Carrier 19 Series - Manual (Page 44)

Control sequence
Before the chiller can start, the condenser and chilled water
pumps must be operating. Field-supplied pilot relays for
pumps and fan are normally applied as shown in the Typical
Control Wiring Schematic.
Closing the chiller ON-STOP switch energizes the tem­
perature control circuits in the compressor safety system.
Once these temperature sensing circuits have been ener­
gized, the circuits containing pressure sensing devices can
be energized. If condenser pressure and cooler refrigerant
pressure/temperature are satisfactory, the machine control
circuit can then be energized by pressing chiller START
button. A program timer now begins a series of 4 timed
steps (PT-1 thru -4) to ensure the proper sequencing of the
oil pump and compressor start.
When the compressor motor reaches RUN condition, a
normally open contact closes to energize holding relay K3.
This locks in control circuit power to oil pump and water
pump motors whenever the chiller compressor is operating.
Energizing the K2 and K3 relays also permits the capacity
control circuit to position the compressor guide vanes as
required to maintain the selected chilled water temperature.
To guard against stress or damage to the compressor
motor, the program timer keeps the chiller control circuit
de-energized for approximately 15 minutes after a com­
pressor stop.
The chiller is stopped by pressing the ON-STOP switch.
The auxiliary water pumps and fan motor are stopped by
pressing the field-supplied STOP button.
Typical control components
Condenser high-pressure cutout (manual reset) —
Shuts down compressor if condenser pressure rises above
cutout set point.
Bearing high-temperature cutout — Prevents damage
to motor and compressor bearings from excessive tem­
perature. Keeps compressor from starting or shuts com­
pressor off if bearing temperature reaches set point. Chiller
ON-STOP button must be opened and reclosed to reset this
safety circuit.
Motor winding high-temperature cutout — Prevents
compressor start or shuts compressor off if motor
winding temperature reaches set point. Requires opening
and reclosing chiller ON-STOP button to reset.
Cooler low-refrigerant cutout — Switch trips when
refrigerant charge is low, shutting off compressor. Switch
protects the cooler tubes from freeze-up if water flow drops
off or chilled water thermostat is set too low. Switch re­
quires manual reset.
Oil low-pressure cutout — Prevents compressor start
until oil pressure is adequate for good bearing lubrication.
Automatically stops compressor if oil pressure falls to
set point. Coastdown lubrication is provided in the
compressor.
Chilled water low-temperature recycle switch — Stops
compressor when chilled water temperature drops to a
point indicating minimum refrigeration load. Allows
chiller to recycle automatically when water temperature
rises to a point that indicates need for further cooling. Also
provides protection against tube freeze-up.
Vane closed switch — Prevents compressor start unless
compressor inlet guide vanes are in closed position. Ensures
no-load starting.
Capacity control module (solid state) — Transmits
signals from temperature sensing element in the chilled
water line to compressor guide vane actuator. Provides
precise control of vane position, and hence, machine
capacity at all loads. Module contains a motor load control
that overrides the chilled water temperature control and
closes the guide vanes to prevent motor overload.
Guide vane actuator — Motor opens and closes com­
pressor guide vanes in response to signals from capacity
control module.
Program timer — Sequences the start of oil pump and
compressor motor to ensure adequate lubrication before
compressor starts during operation and as the compressor
coasts down at shutdown. The program timer also provides
a 15-minute delay between any compressor stop and
subsequent start-up.
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