Protocol
Control
System
Users
Manual
2/12/01
8. Controller Operation
The basic operation of the controller is described in the
following paragraphs. An overview of the menu
structure is given, the more detailed information for each
screen is presented. Where applicable, additional
information about the control algorithms, etc. are
provided.
8.1 Circuit Control
The PSC controller provides circuit control for up to 10
circuits. Each circuit can have up to 3 temperature
probes for temperature monitoring. A relay can be
assigned for temperature control. Each circuit also has a
defrost schedule that controls defrost.
8.1.1 Defrost
There are three types of defrost supported by the
controller, HOT GAS, ELECTRIC and OFF
CYCLE. Refer to the Protocol schematics for
details of the wiring for each type of defrost.
8.2 Pressure Control
The PCS controller provides control of suction pressure
by cycling compressors on and off to meet the load
requirements. Up to 6 compressors are supported for a
suction group. The controller will support 1 or 2 suction
groups.
8.2.1 Switchback
Switchback is a mechanical backup mode of the
PCS. This occurs when a hard failure occurs in
the controller and it is unable to run. When this
occurs, the switchback relay (controller point 7)
will open (normally, this relay is held close).
This willcause the SBA relay to open and some
of the compressors will be powered through the
Normally Closed contacts of the SBA relay and
the Low Pressure Control. Refer to the Protocol
schematics for more detail.
8.2.2 Pressure Control Algorithm
There are two basic pressure control algorithms
used in the Controller. If all compressors have
the same horsepower, an equal horsepower
algorithm is used that equalizes run times
between compressors. If there are different
horsepowers, a different algorithm is used. The
algorithms are described in the following
paragraphs.
8.2.2.1 Equal Horsepower compressors
If the compressors all have the same
horsepower, the controller uses the deadband
and setpoint to determine what compressors
should be on. “Bins” are created which
determine when a compressor is turned on and
off. One Bin is created for each compressor.
The size of the Bin is determined by the
deadband divided by the number of
compressors. An example is shown in Figure
17. In this example, the setpoint is 50 psi, the
deadband is 16 psi and there are four
compressors. As the pressure increases (going
toward the right side), more compressors are
turned on. As the pressure decreases (going
toward the left side) compressors are turned off.
Figure 17-Equal Horsepower Example
Note that there are other setpoints which affect
the minimum on and off times for the
compressors (See Figure 30 and Figure 31).
The above example shows the effect of the
Setpoint and Deadband in controlling the
suction pressure. This is basically
“Proportional” control. The controller
incorporates an additional parameter that is used
for PI (Propor Integral) control (See
Figure 25). This parameter is called
“Integration time” (See Figure 54). For PI
control, the controller will add (or subtract) a
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