Compressor Cooling Lockout; Minimum On And Off Time; Compressor Start Delay; Outdoor Air Fan Operation - McQuay MicroTech II Operation & Maintenance Manual

Figure 25. Compressor Envelope in Self-Contained Units
Area for compressor operation envelope.
Area where liquid slugging could occur.
Area where starving could occur.

COMPRESSOR COOLING LOCKOUT

The UVC is configured to lockout compressor cooling when the OA
temperature falls below the Compressor Cooling Lockout setpoint
O O
(60.8 F/16.0 C). Below this OA temperature setpoint only economizer
cooling will be available.

MINIMUM ON AND OFF TIME

The UVC is provided with minimum On (3-minutes default) and
minimum Off (3-minutes 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 provides a compressor delay (0-
seconds default) used to delay compressor operation each time the
compressor is required.
NOTICE
To prevent multiple unit compressors from all starting at the same
time after a power failure, or after an unoccupied-to-occupied
changeover, it is strongly recommended that each unit be configured
at start-up with a slightly different (random) delay, or that groups of
units be provided with different delays.

OUTDOOR AIR FAN OPERATION

For self-contained units, the UVC has been configured with a fan on
delay which delays OA fan operation for a time period (30-seconds
default) after the compressor starts. The OA fan will stop with the
compressor.
OM 750
DX Cooling
2
1
Condenser Temperature
(Outdoor Air Coil)
Floating-point Actuator Auto-Zero,
Overdrive and Sync
The UVC at power-up will auto-zero all floating-point actuators (OA
damper) before going into normal operation to ensure proper positioning,
this can take as long as 150-seconds after power-up. During auto-zero
the unit will remain Off, the actuators will open approximately 30%
and then be driven full closed, the overdrive feature is then used to
continue forcing the actuators closed for one full stroke period. Once
the zeroing process is complete normal unit operation will begin.
The UVC is configured such that whenever a floating-point actuator is
commanded to go to 0% or 100%, the UVC will overdrive 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 12-hours of operation. To do this the UVC will force the
actuator to the nearest rail position (0% or 100%), use the overdrive
feature, and then return to its normal position. For example, if the
actuator is at 20% when the 12-hour limit is reached, then the UVC will
force the actuator to 0%, use overdrive, and then return to the 20%
position.

External Binary Inputs

The UVC is provided with three (3) binary inputs which can provide the
following functions. These inputs each allow a single set of dry-
contacts (no voltage source) to be used as a signal to the UVC, multiple
units can be connected to a single set of dry-contacts. For wiring see
MicroTech II Unit Ventilator Controller IM747.
NOTICE
Not all of the functions listed can be used at the same time. The
UVC is provided with configuration parameters which can be
adjusted to select which function will be used for these inputs when
multiple functions are indicated below.

EXTERNAL BINARY INPUT 1

This binary input can be configured as an unoccupied (default) or
dewpoint/humidity signal.

Unoccupied Input Signal

This input allows a single set of dry-contacts to be used to signal the
UVC to go into unoccupied or occupied mode. When the contacts close
(unoccupied) the UVC will go into unoccupied mode. When the
contacts open (occupied) the UVC will go into occupied mode.
Additional variables can effect occupancy mode and override this
binary input. See Occupancy Modes.
Dewpoint/Humidity Input Signal (optional for reheat type
units)
This input allows a single set of dry-contacts to be used to signal the
UVC to go into active dehumidification (see Active Dehum State for
reheat type units.) When the contacts close (high humidity) the UVC
will go into active dehumidification, when the contacts open (low
humidity) the UVC will stop active dehumidification. The device used
must incorporate its own differential dewpoint or differential humidity.
See Cool Mode.

EXTERNAL BINARY INPUT 2

This input can only be used for remote shutdown.
Page 25 of 32