Radio Interference In General - Danfoss VLT 2000 Series Manual

Radio interference in general

(emission)
Electrical interference from the cable network, cable-
borne interference, 150 kHz-30 MHz, and airborne
interference from the drive system, 30 MHz-1 GHz, is
produced in particular by the inverter, the motor cable
and the motor system for frequencies under approx.
50 MHz.
As the sketch below shows, capacitive currents in the
motor cable together with high du/dt from the motor
voltage will produce interference.
With regard to installation, it is generally less
complicated to use unscreened cables than screened
cables. For VLT 2000 an RFI and motor filter as well
as a LC and RFI filter have been developed making it
possible to observe the EMC emission requirements
when unscreened motor cables are used. The motor
filter reduces the leak current I
VLT 2000 frequency converters with built-in RFI filter
and without motor coils fulfil the EMC emission
requirements.
MG.20.B6.02 – VLT is a registered Danfoss trademark
(see figure below).
1
VLT ® 2000 Series
Use of screened motor cable increases the leakage
current. 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 leakage range
under approx. 5 MHz. Since the leakage current is
carried back to units through the screen, in principle
this will give only a small electromagnetic field from
the screened motor cable.
The screen reduces the radiated interference, but
increases the low frequency interference on the
mains.
In order to reduce the interference level from the sy-
stem 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 may not be led
together with motor and brake cables.
Radio interference higher than 50 MHz (airborne) will
particularly affect the control electronics.
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