Specifications

46
b. Saturated suction temperature or saturated discharge
temperature is less than 41 F (5 C).
If any of these conditions 1, 2, 3 or 4 are met, the appropri-
ate operating mode, 21 (Circuit A) or 22 (Circuit B) will be in
effect.
Override #41: Circuit A High Current Override
Override #42: Circuit B High Current Override — This
override attempts to avoid an overcurrent failure. The algo-
rithm is run every 4 seconds. If the compressor current is great-
er than 79% of must trip amps (MTA) but less than 85% MTA
then the capacity will be held at current capacity. If the com-
pressor current is greater than 85% MTA then capacity will be
reduced by repositioning the slide valve until the current is less
than 85% MTA (Must Trip Amps, MTA.X).
Override #44: Circuit A High Suction Superheat at Part
Load
Override #45: Circuit B High Suction Superheat at Part
Load — If the compressor of the circuit is on, the compressor
current is no more than 30% of the MTA, main EXV is more
than 90% open and the suction superheat is higher than the
superheat control point for more than 5 minutes, then the cir-
cuit will be shut down.
Override #53: Circuit A Delay for Unloading the Slide
Val ve
Override #54: Circuit B Delay for Unloading the Slide
Val ve — This override prevents the compressor from re-start-
ing with locked rotor failure after being shut down due to an
alarm or power cycle. A delay of 20 minutes will elapse for all
units. The delay allows the slide valve of the compressor to
move back to its fully unloaded position. The delay is adjusted
according to the percent of the compressor running capacity
before it is shut down. If the compressor is stopped normally,
no delay will be applied. If the compressor is shut down by the
locked rotor alarm, a full delay will be applied before the com-
pressor is allowed to re-start.
Override #59: Circuit A Low Oil Level
Override #60: Circuit B Low Oil Level — This override is
only effective when the circuit is not running. The override will
prevent the circuit from starting up with a low oil level. If this
override occurs three times, the low oil level alarm will be
tripped.
Override #62: Circuit A High Motor Temperature Override
Override #63: Circuit B High Motor Temperature Override
— This override prevents the compressor motor temperature
from rising above the high temperature limit, but still allows
the chiller to run close to the high temperature limit by unload-
ing the compressor. If the motor temperature is greater than
214 F (101.1 C), the compressor will not load. This override
will remain active until the temperature drops below 214 F
(101.1 C). If the motor temperature is greater than 225 F
(107.2 C) for 60 seconds, the circuit capacity will decrease by
one stage. If the motor temperature is greater than 228 F
(108.9), the circuit capacity will decrease by one stage
immediately.
Override #66: Circuit A High Discharge Gas Override
Override #67: Circuit B High Discharge Gas Override —
There are two control methods possible for this override:
1. If the leaving fluid temperature exceeds the freeze set-
point (LOSP) + 5.4 °F (3 °C) and the compressor is not at
full load, the compressor capacity will be increased when
discharge gas temperature exceeds 190 F (87.8 C). Com-
pressor capacity will be allowed to continue increasing
until discharge gas temperature falls below 175 F
(79.4 C). When the temperature is above the limit minus
2° F (1.1° C) increase in capacity will not be allowed.
This override will remain active until the discharge gas
temperature drops below the limit by –3° F (–1.7° C).
2. If the leaving fluid temperature is 3.6 °F (2 °C) or more
below the freeze setpoint (LOSP), the compressor capaci-
ty will be decreased when discharge gas temperature ex-
ceeds 190 F (87.8 C). If after five minutes, the discharge
gas temperature falls below 175 F (79.4 C), the compres-
sor will be allowed to load again.
Override #69: Water-Cooled Heating Mode Low Entering
Water Temperature — This override decreases capacity if the
unit is in heating mode and the cooler leaving water tempera-
ture is below one of the two cooling setpoints. This protection
is to avoid low water temperature on cooler in heating mode.
Override #70: Water-Cooled Max Condenser LWT = 113 F
(45 C) — This override is activated only if the water-cooled
unit option “Maximum Condenser LWT = 45degC (113 F)” is
activated. If condenser leaving water temperature is above
111.2 F (44 C), then the compressor capacity cannot increase.
If condenser leaving water temperature is above 113.0 F
(45 C), then the compressor capacity will decrease.
Override #71: Wait Until Pump Turns On Override
— This
override is activated only if the water-cooled unit option “turn
cooler/condenser pump off in heating:cooling mode” is activat-
ed. When unit is not running and about to start, wait until the
pump turns on before starting compressor to avoid HP trips or
cooler freeze.
Override #77: Boostload Function Active
— This override
can be present when boostload function is enabled. It is set in
the following conditions:
If fastload = BOOSTLOAD and cooler leaving water tem-
perature > control point + 5.4° F and cooler entering water tem-
perature > control point + 9.0° F demand limit > 99 and current
limit select = NO.
Override #80: Circuit A, 81 Circuit B: Long Time Minload
Run — If circuit runs for more than 30 minutes at minimum
load, the circuit is stopped. This is to prevent loss of oil.
Head Pressure Control — The Main Base Board
(MBB) uses the saturated condensing temperature input from
the discharge pressure transducer to control the head pressure
control signal. The control will automatically reduce the unit
capacity as the saturated condensing temperature approaches
an upper limit. See capacity overrides #16-18. The control will
indicate through an operating mode that high ambient unload-
ing is in effect. If the saturated condensing temperature in a cir-
cuit exceeds the calculated maximum, the circuit will be
stopped. The control will modulate the 0 to 10v head pressure
control output signal when condensing temperature is below
the minimum head pressure requirement for the compressor. In
addition, the Condenser Valve Select item under factory con-
figuration must be set to Yes and 0-10 V signal wiring connec-
tions must be made to TB7 terminals 1 and 2.
LOW CONDENSER FLUID TEMPERATURE HEAD
PRESSURE CONTROL OPTION — Units will start and op-
erate down to 65 F (18.3 C) entering condenser water tempera-
ture as standard. Operation with entering condenser water tem-
peratures below 65 F (18.3 C) requires a field-supplied and in-
stalled condenser fluid control valve.
Sequence of Operation
— Valve position is controlled
through a 0 to 10 vdc signal provided by the MLV/COND
board, channel 9, to maintain the head pressure set point. Unit
sizes 325-400 use a common condenser water valve output so
the MBB uses the highest saturated condensing temperature of
either circuit. As a safety feature, if the circuit is on and if the
saturated condensing temperature reaches the condensing set
point +10° F, the valve is opened to its maximum position to
avoid a high pressure alarm. The water valve is fully closed
when the circuit is OFF on unit sizes 150-300 and if both cir-
cuits are off on unit sizes 325-400.