30
PKG-SVX029A-EN
voltage output is linearly proportional to the sensed
pressure.
The discharge pressure sensor is used by the unit
controller to do the following:
• Compute circuit differential pressure.
• Warn the user if a high discharge pressure condition
occurs.
The suction pressure sensor is used by the unit controller
to do the following:
• Detect low suction pressures.
• Compute circuit suction superheat.
• Compute circuit differential pressure.
Compressor Suction Temperature
Each circuit has its own suction temperature thermistor. It is
used to compute circuit suction superheat. If superheat is
less than 4 degrees for 5 minutes, or more than 25 degrees
for 1 O minutes, the circuit is locked out on a fault and a
diagnostic is annunciated.
Fixed Speed Compressor (FS)
Each FS compressor has two 24 Vac, 30W crankcase
heaters. The power for these heaters is routed through a
NC auxiliary contact on it's respective start contactor. This
way the heater is energized whenever the compressor is
Off.
In each FS compressor motor junction box is a 24 Vac
powered CoreSense™ compressor protection module
(MCSP). This module displays compressor status and
diagnostics and provides the following protections:
• Motor temperature protection.
• Missing phase protection.
• Reverse phase protection.
• Low control circuit voltage protection.
• Short cycling detection and alert.
If the CoreSense module detects a fault condition it opens
a dry contact on the module interrupting power to that
compressors run contactor coil. The power interruption is
detected by the Unit Controller, which disables the
compressor and annunciates a Compressor Safety Circuit
diagnostic. After the cause of the fault is remedied, a
latching fault can be cleared by cycling 24 Vac module
power.
Compressor Safety Circuit
Each fixed speed compressor has a Compressor Safety
Circuit intended to protect the circuit from a variety of fault
conditions and to distinguish those failures from HPC trips.
For each FS compressor, 24 Vac power is routed in order
from the control power transformer (CPT) through its
respective:
1. HPC
2. MCSP dry contact
3. Circuit breaker auxiliary - 1CB1, 1CB2, 1CB3 or 1CB4
4. Start/Stop NO contact on the Symbio™ 500 binary
output module
5. Compressor start contactor coil - 1K1, 1K2, 1K3 or 1K4
6. From the compressor start contactor coil, the same 24
Vac signal is routed through the contactor NO auxiliary
to an isolating relay - 2K2, 2K4, or 2K8
If any of these devices contacts should open, 24 Vac is
interrupted from the compressor start contactor and the
compressor will shut-off.
The unit controller can distinguish between an HPC switch
opening and any of the other devices in the circuit opening.
It will annunciate either High Pressure Cutout Compressor
X or Compressor Safety Circuit X.
Compressor Limit Conditions
Because the SWUD uses suction and discharge pressure
sensors rather than limit switches, the Unit Controller can
sense when a refrigerant circuit is approaching a high or
low pressure limit condition.
Low Pressure Limit conditions can be caused by the
following:
• Low discharge air temperature
• Low airflow
• Dirty air filters
• TXV malfunction
• Low refrigerant charge
• Faulty suction pressure sensor
High Pressure Limit conditions can be caused by the
following:
• Low condenser water flow
• High entering condenser water temperature
• Fouled entering condenser water strainer
• TXV malfunction
• Refrigerant over charge
• Faulty discharge pressure sensor
The Unit Controller TD-7 will annunciate a Limit Condition if
an FS compressor circuit enters one.
Operating Principles
To Next Page
Loading...
