VLT® 5000 Crane
■
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 can be 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 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.
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, VLT 5000 Crane has
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 frequency in question. The total
voltage distortion THD is calculated on the basis of the
individual voltage harmonics using the following formula:
■
Power Factor
The power factor is the relation between I
1
and I
RMS
.
The power factor for 3-phase control:
The power factor indicates the extent to
which the frequency 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
different harmonic currents are low.
■
General aspects of EMC emissions
Electrical interference at frequences in the range 150
kHz to 30 MHz are usually conducted. Airborne
interference from the drive system in the range
30 MHz to 50 MHz is generated from the inverter,
the motor cable and the motor.
As the sketch below shows, capacitive currents in
the motor cable together with a high dV/dt from the
motor voltage generate leakage currents.
The use of a screened motor cable increases the
leakage current (see figure below). This is because
screened cables have higher capacitance to earth
MG.50.N4.02 - VLT is a registered Danfoss trademark
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