SCZF1 SSR Power Controller
Operation
Rev 2
Ametek HDR Power Systems
7
CHAPTER 3 - Operation
3.1 General
The SCZF1 controls power by the switching action of two SCRs connected in a back to back
configuration. The gating of these SCRs is synchronized with the line frequency (either 50 or 60 Hz)
by the zero crossing detector built into the solid-state relay module. 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 SCZF1 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 SCZF1 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
Power On Indicator
The Power On Indicator is a red Light Emitting Diode (LED) located on the front. The LED lights when
power is applied.
3.3 Command
Indicator
The Command Indicator is a green Light Emitting Diode (LED) located on the front. The flash rate 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
3.4
Zero and Span Adjustments
The Zero potentiometer has both positive and negative voltages available making it usable as a
manual or zero control. By turning the Zero control clockwise the unit's output voltage will increase
proportionally to the adjustment. Turning it counter-clockwise will decrease or zero the output for any
non-zero based command signal.
The Span potentiometer is used to adjust the maximum output. It will adjust for either a remote
manual control or a command signal input. Clockwise adjustment increases the output while counter-
clockwise adjustment decreases the output. Due to some interaction between controls, it may be
necessary to repeat these adjustments.
3.5
Command Input Control
Connect the control signal per the schematic in Appendix A. The SCZF1 Zero and Span is calibrated
at the Factory for the specified input type and normally does
NOT
need adjustment. However, should
