ECKELMANN VS 300 - Loading and Unloading Fixed-Speed Compressors

Function of VS 300
30
Doku-Version 2.05 - Firmware 2.10 - 15. September 2017
4.3.8 Loading and unloading fixed-speed compressors
If the required capacity cannot be delivered by varying the compressor speed, fixed-speed compressors can be
loaded or unloaded allowing for the compressor actuating times (see section 4.3.9 Compressor Actuating
Times).
After the variable-speed compressor has attained maximum speed, the fixed-speed compressor having the
shortest run time is loaded. The variable-speed compressor is unloaded to the level at which the pack capacity
corresponds to the capacity prior to loading the supplementary fixed-speed compressor.
After the variable-speed compressor has attained minimum speed, the fixed-speed compressor having the long
est run time is unloaded. The variable-speed compressor is loaded up to the level at which the pack capacity
corresponds to the capacity prior to unloading the supplementary fixed-speed compressor.
The following diagram shows the control sequence for a two-compressor pack:
Compressor control
Control of speed-controlled
compressors [V]
Relative refrigeration capacity of compressor [%]
10 V10 V
8 V
0 V0 V
1,3 V
0 V
10 V
5 V
ZNR. 51203
81 330 E0
The characteristic of the frequency changer must be parameterisable for this so that an output signal of
0 V at the analogue output for compressor control corresponds to the minimum frequency and an output signal
of 10 V to the maximum frequency. The control signal of the frequency changer must be parameterised as 0 V -
10 V interface.
4.3.9 Compressor actuating times
Compressor actuation with step control:
Compressor actuation takes place only outside the neutral zone, when a defined time load/unload time has
elapsed and the control error has exceeded a defined value (neutral zone).
Compressor actuation with combined control:
If suction pressure is greater than the setpoint, the variable-speed compressor is loaded immediately. Loading
or unloading of supplementary fixed-speed compressors takes place when maximum or minimum compressor
speed is attained and a certain time for loading or unloading has passed.
The delay is a function of the actual control error. With a large control error, actuation takes place quicker than
with a small control error. The actuating time is calculated from basic time t
b
plus variable time t
v
. Differentiation
is made between loading and unloading by the step controller.
The variable time is inversely proportional to the control error. At maximum control error the variable time t
v
is 0. As
the control error becomes smaller, time t
v
is automatically increased up to a defined maximum.