Filter
Top Products
76
Table 41 — Temperature Limits for
Standard 30RA Units
LEGEND
*For sustained operation, EWT should not exceed 85 F (29.4 C).
†Unit requires modification below this temperature.
LOW-AMBIENT OPERATION — If operating temperatures
below 45 F (7 C) for sizes 010-030 or below 32 F (0° C) for
sizes 032-055 are expected, accessory Motormaster® V con-
trol must be installed. Refer to separate installation instructions
for operation using this accessory. Contact your Sterling repre-
sentative for details.
VOLTAGE — ALL UNITS
Main Power Supply
— Minimum and maximum acceptable
supply voltages are listed in the Installation Instructions.
Unbalanced 3-Phase Supply Voltage — Never operate a motor
where a phase imbalance between phases is greater than 2%.
To determine percent voltage imbalance:
The maximum voltage deviation is the largest difference
between a voltage measurement across 2 legs and the average
across all 3 legs.
Example: Supply voltage is 240-3-60.
AB = 243 v
BC = 236 v
AC = 238 v
1. Determine average voltage:
2. Determine maximum deviation from average voltage:
(AB) 243 – 239 = 4 v
(BC) 239 – 236 = 3 v
(AC) 239 – 238 = 1 v
Maximum deviation is 4 v.
3. Determine percent voltage imbalance:
This voltage imbalance is satisfactory as it is below the
maximum allowable of 2%.
Control Circuit Power
— Power for the control circuit is
supplied from the main incoming power through a factory-
installed control power transformer (TRAN1) for all models.
Field wiring connections are made to either terminal block
TB5 or TB6.
OPERATION SEQUENCE
During unit off cycle, the control monitors the outdoor air
temperature. If the ambient temperature drops below 40 F
(4.4 C), cooler and hydronic system heaters (if either are facto-
ry installed) are energized. If power is maintained to the chiller
and the EMERGENCY ON/OFF switch is left in the OFF po-
sition, these heaters are also energized.
The unit is started by putting the ENABLE/OFF/REMOTE
CONTACT switch in the ENABLE or REMOTE CONTACT
position. When the unit receives a call for cooling (either from
the internal control or SCN network command or remote con-
tact closure), the unit stages up in capacity to maintain the leav-
ing fluid set point. The first compressor starts 1
1
/
2
to 3 minutes
after the call for cooling.
The lead circuit can be specifically designated on all models
or selected based on compressor run hours and starts depend-
ing on field configuration. The unit control will override this
selection under certain starting conditions to properly maintain
oil return to the compressors. In general, on dual compressor
circuits, the control will most often start the A1 or B1 compres-
sor first, especially after long off periods. The MBB controls
fan stages to maintain the head pressure set point and will auto-
matically adjust unit capacity as required to keep compressors
from operating outside of the specified envelope. There are no
pumpout or pumpdown sequences on these chillers.
For all units, if temperature reset is being used, the unit con-
trols to a higher leaving-fluid temperature as the building load
reduces. If demand limit is used, the unit may temporarily be
unable to maintain the desired leaving-fluid temperature be-
cause of imposed power limitations. Loading sequence for
compressors is shown in Tables 6 and 7.
UNIT SIZE 30RA 010-030 032-055
Temperature F C F C
Maximum Ambient
Temperature
120 49 120 49
Minimum Ambient
Temperature
45 7 32 0
Maximum Cooler EWT*
95 35 95 35
Maximum Cooler LWT
70 21 70 21
Minimum Cooler LWT†
40 4.4 40 4.4
EWT —
Entering Fluid (Water) Temperature
LWT —
Leaving Fluid (Water) Temperature
Brine duty application (below 40 F [4.4 C] LCWT) for
chiller normally requires factory modification. Contact
your Sterling representative for applicable LCWT range for
standard water-cooled chiller in a specific application.
% Voltage Imbalance = 100 x
max voltage deviation
from avg voltage
average voltage
Average voltage =
243 + 236 + 238
3
=
717
3
= 239
% Voltage Imbalance = 100 x
4
239
= 1.7%
IMPORTANT: If the supply voltage phase imbalance is
more than 2%, contact your local electric utility company
immediately. Do not operate unit until imbalance condition
is corrected.