Drive, Input Side; Dc Link - Danfoss VLT CDS 803 Manual

VLT® Compressor Drive CDS 803
Design Guide
duced by the drive. More advanced passive solutions combine capacitors and inductors in trap arrangement specially tuned to
eliminate harmonics starting from, for example, the 5
For more details on the Danfoss passive solutions, refer to VLT® Advanced Harmonic Filters AHF 005/AHF 010 Design Guide.
The active solutions determine the exact current that cancels the harmonics present in the circuit and synthesizes and injects that
current into the system. Thus, the active solution mitigates the real-time harmonic disturbances, which makes these solutions effec-
tive at any load profile.
For more details on the Danfoss active solutions, refer to VLT® Low Harmonic Drive Operating Instructions, and VLT® Advanced Ac-
tive Filter AAF 006 Operating Instructions.

3.4.3 Drive, Input Side

RFI (radio frequency interference)
Drives generate radio frequency interference (RFI) due to their variable-width current pulses. Drives and motor cables radiate these
components and conduct them into the mains system.
RFI filters are used to reduce this interference on the mains according to IEC 61800-3 in order not to disturb radio services. Maxi-
mum allowed emission depends on the environment where the PDS is used.
The need for reducing the interferences and the losses created by the coils is a trade-off that is hard to influence in the use of drives.
Even though the losses exist, it is important to fulfill the legislation demands for the installation environment.
RFI filter on IT grid
If the drive is supplied from an isolated mains source (IT mains, floating delta) or TT/TN-S mains with grounded leg (grounded delta),
the RFI filter must be turned off.
In the OFF position, the internal capacitors between the chassis (ground), the input RFI filter, and the DC link are cut off. As the RFI
switch is turned off, the drive is not able to meet optimum EMC performance.
By opening the RFI filter switch, the ground leakage currents are also reduced, but not the high-frequency leakage currents caused
by the switching frequency of the drive. It is important to use isolation monitors that are designed for use with power electronics
(IEC 61557-8). For example, Deif type SIMQ, Bender type IRDH 275/375, or similar.
The Danfoss VLT® drives can be ordered with different types of RFI filters. See more details on RFI, the use of RFI filters, and EMC
compliance in 6.5 Electromagnetic
Passive diode rectifier input
The use of diode rectifiers on the input side of the drives are the most cost-effective design. The energy flow goes from the mains to
the load and have low losses. On the other hand, diodes create harmonics in the mains when rectifying and thereby create losses.
These harmonics can be reduced by having DC-link coils, which are used in the Danfoss VLT® drives.
An energy flow from the drive back to the grid is not possible with this design as the energy is generated back from the application
to the DC link. Use a DC chopper and a connected resistor to absorb the energy. This reduces the energy efficiency significantly.

3.4.4 DC Link

The DC link is a power storage facility for the output section of the drive. There are 2 major components to the DC-link section:
• Capacitors
• Coils
In Illustration 16 only 1 capacitor is shown, but it is always a series of capacitors.
24 | Danfoss A/S © 2021.05
th
harmonic.
Compatibility.
Product Overview
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