Enclosure size
Power [kW (hp)]
Non-UL fuse
Non-UL circuit breaker
UL fuse
K1
0.37 (0.5)
gG-10
PKZM0-16
JJS-3
0.55–0.75
(0.74–1.0)
JJS-6
1.1–1.5
(1.48–2.0)
gG-20
JJS-10
2.2 (3.0)
JJS-15
K2
3.0–5.5
(4.0–7.5)
gG-25
PKZM0-20
JJS-25
K3
7.5
(10)
gG-25
PKZM0-25
JJS-25
K4
11–15
(15–20)
gG-50
–
JJS-50
K5
18.5–22
(25–30)
gG-80
–
JJS-80
Table 7.4 Fuse and Circuit Breaker, 380–480 V
7.9 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
. This rule also applies even if
the motor supplies 100% of the rated shaft torque or only
75%, for example if there is 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. If the
mains voltage is 480 V, or if the motor cable is longer than
30 m (98.4 ft), 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
by the specific efficiency factor listed in
the specification tables in
:
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 7.2 Typical Efficiency Curves
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 type of motor.
In the range of 75–100% of the rated torque, the efficiency
of the motor is practically constant, both when controlled
by the frequency converter and when running directly on
mains.
In small motors, the influence from the U/f characteristic
on efficiency is marginal. However, in motors from 11 kW
(14.8 hp) and up, the advantages are significant.
In general, the switching frequency does not affect the
efficiency of small motors. Motors from 11 kW (14.8 hp)
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
Specifications
VLT
®
Midi Drive FC 280
82
Danfoss A/S © 03/2016 All rights reserved.
MG07B102
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