Mains supply interference/harmonics
A frequency converter takes up a non-sinusoidal cur-
rent from mains, which increases the input current
I
RMS
. A non-sinusoidal current can be transformed by
means of a Fourier analysis and split up into sine wave
currents with different frequencies, i.e. different har-
monic currents 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 consumption
directly, but increase the heat losses in the installation
(transformer, cables). Consequently, in plants with a
rather high percentage of rectifier load, it is important
to maintain harmonic currents at a low level to avoid
overload of the transformer and high temperature in
the cables.
Harmonic currents compared to the RMS input cur-
rent:
Input current
I
RMS
1.0
I
1
0.9
I
5
0.4
I
7
0.3
I
11-49
<0.1
To ensure low, harmonic currents, VLT 6000 HVAC
has intermediate circuit coils as standard. This nor-
mally reduces the input current I
RMS
by 40%, down to
40-45% ThiD.
In some cases, there is a need for further suppression
(e.g. retrofit with frequency converters). For this pur-
pose Danfoss can offer two acvanced harmonic filters
AHF05 and AHF10, bringing the harmonic current
down to around 5% and 10% respectively. For further
details, see the operating instructions MG.80.BX.YY.
For calculation of harmonic, Danfoss offers the soft-
ware tool MCT31.
Some of the harmonic currents might disturb commu-
nication equipment connected to the same transform-
er or cause resonance in connection with power-factor
correction batteries. VLT 6000 HVAC has been de-
signed in accordance with the following standards:
- IEC 1000-3-2
- IEEE 519-1992
- IEC 22G/WG4
- EN 50178
- VDE 160, 5.3.1.1.2
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 the following
formula:
THD
% =
U
2
5
+
U
2
7
+ ..... +
U
2
N
U
1
(
U
N
%
of U
)
Power factor
The power factor is the relation between I
1
and I
RMS
.
The power factor for 3-phase control
=
3 ×
U
×
I
1
×
cos
ϕ
1
3 ×
U
×
I
RMS
Power factor
=
I
1
×
cos
ϕ
1
I
RMS
=
I
1
I
RMS
since cos
ϕ
= 1
The power factor indicates the extent to which the fre-
quency converter imposes a load on the mains supply.
The lower the power factor, the higher the I
RMS
for the
same kW performance.
In addition, a high power factor indicates that the dif-
ferent harmonic currents are low.
I
RMS
=
I
2
1 +
I
2
5 +
I
2
7 + ..... +
I
2
n
VLT
®
6000 HVAC Series
MG.61.B5.02 - VLT
®
is a registered Danfoss trademark
203
A
ll
ab
out VL
T
60
00
HV
AC
Содержание VLT 6000 HVAC Series
Страница 31: ...Ordering form VLT 6000 HVAC Series MG 61 B5 02 VLT is a registered Danfoss trademark 31 Introduction to HVAC ...
Страница 63: ...Mechanical dimensions VLT 6000 HVAC Series MG 61 B5 02 VLT is a registered Danfoss trademark 63 Installation ...
Страница 75: ...VLT 6000 HVAC Series MG 61 B5 02 VLT is a registered Danfoss trademark 75 Installation ...
Страница 204: ...VLT 6000 HVAC Series 204 MG 61 B5 02 VLT is a registered Danfoss trademark ...