3.5 General Aspects of EMC
3.5.1 General Aspects of EMC Emissions
Electrical interference is usually conducted at frequencies in the range 150kHz to 30MHz. Airborne interference from the
frequency converter system in the range 30MHz to 1GHz is generated from the inverter, motor cable, and the motor.
As shown in the illustration below, capacitive currents in the motor cable coupled with a high dU/dt from the motor
voltage generate leakage currents.
The use of a screened motor cable increases the leakage current (see illustration below) 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 approximately 5MHz. 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. This is best done 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 fieldbus, 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.
Earth Plane
LINE
FREQUENCY
MOTOR CABLE SCREENED
MOTOR
CONVERTER
Earth wire
Screen
z
z
z
L1
L2
L3
PE
U
V
W
PE
175ZA062.11
C
S
I
2
I
1
I
3
I
4
C
S
C
S
C
S
C
S
I
4
C
S
z
PE
If 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 have to be conveyed back to the unit. Moreover, ensure good electrical contact from the
mounting plate through the mounting screws to the frequency converter chassis.
When unscreened cables are used, some emission requirements are not complied with, although the immunity
requirements are observed.
In order to reduce the interference level from the entire system (unit + installation), make motor and brake cables as short
as possible. Avoid placing cables with a sensitive signal level alongside motor and brake cables. Radio interference higher
than 50MHz (airborne) is especially generated by the control electronics. Please see for more information on EMC.
Introduction to FC 300
FC 300 Design Guide
MG.33.BD.02 - VLT
®
is a registered Danfoss trademark
37
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3