Watlow PM PLUS™ 6 • 92 • Chapter 8: Features
Variable Time Base
Variable time base is the preferred method for controlling a resistive load, providing a very short time base
for longer heater life. Unlike phase-angle firing, variable-time-base switching does not limit the current and
voltage applied to the heater. With variable time base outputs, the PID algorithm calculates an output be-
tween 0 and 100%, but the output is distributed in groupings of three ac line cycles. For each group of three
ac line cycles, the controller decides whether the power should be on or off. There is no fixed cycle time
since the decision is made for each group of cycles. When used in conjunction with a zero cross (burst fire)
device, such as a solid-state power controller, switching is done only at the zero cross of the ac line, which
helps reduce electrical noise (RFI). Variable time base should be used with solid-state power controllers,
such as a solid-state relay (SSR) or silicon controlled rectifier (SCR) power controller. Do not use a variable
time base output for controlling electromechanical relays, mercury displacement relays, inductive loads or
heaters with unusual resistance characteristics.
The combination of variable time base output and a solid-state relay can inexpensively approach the effect
of analog, phase-angle fired control. Select the AC Line Frequency at Operations — Setup - Global - AC Line
Frequency), 50 or 60 Hz.
Single Set Point Ramping
Ramping protects materials and systems that cannot tolerate rapid temperature changes. The value of the
ramp rate is the maximum degrees per minute or hour that the system temperature can change.
Select Ramp Action (Operations — Setup - Control Loop - Ramp Action):
• Off ramping not active.
• Start ramp at startup.
• Set Point ramp at a set point change.
• both ramp at startup or when the set point changes.
Time
Temperature
Negative Dead Band
Set Point
Heat Output Active
Cool Output Active
50 percent output
3 ON, 3 OFF
100 percent output
10 ON, 0 OFF
66 percent output
6 ON, 3 OFF
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