ARTC-SVX007A-EN
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Restart State
Note: Not needed on Thermafit TACW.
It is assumed in the stage-in state that the pressure ratio
will come down below the threshold where another
compressor can be started. However, it must be assumed
that this is not always the case (although this should not
happen in a well-designed system unless it is
malfunctioning). In that case, all compressors will be
stopped and restarted together with one additional
compressor. In the restart state, all compressors are
signaled to stop. As soon as they have all reached full stop,
the controller signals start to all compressors that need to
be started and switches into the start state.
Stage-out State
Note: Not needed on Thermafit TACW.
In the stage-out state, one compressor is instructed to stop
in order to reduce capacity. When it has reached full stop,
the state switches automatically back to the operational
state.
Shutdown State
The shutdown state is similar to the stage-out state with the
sole exception that in this case the last running compressor
is being stopped. The shutdown state therefore switches
into the idle stage when this compressor reaches full stop.
Compressor Control States
Figure 45. 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 (0% demand)
and is in the process of slowing down. The state
automatically switches into the idle state when the
compressor reaches full stop.
Retreating State
Note: Not needed on Thermafit™ TACW.
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
Note: Not needed on Thermafit TACW.
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:
Sequence of Operations
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