4.4 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%, i.e. 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 above the default value. The
efficiency is also slightly reduced if the mains voltage is
480 V, or if the motor cable is longer than 30 m.
Frequency converter efficiency calculation
Calculate the efficiency of the frequency converter at
different loads based on
. The factor in this
graph must be multiplied 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 4.2 Typical Efficiency Curves
Efficiency of the motor (η
MOTOR
)
The efficiency of a motor connected to the frequency
converter depends on the magnetising level. In general,
the efficiency is just as good as with mains operation. The
efficiency of the motor depends on the type of motor.
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 small motors, the influence from the U/f characteristic
on efficiency is marginal. However, in motors from 11 kW
and up, the advantages are significant.
In general, the switching frequency does not affect the
efficiency of small motors. Motors from 11 kW and up have
their efficiency improved 1–2% because the sine shape of
the motor current is almost perfect 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
4.5 Acoustic Noise
The acoustic noise from the frequency converter comes
from 3 sources:
•
DC intermediate circuit coils.
•
Integral fan.
•
RFI filter choke.
The typical values measured at a distance of 1 m from the
unit:
Enclosure size
50% fan speed
[dBA]
Full fan speed
[dBA]
J1 (0.37–2.2 kW)
N.A.
1)
51
J2 (3.0–5.5 kW)
N.A.
1)
55
J3 (7.5 kW)
N.A.
1)
54
J4 (11–15 kW)
52
66
J5 (18.5–22 kW)
57.5
63
J6 (30–45 kW)
56
71
J7 (55–75 kW)
63
72
Table 4.5 Typical Measured Values
1) For J1–J3, the fan speed is fixed.
4.6 dU/dt Conditions
When a transistor in the frequency converter bridge
switches, the voltage across the motor increases by a
dU/dt ratio depending on the following factors:
•
The motor cable type.
•
The cross-section of the motor cable.
•
The length of the motor cable.
•
Whether the motor cable is screened or not.
•
Inductance
The natural induction causes an overshoot U
PEAK
in the
motor voltage before it stabilises itself at a level
depending on the voltage in the intermediate circuit. The
rise time and the peak voltage U
PEAK
affect the service life
of the motor. If the peak voltage is too high, motors
without phase coil insulation are affected. The longer the
motor cable, the higher the rise time and peak voltage.
Peak voltage on the motor terminals is caused by the
switching of the IGBTs. The FC 360 complies with IEC
60034-25 regarding motors designed to be controlled by
frequency converters. The FC 360 also complies with IEC
60034-17 regarding Norm motors controlled by frequency
converters.
Specifications
Design Guide
MG06B402
Danfoss A/S © 09/2014 All rights reserved.
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