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. (Only for Low
Ambient Option)
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, 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.
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 1 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 compressor arrangement and the fixed speed
compressor is running, and there is a low oil indication. Upon
entering the oil balance mode the fixed speed compressor is
turned off and the VFD compressor speed is increased to the
oil boost value shown in Table 20. The VFD compressor runs
at this condition until the optical oil sensor verifies that oil is
present for 3 continuous minutes. Unit Controller default is set
for a 10 minute max balance.
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IM 1058-8 • MAVERICK II ROOFTOP SYSTEMS
