Mains Supply Interference/Harmonics; Power Factor - Danfoss VLT 2800 Manual

Efficiency of the system (•
To calculate the system efficiency, the efficiency of the
frequency converters (•
INV
efficiency of the motor (•
MOTOR
• = • x • .
SYSTEM INV MOTOR
Based on the graph above, it is possible to calculate
the system efficiency at different loads.

Mains supply interference/harmonics

A frequency converter takes up a non-sinusoidal cur-
rent from mains, which increases the input current
I . A non-sinusoidal current can be transformed by
RMS
means of a Fourier analysis and split up into sinusoidal
currents with different frequencies, i.e. different har-
monic currents I with 50 Hz as the basic frequency:
n
Harmonic currents
Frequency [Hz]
The harmonic currents do not affect power consump-
tion directly, but they 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 commu-
nication equipment connected to the same transform-
er or cause resonance in connection with power-factor
correction batteries.

Power factor

The power factor (Pf) is the relation between I
.
RMS
The power factor for 3-phase supply:
U I
3 × ×
P f
=
U
3 ×
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
same kW performance. In addition, a high power factor
indicates that the different harmonic currents are low.
138
)
SYSTEM
) should be multiplied by the
):
I I I
1 5 7
50 250 350
0,9 0,4 0,3
and I
1
cos
1 × ϕ
I
×
RMS
for the
RMS
MG.27.E2.02 - VLT is a registered Danfoss trademark