19
VLT
®
3500 HVAC
MG.35.B1.02 – VLT is a registered Danfoss trademark
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General aspects of radio noise
In general, electrical noise can be divided into two
forms: the wire-borne noise and the radiated noise.
Wire-borne noise occurs in the frequency range: 150
kHz-30 MHz. Radiated noise in the range 30 MHz-1
GHz radiates from the entire frequency converter sy-
stem. Radiated electrical noise below 50 MHz is
generated especially by the frequency converter, the
motor and the motor cables.
As shown in the sketch below, the discharge capacity
of the motor cable - together with a high du/dt from
the motor voltage - generates noise.
The use of a screened motor cable increases the
noise current I
1
(see fig. below). This is because
screened cables have a greater discharge capacity
than unscreened cables. If the noise current is not
filtered, there will be more noise on the mains in the
radio noise range below approx. 5 MHz. Since the
noise current I
1
is taken back to the unit through the
screen (I
3
), only a small electromagnetic field will in
principle be generated from the screened motor
cable, see the figure below.
The screen reduces the radiated noise, but increases
the low-frequency noise on the mains. With an RFI fil-
ter, the noise level on the mains will be reduced to
about the same level for screened and unscreened
cables alike.
The motor cable screen must be mounted on the
enclosure of the frequency converter as well as the
motor enclosure. The best way of doing this is by
using screen brackets to avoid twisted screen ends.
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2.6 EMC-corr
ect installation
Such twisting increases the screen impedance at
higher frequencies, which reduces the screen effect
and increases the noise current.
If a screened cable is used for PROFIBUS, control
cable and signal interface, the screen must be
mounted on the enclosure at both ends. In certain
situations, however, it will be necessary to break the
screen to avoid current loops.
In those cases where the screen is to be connected
to a mounting plate for VLT 3500 HVAC, the mounting
plate must be made of metal, because the screen
currents are to be taken back to the unit. It is also im-
portant to ensure good, electrical contact from the
mounting plate through the mounting screws to the
chassis of VLT 3500 HVAC.
With respect to installation, it is generally less com-
plicated to use unscreened motor cables than
screened ones. If unscreened cables are used, the
EMC requirements are not met.
In order to reduce the noise level from the total sy-
stem (frequency con installation) to the grea-
test possible extent, it is important to make the motor
cables as short as possible.
Cables with a sensitive signal level are not to be placed
together with motor cables.
Radio noise above 50 MHz (air-borne), in particular,
will be disturbed by the control electronics.
EMC-correct installation
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