VLT
®
2800 Series
All about VLT
2800
■ 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 sinusoidal
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
Frequency [Hz] 50 25 0 350
0,9 0,4 0,3
The harmonic currents do not affect power
consumption directly, but they increase the heat losses
in the installation ( trans fo
rmer, cable s). Consequently,
in plants w ith a rather high percentage of rectifier
load, it is im portant to maintain harmonic currents
at a low level to avoid ove
rload of the transformer
and high temperature in the cables.
Some of the harmonic currents might disturb
communication equi
pment connected to the same
transformer or cause resonance in connection
with power-factor correction batteries.
■ Power factor
The power facto
r(Pf)istherelationbetween
I
1
and I
RMS
.
The power factor for 3
-phase supply:
The power factor indicates the extent to which
the frequency converter imposes a load on the
mains supply. The lower the pow er factor, the
higher the I
RMS
for the same kW performance. In
addition, a high power factor indicates that the
different harmonic currents are low.
MG.28.E9.02 - VLT is a registered Danfoss trademark
135
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