General Aspects Of Emc - Danfoss VLT DriveMotor FCM 106 Design Manual

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2.5 General Aspects of EMC

Electrical interference is conducted at frequencies in the range 150 kHz to 30 MHz. Airborne interference from the frequency
2 2
converter system in the range 30 MHz to 1 GHz is generated from the inverter, motor cable, and the motor.
Capacitance 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 2.13) 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
through the screen (I
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 connection is best
done by using integrated screen clamps to avoid twisted screen ends (pigtails). Pigtails increase the screen impedance at
higher frequencies, which reduces the screen effect and increases the leakage current (I
If a screened cable is used for relay, control cable, signal interface, and brake, the screen must be mounted on the enclosure
at both ends. In some situations, however, it is necessary to break the screen to avoid current loops.
Illustration 2.13 Equivalent Diagram: Coupling of Capacitors, which Generates Leakage Currents
1 Ground wire
2 Screen
3 AC mains supply
Table 2.4 Legend to Illustration 2.13
When positioning a screen on a frequency converter mounting plate, the mounting plate must be made of metal, to ensure
that the screen currents are conveyed back to the unit. Moreover, ensure good electrical contact from the mounting plate
through the mounting screws to the frequency converter enclosure.
When unscreened cables are used, some emission requirements are not complied with, although most immunity
requirements are observed.
To reduce the interference level from the entire system (unit+installation), make motor cables as short as possible. Avoid
placing cables with a sensitive signal level alongside motor cables. Radio interference higher than 50 MHz (airborne) is
especially generated by the control electronics. See chapter 2.5.1 EMC-compliant Electrical Installation for more information
on EMC.
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VLT
), there is only a small electro-magnetic field (I
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z C
L1
S
U
z
L2
V
z
L3
W
z PE
PE
C
S
I
4
3
4
®
DriveMotor FCP 106 and FCM 106 Design Guide
) from the screened motor cable.
4
I
1
I
C
2
S
1
I
3
2
5
4 Frequency converter
5 Screened motor cable
6 Motor
MG03M102 - 08/2015
) is carried back to the unit
1
).
4
C
S
C
C
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S
I
4
6