Daikin RDT Installation And Maintenance Manual page 95

VFD Compressor Control
Control of the VFD compressor is accomplished with a digital
output enable signal and a 0-10VDC analog modulating
control signal.
General VFD Compressor Start Sequence
On a call for VFD compressor operation the VFD enable
output is energized (on) and the 0-10VDC analog control
signal is set to 3.33VDC (50 rps) for 10 seconds. 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.
NOTE: In addition to enabling VFD compressor operation
the VFD enable output is used to energize the
liquid line drop solenoid on the VFD circuit and
also starts the auxiliary ventilation fans in the VFD
compressor panel.
Compressor Stage Up Transition
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 each fixed compressor stage UP sequence, the VFD
compressor speed is reduced to its minimum (25 rps) 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 a fixed
compressor. The VFD is held at minimum speed for 30
seconds before normal modulation resumes.
Compressor Stage Down Transition
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 each 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 a 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 a fixed compressor.
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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) resumes.
• 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 1 fixed compressor and modulate the VFD
compressor speed to achieve the DAT set point.
95 IM 893-10 • ROOFPAK SINGLEZONE UNITS
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