5.2.15 Efficiency
To reduce energy consumption it is very important to optimize the effi-
ciency of a system. The efficiency of each single element in the system
should be as high as possible.
Efficiency of frequency converters (
η
INV
)
The load on the frequency converter has little effect on its efficiency. In
general, the efficiency is the same at the rated motor frequency f
M,N
,
regardless of whether the motor supplies 100% rated shaft torque or only
75%, e.g. in case of part loads.
This also means that the efficiency of the frequency converter does not change even if other U/f characteristics are chosen. However, the U/f characteristics
influence the efficiency of the motor.
The efficiency declines a little when the switching frequency is set to a value above 4.5 kHz (parameter 411
Switching frequency
). The rate of efficiency
will also be slightly reduced at a high mains voltage (480 V).
Efficiency of the motor (
η
MOTOR
)
The efficiency of a motor connected to the frequency converter depends on the sine shape of the current. In general, the efficiency is just as good as in
mains operation. The efficiency of the motor depends on the motor type.
In the range of 75-100% of the rated torque, the efficiency of the motor is practically constant, both when it is controlled by the frequency converter
and when it runs directly on mains.
In general, the switching frequency does not affect the efficiency of small motors.
Efficiency of the system (
η
SYSTEM
)
To calculate the system efficiency, the efficiency of the frequency converters (
η
INV
) should be multiplied by the efficiency of the motor (
η
MOTOR
):
η
SYSTEM
=
η
INV
x
η
MOTOR
.
Based on the graph above, it is possible to calculate the system efficiency at different loads.
5.2.16 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 sinusoidal 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
Frequency [Hz]
50
250
350
0,9
0,4
0,3
The harmonic currents do not affect power consumption 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 communication equipment connected to the same transformer or cause resonance in connection with power-
factor correction batteries.
5 All About FCD 300
VLT
®
Decentral FCD 300 Design Guide
130
MG.90.S1.02 - VLT
®
is a registered Danfoss trademark
5