2.8.10 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 controlling speed overshoot.
2.8.11 Manual PID Adjustment
1.
Start the motor
2.
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 set-point 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%.
3.
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 set-point reference to attempt to cause oscillation. Next, increase the PID Integral Time until the feedback signal
stabilizes. Then increase of the Integral Time by 15-50%.
4.
Par. 20-95 (PID Differential Time) should only be used for very fast-acting systems. The typical value is 25% of the PID Integral Time (par.
20-94). The differential function 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 dampened by the low-pass filter for the feedback signal (par 6 16, 6 26, 5 54
or 5 59, as required).
2.9 General aspects of EMC
2.9.1 General Aspects of EMC Emissions
Electrical interference is usually conducted at frequences in the range 150 kHz to 30 MHz. Airborne interference from the drive system in the range 30
MHz to 1 GHz is generated from the inverter, motor cable, and the motor.
As shown in the illustration below, capacitive currents in the motor cable coupled with a high dV/dt from the motor voltage generate leakage currents.
The use of a screened motor cable increases the leakage current (see illustration below) because screened cables have higher capacitance to earth than
unscreened cables. If the leakage current is not filtered, it will cause greater interference on the mains in the radio frequency range below approx. 5
MHz. Since the leakage current (I
1
) is carried back to the unit through the screen (I
3
), there will in principle only be a small electro-magnetic field (I
4
)
from the screened motor cable according to the below figure.
The screen reduces the radiated interference but increases the low-frequency interference on the mains. The motor cable screen must be connected to
the frequency converter enclosure as well as on the motor enclosure. This is best done by using integrated screen clamps so as to avoid twisted screen
ends (pigtails). These increase the screen impedance at higher frequencies, which reduces the screen effect and increases the leakage current (I
4
).
If a screened cable is used for Fieldbus, relay, control cable, signal interface and brake, the screen must be mounted on the enclosure at both ends. In
some situations, however, it will be necessary to break the screen to avoid current loops.
2 Introduction to VLT AQUA Drive
VLT
®
AQUA Drive Design Guide
26
MG.20.N5.02 - VLT
®
is a registered Danfoss trademark
2
