2.6 General Aspects of EMC
2.6.1 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 1G Hz is generated from the inverter, motor cable and the motor.
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
) 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 only be a small electro-magnetic field (I
4
) from the screened motor cable.
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 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.
1
2
z
z
z
L1
L2
L3
PE
U
V
W
C
S
I
2
I
1
I
3
I
4
C
S
C
S
C
S
C
S
I
4
C
S
z
PE
3
4
5
6
175ZA062.12
Illustration 2.17 EMC Emission
1 Earth wire
2 Screen
3 AC mains supply
4 Frequency converter
5 Screened motor cable
6 Motor
Table 2.10 Legend to
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.
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
50 MHz (airborne) is especially generated by the control electronics.
Product Overview
VLT
®
AutomationDrive FC 360 Design Guide
32
MG06B202 - VLT
®
is a registered Danfoss trademark
2
2
