Accessory Controls; Ground Current Protection; Abnormal Conditions; Capacity Control - Carrier 30GTN Product Data

Accessory controls

Demand can be further limited by keeping a selected number of
compressors from turning on by utilizing demand limit control
(the Energy Management Module is required for this function).
This FIOP/accessory interfaces with microprocessor to control
unit so that chiller's kW demand does not exceed its setting. It is
activated from an external switch.
The standard ComfortLink™ control is programmed to accept
various accessory temperature reset options (based on return-fluid
temperature, outdoor-air temperature, or space temperature), that
reset the LCWT. An accessory thermistor (T9 or T10) is required
if outdoor-air temperature or space temperature reset is selected.
The Energy Management Module (EMM) is only required for
temperature reset that is initiated by a 4 to 20 mA signal.

Ground current protection

The 070-400 sizes have ground current protection that shuts off
compressor(s) if a 2 to 3 amp ground current is sensed by a toroid
around the compressor power leads.
30GTN,GTR,GBN,GBR ComfortLink controls with
Scrolling Marquee display module
A standard four-digit alphanumeric display shows all of the
ComfortLink control codes (with expandable clear language),
plus set points, time of day, temperatures, pressures, and
superheat. Additional information can be displayed all at once
with the Navigator display.
Control sequence
Off cycle — During unit off cycle, crankcase heater is energized.
If ambient temperature is below 36 F (2 C), cooler heaters (if
equipped) are also energized. Electronic expansion valves are
closed.
Start-up — After control circuit switches on, prestart process
takes place, then microprocessor checks itself and waits for
temperature to stabilize. First circuit to start may be A or B
(automatic lead/lag feature). The controlled pulldown feature
limits compressor loading on start-up to reduce demand on start-
up and unnecessary compressor usage. The microprocessor limits
supply-fluid temperature decrease (start-up only) to 1° F (0.6° C)
per minute.

Capacity control

On first call for cooling, microprocessor starts initial compressor
and fan stage on lead circuit. The EXV
permitting a pumpout on startup. After pumpout, the valves open
and, if necessary, additional outdoor fans are energized.
Crankcase heaters are deenergized when a compressor is started.
As additional cooling is required, lag circuit starts. If further
cooling is needed, compressors are added, alternating between
lead and lag circuits. Speed at which capacity is added or
decreased is controlled by temperature deviation from set point
and rate of temperature change of chilled fluid. As less cooling is
required, circuits shut down (or unload) in an order that balances
each circuit's compressor run
time (depending upon configuration). When no further cooling is
called for (in each compressor circuit), EXV closes and
compressor and fans continue to run while pumping down cooler.
Control features
Low-temperature override — This feature prevents LCWT
from overshooting the set point and possibly causing a nuisance
trip-out by the freeze protection.
High-temperature override — This feature allows chiller to add
capacity quickly during rapid load variations.
Demand limit — If applied, limits the total power draw of unit to
selected point by controlling number of operational compressors
during periods of peak electrical demand. The Energy
Management Module is required for either 2-stage or 4 to 20 mA
demand limit.
Temperature reset — If applied, microprocessor compares
either return fluid, space temperature, or outdoor-air
temperature with the accessory board settings, and adjusts
LCWT appropriately. The Energy Management Module can
also be added for 4 to 20 mA reset.
Electronic expansion valve and condenser-fan control —
The EXV opens and closes on signal from microprocessor to
maintain an approximate 29° F (16° C) refrigerant superheat
entering the compressor cylinders. (The compressor motor
increases the refrigerant superheat from the approximate 5° F
[3° C] leaving the cooler to that entering the cylinders.)
Condenser fans (operated by microprocessor) run to as low
an ambient as possible to maintain a minimum EXV pressure
differential.

Abnormal conditions

All control safeties in chiller operate through compressor
protection board or control relay and microprocessor. High-
pressure switch directly shuts down compressor(s) through
compressor protection board or control relay. For other
safeties, microprocessor makes appropriate decision to shut
down a compressor due to a safety trip or bad sensor reading
and displays appropriate failure code on the display. Chiller
holds in safety mode until reset. It then reverts to normal
control when unit is reset.
Oil pressure safety — Safety cuts out if pressure differential
is below minimum. Safety is bypassed on start-up for 2
minutes.
Loss-of-charge safety — Safety cuts out if system pressure
drops below minimum.
High-pressure cutout — Switch shuts down compressors
if compressor discharge pressure increases to
426 psig (2918 kPa).
Ground current safety — Safety opens on sensing a
current-to-ground in compressor windings in excess of 2.5
amps.
Compressor anti-cycling — This feature limits compressor
cycling.
Loss of flow protection — Additional protection is provided
by temperature differences between entering and leaving
fluid temperature sensors if cooler temperature drops to 34 F
(1.1 C). Proof of flow switches are recommended.
Sensor
remains closed,
microprocessor.
Diagnostics
Microprocessor may be put through service test (see
Controls, Start-Up, Operation, Service and Troubleshooting
literature) without additional equipment or tools. Service test
confirms microprocessor is functional, informs observer
through display the condition of each sensor and switch in
chiller, and allows observer to check for proper operation of
fans and compressor(s).
Default settings
To facilitate quick start-ups, all 30GTN,GTR,GBN,GBR
chillers with ComfortLink™ controls are pre-configured with
a default setting that assumes stand-alone operation
supplying 44 F (6.7 C) chilled water.
Configuration setting will be based on any options or
accessories included with the unit at the time of
manufacturing. Date and time will need reconfiguring based
on location and local time zone. If operation based on
occupancy scheduling is desired, this will also need to be set
during installation.
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failures — Failures are detected by the