VFD Compressor Control
Control of the VFD compressor is accomplished with a digital
output enable signal and Modbus control signal.
General VFD Compressor Start Sequence
On a call for VFD compressor operation the VFD enable
output is energized (ON) Modbus signal is set. During this 10
second initial period the VFD compressor’s internal logic ramps
the compressor to 50 rps to insure compressor startup oil
lubrication. After 10 seconds the VFD compressor control signal
begins modulation to maintain the cooling discharge set point.
Compressor Stage Up Transition (DPS 025 & 028 Only)
When the VFD compressor has been operating at maximum
capacity for the cooling stage time period and there is a call
for more cooling capacity the following transition sequence is
followed when staging up.
During the fixed compressor stage UP sequence, the VFD
compressor speed is reduced to its minimum, as a fixed speed
compressor is turned on. Note that the VFD compressor speed
range is extended for these staging points to assure smooth
transition and to minimize capacity gaps. Typically, the VFD
compressor is overdriven (higher speed than normal full load
rating speed) before staging up the fixed compressor. The
VFD is held at minimum speed for 30 seconds before normal
modulation resumes .
Compressor Stage Down Transition (DPS 025 & 028 Only)
When the VFD compressor has been operating at minimum
capacity for the cooling stage time period and there is a call
for less capacity the following transition sequence is followed
when staging down.
During the fixed speed compressor stage DOWN sequence,
the VFD compressor speed is increased to maximum speed
(which varies with unit size and number of operating fixed
compressors) as the fixed speed compressor is turned
off. Note that the VFD compressor speed range has been
extended for these staging points to assure smooth transition
and to minimize capacity gaps. Typically, the VFD compressor
will be overdriven (higher speed than normal full load rating
speed) when staging down the fixed compressor.
Dehumidification Transition During Cooling State
When dehumidification operation becomes active while the
unit is in the Cooling operating state, The VFD compressor
is ramped to its maximum capacity. If the VFD capacity at
this point is already above 75% of its full modulation a fixed
compressor is also turned on. The compressors are held at this
capacity for 1 minute before normal modulation resumes, to
maintain leaving coil temperature (LCT).
• VFD compressor will load up completely before starting
any fixed speed compressors to achieve LCT of 45F
(default) with the VFD compressor option. LCT may be
set between 45F to 52F.
• If reheat signal is at 100% for 10 minutes and the unit is
unable to raise the DAT to desired point, the controller
will stage off the fixed compressor and modulate the VFD
compressor speed to achieve the DAT set point.
Oil Balance/Boost Operational Sequence
When a low oil level is indicated in the VFD compressor sump,
the unit switches to either an oil balance or oil boost state. The
VFD compressor speed is increased during these modes to
promote the return of refrigerant oil to the VFD compressor.
To avoid short cycling of the oil balance/boost sequence, no
action is taken until a low oil indication has been present for 5
consecutive run minutes.
The unit determines whether to enter the oil balance or oil
boost mode based on the running conditions when a low oil
indication is experienced. The balance mode is only used when
a VFD compressor is part of a tandem compressor set. The
balance mode is usually entered first, and is utilized to move
oil from the fixed speed compressor to the VFD compressor. If
this mode fails to resolve the low oil indication issue, the unit
will then go into the boost mode. The boost mode is utilized
to return oil from the refrigerant system to the compressors.
VFD compressors that are not part of a tandem compressor
arrangement will skip the balance mode and only utilize the
boost mode .
The balance mode will be entered if the VFD compressor is part
of a tandem arrangement and there is a low oil indication with
the fixed speed compressor is running. Oil balance mode will
turn OFF the fixed speed compressor and the VFD compressor
speed is increased to the oil boost speed. The VFD compressor
will operate at this condition until the optical oil sensor verifies
that oil is present for 3 uninterrupted minutes.
IM-1125-13 • REBEL ROOFTOPS
34
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Refrigeration System
