Measure
P
u
when the amplitude of oscillation is quite small. Then “back off” from this gain again, as shown in
K
u
is the gain at which the oscillation is obtained.
Type of control
Proportional gain
Integral time
Differentiation time
PI-control
0.45 *
K
u
0.833 *
P
u
-
PID tight control
0.6 *
K
u
0.5 *
P
u
0.125 *
P
u
PID some overshoot
0.33 *
K
u
0.5 *
P
u
0.33 *
P
u
Table 3.12 Ziegler Nichols Tuning for Regulator, Based on a Stability Boundary
Experience has shown that the control setting following the Ziegler Nichols rule described in the steps below, provides a
good closed-loop response for many systems. The process operator can do the final tuning of the control repeatedly to
yield satisfactory control.
Step-by-Step Description
1.
Select only Proportional Control (Integral time is selected to the maximum value, while the differentiation time is
selected to zero).
2.
Increase the value of the proportional gain until the point of instability is reached (sustained oscillations) and the
critical value of gain,
K
u
, is reached.
3.
Measure the period of oscillation to obtain the critical time constant,
P
u
.
4.
Use
to calculate the necessary PID control parameters.
3.5 General Aspects of EMC
3.5.1 General Aspects of EMC Emissions
Electrical interference is most commonly found at frequencies in the range 150 kHz to 30 MHz. Airborne interference from
the adjustable frequency drive system in the range 30 MHz to 1 GHz is generated from the inverter, motor cable, and the
motor.
Capacitive currents in the motor cable coupled with a high dU/dt from the motor voltage generate leakage currents.
Shielded motor cables increase the leakage current (see
) because they have higher capacitance to ground than
non-shielded cables. If the leakage current is not filtered, it causes greater interference on line power in the radio frequency
range below 5 MHz. Since the leakage current (I1) is carried back to the unit through the shield (I 3), there is only a small
electromagnetic field (I4) from the shielded motor cable.
While the shield reduces the radiated interference, it increases the low-frequency interference on line power. Connect the
motor cable shield to the adjustable frequency drive enclosure as well as the motor enclosure. To connect the shield, use
integrated shield clamps to avoid twisted shield ends. The twisted shield ends increase the shield impedance at higher
frequencies, which reduces the shield effect and increases the leakage current (I
4
).
If a shielded cable is used for serial communication bus, relay, control cable, signal interface, or brake, mount the shield on
the enclosure at both ends. In some situations, however, it is necessary to break the shield to avoid current loops.
Product Introduction
Design Guide
46
Danfoss A/S © Rev. 2014-02-10 All rights reserved.
MG34S222
3
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