9 EMC and Harmonics
9.1 General Aspects of EMC Emissions
Electrical interference is most commonly found 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.
Capacitive currents in the motor cable coupled with a high dU/dt from the motor voltage generate leakage currents.
Shielded motor cables increase the leakage current (see
) because they have higher capacitance to ground
than unshielded cables. If the leakage current is not filtered, it causes greater interference on the mains in the radio
frequency range below 5 MHz. Since the leakage current (I
1
) is carried back to the unit through the shield (I
3
), there is only
a small electromagnetic field (I
4
) from the shielded motor cable.
While the shield reduces the radiated interference, it increases the low-frequency interference on the mains. Connect the
motor cable shield to the frequency converter enclosure and to the motor enclosure. To connect the shield, use integrated
shield clamps to avoid twisted shield ends. The twisted shield ends increase the shield impedance at higher frequencies,
which reduces the shield effect and increases the leakage current (I
4
).
If a shielded cable is used for fieldbus, relay, control cable, signal interface, or brake, mount the shield on the enclosure at
both ends. In some situations, however, it is necessary to break the shield 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
1
Ground wire
2
Shield
3
AC mains supply
4
Frequency converter
5
Shielded motor cable
6
Motor
Illustration 9.1 Leakage Currents
shows an example of a 6-pulse frequency converter, but could be applicable to a 12-pulse as well.
EMC and Harmonics
Design Guide
MG37N102
Danfoss A/S © 6/2016 All rights reserved.
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