions.

According to equation described in the Current Sense inputs

section:

R1 = R2 = (IL / 16µA ) x RSH / 2

= 80A /2500 / 16µA x 2.7W / 2

= 2.7kW

IL = Line current / CT Ratio

The three current channels are identical so R1 = R2 = R3 = R4

= R5 = R6.

VOLTAGE DIVIDER

The voltage divider is calculated for a voltage drop of 14V.

Equations for the voltage divider in figure 5 are:

RA = R16 + R19 + R22

RB = R8 || R13)

Combining the two equations gives:

( RA + RB ) / 230V = RB / 14V

A 24k resistor is chosen for R13 and a 1M resistor is used for

R8.

Substituting the values result in:

RB = 23.44k

RA = RB x (230V / 14V - 1)

RA = 361.6k.

Resistor values of R16, R19 and R22 is chosen to be 120k

each.

The three voltage channels are identical so R14= R15= R16 =

R17 = R18 = R19 and R20 = R21= R22.

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