17
SYSTEM DESIGN RECOMMENDATIONS
Low ambient temperature
operation
Operating voltage range
Operating voltage and
cycle rate
|V
AVG
- V
1-2
|+|V
AVG
- V
1-3
|+|V
AVG
- V
2-3
|
2
X
V
AVG
%
VOLTAGE
UNBALANCE
:
X
100
Cycle rate limit
At low ambient temperatures,
the condensing temperature and
condensing pressure in air cooled
condensers will decrease.
This low pressure may be insuffi
cient to
supply enough liquid refrigerant to the
evaporator. As a result the evaporator
temperature will strongly decrease
with the risk of frosting. At compressor
start-up, the compressor can pull a
deep vacuum and it can be switched
off by the low pressure protection.
Depending on the low pressure switch
setting and delay timer, short cycling
can occur. To avoid these problems,
several solutions are possible, based
on reducing condenser capacity:
• Indoor location of condensers
• Liquid fl ooding of condensers (note:
this solution requires extra refrigerant
charge, which can introduce other
problems. A non-return valve in the
discharge line is required and special
care should be taken when designing
the discharge line.)
• Reduce air fl ow to condensers.
Other problems can also occur when
the compressor is operating at low
ambient temperature. During shut
down periods, liquid refrigerant can
migrate to a cold compressor.
For such conditions a belt-type
crankcase heater is strongly
recommended.
Note that with 100% suction gas cooled
motors, Danfoss MPZ compressors can
be externally insulated.
Refer to section "Liquid refrigerant
migration & charge limits" for more
details.
The operating voltage limits are shown
in the table on page 14. The voltage
applied to the motor terminals must
always be within these table limits.
The maximum allowable voltage
unbalance for 3-phase compressors
is 2%. Voltage unbalance causes high
current draw on one or more phases,
which in turn leads to overheating and
possible motor damage.
Voltage unbalance is given by the
formula:
There may be no more than 12 starts
per hour (6 when a soft start accessory
is used). A higher number reduces the
service life of the motor-compressor
unit. If necessary, use an anti-short-
cycle timer in the control circuit.
A time-out of three minutes is
recommended. The system must be
designed in such a way to guarantee a
minimum compressor running time in
order to provide proper oil return and
suffi
cient motor cooling after starting.
Note that the oil return rate varies as a
function of the system design.
Vavg = Mean voltage of phases 1, 2 and 3
V1-2 = Voltage between phases 1 and 2
V1-3 = Voltage between phases 1 and 3
V2-3 = Voltage between phases 2 and 3