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circuit is locked out on a fault and a diagnostic is
annunciated.
Fixed Speed Compressor (FS)
Each FS compressor has two 24 VAC, 30 W 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
See Figures 1 for module status and diagnostics.
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, 24VAC 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. It will annunciate either High Pressure Cutout
Compressor X or Compressor Safety Circuit X.
PKG-SVX027A-EN
Variable Speed Compressor (VS)
The VS compressor does not have an external crankcase
heater (like FS compressors). Instead, whenever the VS
compressor is off, the VS drive applies a voltage to the
compressor motor stator such that the compressor
doesn't rotate but heats the oil.
The VS drive, 1U1, has its own software to control and
protect the VS compressor. See
Input/Output terminals.
The drive receives the following signals from the Unit
Controller:
•
Controlled Start/Stop
•
Speed signal
•
Drive Reset
•
Stator Heat Enable
•
Emergency stop
Note: The 24 VDC voltage used in each of the circuits
above -except the speed signal-originate from the
VS drive.
The drive provides the following signals to the Unit
Controller:
•
Compressor On status
•
Drive Fault status
Upon a call for mechanical cooling, the Unit Controller
issues the command to start the compressor via a dry
contact closure on XM32.3. When the 1U1 drive receives
a Controlled Start signal it runs the compressor at 3600
rpm for 60 seconds. The compressor then runs at
whatever speed commanded by the Unit Controller.
Similarly, when the 1U1 drive receives a Controlled Stop
signal it runs the compressor at 3600 rpm for 60 seconds
then turns the compressor off.
The VS compressor HPC is wired in series with the 1U1 E-
stop circuit. A VS compressor HPC event or upon a Unit
Controller remote Emergency Stop input will immediately
stop the VS compressor and lockout its operation on a
manually resettable diagnostic.
Compressor Limit Conditions
Because the MSC 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
Operating Principles
Figure 38, p. 47
for VSD
45
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