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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.
8.7.4. 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 required
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 short time. If this occurs more than three
times within an hour the compressors are blocked. Before a low-pressure fault is triggered a low-
pressure warning is signalled.
8.7.5. Fan speed control
Due to the continuous adjustment in the number of fans, the condensing pressure is kept
constant depending on the ambient temperature by means of a PI controller. As the condensing
pressure at the moment in which the compressor is switched on rises very fast, the PI controller is
superimposed by a P controller with limiting setpoint. This only intervenes if the PI controller is too
slow.
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