Using RCDs
Where residual current devices (RCDs), also known as
ground leakage circuit breakers, are used, comply with the
following:
•
Use RCDs of type B only as they can detect AC
and DC currents.
•
Use RCDs with a delay to prevent faults due to
transient ground currents.
•
Dimension RCDs according to the system configu-
ration and environmental considerations.
The leakage current includes several frequencies
originating from both the mains frequency and the
switching frequency. Whether the switching frequency is
detected depends on the type of RCD used.
130BB958.12
f
sw
Cable
150 Hz
3rd harmonics
50 Hz
Mains
RCD with low f
cut-
RCD with high f
cut-
Leakage current
Frequency
Illustration 10.14 Main Contributions to Leakage Current
The amount of leakage current detected by the RCD
depends on the cut-off frequency of the RCD.
130BB957.11
Leakage current [mA]
100 Hz
2 kHz
100 kHz
Illustration 10.15 Influence of the RCD Cut-off Frequency on
Leakage Current
10.10 IT Mains
Mains supply isolated from ground
If the drive is supplied from an isolated mains source (IT
mains, floating delta, or grounded delta) or TT/TN-S mains
with grounded leg, the RFI switch is recommended to be
turned off via
parameter 14-50 RFI Filter
on the drive and
parameter 14-50 RFI Filter
on the filter. For more detail, see
IEC 364-3. In the off position, the filter capacitors between
the chassis and the DC link are cut off to avoid damage to
the DC link and to reduce the ground capacity currents,
according to IEC 61800-3.
If optimum EMC performance is needed, or parallel motors
are connected, or the motor cable length is above 25 m
(82 ft), Danfoss recommends setting
parameter 14-50 RFI
Filter
to [ON]. Refer also to the
Application Note, VLT on IT
Mains
. It is important to use isolation monitors that are
rated for use together with power electronics (IEC
61557-8).
Danfoss does not recommend using an output contactor
for 525–690 V drives connected to an IT mains network.
10.11 Efficiency
Efficiency of the drive (η
VLT
)
The load on the drive has little effect on its efficiency. In
general, the efficiency is the same at the rated motor
frequency f
M,N
, whether the motor supplies 100% of the
rated shaft torque or only 75%, in case of part loads.
The efficiency of the drive does not change even if other
U/f characteristics are selected. However, the U/f character-
istics influence the efficiency of the motor.
The efficiency declines slightly when the switching
frequency is set to a value of above 5 kHz. The efficiency is
slightly reduced when the mains voltage is 480 V, or if the
motor cable is longer than 30 m (98 ft).
Drive efficiency calculation
Calculate the efficiency of the drive at different speeds and
loads based on
. The factor in this graph
must be multiplied by the specific efficiency factor listed in
the specification tables in
chapter 7.1 Electrical Data, 380–
and
chapter 7.2 Electrical Data, 525–690 V
.
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 10.16 Typical Efficiency Curves
Electrical Installation Con...
Design Guide
MG38C202
Danfoss A/S © 01/2018 All rights reserved.
159
10
10
