PKG-SVX027C-EN
51
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.
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
for
VSD 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
N
No
otte
e:: 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
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
•
water regulating valve (WRV) malfunction
•
faulty discharge pressure sensor
The Unit Controller's TD7 will annunciate a Limit
Condition if an FS compressor circuit enters one. If the
unit is equipped with WRV, the Unit Controller can
open the valve to allow more flow through the
condenser to help mitigate HPC trip.
If the VS circuit enters either a High or Low Pressure
limit condition the Unit Controller can reduce
compressor speed to help mitigate either a Low or High
pressure diagnostic trip.
Figure 36.
Fixed speed compressor protection
module (MCSP)
