Compressor Control For Roofpak - Daikin MicroTech III Operation Manuals

Compressor Control for RoofPak

VFD Compressor Operation
When an RTU is equipped with the VFD compressor option
there are two compressor circuits, one VFD controlled
compressor in circuit #2 with up to 3 fixed speed compressors
in circuit #1 depending on unit model . Circuit #2 (containing
the VFD compressor) is always the "lead" circuit . The VFD
compressor is controlled via a 0-10VDC analog output signal
from the MT III controller that varies the VFD frequency
Table 51: VFD Compressor Size Range
RPS/RDT Unit
VFDMinrps/
Model
VFDMinV
25 rps
016
0 Vdc
25 rps
021
0 Vdc
25 rps
026
0 Vdc
25 rps
031
0 Vdc
25 rps
042
0 Vdc
25 rps
045
0 Vdc
25 rps
051
0 Vdc
25 rps
063
0 Vdc
25 rps
074
0 Vdc
The basic compressor control sequence is to first start the VFD
compressor and modulate it with a PI control loop to maintain
discharge temperature . When the VFD compressor is at its
maximum speed and more capacity is required, the available
fixed compressor with the fewest run hour total is started (any
fixed compressors on the VFD circuit are started first) while
the VFD compressor is reduced to minimum speed . When the
VFD compressor is at its minimum speed and less capacity is
required the fixed compressor with the highest run hour total
is stopped (any fixed compressors on the circuit opposite the
VFD circuit are stopped first) while the VFD compressor is
increased to maximum speed .
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VFD Modulation Range
VFD Max rps
VFD and Fixed Comp(s) On
1 Fixed On 2 Fixed On
VFD1Maxrps/ VFD2Maxrps/
VFD1MaxV VFD2MaxV
55 rps
NA
4 .0 V
70 rps
NA
6 .0 V
85 rps
NA
8 .0 V
70 rps
NA
6 .0 V
100 rps 95 rps
10 .0 V 9 .33 V
75 rps 75 rps
6 .7 V 6 .7 V
85 rps 85 rps
8 .0 V 8 .0 V
95 rps 95 rps
9 .33 V 9 .33 V
70 rps 50 rps
6 .0 V 3 .3 V
113
between 25rps (0VDC) and 100rps (10VDC) . The minimum
and maximum rps (VDC) values actually used vary depending
on unit size, whether or not a fixed speed compressor is
running and whether or not a special oil return boost mode of
operation is active . The following table shows the relationship
between the minimums and maximums and unit size .
VFD Comp Only
VFDMaxrps/
3 Fixed On
VFDMaxV
VFD3Maxrps/
VFD3MaxV
100 rps
10 .0 V
100 rps
NA
10 .0 V
100 rps
NA
10 .0 V
100 rps
NA
10 .0 V
100 rps
NA
10 .0 V
75 rps
NA
6 .7 V
85 rps
NA
8 .0 V
95 rps
NA
9 .33 V
100 rps 75 rps
10 .0 V 6 .7 V
VFD Compressor Start Sequence
On a call for VFD compressor operation the VFD Enable
output (MCB DO3) is energized (on) and the 0-10VDC analog
control signal is set to 3 .33VDC (50rps) for 10 seconds . During
this 10 second initial period the VFD compressor's internal
logic ramps the compressor to 50rps (this aids in starting oil
circulation) . After 10 seconds the VFD compressor control
signal begins modulating based on a PI loop to maintain the
cooling discharge setpoint .
NOTE: In addition to enabling VFD compressor operation
the VFD Enable output is used to energize a liquid
line drop solenoid on the VFD circuit and to turn on
auxiliary ventilation fans in the VFD compressor
enclosure .
Compressor Stage Up Transition
When the VFD compressor has been operating at maximum
capacity for the cooling stage time period (default 5 Minutes)
and more capacity is required, the fixed compressor with the
fewest run hours is started (any fixed speed compressors on
the same circuit as the VFD compressor is started first). The
VFD compressor is held at is minimum value for 30 seconds .
OM 920-6 • MICROTECH UNIT CONTROLLER
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peraTor s uIde
LoOilBoostrps/ HiOilBoostrps/
LoOilBoostV HiOilBoostV
70 rps 100 rps
6 .0 V 10 .0 V
70 rps 100 rps
6 .0 V 10 .0 V
70 rps 100 rps
6 .0 V 10 .0 V
70 rps 100 rps
6 .0 V 10 .0 V
70 rps 100 rps
6 .0 V 10 .0 V
70 rps 75 rps
6 .0 V 6 .7 V
70 rps 85 rps
6 .0 V 8 .0 V
70 rps 95 rps
6 .0 V 9 .33 V
70 rps 100 rps
6 .0 V
10 .0 V