Danfoss VLT AutomationDrive FC 300 Design Manual page 37

FC 300 Design Guide
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,
reached.
Step 3: Measure the period of oscillation to obtain the critical time constant,
Step 4: Use the table above to calculate the necessary PID control parameters.
3.4.1. 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
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
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.
If the screen is to be placed on a mounting plate for the frequency converter, the mounting plate must be made of metal, because the screen currents
have to be conveyed back to the unit. Moreover, ensure good electrical contact from the mounting plate through the mounting screws to the frequency
converter chassis.
NB!
When unscreened cables are used, some emission requirements are not complied with, although the immunity requirements are ob-
served.
In order to reduce the interference level from the entire system (unit + installation), make motor and brake cables as short as possible. Avoid placing
cables with a sensitive signal level alongside motor and brake cables. Radio interference higher than 50 MHz (airborne) is especially generated by the
control electronics.
) is carried back to the unit through the screen (I
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3. Introduction to FC 300
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