Version 2.02 15.05.2007
Function of VS 3000
11
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3.4 COP optimization
The system’s coefficient of performance is a function of compressor s peed. So as to prevent the system running
continuously at a poor COP, the controller switches to an operating point of better energy efficiency. If com-
pressor speed exceeds 78 Hz or 52 Hz for more than 3 minutes with capacity-controlled compressors, com-
pressor capacity is loaded up by one stage. Compressor s peed is reduced at the same time.
Speed without capacity control: Controller output = (Current controller output - 78 Hz) : 2
Speed with capacity control: Controller output = (Current controller output - 52 Hz) : 4
Controller output = 0 to 10 V signal for speed adjuster
Current controller output = Controller output prior to COP optimization.
3.5 Compressor control times
Compressor actuation with step control:
Compressor actuation takes place only outside the neutral zone, after a certain time for loading or unloading has
passed and when the control error has exceeded a defined level (neutral zone).
Compressor actuation with combined control:
If low pressure is greater than the setpoint, the variable-speed compressor is cut in immediately.
Loading or unloading of additional fixed-speed compressors takes place when maximum or minimum speed of
the compressor is attained and a certain time for loading or unloading has passed.
The time delay is dependent on the actual control error. With a large control error, actuation takes place in a
shorter time than when the control error is smaller. The control time is calculated as the sum of basic time t
b
and
variabletimet
v
. Differentiation is made between up and down stepping of the step controller.
The variable time is inversely proportional to the control error. At maximum c ontrol e rror the variable time is
t
v
= 0. As the control error decreases, time t
v
automatically increases up to a defined maximum. Basic time and
maximum variable time for loading (starting) and unloading (stopping) can be programmed as parameters for
each capacity stage.
With combined control, speed control is not enabled until the basic time of the first compressor capacity stage
has expired. Until this time, the variable-speed compressor is operated at lowest speed. Relationships for deter-
mining control times are as follows:
t = t
b
+ t
v
t
b
= Basic time. Can be programmed for each loading of a compressor capacity stage.
t
v
= Variable control time
For t
v
t
v
= t
v_max
−
t
v_max
⋅ d
t
d
t_max
The following applies:
d
t
≻ d
t_max
is equal to d
t
= d
t_max
t
v
= Variable control time
t
v_max
= Maximum variable control time (definable for each capacity stage)
d
t
= Control error
d
t_max
= Maximum control error/constant (definable)
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