Rotek 800A - 3.2 System Operation

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CAUTION
To prevent an overload condition, the external
four wire connections should be made prior to
switching to the external sense configuration.
3.1.2 Current
Figure 3-2 shows the arrangement of components for the generation of AC current output.
The basic components in this block are:
• Precision source of DC voltage, the I Reference (this is a separate Reference from
that referred to in 3.1.1 above).
• Voltage controlled oscillator with self contained amplitude control, similar to that in
3.1.1 above.
• An AC D to A converter, the I Attenuator.
• Power amplifier.
• Precision current shunts.
• Sense amplifier system.
• Current source.
The I oscillator, which has a self contained amplitude control circuit similar to that of the
voltage oscillator described in 3.1.1, maintains a constant amplitude of 7.0711 volts. This signal is
injected to the scaling amplifier which consists of the I Attenuator and AR3. The output of the scaling
amplifier (which has a value of 0.5V to 5V) is then fed to the current source via AR4. The current
source (which consists of AR9, Q1 and the output stages) then feeds the current shunts. The current
then is fed to either the output terminals or to the current transformer T5 (10-50A). AR8 also senses
the voltage developed across the current shunts and feeds it back to AR4 which provides a correction
signal to the current source so that a 1 volt full scale voltage is maintained a cross the current shunts
independent of the voltage at the output terminals.
3.1.3 Phase Control
Phase control is accomplished by measuring the area under a sine wave between axis
crossings of the current and voltage waveforms. The area is caused to be equal to the power factor
input by summation at the input of an operational amplifier integrator. The integrator controls the
relative phase of the I oscillator.
This is illustrated in Figure 3-3. The I oscillator output and the Voltage output are sliced to
detect their axis crossings. The phase sensor generates a pulse train, in which the pulse length is
precisely the phase delay or advance. If the current lags the voltage, the train appears in the LAG
line, otherwise the train appears on the LEAD line. These trains close switches which apply part of
the I oscillator output or its inverse to the integrator input which is then balanced against the decoded
lead or lag PF input voltage.
3.2 System Operation
The system operation is described by reference to three drawings 08-800-002 1/2/3, which
show the components, connections and waveforms for the voltage, current and phase functions.