FCM 300 Series
MG.03.B6.02 - VLT is a registered Danfoss trademark
88
To ensure low, harmonic currents, FCM 300 has
intermediate circuit coils as standard..
THD (current)
≤
54%
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
1
■
Mains supply interference/harmonics
A FC motor 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
I
11
Hz
50 Hz
250 Hz 350 Hz 550 Hz
In/I1 [%]
100%
44% 29% 8%
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.
Special conditions
Power factor =
=
■
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 FC
motor 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.
3 x U x I
1
x cos
ϕ
1
3 x U x I
RMS
I
RMS
I
1
I
RMS
I
RMS
= I
1
2
+ I
5
2
+ I
7
2
+ . . . + I
n
2
I
1
x cos
ϕ
1
≈
0.9 since cos
ϕ
= 1
■
What is CE labelling?
The purpose of CE labelling is to avoid technical
obstacles to trade within EFTA and the EU. The EU
has introduced the CE label as a simple way of
showing whether a product complies with the relevant
EU directives. The CE label says nothing about the
specifications or quality of the product. Frequency
converters are regulated by three EU directives:
■
The machinery directive (89/392/EEC)
All machines with critical moving parts are covered by
the machinery directive, which came into force on 1
January 1995. Since a frequency converter is largely
electrical, and the motor always will be placed in
connection with other machines, it does not fall under
the machinery directive. However, if a FC motor is
supplied for use in a machine, we provide information
on safety aspects relating to the FC motor. We do this
by means of a manufacturer's declaration.
■
The low-voltage directive (73/23/EEC)
Frequency converters must be CE labelled in
accordance with the low-voltage directive. The
directive applies to all electrical equipment and
appliances used in the voltage range of 50-1000 V AC
and 75-1500 V DC.
√
U
2
2
+ . . .
+ U
N
2
