VLT
®
6000 HVAC Series
All about VLT 6000
HVAC
■ Mains supply interference/harmonics
A frequency converter takes up a non-sinusoidal
current from mains, which increases the input current
I
RMS
. A non-sinusoidal current can be transformed
by means of a Fourier analysis and split up into sine
wave currents with different frequencies, i.e. different
harmonic currents I
N
with 50 Hz as the basic frequency:
Harmonic currents I
1
I
5
I
7
Hz 50 Hz 250 Hz 350 Hz
The harmonics do not affect the power consumption
directly, but increase the heat losses in the installation
(transformer, cables). Con
sequently, in plants
with a rather high percentage of rectifier load, it
is important to maintain harmonic currents at a
low level to avoid over
load of the transformer and
high temperature in the cables.
Harmonic currents compared to the RMS input current:
Input current
I
RMS
1.0
I
1
0.9
I
5
0.4
I
7
0.3
I
11-49
<0.1
To ensure low, harmonic currents, VLT 6000
HVAC has intermediate circuit coils as standard.
This normally reduces the input current I
RMS
by
40%, down to 40-45% ThiD.
In some cases, there is a need for further suppression
(e.g. retrofit with frequency converters). For this
purpose Danfoss can offer two acvanced harmonic
filters AHF05 and AHF10, bringing the harmonic
current down to around 5% and 10% respectively.
For further details, see the operating instructions
MG.80.BX.YY. For calculation of harmonic, Danfoss
offers the software tool MCT31.
Some of the harmonic currents might disturb
communication equipment connected to the same
transformer or cause resonance in connection
with power-factor correction batteries. VLT
6000 HVAC has been designed in accordance
with the following standards:
- IEC 1000-3-2
- IEEE 519-1992
-IEC22G/WG4
- EN 50178
- VDE 160, 5.3.1.1.2
The voltage distortion on the mains supply depends on
the size of the harmonic currents multiplied by the mains
impedance for the frequency in question. The total
voltage distortion THD is calculated on the basis of the
individual voltage harmonics using the following formula:
■ Power factor
The power factor is the relation between I
1
and I
RMS
.
The power factor for 3-phase control
The power factor
indicates the extent to which the
frequency converter imposes a load on the mains
supply. Thelowerthepowerfactor,thehigherthe
I
RMS
for the same kW performance.
In addition, a high power factor indicates that the
different harmonic currents are low.
MG.61.A5.02 - VLT is a registered Danfoss trademark
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