Danfoss VLT Inverter Scroll CDS 801, Operating Instructions Manual, Page: 38 / 56

1 0 100
15
70
Compressor speed
[Hz ]
Speed request
[%]
130ZB124.10
Illustration 6.6 Compressor Speed vs. Speed Request
6.3.4 Condensing Pressure Control
The outdoor fan speed control can be done through different strategies. The fan control strategy is chosen by setting the
THERMO_IN_FAN_MODE register. The fan speed control strategy depends also on the system mode: AC vs. HP. If the system
is in AC mode, the outdoor fan controls the condensing pressure/temperature. This temperature has to be higher than the
ambient temperature, so that the condenser can reject heat. If the system is in HP mode, the outdoor fan controls the
evaporator pressure/temperature. This temperature has to be lower than the ambient temperature, so that the evaporator
can absorb heat. Fan speed control is illustrated in
Table 6.1 .
Mode THERMO_IN_FAN_MODE System mode = AC (and defrost) System mode = HP
Optimal COP 0 Pc,ref = f(P0) P0 = f(Pc)
Delta T 1 Tc,ref = Tamb + delta T
(delta T = 7K)
T0,ref = Tamb – delta T
(delta T = f(T0min, Tamb))
External Speed 2 Fan speed = THERMO_IN_FAN_EXTERNAL_SPEED_PCT
Bypass 3 Fan speed = THERMO_IN_FAN_EXTERNAL_SPEED_PCT
Table 6.1 Condensing Pressure Control
6.3.4.1 AC Mode
In AC mode, the fan controls condensing pressure, since the outdoor unit works as condenser.
Optimal Pressure Ratio Control
In Optimal COP mode, the fan speed control attempts to maintain a constant pressure ratio
Pc/P0 = constant
across the compressor, see Illustration 6.7 . This ratio is the theoretically optimal pressure ratio for the Inverter Scroll CDS 801
compressor.
To run Optimal COP mode:
Set THERMO_IN_FAN_MODE = 0
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Inverter Scroll CDS 801 Operating Instructions
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