1.
2.
Control logic
Safety control logic requirements
Table 33: Safety control logic requirements
Safeties
Tripping conditions
Re-start conditions
Value
Time
Value
Time
HP safety switch
See Pressure settings table
from section
Immediate, no delay. No by-
pass
Conditions back to normal.
Switch closed again.
Manual reset
LP safety switch
Maximum 5 auto reset during a
period of 12 hours, then man-
ual reset.
Electronic module (Motor pro-
tection, DGT)
Contact M1-M2 opened
Cycle rate limit requirements
Danfoss requires a minimum compressor running time of 2 minutes to ensure proper oil return and sufficient motor
cooling.
Additionally, compressor must not exceed 12 starts per hour. 12 starts per hour must not be considered as an
average, this is the maximum number of starts acceptable to keep a good regulation accuracy during low load.
Oil management logic recommendations
In some cases, oil management can be enhanced by control logic:
• If oil return test failed, a function can be integrated in control logic to run all compressors simultaneously during 2
minutes every hour in order to boost oil return. Time and delay can be fine-tuned by oil return test No.1 in section
. During oil boost, pay special attention to superheat management to avoid liquid flood
back.
• In trio system, after running long time in same state with 2 or 3 compressors, (1+2+3) or (1+2), (2+3) or (3+1), oil
unbalance may appears. A function can be implemented in control logic to stop all compressors during one
minute every two hours in order to balance oil. Time and sequence can be fine-tuned during Oil balancing test in
section
Defrost logic recommendations / Reversible systems
In reversible systems, the defrost logic can be worked out to limit liquid flood back effect by:
Running full load during defrost to share liquid refrigerant between all compressors.
Reducing refrigerant flooding to compressor by transferring liquid refrigerant from one exchanger to the other
before reversing valve thanks pressures.
The following defrost logic combines both advantages:
Figure 59: Defrost logic advantages
1
Cp 1
ON
Cp 2
ON
H
100%
2
6
5
8
9
7
3
4
CC-000019
4WV
EXV
Scroll compressors, DSF090 to DSF530 | Application
© Danfoss | Climate Solutions | 2023.06
AB309735209512en-000603 | 52
