32
IOM 1206-1
Unit Controller Operation
Unit States
The unit will always be in one of three states:
Off – Unit is not enabled to run
Auto – Unit is enabled to run
Pumpdown – Unit is doing a normal shutdown
Transitions between these states are shown in the following
diagram.
Diagram explanation on following page.
T1 - Off to Auto
All of the following are required:
• Unit Enable = On
• No Unit Alarm
• A circuit is enabled to start
• If Unit Mode = Ice then Ice Delay not active
T2 - Auto to Pumpdown
Any of the following are required:
• Unit Enable = Off and Unit Switch is closed
• Unit Mode = Ice AND LWT target is reached
• Unit Pumpdown Alarm active
T3 - Pumpdown to Off
Any of the following are required:
• Unit rapid stop alarm active
• All circuits complete pumpdown
• Unit Switch open
T4 - Auto to Off
Any of the following are required:
• Unit rapid stop alarm active
• No circuit enabled and no compressors running
• Unit Switch open
Power Up Start Delay
After powering up the unit, the motor protectors may not be
engaged for up to 150 seconds. Therefore, after the control is
powered up, no compressor can start for 150 seconds. In
addition, the motor protect inputs are ignored during this time
so as to avoid tripping a false alarm.
Ice Mode Start Delay
An adjustable start to start ice delay timer will limit the
frequency with which the chiller may start in Ice mode. The
timer starts when the first compressor starts while the unit is in
ice mode. While this timer is active, the chiller cannot restart
in Ice mode. The time delay is adjustable via the Ice Time
Delay set point.
The ice delay timer may be manually cleared to force a restart
in ice mode. A set point specifically for clearing the ice mode
delay is available. In addition, cycling the power to the
controller should clear the ice delay timer.
Low Ambient Lockout
When the OAT drops below the low ambient lockout set point
and the OAT sensor fault is not active, low ambient lockout is
triggered. The unit should go into the pumpdown state if any
circuits are running. If no circuits are running the unit should
go into the off state. This condition should clear when OAT
rises to the lockout set point plus 2.5°C (4.5°F).
OFF
AUTO
PUMPDOWN
T
3
T 2
T
4
T
1
POWER
ON
