Danfoss VLT Series Design Manual page 122

5. How to Install
5.8. AC Line Supply Interference/Harmonics
5.8.1. AC Line Supply Interference/Harmonics
An adjustable frequency drive takes up a non-
sinusoidal current from the line power, which
increases the input current I
oidal current is transformed by means of a
Fourier analysis and split up into sine-wave
currents with different frequencies, i.e., dif-
ferent harmonic currents I
their basic frequency:
5
The harmonics do not affect the power con-
sumption directly, but they do increase the
heat losses in the installation (transformer,
cables). Consequently, in plants with a high
percentage of rectifier load, maintain harmon-
ic currents at a low level to prevent an over-
load of the transformer and high temperature
in the cables.
Harmonic currents compared to the RMS input
current:
To ensure low harmonic currents, the adjustable frequency drive is standard-equipped with in-
termediate circuit coils. This normally reduces the input current I
The voltage distortion in the line supply de-
pends on the size of the harmonic currents
multiplied by the line power impedance for the
frequency in question. The total voltage dis-
tortion THD is calculated on the basis of the
individual voltage harmonics using this formu-
la:
5.9.1. Residual Current Device
You can use RCD relays, multiple protective grounding or grounding as extra protection, provided
that local safety regulations are complied with.
122
. A non-sinus-
RMS
with 50 Hz as
N
NOTE
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.
MG.11.B4.22 - VLT
Harmonic currents I
Hz 50 Hz
I
RMS
I
1
I
5
I
7
I
11-49
by 40%.
RMS
2 2
THD % = U + U
5 7
(U % of U)
N
® is a registered Danfoss trademark.
VLT ® HVAC Drive Design Guide
I I
1 5 7
250 Hz 350 Hz
Input current
1.0
0.9
0.4
0.2
< 0.1
2
+ ... + U
N