Danfoss VLZ065, Application Manuallines, Page: 30 / 54

System evaluation
Manage operating envelope
Single envelope control: Limit speed range from 3000RPM to 6600RPM
VLZ drive can only protect the compressor from
over current. To manage operating envelop, an
advanced envelope protection principle needs
to be used with variable speed compressors.
This solution offers much better protection than
basic protection, and also offers the possibility
to adjust running conditions to avoid tripping
(for example reduce compressor speed when
reaching high pressure limit).
The advanced protection principle is based
on a permanent measurement of suction
and discharge pressure. Unit controller is
permanently checking that the compressor is
running within the defined envelope.
When compressor reach a limit, controller can act
on different parameter to avoid unit tripping.
On top of suction and discharge pressure
limitations, the discharge T° must remain below
135°C (275°F).
Low pressure switch and high pressure switch
remain necessary as an ultimate protection.
The whole envelope can be used on the whole
speed range, see “Single application envelope”
Depending on speed range needed, two types
of controls to be considered as below (R404A as
examples):
• Speed range limited from 3000RPM to 6600RPM
• Full speed range from 1800RPM to 6600RPM
Protection required
• HP switch*
• LP switch*
• DGT set @ 135°C (275°F) if necessary see “Discharge T°
protection”
• Measurements of suction and discharge pressure,
unit controller with is permanently maintaining
compressor within its envelope.
• drive setting:
parameter 4-11[ Motor speed low limit(rpm)]:
set value as 3000
Controller do not need to manage speed limitation according to operating conditions.
OE-000066
3000-6600RPM
HP switch setting
MOP
Condensing pressure control
Minimum pressure ratio
LP switch setting
3300-6600RPM
DGT
30
AB444226142054en-000101