VLT
®
4000 VT
All about VLT 4000
VT
■
Line supply interference/harmonics
An AFD takes up a non-sinusoidal current from
line, which increases the input current IRMS. 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). Consequently, in plants
with a rather high percentage of rectifier load, it
is important to maintain harmonic currents at a
low level to avoid overload 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 4000 VT has
intermediate circuit coils as standard. This normally
reduces the input current I
RMS
by 40%.
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
4000 VT has been designed in accordance
with the following standards:
- IEC 1000-3-4
- IEEE 519-1992
- IEC 22G/WG4
- EN 50178
- VDE 160, 5.3.1.1.2
The voltage distortion on the line supply depends on
the size of the harmonic currents multiplied by the line
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
AFD imposes a load on the line supply.
The lower the power factor, the higher the I
RMS
for the same HP performance.
In addition, a high power factor indicates that the
different harmonic currents are low.
MG.40.A7.22 - VLT is a registered Danfoss trademark
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