VLT® 5000 Crane
Appendix
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
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. 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
).
In the cases when 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 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 frequency converter chassis.
NB!:
Please note, however, that when unscreened
cables are used, some emission requirements
are not complied with, although the immunity
requirements are complied with.
In order to reduce the interference level from the
system 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
interference higher than 50 MHz (airborne) will be
generated especially by the control electronics.
MG.50.N4.02 - VLT is a registered Danfoss trademark
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