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PKG-SVX027D-EN
Important: The HPC is wired into each compressors
Enable circuit, and the VS and FS compressor
enable circuit are unique. The FS compressor
enable circuit uses 24 Vac power sourced
from the Control Power Transformer, where
the VS compressor enable circuit uses 24 Vdc
power sourced from the VS compressor drive.
Inadvertently applying 24 Vac to the VS drive
terminal block will permanently damage the
drive.
On the fixed speed compressors, if an HPC opens, the 24
Vac power that energizes the compressor start contactor is
interrupted and the compressor will turn off. The Unit
Controller will detect this and lock the circuit out on a High
Pressure Cutout.
On the variable speed compressor, a 24 Vdc signal
originates from terminal #22 on the 1U1 and is routed
through the NC contact of the VS compressor HPC. That
signals return must be sensed on terminal #31 of the 1U1.
If that signal is lost due to an HPC or Emergency Stop
input, the compressor will immediately shut down. The Unit
Controller will detect HPC trip and lock the circuit out on a
High Pressure Cutout.
Compressor Input/Output Module
Each compressor has a dedicated XM30 input/output
module that communicates digitally with the Unit Controller
(XM30.8, 9, 10, and 11 for compressors A, B, C, and D,
respectively). The XM30 module is used to sense its
respective suction and discharge pressure sensor, suction
temperature sensor, and control the condenser water
regulating valve via a 0-10 Vdc output.
Compressor Pressure Sensors
Each compressor has its own set of suction and discharge
pressure sensors. The sensors are powered by the 5 Vdc
power supply 2U1. Their range is 0-600 psig and their
voltage output is linearly proportional to the sensed
pressure.
The discharge pressure sensor is used by the Unit
Controller to do the following:
• Control the optional WRV (if equipped).
• Compute compressor differential pressure.
• Warn the user if a high discharge pressure condition
occurs.
• Activate the VS compressor Limit Condition control
function.
The suction pressure sensor is used by the Unit Controller
to do the following:
• Detect low suction pressures.
• Compute compressor suction superheat.
• Compute compressor differential pressure.
• Activate the VS compressor Limit Condition control
function.
Compressor Suction Temperature
Each compressor has its own suction temperature
thermistor. It is used to compute compressor suction
superheat. If superheat is less than 4 degrees for 5
minutes, or more than 25 degrees for 10 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, or 1CB3
4. Start/Stop NO contact on the XM32 binary output
module
5. Compressor start contactor coil-1K1, 1K2 or 1K3
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.
Operating Principles
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