Carrier 17EX Series Start-Up, Operation And Maintenance Instructions Manual Download Page 35

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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) or 1 (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 point is 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