Fridgewatch 2100 Controller Mk4
Publication 3-58
Section 3
Page 32 of 152
Issue 1.1 : 01/08
5.9.1.
Control Strategy for Limiting Inputs
A compressor undergoing limiting is considered by the Fridgewatch 2100
to be at maximum capacity.
In a multi-compressor application, if the PID algorithm calls for more duty,
the procedure for starting the next compressor is initiated. If limiting
ceases, the appropriate timers are reset. However, if the situation
continues and the conditions for starting the next compressor are met,
unless it is inhibited from starting as described under 5.6. Starting and
Stopping the First Compressor, this compressor will start, i.e. behave as
if the previous compressor was at maximum capacity. When the limiting
conditions pass, the first compressor to be started once more takes
priority for loading until it reaches maximum capacity, after which capacity
modulation reverts to the second compressor.
In a master/slave configuration, if the PID algorithm calls for more duty,
the procedure for starting the slave compressor is initiated. If limiting
ceases, the appropriate timers are reset. However, if limiting continues
and the conditions for starting the slave compressor are met, unless it is
inhibited from starting (as described under 5.6. Starting and Stopping the
First Compressor), the slave compressor will start, i.e. behave as if the
master compressor were at maximum capacity, or in a multi-compressor
application, all the compressors on the master package unit were at
maximum capacity. When the limiting conditions pass, the master
compressor once more takes priority for loading until it reaches maximum
capacity, after which capacity modulation reverts to the slave
compressor.
In a multi-compressor application, when the limiting conditions pass, the
compressors on the master package unit once more take priority for
loading until all the compressors on the master unit reach maximum
capacity, after which capacity modulation reverts to the compressors on
the slave package unit.
5.9.2. Current
Limiting
Current limiting, the type of limiting input most commonly used, protects
the compressor motor and switch gear from damage which may occur if
the motor tries to exceed the design full load current. Motor current is
displayed as monitoring point P5.
When current limiting is implemented, Fridgewatch 2100 senses motor
current via a 1 V ac analogue input from a 1 amp current transformer,
sensing current in the line conductor, with a 1
Ω
resistor connected
across its terminals.
If the measured motor current exceeds the current limit 1 (inhibit loading)
value defined by configuration settings S65, S68 and S71 for
compressors 1, 2 and 3, Fridgewatch 2100 inhibits load pulses from the
control algorithm, preventing the compressor from loading. If, however,
the motor current continues to increase and the second, higher, current
limit 2 value (force unload) is exceeded, defined by settings S66, S69 and
S71, Fridgewatch 2100 unloads the compressor until the measured
current falls below the higher value. The display shows ‘Limiting – P5
Motor Current High’ next to the compressor number, when limiting is
taking place.
5.10.
Limiting on P4 Input
This feature can only be used when the P4 analogue inputs are used for
‘other analogues’ instead of slide valve position transmitters.
This control philosophy is best illustrated by an example:
