Variable Time Base - Watlow ez-zone pm User Manual

continues to bring the process temperature to the set point. Using a positive dead band
value keeps the two systems from fighting each other.
Positive Dead Band
Cool Output Active
Set Point
Heat Output Active
When the dead band value is zero, the heating output activates when the temperature
drops below the set point, and the cooling output switches on when the temperature ex-
ceeds the set point.
Zero Dead Band
Cool Output Active
Set Point
Heat Output Active
Time
When the dead band value is a negative value, both heating and cooling outputs are active
when the temperature is near the set point. Adjust the dead band with Dead Band
erations Page, Loop Menu).
Negative Dead Band
Cool Output Active
Set Point
Heat Output Active
Time

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 out-
puts, the PID algorithm calculates an output between 0 and 100%, but the output is distrib-
uted in groupings of three ac line cycles. For each group of three ac line cycles, the control-
ler 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
Wat lo w EZ-ZO N E ® PM PI D C o n tr o lle r
Time
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Chapter 9 F e atur e s