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STANDARD IPLV CAPACITY CONTROL (CON'T)
(Loading/Unloading and starting additional compressors)
In this example, one compressor will be shut down when
the speed of the compressors drops to 200 Hz x (2-1)/2
= 100 Hz-20 Hz = 80 Hz.
The restart frequency for the compressor(s) after remov-
ing a lag compressor is the OFF FREQ. The OFF FREQ
is designated as:
OFF FREQ = Current VSD Freq x (Number of compressors enabled +1)
Number of Compressors enabled
For example: 80 Hz = current freq of the chiller in the example
above.
Number of compressors enabled at shutdown = 1
In the example above, one compressor will restart at 160
Hz as calculated in the formula below:
80 Hz x (1+1) = 160 Hz
1
The load timer will also be set to 30 seconds and the
unload timer will be set to 10 seconds.
On 3 and 4 compressor chillers, if frequency (speed)
drops below the LESS COMP FREQ – 20 Hz or
the minimum VSD frequency, whichever is higher,
another lag compressor will be shut down using the
same guidelines.
When the system is only operating a single (lead)
compressor, if temperature continues to stay below the
control range (Setpoint – CR) or continues to drop while
in the CONTROL RANGE, the Chiller Control Board
microprocessor will unload the compressor at the rate of
0.1-1 Hz every 2 seconds. This will continue until the
frequency drops below the Minimum VSD Frequency
determined by the ambient temperature. At this point,
the lead compressor will be shut down, if temperature
is below the Setpoint - CR.
JOHNSON CONTROLS
FUZZY LOGIC CONTROL
The fuzzy logic control in software makes decisions
to increase or decrease speed according to the error or
deviation from Setpoint, and the rate of change of chilled
liquid temperature. Before making a change in speed,
the Chiller Control Board microprocessor will look at
the load and unload timers to assure they are timed out.
It also looks to assure there is no load limiting in effect.
Each time a change is made, the incremental change in
speed is still between 0.1-1 Hz, unless temperatures fall
near the leaving chilled liquid cutout.
In most situations, when the chilled liquid temperature
is above the Setpoint + CR, the Chiller Control Board
microprocessor will continue to increase the speed of
the compressor(s) to load the chiller until temperature
drops in the general range of the Setpoint High Limit
(Setpoint + CR). If the rate of change is dropping too
fast and there is potential for overshoot, the Chiller
Control Board microprocessor may elect not to continue
to increase speed.
In cases where temperature is dropping too fast when
temperature is within the desired CONTROL RANGE,
the micro will be required to make decisions regarding
speed changes under conditions where the "error" and
"rate" conflict. For example, the micro may elect to
decrease the speed of the compressor(s) if the error is
"0" (temperature is at Setpoint), while the rate of change
of chilled liquid temperature is falling (negative). The
Chiller Control Board microprocessor may also elect to
hold the speed when error is "positive" (temperature is
above Setpoint, but not above Setpoint + CR) because
the rate of change of chilled liquid is "negative" (falling).
TABLE 4 illustrates these conditions.
FORM 201.21-NM3 (616)
7
213
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