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2. The motor load ramp loading rate is an operator-configured
value that limits the rate at which the compressor motor
current or compressor motor load increases. (LOAD PULL-
DOWN %/MIN on the CONFIG screen).
To select the ramp type, highlight the SELECT RAMP
TYPE parameter on the CONFIG screen and select either
0(TEMP)or1(LOAD). Motor load (1) is the default ramp
loading control type.
Capacity Override (See Table 4) — Capacity over-
rides can prevent some safety shutdowns caused by exceed-
ing the motor amperage limit, refrigerant low temperature
safety limit, motor high temperature safety limit, and con-
denser high pressure limit. In all of these cases, there are
2 stages of compressor vane control.
1. The guide vanes are kept from opening further, and the
status line on the LID displays the reason for the
override.
2. The guide vanes are closed until the condition decreases
below the first step set point. Then, the vanes are released
to normal capacity control.
Whenever the motor current demand limit set pointis reached,
it activates a capacity override, again using the 2-step pro-
cess. Exceeding 110% of the rated load amps for more than
30 seconds initiates a safety shutdown.
The compressor high lift (surge prevention) set point causes
a capacity override as well. When the surge prevention set
point is reached, the PIC normally prevents the guide vanes
from opening. See the Surge Prevention Algorithm section,
page 37. If the chiller is equipped with the hot gas bypass
option, the PIC opens the hot gas bypass valve instead of
holding the guide vanes.
High Discharge Temperature Control — If the dis-
charge temperature increases above 200 F (93 C), the guide
vanes are proportionally opened to increase gas flow through
the compressor. If the leaving chilled water temperature drops
5° F (2.8° C) below the control point temperature, chiller
enters the RECYCLE mode.
Oil Sump Temperature Control — The oil sump tem-
perature control is regulated by the PIC which uses the oil
heater relay when the chiller is shut down.
As part of the pre-start checks executed by the controls,
the PIC compares the oil sump temperature to the evapora-
tor refrigerant temperature. If the difference between these 2
temperatures is 50 F (27.8 C) or less, the start-up is delayed
until the oil temperature difference is 50 F (27.8 C) or more.
Once this temperature is confirmed, the start-up continues.
The oil heater relay is energized whenever the chiller com-
pressor is off and the oil sump temperature is less than
150 F (65.6 C) or the oil sump temperature is less than the
cooler refrigerant temperature plus 70° F (39° C). The oil
heater is turned off when the oil sump temperature is either
• more than 160 F (71.1 C)
• or the oil sump temperature is more than 155 F (68.3 C)
and more than the cooler refrigerant temperature plus
75° F (41.6° C).
The oil heater is always off during start-up or when the
compressor is running.
When a power failure to the PSIO module has occurred
for more than 3 hours (i.e., initial start-up), the compressor
guide vane opening is slowed down to prevent excessive oil
foaming that may result from refrigerant migration into the
oil sump during the power failure. The vane opening is slowed
via temperature ramp loading to a value of 2° F (1.1° C) per
minute.
Table 4 — Capacity Overrides
OVERRIDE
CAPACITY
CONTROL
FIRST STAGE SETPOINT
SECOND STAGE
SETPOINT
OVERRIDE
TERMINATION
View/Modify
on LID Screen
Default Value Configurable Range Value Value
HIGH CONDENSER
PRESSURE
Equipment
Service1
125 psig
(862 kPa)
90 to 200 psig
(620 to 1379 kPa)
ϾOverride
Set Point
+ 4 psid (28 kPad)
ϽOverride
Set Point
LOW REFRIGERANT
TEMPERATURE
(Refrigerant Override
Delta Temperature)
Equipment
Service1
Ͻ3° F (1.6° C)
(Above Trippoint)
2° to 5° F
(1° to 3° C)
рTrippoint
+ Override
⌬T –1° F
(0.56° C)
ϾTrippoint
+ Override
⌬T +2° F
(1.2° C)
HIGH COMPRESSOR
LIFT
(Surge Prevention)
Equipment
Service1
Min: T1 — 1.5° F
(0.8° C)
P1 — 50 psid
(345 kPad)
Max: T2 — 10° F
(5.6° C)
P2 — 85 psid
(586 kPad)
0.5° to 15° F
(0.3° to 8.3° C)
30 to 170 psid
(207 to1172 kPad)
0.5° to 15° F
(0.3° to 8.3° C)
30 to 170 psid
(207 to 1172 kPad)
None
Within
Lift Limits
Plus Surge/
HGBP
Deadband
Setting
MANUAL
GUIDE VANE
TARGET
Control
Algorithm
Maint01
Automatic 0 to 100% None
Release of
Manual
Control
MOTOR LOAD —
ACTIVE
DEMAND LIMIT
Status01 100% 40 to 100%
у5% of
Set Point
2% Lower
Than
Set Point
LEGEND
HGBP — High Gas Bypass
P1 — Minimum Pressure Load
P2 — Maximum Pressure Load
T1 — Minimum Temperature Load
T2 — Maximum Temperature Load
35