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Theory of Operation
4-12.
SOFT START RESET CIRCUIT.
4-13.
Inputs to the Soft Start Reset circuit are supplied from the Power Supply circuitry. A regulated +12 V
dc and various pulsating DC Voltages are used to power the circuit. The function of the circuit is to reset the
Soft Start circuit when a short interruption of the line Voltage occurs. After periods of momentary input Voltage
loss, the Soft Start circuit allows the power supplied to the load from the PHASE/AMP to be brought up slowly.
This feature prevents the possible clearing of the fuses when the Voltage on the line returns to normal. The
output of the Soft Start Reset circuit is applied to the Soft Start circuit.
4-14.
SCR FIRING CIRCUIT.
4-15.
Inputs to the SCR Firing circuit are supplied from the Power Supply circuitry, Soft Start circuit, and AC
line. The SCR Firing circuit produces SCR firing pulses synchronized with each half cycle of the AC line. The
pulses are advanced from 180 degrees toward 0 degrees of each half cycle by an amount proportional to the
DC output of the Summing Amplifier circuit. The output of the SCR Firing circuit is supplied to the two Pulse
Shaper circuits.
4-16.
PULSE SHAPER CIRCUITS.
4-17.
There are two basically identical Pulse Shaper circuits. The inputs to the circuits are supplied from the
SCR Firing circuit and AC line. The AC Voltage from the line is used to power the circuit and the output from
the SCR Firing circuit is used to control the circuit. The function of the Pulse Shaper circuit is to amplify and
generate extended SCR firing pulses from the output of the SCR Firing circuit. The duration of the extended
gate pulses is directly proportional to the degree of firing advance. The extended gate pulses have a quasi-
rectangular wave shape characterized by a fast rise time spike at the leading edge and a "back porch" that
continues for the remainder of the half cycle. The outputs of the Pulse Shaper circuits are supplied to the
gates of the power SCR's.
4-18.
VOLTAGE FEEDBACK CIRCUIT.
4-19.
The input to the Voltage Feedback circuit is supplied from the Power SCR circuits.
samples the output Voltage, performs square law detection (Voltage squared is proportional to power), and
feeds the resultant output signal to the summing point feeding the Summing Amplifier circuit. The Voltage
feedback holds the RMS Voltage output of the PHASE/AMP constant with respect to the control signal from
the process controller regardless of line Voltage variations, temperature changes, and other variable factors.
The Voltage feedback also forces linearity between the control signal from the process controller and the
power output of the PHASE/AMP.
4-20.
OPTIONAL CURRENT LIMIT.
4-21.
The Current Limit option consists of a Current Transformer, Threshold circuitry, and Current
Rectification circuit. The RMS current in the load line is coupled to the input of the Current Limit by the Current
Transformer. The Current Rectification circuit applies a DC signal to the summing point feeding the Summing
Amplifier circuit. When the current in the load line approaches the maximum value set by the CURRENT
LIMIT control on the Current Limit Module, a Voltage proportional to the over-current will be fed back to the
summing point. The feedback signal is subtractive and will cause the Summing Amplifier to reduce its DC
output signal so that the output current of the PHASE/AMP will remain at the value set by the CURRENT LIMIT
control.
24
AMETEK HDR Power Systems
Phase/AMP Series DPA1/DCPA1
The circuit
Document 9100030
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Troubleshooting
