Defrost Mode; Auxiliary Functions; Start-Up Sequence - Danfoss VLT Inverter Scroll CDS 801 Operating Instructions Manual

Embedded Application Contro...
To transfer heat to the evaporator, a temperature
difference is needed (evaporator temperature must be
lower than the ambient temperature). Under cold ambient
conditions this brings the suction pressure close(r) to the
edge of the operating envelope. It is therefore important
to make the temperature difference between evaporator
and ambient as small as possible under cold conditions.
The Delta T, (or the T0 ) is therefore made dependent on
ref
the ambient temperature, see Illustration 6.13.
Illustration 6.13 shows how the set point for the Delta T
control mode in HP system mode is a function of ambient
temperature and T0 . As the ambient temperature
min
approaches the T0 line (see Illustration 6.14), the Delta T
min
is lowered. This is done to prevent too low evaporator
temperatures when the ambient temperature is low.
T0, ref
T ambient
T ambient -
DeltaTemp
T0 min + 10K T0 Min + 20K
Illustration 6.13 T0 = f(Tambient) in HP mode (Delta T)
ref
Performer VSD Operating Envelope
° C
-40 -36 -32 -28 -24 -20 -16 -12 -8 -4 0 4
155
145
135
T 0MinLine
125
115
Line 1
105
95
85
75
65
55
45
35
-40 -30 -20 -10 0 10 20
Saturated evaporating temp . (°F)
Illustration 6.14 Definition of T0
leftmost line(s) of the envelope
Open Loop/ External Speed Control
In external speed control, the fan speed is set externally
from the OEM controller through the Modbus. The speed
can be overruled by the fan protection functions like min
and max condensing pressure derating, see Illustration 6.15.
®
VLT Inverter Scroll CDS 801 Operating Instructions
T ambient
8 12 16 20 24
70
Line 14
66
62
58
54
50
Line 7
46
42
900-4200
38
34
Line 5 30
26
22
18
Line 6
14
10
6
A10
2
30 40 50 60 70 80
line. The T0 line is the
min min
®
MG10V402 - VLT is a registered Danfoss trademark
To set the fan speed control to external speed control:
Set THERMO_IN_FAN_MODE = 2
The external speed % is set in the
THERMO_IN_FAN_EXTERNAL_SPEED_PCT register.
Fan
External
FanMotor
speed setpoint
Protection
Illustration 6.15 Open Loop/External Speed Control
Bypass fan speed control
In Bypass speed control, the fan speed is set externally
from the OEM controller through the Modbus. In bypass
mode, the external speed is bypassing the fan protection
functions, and will never be overruled by functions like
min and max condensing pressure derating, see
Illustration 6.16. To set the fan speed control in to Bypass:
Set THERMO_IN_FAN_MODE = 3
The external speed % is set in the
THERMO_IN_FAN_EXTERNAL_SPEED_PCT register.
External
FanMotor
speed command
Illustration 6.16 Bypass Fan Speed Control

6.3.4.3 Defrost Mode

In Defrost mode the fan controls condensing pressure,
since the outdoor unit works as a condenser. The same fan
control options like in AC are available, when defrosting
the outdoor coil.

6.3.5 Auxiliary Functions

6.3.5.1 Start-Up Sequence

When the system starts up from idle in AC or HP mode,
the compressor follows a start up sequence, see
Illustration 6.17. This sequence is defined by a ramp (2.7
Hz/s), start speed (THERMO_SETUP_START_UP_SPEED_HZ)
and hold time for start speed
(THERMO_SETUP_START_UP_HOLD_TIME_SEC).
Evaporating
P
Unit
0
6
Evaporator
P
0
39
6