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UC800 AFD Operation
Figure 12. Pressure coefficient surge boundary
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.0
10.0
AFD Speed Control
UC800 control utilizes an enhanced control method
capable of simultaneously adjusting compressor speed
and inlet guide vane position to achieve the desired chiller
capacity and pressure coefficient. At the heart of the
control is a match model that describes the relationship
between control parameters and actuators. This model
has converted a complicated multi variable control
problem to a system of algebraic equations. The equations
cannot be solved directly, so a binomial search algorithm
is used iteratively to find a solution. A new solution is
found every 5 seconds. This is possible because of the
increased performance of the microprocessor available
with the UC800.
Startup
The starting speed for AFD under UC800 control will vary
depending upon the pressure ratio across the compressor.
The UC800 predicts the condensing pressure during
startup and sets the AFD at a speed that will support the
predicted pressure ratio across the compressor. The
UC800 makes this correction by converting condenser
water temperature to a pressure and comparing it to a
measured pressure. The UC800 uses the greater of the two
pressures for the initial speed command. After the
compressor runs for a few minutes, the actual condenser
pressure is used. The speed will be adjusted every
5 seconds in response to changing pressure ratio and load
requirements.
26
Surge Boundary
Surge
20.0
30.0
40.0
50.0
IGV Position
60.0
70.0
80.0
90.0
On startup, shell pressures and temperatures may not
correspond to saturated conditions. To avoid potential
surge on start, the boundary pressure coefficient will be
reduced by 0.2 below the last running condition, and over
40 minutes adjusts itself towards the last running
condition. This allows for the stabilization of pressures
and water loop conditions. After reaching this condition
the control will do a re-optimization.
100.0
AFDG-SVU01L-EN
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