Unit installation

24 OMRCPMICRO
Compressor Control
The compressor staging logic uses an adjustable control band and interstage timer to determine the correct
number of cooling or heating stages to activate. A project-ahead temperature calculation and a maximum pull
down rate provide stable operation. Compressor set points are shown on Menu 16.
The Control Band is the temperature range on either side of the active leaving water set point that must be
exceeded for a stage change to occur. When in chiller mode and after the unit has started, the controller will
stage-up when the leaving water temperature rises to the Leaving Chilled Water Set Point plus half of the
Control Band. The controller will stage-down when the leaving water temperature lowers to the Leaving
Chilled Water Set Point minus half the Control Band. The THR, controlled from leaving condenser water,
stages up and down in the reverse order of a unit in chiller operation.
The adjustable compressor Interstage Timer set point ("InterStg", see Menu #16) sets the time delay between
the current cooling or heating stage and the next stage-up request. The compressor stage-down time delay is
fixed at a 1/3 ratio of the stage-up setting. For more information on staging, refer to the "Normal Sequence of
Operation" section on page 37 of this manual.
The controller performs a project-ahead temperature calculation to protect against an overshoot condition
when the leaving water temperature is outside the control band. Project-ahead calculation also moderates the
controller's response to a rapid increase and decrease in leaving water temperature. During cooling mode, if
the chilled water temperature is above the control band and the project-ahead calculation has determined that
the chilled water temperature will fall below the control band within 240 seconds (4 minutes), then the
controller will unload the compressors and stage-down the unit until the condition is no longer true. For THRs
in the heating mode, if the project-ahead calculation has determined that the leaving condenser water
temperature will rise above the control band wishing 240 seconds (4 minutes), then the controller will unload
the compressors and stage-down the unit until the condition is no longer true.
As additional protection against overshooting the unit set point, the controller uses the maximum pull down
rate ("MaxPullDn", on Menu #14) to determine if the leaving water temperature is pulling down or up too
rapidly. Every minute, the controller checks the leaving water temperature and compares the temperature to the
last reading. If the Pull Down Rate is exceeded, the controller delays additional stages. Maximum Pull Down
("MaxPullDn", on Menu #14) default setting is 0.5°F. This setting may be changed on Menu 14, Lvg Evap
Spts,.
The MicroTech soft loading feature will also guard against problems of overshooting the set point. The Soft
Load ("SoftLoad" on Menu #15) default setting is 20 minutes up to stage 4 of cooling and heating stages. See
"Soft Loading" section of this manual for more information.
The Interstage Timer ("InterStg" on Menu #16) default setting is 180 seconds for cooling and heating stages.
Interstage timers (anti-cycle) for compressors are 5 minute stop-to-start and 15 minute start-to-start.