FC 300 Design Guide
How to Install
" Mains Supply Interference/Harmonics
An adjustable frequency drive draws a
non-sinusoidal current from mains, which increa ses
the input current I
RMS
. A non-sinusoidal current
is transformed b y 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 fre quency:
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, c ab les). Consequently,
in plants with a high percentage of rectifier lo ad,
minimize harmonic current levels to avoid overload of
the transformer and high temperature in t h e cables.
NOTE
Some of the harmonic currents might disturb communication equipment connected to the same
transformerorcauseresonancein
connection with power-factor correction batteries.
Harmonic currents compared to the RMS
input current:
Input cu rrent
I
RMS
1.0
I
1
0.9
I
5
0.4
I
7
0.2
I
11-49
<0.1
To ensure low harmonic currents, the adjustable frequency drive is equipped with intermediate circuit
coils as standard. This normally reduces the input current I
RMS
by 40%.
The voltage distortion on the mains supply de pend s
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:
110
MG.33.B3.22 - VLT is a registered Danfoss trademark
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