Mains Supply Interference/Harmonics - Danfoss FC 300 Design Manual

FC 300 Design Guide

Mains Supply Interference/Harmonics

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An adjustable frequency drive takes up a
non-sinusoidal current from mains, which increases
the input current I . A non-sinusoidal current
RMS
is transformed 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
The harmonics do not affect the power consumption
directly but increase the heat losses in the
installation (transformer, cables). Consequently,
in plants with a high percentage of rectifier
load, maintain harmonic currents at a low
level to avoid overload of the transformer and
high temperature in the cables.
NOTE
Some of the harmonic currents might disturb communication equipment connected to the same
transformer or cause resonance in connection with power-factor correction batteries.
Harmonic currents compared to the RMS
input current:
To ensure low harmonic currents, the adjustable frequency drive is equipped with intermediate circuit
coils as standard. This normally reduces the input current I
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 this formula:
124
How to Install
MG.33.B6.22 - VLT is a registered Danfoss trademark
Harmonic currents I
1
Hz 50 Hz
I
RMS
I
1
I
5
I
7
I
11-49
by 40%.
RMS
I I
5 7
250 Hz 350 Hz
Input current
1.0
0.9
0.4
0.2
< 0.1