6.12 Efficiency
Efficiency of the frequency converter (η
VLT
)
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
, even if the motor supplies
100% of the rated shaft torque or only 75%, that is 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 selected.
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 of above 5 kHz. If the
mains voltage is 480 V, the efficiency is also slightly
reduced.
Frequency converter efficiency calculation
Calculate the efficiency of the frequency converter at
different loads based on
. Multiply the factor
in this graph with the specific efficiency factor listed in the
specification tables.
1.0
0.99
0.98
0.97
0.96
0.95
0.93
0.92
0%
50%
100%
200%
0.94
Rela
tiv
e Efficienc
y
130BB252.11
1.01
150%
% Speed
100% load 75% load 50% load 25% load
Illustration 6.6 Typical Efficiency Curves
Example: Assume a 22 kW (30 hp), 380–480 V AC
frequency converter runs at 25% load at 50% speed. The
graph shows 0.97, whereas rated efficiency for a 22 kW (30
hp) frequency converter is 0.98. The actual efficiency is
then: 0.97 x 0.98 = 0.95.
Efficiency of the motor (η
MOTOR
)
The efficiency of a motor connected to the frequency
converter depends on the magnetizing level. In general,
the efficiency is as good as with 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. The constant efficiency
applies both when a frequency converter controls the
motor, and when the motor runs directly on mains.
In small motors, the influence from the U/f characteristic
on efficiency is marginal. However, in motors from 11 kW
(15 hp) and up, the advantages are significant.
In general, the switching frequency does not affect the
efficiency of small motors. Motors from 11 kW (15 hp) and
up have their efficiency improved (1–2%). This
improvement is due to an almost perfect sine shape of the
motor current at high switching frequency.
Efficiency of the system (
η
SYSTEM
)
To calculate the system efficiency, the efficiency of the
frequency converter (η
VLT
) is multiplied by the efficiency of
the motor (
η
MOTOR
):
η
SYSTEM
=
η
VLT
x
η
MOTOR
Specifications
Design Guide
MG03M202
Danfoss A/S © 06/2016 All rights reserved.
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