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
ref
) is therefore made dependent on
the ambient temperature, see
.
shows how the set point for the Delta T
control mode in HP system mode is a function of ambient
temperature and T0
min
. As the ambient temperature
approaches the T0
min
line (see
), the Delta T
is lowered. This is done to prevent too low evaporator
temperatures when the ambient temperature is low.
130ZB126.10
T0,ref
T ambient
T0 min + 10K
T0 Min + 20K
T ambient
T ambient -
DeltaTemp
D
elt
a T
Illustration 6.13 T0
ref
= f(Tambient) in HP mode (Delta T)
Performer VSD Operating Envelope
35
45
55
65
75
85
95
105
115
125
135
145
155
-40 -30 -20 -10
0
10
20 30 40
50 60
70 80
Saturated evaporating temp . (°F)
Sa
tu
ra
te
d
co
nd
en
si
ng
te
m
p
. (
°F
)
2
6
10
14
18
22
26
30
34
38
42
46
50
54
58
62
66
70
-40 -36 -32 -28 -24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24
°
C
900-4200
Line 1
Line 5
Line 6
Line 7
Line 14
A10
T 0MinLine
175ZB127.10
°C
Illustration 6.14 Definition of T0
min
line. The T0
min
line is the
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
.
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.
175ZB125.10
FanMotor
Evaporating
Unit
External
speed setpoint
P
Fan
Protection
0
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
. 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.
FanMotor
Evaporator
External
speed command
P
0
175ZB114.10
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
. 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).
Embedded Application Contro...
VLT
®
Inverter Scroll CDS 801 Operating Instructions
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®
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39
6
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