
ZF1 SCR Power Controller Operation
Rev 3 Ametek HDR Power Systems 7
CHAPTER 3 - Operation
3.1 General
The ZF1 controls power by the switching action of two SCRs connected in a back to back
configuration. The zero crossing detector built into the solid-state relay module synchronizes the
gating of these SCRs with the line frequency (either 50 or 60 Hz). The firing circuit provides timing
pulses to the SSR proportional to the command signal. The power controller regulates the output
voltage by proportionally controlling the number of AC cycles “on” versus the number “off”. Because
the voltage is “turned on” at zero crossing, very little radio frequency interference (RFI) is generated
and the power factor (PF) is near unity. The output may be adjusted by a voltage or current signal
from a process controller. Zero and Span controls allow the user to calibrate the power controller’s
output to the process signal.
The firing control circuit of the ZF1 operates on a variable rather than fixed time base. Operating on a
fixed-time base of 1/3 second (20 cycles), for example, at 50% power the SCRs would be on for 10
cycles, off for 10 cycles, on for 10 cycles, and so on. Operating on a variable-time-base, at 50%
power, the SCRs are on for one cycle and off for the next. At 75% power, the controller, with a fixed-
time-base of one-third second (20 cycles), would be on for 15 cycles and off for 5. The variable-time-
base, however, would be on for three cycles, off for one, and so on. The time required to describe the
operation of the ZF1 would be the period of four cycles in this case – three cycles on and one cycle off.
For 50% power, the time required to describe the operation of the variable-time-base would be the
period of two cycles – one cycle on and one cycle off. Hence the time-base of the ZF1 would be the
period of two cycles for 50% power and four cycles for 75% power. The time-base varies for other
power levels as well, so its period is always that of a whole number of cycles
Keep in mind that ZF units can only be used on resistive loads. If an inductive load or a load with a
large resistance change (instantaneously or cold to hot) is being used, a phase fired (PF) power
control should be used.
3.2 Command Signal Indicator
The Command Signal Indicator is a green Light Emitting Diode (LED) located on the Firing board. The
brilliance of this LED will change with a Command Signal. The flash rate increases with an increased
Command Signal and decreases with a decreased Command Signal
3.3 Command Input Control
Connect the control signal per the schematic in Appendix A. The ZF1 Zero and Span is calibrated at
the Factory for the specified input type and normally does NOT need adjustment. However, should a
setup be required use the following methods. (Note: Adjustments requires a jeweler’s screw driver for
multi-turn bias & gain adjustments.)
The output voltage linearity for any given command is affected by the setting of the Zero and Span
adjustments.
a. Set the ZERO and SPAN controls fully CCW (counterclockwise).
b. Connect a voltmeter across terminals LINE and LOAD of the ZF1. Set the voltmeter range to
measure full input voltage.
c. Close the fused disconnect or circuit breaker to apply power to the ZF1. Turn the Process
controller on and set to demand zero power output or remote potentiometer fully CCW.
d. Adjust the ZERO control CW until the output just turns on, then turn the ZERO control CCW just to
the point where the output voltage reading is zero.