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Figure 20: Cooling and heating sequence
Different operating states reduce the number of cooling levels required by one cooling level:
High pressure warning
Low pressure warning
Difference between high and low pressure is too large.
4.5.4.
Compressor control
The control is defined for a refrigeration circuit with a maximum of four compressors. The number of
compressors is selected by means of coding resistance. The number of compressors is determined once
when the control is restarted (switching on of the operating voltage) and is compared with the number of the
last restart. If these are not the same the "Coding resistance changed" alarm is generated. When the alarm
is acknowledged the new number of compressors is stored in the E-EPROM and the factory setting is accepted
as the new number of compressors.
The compressors are requested depending on the controller output signal. The compressor with the least
number of operating hours switches on first. If no more refrigerating capacity is reqstuired the compressor
that switched on first also switches off first. The compressors are switched on with a time delay (comment:
only applies if several compressors exist)
In the event of a fault in a compressor (motor protection switch) a fault changeover is activated. The high
pressure is monitored on the hardware side by means of the high-pressure limiter. If it is triggered all
compressors switch off immediately. In addition, the high pressure is controlled by means of a pressure
transmitter. This also switches off the compressors if the setpoint is exceeded, but releases them again if the
value falls below the release value. Before switching off the, high-pressure transmitter signals a warning.
Low pressure is also monitored by pressure transmitters. If the value falls below the setpoint the compressors
switch off. If the switch-off value is exceeded by the hysteresis, the compressors are released again. The
triggering of the alarm is delayed by two timing elements. The first timing element is started with the request
for the compressor. If no intake pressure has built up by the time the time expires the low-pressure fault
alarm is triggered. After the start time has expired the low pressure may exceed the switch-off value for a
Y
controller
Temperature
100 %
- 100 %
Heating =
electric
Y- cooling
Compressor 1 off
Set - heating
Hy-heating
Set - cooling
P-Band
cooling
Compressor
2 off
Compressor 3 off
Compressor 4 off
Compressor 4 on
Compressor 3 on
Compressor
2 on
Compressor 1 on
75 %
50 %
25 %
- 75 %
- 50 %
- 25 %