2.6 EMC Emission and Immunity
2.6.1 General Aspects of EMC Emission
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 frequency converter, motor cable, and
motor.
Capacitive currents in the motor cable coupled with a high dU/dt from the motor voltage generate leakage currents.
Using a screened motor cable increases the leakage current (see
) because screened cables have higher
capacitance to ground than unscreened cables. If the leakage current is not filtered, it causes 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 is only 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. Connect the motor
cable screen to the frequency converter enclosure as well as the motor enclosure. This is best done by using integrated
screen clamps 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
).
Mount the screen on the enclosure at both ends if a screened cable is used for the following purposes:
•
Fieldbus.
•
Network.
•
Relay.
•
Control cable.
•
Signal interface.
•
Brake.
In some situations, however, it is 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
1
Ground cable
2
Screen
3
AC mains supply
4
Frequency converter
5
Screened motor cable
6
Motor
Illustration 2.29 EMC Emission
Product Overview
Design Guide
MG06B402
Danfoss A/S © 09/2014 All rights reserved.
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2
