General Aspects Of Emc; General Aspects Of Emc Emissions - Danfoss VLT Compressor Drive CDS 803 Design Manual

Product Overview Design Guide

2.7 General Aspects of EMC

2.7.1 General Aspects of EMC Emissions

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Frequency converter (and other electrical devices) generate electronic or magnetic fields that may interfere with their
environment. The electromagnetic compatibility (EMC) of these effects depends on the power and the harmonic character-
istics of the devices.
Uncontrolled interaction between electrical devices in a system can degrade compatibility and impair reliable operation.
Interference may take the form of mains harmonics distortion, electrostatic discharges, rapid voltage fluctuations, or high
frequency interference. Electrical devices generate interference along with being affected by interference from other
generated sources.
Burst transient usually occur 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 compressor.
Capacitive currents in the motor cable coupled with a high dU/dt from the compressor voltage generate leakage currents,
as shown in Illustration 2.7.
The use of a screened motor cable increases the leakage current (see Illustration 2.7) 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 ) is carried back to the unit
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through the screen (I ), there is in principle only a small electro-magnetic field (I ) from the screened motor cable according
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to Illustration 2.7.
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 and on the compressor enclosure. This 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 ).
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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.
If the screen is to be placed on a mounting plate for the frequency converter, use a metal mounting plate 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 compressor and brake cables as short as
possible. Avoid placing cables with a sensitive signal level alongside compressor and brake cables. Radio interference higher
than 50 MHz (airborne) is especially generated by the control electronics.
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