Danfoss VLT Series Design Manual page 41

VLT ® HVAC Drive Design Guide
2.8.5. Tuning the Drive Closed-loop Controller
Once the drive's closed-loop controller has been set up, the performance of the controller should
be tested. In many cases, its performance may be acceptable using the default values of PID
Proportional Gain (par. 20-93) and PID Integral Time (par. 20-94). However, in some cases it may
be helpful to optimize these parameter values to provide faster system response while still con-
trolling speed overshoot. In many situations, this can be done by following the procedure below:
1.
2.
3.
4.
2.8.6. Ziegler Nichols Tuning Method
In general, the above procedure is sufficient for HVAC applications. However, other, more so-
phisticated procedures can also be used. The Ziegler Nichols tuning method is a technique that
was developed in the 1940s and is still commonly used today. It generally provides acceptable
control performance using a simple experiment and parameter calculation.
Start the motor.
Set par. 20-93 (PID Proportional Gain) to 0.3 and increase it until the feedback signal
begins to oscillate. If necessary, start and stop the drive, or make step changes in the
setpoint reference to attempt to cause oscillation. Next, reduce the PID Proportional Gain
until the feedback signal stabilizes. Then reduce the proportional gain by 40-60%.
Set par. 20-94 (PID Integral Time) to 20 sec. and reduce it until the feedback signal
begins to oscillate. If necessary, start and stop the drive, or make step changes in the
setpoint reference to attempt to cause oscillation. Next, increase the PID Integral Time
until the feedback signal stabilizes. Then increase the Integral Time by 15-50%.
Par. 20-95 (PID Differentiation Time) should only be used for very fast-acting systems.
The typical value is 25% of the PID Integral Time (par. 20-94). The differentiator should
only be used when the setting of the proportional gain and the integral time has been
fully optimized. Make sure that oscillations of the feedback signal are sufficiently damp-
ened by the low-pass filter for the feedback signal (par 6 16, 6 26, 5 54 or 5 59, as
required).
NOTE
This method must not be used on applications that could be damaged by oscillations
created by marginally stable control settings.
MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
2. Introduction to the VLT HVAC Drive
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