5.8. Mains supply interference/Harmonics
5.8.1. Mains Supply Interference/Harmonics
A frequency converter takes up a non-sinus-
oidal current from mains, which increases the
input current I
RMS
. A non-sinusoidal current is
transformed by means of a Fourier analysis
and split up into sine wave currents with dif-
ferent frequencies, i.e. different harmonic cur-
rents 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 con-
sumption directly but increase the heat losses
in the installation (transformer, cables). Con-
sequently, 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.
NB!
Some of the harmonic currents might disturb communication equipment connected
to the same transformer or cause resonance in connection with power-factor cor-
rection batteries.
Harmonic currents compared to the RMS input
current:
Input current
I
RMS
1.0
I
1
0.9
I
5
0.4
I
7
0.2
I
11-49
< 0.1
To ensure low harmonic currents, the frequency converter 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
depends on the size of the harmonic currents
multiplied by the mains impedance for the fre-
quency in question. The total voltage distor-
tion THD is calculated on the basis of the
individual voltage harmonics using this formu-
la:
THD
% =
U
2
5 +
U
2
7 + ... +
U
2
N
(
U
N
%
of U
)
5.9.1. Residual Current Device
You can use RCD relays, multiple protective earthing or earthing as extra protection, provided that
local safety regulations are complied with.
If an earth fault appears, a DC content may develop in the faulty current.
If RCD relays are used, you must observe local regulations. Relays must be suitable for protection
of 3-phase equipment with a bridge rectifier and for a brief discharge on power-up see section
Earth Leakage Current
for further information.
5. How to Install
VLT
®
HVAC Drive Design Guide
110
MG.11.B2.02 - VLT
®
is a registered Danfoss trademark
5
Содержание VLT series
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