7.10.1 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 is transformed by
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, 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.
NB!
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:
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 har-
monic 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:
THD
% =
U
5
2 +
U
7
2 + ... +
UN
2
(U
N
% of U)
7.11.1 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.
AF-650 GP Design Guide
156
7
Summary of Contents for AF-650 GP Series
Page 5: ...AF 650 GP Design Guide 4 1...
Page 53: ...AF 650 GP Design Guide 52 4...
Page 97: ...Please note airflow directions 6 1 5 Mechanical Dimensions AF 650 GP Design Guide 96 6...
Page 98: ...Please note airflow directions AF 650 GP Design Guide 97 6...
Page 99: ...Please note airflow directions AF 650 GP Design Guide 98 6...
Page 100: ...Please note airflow directions AF 650 GP Design Guide 99 6...
Page 173: ...Max cable length 150 m AF 650 GP Design Guide 172 9...
Page 179: ...AF 650 GP Design Guide 178 9...