Efficiency of the system (ŋ
SYSTEM
)
To calculate the system efficiency, the efficiency of the
frequency converters (ŋ
INV
) should be multiplied by the
efficiency of the motor (ŋ
MOTOR
):
ŋ
SYSTEM
= ŋ
INV
x ŋ
MOTOR
.
Based on
, it is possible to calculate the
system efficiency at different loads.
5.1.18 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 with a Fourier
analysis and split up into sinusoidal currents with different
frequencies, that is, different harmonic currents I
n
with 50
Hz as the basic frequency:
Harmonic currents
I
1
I
5
I
7
Frequency [Hz]
50
250
350
0,9
0,4
0,3
Table 5.4 Harmonic Currents
The harmonic currents do not affect power consumption
directly, but they increase the heat losses in the installation
(transformer, cables). Therefore, in plants with a rather high
percentage of rectifier load, maintain harmonic currents at
a low level to avoid overload of the transformer and high
temperature in the cables.
Some of the harmonic currents disturb communication
equipment connected to the same transformer or cause
resonance with power-factor correction batteries.
5.1.19 Power Factor
The power factor (Pf) is the relation between I
1
and I
RMS
.
The power factor for 3-phase supply:
Pf
= 3 ×
U
×
I
×
cos
ϕ
3 ×
U
×
I
RMS
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.
All about VLT 2800
Design Guide
132
Danfoss A/S © Rev. May/2014 All rights reserved.
MG27E402
5
5
