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Suction Pressure Transducer
The Suction Pressure Transducer (PTS) is a refrigerant
pressure sensor that screws onto a Schrader fitting on the
suction line of the compressor deck. On single compressor
units (DPS 016–020) this sensor is located on the suction line.
On tandem, two compressor units (DPS 025–028), the PTS is
located upstream of the joint suction.
This sensor is used to ensure that the compressor does not
leave the operating envelope and is used to regulate the super
heat leaving the indoor coil and entering the compressor.
Discharge Pressure Transducer
The Discharge Pressure Transducer (PTD) is a refrigerant
pressure sensor that screws onto a Schrader fitting on
the discharge line of the compressor system. On single
compressor units (DPS 016–020) this sensor is located on the
discharge line. On tandem, two compressor units (DPS 025–
028), the PTD is located downstream of the joint discharge.
This sensor is used to ensure that the compressor does not
leave the operating envelope and is used to regulate the
outdoor fan speed and maintain head pressure.
Discharge Refrigerant Temperature
All Rebel units will have a Discharge Refrigerant Temperature
Sensor (DRT1/DRT3) on the discharge line of each
compressor. This sensor is attached the piping with a metal
clip and wrapped in insulation. The purpose of this device is to
increase compressor life by preventing it from running outside
of the operating envelope.
Suction Refrigerant Temperature
All Rebel units will have a Suction Refrigerant Temperature
Sensor (SRT). This sensor is located on the suction line. Unlike
DRT1 or 3 there is only one SRT for tandem compressor
units. This sensor is used to determine the suction super heat
entering the compressor and is the control input for the EVI in
cooling mode.
VFD Compressor Operation
(DPS 016–028 only)
VFD compressor modulation is controlled by a Mobus
®
signal
from the unit controller. The minimum VFD compressor speed
is 25 rps (1500 rpm) and the maximum VFD compressor speed
is 100 rps (6000 rpm), but the minimum and maximum limits
per unit may vary depending on operating conditions and unit
model size .
The VFD compressor is a 4 pole motor design that operates
off a frequency signal from the VFD between 50Hz and 200Hz.
At start-up the VFD compressor has a 65 second initiation
period where it will ramp to 50 rps for 10 seconds, aiding in oil
circulation, then ramp back down to 25 rps for the remainder of
the initiation period.
Crankcase heating for VFD Compressor is performed by the
VFD via DC-holding current through the motor windings.
VFD compressor modulation is additionally monitored and
adjusted in order to maintain operation within the approved
compressor operating envelope.
Table 12: VFD Compressor Modulation Ranges
DPS Unit
Model
VFD Modulation Range
VFD Min RPS
VFD Max RPS
VFD and Fixed
Comp(s) ON
VFD Comp Only
VFDMax RPS
1 Fixed ON
VFD1Max RPS
016
25 rps
NA
92 rps
018
25 rps
NA
100 rps
020
25 rps
NA
100 rps
025
25 rps
78 rps
100 rps
028
25 rps
100 rps
100 rps
DPS Size 025 & 028, Two Compressor Units
If the VFD compressor were to become inoperative, the
unit can continue to operate on the remaining fixed speed
compressor until the unit can be serviced.
When the VFD compressor is at its maximum speed and
more capacity is required, a fixed speed compressor is started
while the VFD compressor is reduced to minimum speed at
which point it resumes modulating to maintain the discharge
temperature. When the VFD compressor is at its minimum
speed and less capacity is required, a fixed speed compressor
is turned OFF while the VFD compressor is increased to
maximum speed at which point it resumes modulating to
maintain discharge temperature.
Refrigeration System
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IM 1125-13 • REBEL ROOFTOPS