Danfoss vlt fcd 302, Design Manual, Page: 15 / 93

2.2 EMC
2.2.1 General Aspects of EMC Emissions
Electrical interference is usually conducted at frequencies
in the range 150 kHz to 30 MHz. Airborne interference
from the frequency converter system in the range 30 MHz
to 1 GHz is generated from the inverter, motor cable, and
the motor.
As shown in
Illustration 2.8 , 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 2.8 ) 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 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. Connect the
motor cable screen to the frequency converter and motor
enclosures. Use integrated screen clamps to avoid twisted
screen ends (pigtails). Twisted screen ends will increase the
screen impedance at higher frequencies, which reduces the
screen effect and increases the leakage current (I
4
).
When a screened cable is used for fieldbus relay, control
cable, signal interface or brake, ensure the screen is
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
Illustration 2.8 Example - Leakage Current
Mounting plates, when used, must be constructed of
metal, to ensure the screen currents are conveyed back to
the unit. Ensure good electrical contact from the mounting
plate through the mounting screws to the chassis of the
frequency converter.
When unscreened cables are used, some emission
requirements are not fulfilled. However 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 frequency above 50 MHz (airborne) is
generated by the control electronics in particular.
2.2.2 EMC Test Results
The following test results have been obtained using a
system with a frequency converter (with options if
relevant), a screened control cable, a control box with
potentiometer, as well as a motor and motor screened
cable.
Product Overview
VLT
®
Decentral Drive FCD 302
14 MG04H102 - VLT
®
is a registered Danfoss trademark
2 2