Fuji Electric FRENIC-HVAC User Manual page 541

6.3 Details of Function Codes
 D differential time (External PID control 1: J512, External PID control 2: J612,
External PID control 3: J662)
J512/J612/J662 specifies the differential time for the external PID processor.
- Data setting range: 0.00 to 600.00 (s)
0.00 means that the differential component is ineffective.
D (Differential) action
An operation in which the MV (manipulated value: output frequency) is proportional to the
differential value of the deviation is called D action, which outputs the MV that differentiates
the deviation. D action makes the inverter quickly react to a rapid change of deviation.
The effectiveness of D action is expressed by differential time as a parameter. Setting a long
differential time will quickly suppress oscillation caused by P action when a deviation occurs.
Too long differential time makes the inverter output oscillation more. Setting short
differential time weakens the suppression effect when the deviation occurs.
The combined uses of P, I, and D actions are described below.
(1) PI control
PI control, which is a combination of P and I actions, is generally used to minimize the
remaining deviation caused by P action. PI control always acts to minimize the deviation
even if a commanded value changes or external disturbance steadily occurs. However, the
longer the integral time, the slower the system response to quick-changed control.
P action can be used alone for loads with very large part of integral components.
(2) PD control
Under PD control, the moment that a deviation occurs, the control rapidly generates greater
MV (manipulated value) than that generated by D action alone, to suppress the deviation
increase. When the deviation becomes small, the behavior of P action becomes small.
A load including the integral component in the controlled system may oscillate due to the
action of the integral component if P action alone is applied. In such a case, use PD control to
reduce the oscillation caused by P action, for keeping the system stable. That is, PD control is
applied to a system that does not contain any damping actions in its process.
(3) PID control
PID control is implemented by combining P action with the deviation suppression of I action
and the oscillation suppression of D action. PID control features minimal control deviation,
high precision and high stability.
In particular, PID control is effective to a system that has a long response time to the
occurrence of deviation.
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