Machine Protection
and Adaptive
Control
Differential to Start or Stop
The Differential to Start setpoint is adjustable from 1 to 10°F (0.55 to 5.55°C) and the Differential to Stop setpoint adjustable from 1 to 10°F (0.55 to 5.55°C). Both setpoints are with respect to the Active Chilled Water Setpoint. When the chiller is running and the LWT (Leaving Water Temperature) reaches the Differential to Stop setpoint the chiller will go through its shutdown sequence to AUTO. (Refer to Figure 10.)
SoftLoading
Softloading stabilizes the startup control during the initial chiller pulldown. Soft loading is used to bring the building loop temperature from its start value to the Chilled Water or Hot Water Setpoint in a controlled manner. Without soft loading, the chiller controls will load the chiller rapidly and use the full chiller capacity to bring the loop temperature to setpoint. Although the start temperature of loop may have been high, the actual system load may be low. Thus, when the setpoint is met the chiller must unload quickly to the system load value. If it is not able to unload quickly enough, the supply water temperature will
drop below setpoint and may even cause the chiller to cycle off. Soft loading prevents the chiller from going to full capacity during the pulldown period. After the compressor has been started, the starting point of the filtered setpoint is initialized to the value of the Evaporator Leaving Water temperature and the percent RLA.
There are three independent Softload setpoints:
•Capacity Control Softload Time (default to 10 minutes,
•Current Limit Control Softload Time (default 10 minutes;
•Current Limit Softload Starting Percent (default is 40 percent RLA;
Service tool provides access to these three setpoints, if it is determined necessary to change from the defaults.
Softloading is not active during Ice Making or during the Ice To normal Transition. Softloading will be enabled after the Ice to normal Transition timer has expired.
Minimum and Maximum Capacity Limit
A Minimum Capacity can be set to limit the unloading ability of the compressor thus forcing differential to stop to be reached cycling the chillers. Minimum capacity limit will be displayed when in this limit mode. This indicates when the chiller is running fully unloaded.
Similarly a maximum capacity can be set to limit normal chilled water temperature control, the maximum capacity relay is energized which is a signal used by generic BAS systems to start another chiller.
The minimum (default at 0 percent) and maximum (default at 100 percent) capacity are adjustable via the service tool.
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