General Aspects Of Emc - Danfoss FC 102 Design Manual

Introduction to VLT® HVAC D...

2.9 General Aspects of EMC

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 1 GHz is generated from the inverter, motor cable, and the motor.
As shown in Illustration 2.32, 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 2.32) because screened cables 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 approximately 5 MHz. Since the leakage current (I
through the screen (I ), there is in principle only a small electro-magnetic field (I
3
to Illustration 2.32.
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 on the motor enclosure. This is best done by using integrated
screen clamps so as 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, mount the screen on the enclosure at both
ends. In some situations, however, it is necessary to break the screen to avoid current loops.
z
L1
z
L2
z
L3
z
PE
PE
3
Illustration 2.32 Situation that Generates Leakage Currents
1 Earth wire
2 Screen
3 AC mains supply
Table 2.16 Legend to Illustration 2.32
If the screen is to be placed on a mounting plate for the frequency converter, the mounting plate must be made of metal,
to convey the screen currents 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 most 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. See for more information on EMC.
MG11BC02
Design Guide
C
S
U
I
1
V
W
I
2
I
3
C
S
I
4
4
4 Frequency converter
5 Screened motor cable
6 Motor
Danfoss A/S © Rev. 06/2014 All rights reserved.
) is carried back to the unit
1
) from the screened motor cable according
4
).
4
C
S
1
2
I
4
5
C
S
C
C
S
S
6
41
2 2