Danfoss VLT Series Design Manual page 42

2. Introduction to the VLT HVAC Drive
2
2.1: Marginally stable system
1.
2.
3.
4.
Type of Control
PI-control
PID tight control
PID some overshoot
Ziegler Nichols tuning for regulator, based on a stability boundary
Experience has shown that the control setting according to the Ziegler Nichols rule provides a
good closed-loop response for many systems. If necessary, the operator can perform the final
tuning of the control iteratively in order to modify the response of the control loop.
42
Select proportional control only. That is, PID Integral Time (par. 20-94) is set to Off
(10,000 s), and the PID Differentiation Time (par. 20-95) is also set to Off (0 s in this
case).
Increase the value of the PID Proportional Gain (par 20-93) until the point of instability
is reached, as indicated by sustained oscillations of the feedback signal. The PID Pro-
portional Gain that causes sustained oscillations is called the critical gain, K
Measure the period of oscillation, P
NOTE: P should be measured when the amplitude of oscillation is relatively small. The
u
output must not saturate (i.e., the maximum or minimum feedback signal must not be
reached during the test).
Use the table below to calculate the necessary PID control parameters.
Proportional Gain
0.45 * K
u
K
0.6 *
u
K
0.33 *
u
MG.11.B4.22 - VLT
.
u
Integral Time Differentiation
Time
0.833 * P -
u
P
0.5 * 0.125 *
u
P
0.5 * 0.33 *
u
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VLT ® HVAC Drive Design Guide
.
u
P
u
P
u