General Aspects Of Emc - Danfoss VLT FC 102 Series Design Manual

Introduction to VLT® HVAC D...

2.9 General Aspects of EMC

2 2
Electrical interference is usually conducted at frequencies in the range 150 kHz to 30 MHz. Airborne interference from the
adjustable frequency drive system in the range 30 MHz to 1 GHz is generated from the inverter, motor cable, and the
motor.
As shown in Figure 2.32, capacitance in the motor cable coupled with a high dU/dt from the motor voltage generate
leakage currents.
The use of a shielded motor cable increases the leakage current (see Figure 2.32) because shielded cables have higher
capacitance to ground than non-shielded cables. If the leakage current is not filtered, it causes greater interference on the
line power in the radio frequency range below approximately 5 MHz. Since the leakage current (I
unit through the shield (I
according to Figure 2.32.
The shield reduces the radiated interference, but increases the low-frequency interference in the line power supply. Connect
the motor cable shield to the adjustable frequency drive enclosure as well as on the motor enclosure. This is best done by
using integrated shield clamps so as to avoid twisted shield ends (pigtails) Pigtails increase the shield impedance at higher
frequencies, which reduces the shield effect and increases the leakage current (I
If a shielded cable is used for relay, control cable, signal interface and brake, mount the shield on the enclosure at both
ends. In some situations, however, it is necessary to break the shield to avoid current loops.
Figure 2.32 Situation that Generates Leakage Currents
1 Ground wire
2 Shield
3 AC line power supply
Table 2.16 Legend to Figure 2.32
If the shield is to be placed on a mounting plate for the adjustable frequency drive, the mounting plate must be made of
metal, to convey the shield currents back to the unit. Moreover, ensure good electrical contact from the mounting plate
through the mounting screws to the adjustable frequency driver chassis.
When non-shielded cables are used, some emission requirements are not complied with, although most immunity
requirements are observed.
46
Design Guide
), there is in principle only a small electro-magnetic field (I
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Danfoss A/S © Rev. 06/2014 All rights reserved.
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4 Adjustable frequency drive
5 Shielded motor cable
6 Motor
) is carried back to the
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MG11BC22