Drive, Output Side - Danfoss VLT CDS 803 Manual

VLT® Compressor Drive CDS 803
Design Guide
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Illustration 16: Wiring Diagram of the DC Link
1 Direct current (AC ripple)
2 Rectifier
With Danfoss VLT® drives, this intermediate section always uses DC coils, also known as DC line reactors or DC chokes. For cost con-
siderations, most other drive manufacturers do not offer these DC line reactors as standard equipment. Danfoss regards these coils
as essential for 2 main reasons:
• The ability to reduce harmonic noise (interference) by 40%.
• The ability to ride through a temporary loss of power. This allows the drive to avoid numerous unplanned shutdowns.

3.4.5 Drive, Output Side

The output side of the drive contains IGBTs used for generating a variable AC voltage with variable frequency. If no filters are used,
overvoltage spikes, due to reflection of the voltage waveform, can be measured on the motor connection. This situation is often
linked with long motor cables used in the installation and can reach values up to twice the level of the DC-link voltage.
From a user perspective, losses on the output side of the drive can be influenced by using a lower switching frequency, but this also
contributes to higher losses in the motor and filters installed. To optimize energy efficiency, a compromise must be found when
selecting the components used, for example, filters, motor type, and others. Often, output filters are used with the purpose of re-
ducing stress on the insulation.
In the following sections, the aspect of different filter types is discussed in perspective of energy efficiency versus function.
Common-mode filters
Common-mode HF filters are placed between the drive and the motor. They are nanocrystaline cores that mitigate high-frequency
noise in the motor cable (shielded or unshielded) and eliminate bearing currents, and hence Electro Discharge Machining (EDM) or
bearing etching in the motor. Bearing currents caused by drives are also referred to as common-mode currents.
Since the common-mode filters mitigate high frequency, these filters absorb energy and contribute also to losses. Here, the trade-
off is the advantage described compared with the losses.
More information on VLT® Common Mode Filters MCC 105 can be found on www.Danfoss.com.
dU/dt filters
At the IGBTs on the output switch, the voltage is not a clean sinus curve. It contains fast changes in voltage levels over a very short
time. The use of dU/dt filters increases the raise time of the motor voltage to reduce the stress on the motor insulation. If not avoi-
ded, the problem will typically not show at once, but after some time, the insulation breaks and creates problems.
The switching frequency influences the losses in the dU/dt filters. These losses can be up to 1% of the rated power. Here, the trade-
off is the possible damage of the motor over time compared with the cost of energy losses.
Danfoss offers the VLT® dU/dt Filter MCC 102 as a possible solution. Find more information on www.Danfoss.com.
Sine-wave filters
A more advanced, but also more costly solution, is using sine-wave filters.
The VLT® Sine-Wave Filter MCC 101 is a differential-mode low-pass filter that suppresses the switching frequency component com-
ing from the drive and smoothes out the phase-to-phase voltage of the drive to become sinusoidal. This reduces the motor insula-
Danfoss A/S © 2021.05
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3 Direct current
4 Inverter
Product Overview
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