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Operating and service manual
Below figure indicates the ranges for the capacity and compressor speed (frequency).
20 Hz (600 rpm)
line 2
line 3
line 1
Set point
Set point
110 Hz (3300 rpm)
-100%
cooling
100%
heating
-20%
+10%
Compressor frequency, Hz
Capacity
Line 1: The area where cooling is requested, controlled by the frequency converter.
Line 2: The area where the compressor is on/off regulated.
Line 3: The area where heating is requested, controlled by pulsing of heating elements.
The limiter function secures that the controller operates with valid settings to protect the unit in
order to maintain the conditions for the cargo. To maintain the set point temperature, capacity
control constantly monitors and adjusts the capacity. The limiter acts as a brake to the capacity
change requests from capacity control and thereby controlling how fast the capacity can change,
so that safe operation of the unit constantly is ensured.
The limiter monitors the following parameters from the unit and generates a limiter factor for each:
•
IFC, to limit the maximum current draw from the FC.
•
TFC, to limit the maximum internal FC temperature.
•
Tc, to limit the maximum condenser pressure (and temperature).
•
T0, to ensure a minimum evaporator pressure.
•
Teco, to ensure a minimum middle pressure in the compressor.
The largest of the factors is used as the active limiter. If the limiter factor is higher than the re-
quested capacity change, the capacity is actually reduced instead of increased.
If for example the ambient temperature is very high, the requested capacity may increase the FC
temperature over its limits and so the limiter will reduce the capacity until a safe and stable opera-
tion condition for the FC has been reached.
Expansion Valve
This function ensures optimum evaporator superheat (SH) and calculates the percentage of open-
ing (SHVod) and controls the valve. This function is active during compressor operation. Valve is
closed during compressor turn off.
Expansion valve function includes the following sub-functions:
•
MSS (Minimum Stable Superheat search).
•
Superheat control.
•
MOP (Maximum Operating Pressure).
MSS
This function searches for minimum stable superheat within the ranges SHmin and SHmax. With a
stable T0, SHset is reduced and with an unstable T0, SHset is raised.
SHact: = Tsuc – T0
Superheating
Function output is the expansion valve opening rate (Vexp).