General Aspects Of Emc; General Aspects Of Emc Emissions - Danfoss VLT AQUA Drive FC 202 Design Manual

Introduction

2.10 General Aspects of EMC

2.10.1 General Aspects of EMC Emissions

2 2
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.
Screened motor cables increase the leakage current (see Illustration 2.18) because they have higher capacitance to earth
than unscreened 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
a small electromagnetic field (I
While the screen reduces the radiated interference, it increases the low-frequency interference on the mains. Connect the
motor cable screen to the frequency converter enclosure as well as the motor enclosure. To connect the screen, use
integrated screen clamps to avoid twisted screen ends. The twisted screen ends 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 fieldbus, relay, control cable, signal interface, or brake, mount the screen on the enclosure at
both ends. In some situations, however, it is necessary to break the screen to avoid current loops.
Illustration 2.18 Leakage Currents
1 Earth wire
2 Screen
3 AC mains supply
4 Frequency converter
5 Screened motor cable
6 Motor
Table 2.10 Legend to Illustration 2.18
Illustration 2.18 shows an example of a 6-pulse frequency converter, but could be applicable to a 12-pulse as well.
If placing the screen on a mounting plate, use a metal plate because the screen currents must be conveyed back to the
frequency converter. 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 and 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) comes from the control electronics. For more information on EMC, see chapter 5.9 EMC-correct Installation.
30
Design Guide
) from the screened motor cable.
4
z C
L1
S
U
z
L2
V
z
L3
W
z
PE
PE
C
S
I
4
3
4
Danfoss A/S © Rev. 06/2014 All rights reserved.
) is carried back to the unit through the screen (I
1
I
1
I
C
2
S
1
I
3
2
5
3
).
4
C
S
C C
S S
I
4
), there is only
6
MG20Z202