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Mains supply interference/harmonics
A frequency converter takes up a non-sinusoidal
current from mains, which increases the input current
I
. A non-sinusoidal current can be transformed
RMS
by means of a Fourier analysis and split up into sine
wave currents with different frequencies, i.e. different
harmonic currents I
with 50 Hz as the basic frequency:
N
Harmonic currents
I
1
Hz
50 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:
I
RMS
I
1
I
5
I
7
I
11-49
To ensure low, harmonic currents, VLT 7000
Booster has intermediate circuit coils as standard.
This normally reduces the input current I
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.
Power factor
The power factor is the relation between I
The power factor for 3-phase control
The power factor indicates the extent to which the
frequency converter imposes a load on the mains
MG.70.A1.02 - VLT is a registered Danfoss trademark
I
I
5
7
250 Hz
350 Hz
Input current
1.0
0.9
0.4
0.3
<0.1
by
RMS
and I
.
1
RMS
VLT® 7000 Booster
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
7000 Booster has been designed in accordance
with the following standards:
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:
supply. The lower the power factor, the higher the
I
for the same kW performance.
RMS
- IEC 1000-3-2
- IEEE 519-1992
- IEC 22G/WG4
- EN 50178
- VDE 160, 5.3.1.1.2
125
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