ARTC-SVX013A-EN
73
Compressor Control States
Figure 37. Compressor control states flow chart
Absent State
A chiller may contain less than the maximum allowed
number of compressors. This leaves some of the state
machines unused and uninitialized. This is indicated by the
absent state.
Offline State
The offline state indicates that the compressor is not to be
started. This can be due to three reasons:
1. The operator has taken the compressor offline for
maintenance.
2. The compressor encountered a fault state which cannot
be recovered by the controller (the operator needs to
recycle the power manually).
3. The controller lost its Modbus connection to the
compressor.
Idle State
The compressor is currently idle but it is online and ready
for use.
Starting State
The compressor has received a start demand and is in the
process of speeding up. The state automatically switches
into the operational state when the compressor reaches
sufficient speed to operate normally without risk of surging.
Operational State
The compressor is in normal operation.
Stopping State
The compressor has received a stop signal and is in the
process of slowing down. The state automatically switches
into the idle state when the compressor reaches full stop.
Retreating State
Compressors cannot be started against a high-pressure
ratio. Thus, it is sometimes necessary to bring the pressure
ratio down before another compressor can be started.
It is advisable to do this as quickly as possible so that the
interruption of the normal operation is reduced to a
minimum. This involves the entire chiller (for example
utilizing condenser and bypass valves).
The compressors that are already running can help by
reducing their power as much as possible. This is
implemented as the retreating state: power is reduced as
long as the pressure ratio is too high to start another
compressor. It automatically switches into the hold state as
soon as the threshold is reached.
Hold State
The compressor keeps running at low power after the
retreating state has changed into the hold state. It returns
to normal operation after the new compressor has been
started (i.e. when it has reached its operational state).
Fault State
The compressor has shut itself down after it detected a
fault. The controller attempts to clear the fault, with three
possible outcomes:
1. The fault is reset, and the compressor is made
available for normal use again by switching into the idle
state.
2. After overheating faults (motor or electronics have
become too hot), the compressor is given some extra
time to cool down by changing into the timeout state.
This ensures that the compressor cools down well
below its alarm threshold before it can be started again.
3. Some faults cannot be reset by the controller, for
example when the compressor is damaged. The
controller then takes this compressor out of circulation
by switching into the offline state.
Timeout State
The compressor has recovered from an overheat fault. It is
given some additional time to cool down before it becomes
available.
Power Failure
Indication when a power failure has been detected. All
other faults and alarms are suppressed and cleared.
Power Restore
Main power has been restored, if Fast Restart enabled,
system will begin the process.
Fast Restart
Fast Restart is enabled, compressors that were running are
started again.
Sequence of Operations
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