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McQuay OM 751
Description of Operation
Compressor Minimum On and Off Timers
The UVC is provided with minimum On (3-minute default) and minimum Off (5-minute
default) timers to prevent adverse compressor cycling.
Compressor Start Delay
The UVC is provided with a Compressor Start Delay configuration variable, which is intended
to be adjusted as part of the start-up procedure for each unit. This variable is used to delay
compressor operation each time the compressor is required.
Note –
To prevent strain on a building’s electrical supply system from multiple unit compressors
all starting at the same time after a power failure or after an unoccupied-to-occupied
changeover, McQuay strongly recommends configuring each unit or groups of units at
start-up with different start delays.
Outdoor Air Fan Operation
The UVC is configured with a fan on delay that delays OA fan operation for a time period (10
seconds, default) after the compressor starts. The OA fan stops with the compressor.
Floating-Point Actuator Auto-Zero, Overdrive and Sync
The UVC at power-up auto-zeros all floating-point actuators (OA damper) before going into
normal operation to ensure proper positioning. During auto-zero, the unit remains off. The
actuators all open approximately 30% and then are driven full closed. The overdrive feature
then is used to continue forcing the actuators closed for one full stroke period. Once the
zeroing process is complete, normal unit operation begins.
The UVC is configured such that whenever a floating-point actuator is commanded to go to
0% or 100%, the UVC overdrives the actuator one full stroke period past the 0% or 100%
position to ensure proper positioning.
Additionally, the UVC is configured to sync all floating-point actuators once every six hours
of operation. To do this, the UVC forces the actuator to the closest rail position (0% or 100%),
uses the overdrive feature, and then returns to the required position. For example, if the
actuator is at 20% when the six-hour limit is reached, the UVC then forces the actuator to 0%,
overdrive for one full stroke and then returns to the 20% position.
