Internal Current Control; Parameter Download; General Aspects Of Emc Emissions - Danfoss VLT AutomationDrive FC 300 Design Manual

FC 300 Design Guide
Experience has shown that the control setting according to Ziegler Nichols rule provides a
good closed loop response for many systems. The process operator can do the final tuning
of the control iteratively to yield satisfactory control.
Step-by-step Description:
Step 1: Select only Proportional Control, meaning that the Integral time is selected to the
maximum value, while the differentiation time is selected to zero.
Step 2: Increase the value of the proportional gain until the point of instability is reached
(sustained oscillations) and the critical value of gain, K
Step 3: Measure the period of oscillation to obtain the critical time constant, P
Step 4: Use the table above to calculate the necessary PID control parameters.

Internal Current Control

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The frequency converter features an integral current limit control which is activated when the motor
current, and thus the torque, is higher than the torque limits set in par. 4-16 and 4-17.
When frequency converter is at the current limit during motor operation or regenerative
operation, the frequency converter will try to get below the preset torque limits as quickly
as possible without losing control of the motor.
NB!:
While the current control is active, the frequency converter can only be stopped by means of a
digital terminal if set to Coast, inverse [2] or Coast and Reset, inverse [3]. Any signal on terminals
18 to 33 will not be active until the frequency converter is no longer near the current limit.

Parameter Download

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Parameter download is possible via the following:
PC Software MCT 10 tool - see how in FC 300 PC Software Operating Instructions.
•
Fieldbus options - see how in FC 300 Profibus Operating Instructions or FC 300
•
DeviceNet Operating Instructions.
• LCP up- and download as described in par. group 0-5 .

General Aspects of EMC Emissions

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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, the 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
) is carried back to the unit through the screen (I
leakage current (I
1
small electro-magnetic field (I
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
If a screened cable is used for Profibus, standard bus, 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.
40
Introduction to FC 300
) from the screened motor cable according to the below figure.
4
MG.33.B4.02 - VLT is a registered Danfoss trademark
, is reached.
u
u
), there will in principle only be a
3
).
4
.