Danfoss VZH052 Manual page 38

Control logic
Safety control logic
requirements
HP switch
LP safety switch
High pressure
Low pressure
Electronic expansion valve
Reverse rotation protection
38
AB237586440343en-000601
Tripping conditions
Value
See Pressure settings table
Immediate, no delay.
from section "Manage
operating envelope"
According to EN378-2, a high-pressure (HP) safety
switch is required to shut down the compressor.
The high-pressure switch can be set to lower
values depending on the application and
ambient conditions. The HP switch must either be
placed in a lockout circuit or consist of a manual
reset device to prevent cycling around the high-
A low-pressure (LP) safety switch must be
used. Deep vacuum operations of a scroll
compressor can cause internal electrical arcing
and scroll instability. VZH compressors exhibit
high volumetric efficiency and may draw very
low vacuum levels, which could induce such
a problem. The minimum low-pressure safety
switch (loss-of-charge safety switch) setting is
given in the following table. For systems without
pump-down, the LP safety switch must either be
a manual lockout device or an automatic switch
With variable capacity systems, an electronic
expansion valve (EXV) is the strongly
recommended solution to handle refrigerant
mass flow variations. Danfoss recommends the
use of ETS products. Ramp-up and ramp-down
settings, of both EXV and compressor, must be
done with great care.
Ramp-up of the EXV must be shorter than
the ramp-up of the compressor, to avoid any
low pressure operation on suction side of the
Due to drive protection, compressors could work
properly even if the power connection between
the drive and mains is dis-matched. However, the
wires between compressor and drive must be
connected accordingly. To protect compressors
from reverse rotation, pressure difference could
Time Value
Conditions back to normal.
No by- pass Switch closed again
pressure limit. If a discharge valve is used, the HP
switch must be connected to the service valve
gauge port, which must not be isolated. The HP
switch must be connected to the CDS303 input
37 or an external contactor placed before and
after the drive.
wired into an electrical lockout circuit. The LP
switch tolerance must not allow for vacuum
operations of the compressor. LP switch settings
for pump-down cycles with automatic reset are
also listed in the table below. Lock-out circuit or
LP switch or series with other safety devices must
be connected to CDS303 input 27.
OEM need to set port 27 to "coast inverse or
external interlock" to get rid of minimum running
time restriction.
compressor. The EXV can also be opened, up
to a certain degree, before the start up of the
compressor.
Ramp-down of the EXV must be longer than the
ramp-down of the compressor, also to avoid low
pressure operation (except with pump-down).
EXV should be closed, and remain closed,
when the compressor is off, to avoid any liquid
refrigerant entering the compressor.
be checked as a reference value. Use pressure
sensors to monitor pressure difference between
discharge and suction of the compressor, and for
normal operation, discharge pressure should be
at least 1 bar higher than suction pressure within
30 s running after compressor starting.
Re-start conditions
Time
Manual reset
Maximum 5 auto reset during
a period of 12 hours, then
manual reset.