36
IOM 1206-1
Unit Controller Operation
Unit Capacity Overrides
Unit capacity limits can be used to limit total unit capacity in
Cool mode only. Multiple limits may be active at any time, and
the lowest limit is always used in the unit capacity control.
Demand Limit
The maximum unit capacity can be limited by a 4 to 20 mA
signal on the Demand Limit analog input. This function is
only enabled if the Demand Limit set point is set to ON. The
maximum unit capacity stage is determined as shown in the
following tables:
Table 35: Four Compressors:
Network Limit
The maximum unit capacity can be limited by a network
signal. This function is only enabled if the control source is set
to network. The maximum unit capacity stage is based on the
network limit value received from the BAS and is determined
as shown in the following tables:
Table 36: Four compressors:
Maximum LWT Pulldown Rate
The maximum drop rate for the leaving water temperature
shall be limited by the Maximum Pulldown Rate set point only
when the unit mode is Cool.
If the rate exceeds the set point, no more compressors can be
started until the pulldown rate is less than the set point.
Running compressors will not be stopped as a result of
exceeding the maximum pulldown rate.
High Ambient Limit
On units configured with single point power connections, the
maximum load amps could be exceeded at high ambient
temperatures. If all compressors are running on circuit 1 or all
but one compressor on circuit 1, power connection is single
point, and the outdoor air temperature OAT is greater than
46.6°C (115.9°F), circuit 2 is limited to running all but one
compressor. This limit will allow the unit to operate at higher
temperatures than 46.6°C (115.9°F) up to 51.6°C (125°F).
When the limit is active, the unit is allowed to run all but one
compressor. So it will inhibit the unit fromloading if all but one
compressor is on, and it will shut down a compressor if all
compressors are running.
RapidRestore
®
Option
RapidRestore is an option that can be added to AGZ chillers.
The general purpose of the option is to allow the capability to
restart more quickly and to load faster than normal.
Enabling
The RapidRestore option shall be enabled via the
RapidRestore set point. Doing so will require the following to
be true:
• RapidRestore module is present at address 22
• DI1 on the RapidRestore module has a signal
If the DI1 input loses the signal or the RapidRestore module is
not longer communicating, then the option will be disabled in
the chiller.
Operation Following Power Cycle
The chiller will enter RapidRestore upon powering up when
the following conditions are met:
• RapidRestore is enabled
• Power failure lasts less than the value of the Max Power
Failure Time set point
• Power failure lasts at least one second (shorter power
loss may result in unpredictable operation)
• Unit is enabled
When RapidRestore is triggered, the time value used for the
evaporator recirculation time will be limited to 110 seconds or
less. The evaporator recirculation time set point will not be
changed. Only the value used in the evaporator state logic will
be limited, and only if the set point exceeds the 110 second
limit.
This action will ensure that the chiller is ready to start after the
motor protection module delay has expired.
Time to Start
The manufacturer requires a minimum two minute delay after
power on until a compressor should be started, which is to
ensure proper operation of the motor protection modules. Unit
controller boot time is about 10 seconds, so a delay of 110
seconds will start upon completing boot up. After this delay,
the two minute manufacturer requirement will be satisfied.
After the 110 second delay, the first circuit to start will enter
the preopen state, which takes five seconds. The end result is
that the first compressor should start approximately 125
seconds after power is restored to the chiller.
Demand Limit
Signal (%)
Demand Limit (mA)
Stage
Limit
Limit
≥
75%
Limit
≥
16 mA
1
75% > Limit
≥
50%
16 mA > Limit
≥
12 mA
2
50% > Limit
≥
25%
12 mA > Limit
≥
8 mA
3
25% > Limit
8 mA > Limit
4
Network Limit
Stage Limit
Limit
≥
100%
4
100% > Limit
≥
75%
3
75% > Limit
≥
50%
2
50% > Limit
1
