Fridgewatch 2100 Controller Mk4
Section 3
Publication 3-58
Issue 1.1 : 01/08
Page 89 of 152
Before making alterations to S5 Proportional Band, S6 Integral Action
Time or S7 Derivative Action Time, it is essential to have read and
understood the notes under 5. Principles of Control.
S7
Derivative action time (units: second)
This is the time, in seconds, required for the proportional action to equal
the derivative action, given a ramp change in demand. The derivative
term introduces a predictive quantity into the control algorithm, and in
doing so increases both the stability and the response of the control
system to deviations away from the set point. This occurs because the
derivative term works with the proportional term when the measured
value is moving away from the set point, and against the proportional
term when moving towards the set point.
Increasing the derivative action time increases the derivative response,
which to a certain degree, will tend to increase the stability of the control
system at the expense of the initial response time. However, increasing
the derivative action time too far, may lead to instability as it over-
compensates for the proportional response. Setting the derivative action
time to zero has the effect of switching off the derivative function.
Before making alterations to S5 Proportional Band, S6 Integral Action
Time or S7 Derivative Action Time, it is essential to have read and
understood the notes under 5. Principles of Control.
S8
Next compressor start delay (units: second)
This is the time, in seconds, that the currently controlled compressor, i.e.
the compressor whose capacity is being modulated, must run
continuously at maximum load before the next lag compressor in the
running order priority is started (should one be available) (subject to S12
Set Point Deviation to Start Next Compressor).
S9
Next compressor stop delay (units: second)
This is the time, in seconds, that the currently controlled compressor, i.e.
the compressor whose capacity is being modulated, must run
continuously at minimum load before it is stopped and control passed to
the previously started compressor.
S10 Hold last running compressor at 50 % (units: %)
Set to 1 if the compressor left running must not be allowed to unload
below 50 %. This may be desirable if one compressor will just hold the
load on minimum capacity but never actually stop. Using this option can
force the measured variable (pressure or temperature) down to the stop
value. Use the default value of 0 if not required.
S10 is only enabled when ‘auto’ capacity control mode is selected.
S11 Enable unload off maximum on starting next compressor
Selecting a value of 1 forces the compressor at maximum load to unload
by a small amount when the next compressor starts, thus minimising the
effects of a sudden increase in overall compressor capacity. Use the
default value of 0 if not required.
S12 Set point deviation to start next compressor (units: pressure
in bar g or temperature in °C)
In addition to the time delay before starting the next compressor (S8), it is
also possible to set a deviation from the set point which must be
exceeded before the next compressor starts. This has the effect of
permitting the set point to float up by an allowable amount, and can
prevent the next compressor starting unnecessary.
