8.9.4 RFI Switch
Line power supply isolated from ground
If the adjustable frequency drive is supplied from an isolated line power source ( IT line power, floating delta and grounded delta) or TT/TN-S line power
with grounded leg, the RFI switch is recommended to be turned off (OFF)
1)
via par. 14-50
RFI 1
. For further reference, see IEC 364-3. In case optimum
EMC performance is needed, parallel motors are connected or the motor cable length is above 82 ft [25 m],, it is recommended to set par. 14-50
RFI 1
to [ON].
1)
Not available for 525–600/690 V adjustable frequency drives in frame sizes D, E and F.
In OFF, the internal RFI capacities (filter capacitors) between the chassis and the intermediate circuit are cut off to avoid damage to the intermediate
circuit and to reduce the ground capacity currents (according to IEC 61800-3).
Please also refer to the application note
VLT on IT line power, MN.90.CX.02.
It is important to use isolation monitors that are capable for use together
with power electronics (IEC 61557-8).
8.10.1 Line Power Supply Interference/Harmonics
An adjustable frequency drive takes up a non-sinusoidal current from the
line power, which increases the input current I
RMS
. A non-sinusoidal cur-
rent 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 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 high percentage of rectifier load, maintain harmonic cur-
rents at a low level to prevent an overload of the transformer and high
temperature in the cables.
NOTE!
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 adjustable frequency drive is standard-equipped with intermediate circuit coils. This normally reduces the input
current I
RMS
by 40%.
The voltage distortion in the line power supply depends on the size of the
harmonic currents multiplied by the line power impedance for the fre-
quency 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)
FC 300 Design Guide
8 Electrical Installation
MG.33.BC.22 - VLT
®
is a registered Danfoss trademark
8-49
8