3.5 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 3.23, 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 3.23) 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
1
) is carried back to the unit
through the screen (I
3
), there will in principle only be a small electro-magnetic field (I
4
) from the screened motor cable
according to the below figure.
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 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
4
).
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.
Earth Plane
LINE
FREQUENCY
MOTOR CABLE SCREENED MOTOR
CONVERTER
Earth wire
Screen
z
z
z
L1
L2
L3
PE
U
V
W
PE
175ZA062.11
C
S
I
2
I
1
I
3
I
4
C
S
C
S
C
S
C
S
I
4
C
S
z
PE
Illustration 3.23 Situation that Generates Leakage Currents
If the screen is to be placed on a mounting plate for the frequency converter, the mounting plate must be made of metal,
because the screen currents have to be conveyed 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.
In order 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.
3.5.1
EMC Test Results
The following test results have been obtained using a system with a frequency converter, a screened control cable, a control
box with potentiometer, as well as a motor and screened motor cable at nominal switching frequency. In Table 3.11 the
maximum motor cable lengths for compliance are stated.
Introduction
VLT
®
AutomationDrive FC 300 Design Guide, 0.25-75 kW
38 MG33BE02 - VLT
®
is a registered Danfoss trademark
3
3
RSPSupply - 1-888-532-2706 - www.RSPSupply.com
http://www.RSPSupply.com/p-21462-Danfoss-131B8966-VLT-Automation-VT-Drive-VFD-FC301-230V-5-HP.aspx
To Next Page
Loading...
