Danfoss VLT AQUA Design Manual page 111

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VLT AQUA Drive Design Guide
5.8.3. Earthing of Screened/Armoured Control Cables
Generally speaking, control cables must be braided screened/armoured and the screen must be connected by means of a cable clampat both ends to the
metal cabinet of the unit.
The drawing below indicates how correct earthing is carried out and what to do if in doubt.
a. Correct earthing
Control cables and cables for serial communication must be fit-
ted with cable clamps at both ends to ensure the best possible
electrical contact.1
b. Wrong earthing
Do not use twisted cable ends (pigtails). They increase the
screen impedance at high frequencies.1
c. Protection with respect to earth potential between PLC
and
If the earth potential between the frequency converter and the
PLC (etc.) is different, electric noise may occur that will disturb
the entire system. Solve this problem by fitting an equalising
cable, next to the control cable. Minimum cable cross-section:
16 mm 2 .1
d. For 50/60 Hz earth loops
If very long control cables are used, 50/60 Hz earth loops may
occur. Solve this problem by connecting one end of the screen
to earth via a 100nF capacitor (keeping leads short).1
e. Cables for serial communication
Eliminate low-frequency noise currents between two frequency
converters by connecting one end of the screen to terminal 61.
This terminal is connected to earth via an internal RC link. Use
twisted-pair cables to reduce the differential mode interference
between the conductors.1
5.9.1. Mains Supply Interference/Harmonics
A frequency converter takes up a non-sinusoidal current from mains,
which increases the input current I . A non-sinusoidal current is trans-
RMS
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MG.20.N3.02 - VLT is a registered Danfoss trademark
formed by means of a Fourier analysis and split up into sine-wave
currents with different frequencies, i.e. different harmonic currents I
with 50 Hz as the basic frequency:
5. How to Install
5
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111