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8 - Operating P8 - Operating P

8 - Operating P

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The Magtrol 5300 PA uses the three wattmeter method

to measure true RMS power. Voltage and current are

sensed and amplified in an isolated analog front end,

analog to digital converted with 16 bit precision,

processed through an 8 bit microprocessor (MPU) and

output in BCD form for display on the front panel

meters. The PA provides a digital display for each

selected phase of rms volts, amps, and watts; average

rms volts, amps and watts; and total load power factor

(PF).

AMPERES TRANSDUCING

Current measuring is accomplished using a three section

calibrated manganin resistance shunt per phase. The 5,

25 and 100 amp sections are 0.016 Ohm (80 mV), 0.003

Ohm (75 mV) and 0.001 Ohm (100 mV), respectively.

See Figure 14.

The current flow through each shunt is determined by

measuring the voltage drop across the shunt and the

MPU, applying Ohm's Law. Current ranges are set by

the MPU and energize the appropriate shunt bypass

relays CR1 and CR2. The 5 amp range energizes (opens

N/C contacts) of both CR1 and CR2; the 25 Amp. range

energizes CR1, and both relays remain de-energized for

the 100 Amp range. The CRs are very low resistance,

high current mercury wetted relay contacts that bypass

unused sections of the metering shunts.

VOLTAGE SENSING

Each VOLTS input terminal connects a 1.5 Megohm

resistive voltage divider to provide scaled phase voltage

for measurements. The three 1.5 Megohm resistors form

a classic "Y Box" neutral. The resulting phase voltage

signals are amplified and scaled for processing. See

Figure 14.

NOTE: Connecting the Voltage Sense

leads incorrectly (e.g. connecting

the voltage sense from phase 1 to

phase 2 current lines) will cause

the Watts display to show an

incorrect value. If you are

experiencing poor readings, shut

down the system and carefully

review your connections.

ANALOG PROCESSING

Input signals proportional to AMPS and VOLTS are

amplified and calibrated for each range by amplifiers

78B146-U7, U8, U9 and U10. These scaled signals pass

through true rms to DC converters 78B147-U1 & U4,

voltage to frequency converters 78B147-U2 & U5 and

optical isolators 78B147-U3 & U6. The resulting

frequency signals represent true rms current and voltage

(AF

and VF

,) scaled to 0.0707 volts per 1.0 kilohertz.

25A

100A

CR2

CR1

CR2

CR1

CR2

CR1

INPUT

OUTPUT

(LOAD)

VOLTS

(LOW)

VOLTS

(HIGH)

E PH1

E PH2

E PH3

VOLTS

(HIGH)

VOLTS

(HIGH)

Figure 14. 5300 Current Shunt Inputs

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