General aspects of EMC emissions
Electrical interference at frequences in the range 150
kHz to 30 MHz are usually conducted. Airborne inter-
ference from the drive SYSTEM in the range 30 MHz
to 1 GHz is generated from the inverter, the motor ca-
ble and the motor.
As the sketch below shows, capacitive currents in the
motor cable together with a high dV/dt from the motor
voltage generate leakage currents.
The use of a screened motor cable increases the leak-
age current (see figure below). This is 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
1
) is carried back to the
unit through the screen (I
3
), there will in principle only
be a small electro-magnetic field (I
4
) from the
screened motor cable according to the below figure.
The screen reduces the radiated interference, but in-
creases the low-frequency interference on the mains.
The motor cable screen must be connected to the VLT
enclosure as well as on the motor enclosure. The best
way of doing this is 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
4
).
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.
In the cases when the screen is to be placed on a
mounting plate for the VLT frequency converter, the
mounting plate must be made of metal, because the
screen currents are to be conveyed back to the unit. It
is also important to ensure good electrical contact from
the mounting plate through the mounting screws to the
VLT frequency converter chassis.
With respect to installation, it is generally less compli-
cated to use unscreened cables than screened ones.
NOTE
Please note, however, that when
unscreened cables are used, some emis-
sion requirements are not complied with,
although the immunity requirements are
complied with.
In order to reduce the interference level from the sys-
tem overall (unit + installation) as far as possible, it is
important to make motor and brake cables as short as
possible. Cables with a sensitive signal level must not
be alongside motor and brake cables. Radio interfer-
ence higher than 50 MHz (airborne) will be generated
especially by the control electronics.
VLT
®
5000 Series
MG.51.C5.22 - VLT
p
is a registered Danfoss trademark.
179
EC/EMC
Summary of Contents for VLT 5000 Series
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