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Operating Principles
•
The amount of protection the compressor is receiving
against flooding.
The expected range for superheat is 11-15°F at full load
conditions. At part load, expect a properly adjusted
expansion valve to control to 8-12°F superheat.
Systems operating with lower superheat could cause
serious compressor damage due to refrigerant floodback.
Fan Operation
Multiple, identical fans are arranged in the fan cabinet to
meet the customer's airflow and static requirements. Each
fan's speed, start/stop command, and fault status are
controlled and monitored individually by the unit
controller. However, all fans receive the same fan speed
signal to meet airflow and static demand, and all fans are
commanded On upon Unit Controller status transition to
Occupied Mode.
Each fan assembly has its own motor with integrated
variable speed drive (VSD). Each fan VSD varies the speed
of the fan proportional to a 0-10 VDC signal supplied by the
XM70.1 module (and XM30.5 on 70-80 ton units).
The VSD starts and stops the fan according to the Enable/
Disable Binary Output from the XM32.2 module (and
XM32.4 on 70-80 ton units). If the VSD is faulted it
annunciates this via a dry contact that is sensed by the Unit
Controller on the XM70.1 module (XM30.5 on 70-80 ton
units).
Compressors
The MSC uses a combination of one variable speed
compressor (VS) and one, two or three fixed speed
compressors (FS) for mechanical cooling.
The VS compressor is always the first compressor on and
last compressor off, except in the event of a VS
Compressor or Circuit lockout. A balanced starts routine is
used by the Unit Controller when starting FS compressors.
Compressor Oil
All compressors have an Oil Sight Glass. Always ensure oil
is visible in the sight glass when the compressor is off or
running.
High Pressure Cutout Switch (HPC)
Each VS and FS compressor has an HPC installed in its
discharge line. The switch is set to open at 553 psi +/- 15
and re-close at 424 +/-30.
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.
44
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 5VDC
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
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