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UC800 AFD Operation
Figure 14. Boundary re-optimization
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.0
10.0
20.0
Surge Recovery
When surge occurs, the pressures in the evaporator and
condenser shells can become erratic. Surge recovery is
needed to force conditions out of this unstable operating
point. This is accomplished by reducing the pressure
coefficient every 90 seconds of continuous surge. In
addition, when the surge flag is set, the compressor speed
command is increased by 1 Hz every 5 seconds until the
surge condition clears. When the surge flag is removed,
the speed command will relax back to the speed needed to
raise the pressure coefficient to the new surge boundary.
Figure 15. Motor current signature representing surge
Rectified
Motor
Current
Transitory
Pressure
Breakdown
28
Pressure Coefficient Optimization
Surge detected
30.0
40.0
50.0
60.0
IGV % Capacity
Previous
Sampled
Value
1 Surge
Occurrence
70.0
80.0
90.0
Surge Detection
Surge detection control logic monitors changes in
compressor motor current. A surge occurrence leaves a
characteristic motor current signature as shown in
Figure 15
This signature is formed because the transitory
pressure breakdown between the condenser and
evaporator causes a sudden reduction in compressor
motor load. As the pressures equalize, the compressor
begins to quickly load, increasing the motor current.
Present
Sampled
Value
Pressure
Rebuilding
Time
100.0
AFDG-SVU01L-EN
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