Which Filter For Which Purpose; Du/Dt Filters - Danfoss VLT AutomationDrive FC 300 Design Manual

Introduction to Output Filt...

3.6 Which Filter for which Purpose

Table 3.2 shows a comparison of dU/dt, Sine-wave filter, and HF-CMperformance. It can be used to determine which filter to use
with your application.
3 3 Performance criteria
Motor insulation
stress
Motor bearing stress Slightly reduced, only in high-
EMC performance
Max. motor cable
length
Acoustic motor
switching noise
Relative size
Voltage drop
Table 3.2 Comparison of dU/dt and Sine-wave Filters
1) Not 690V.
2) See general specification for formula.

3.6.1 dU/dt Filters

The dU/dt filters consist of inductors and capacitors in a low
pass filter arrangement and their cut off frequency is above
the nominal switching frequency of the frequency converter.
The inductance (L) and capacitance (C) values are shown in
the tables in 4.2 Electrical Data - dU/dt Filters. Compared to
Sine-wave filters they have lower L and C values, thus they
are cheaper and smaller. With a dU/dt filter the voltage wave
form is still pulse shaped but the current is sinusoidal - see
following illustrations.
Features and benefits
dU/dt filters reduce the voltage peaks and dU/dt of the
pulses at the motor terminals. The dU/dt filters reduce dU/dt
to approx. 500V/μs.
12
Output Filters Design Guide
dU/dt filters
Up to 150m cable (screened/
unscreened) complies with the
requirements of IEC 60034-17 1
(general purpose motors). Above
this cable length the risk of "double
pulsing" (two time mains network
voltage) increases.
power motors.
Eliminates motor cable ringing.
Does not change the emission class.
Does not allow longer motor cables
as specified for the frequency
converter's built-in RFI filter.
100m ... 150m
With guaranteed EMC performance:
150m screened.
Without guaranteed EMC
performance: 150m unscreened.
Does not eliminate acoustic
switching noise.
15-50% (depending on power size) 100%
0.5%
MG.90.N5.02 - VLT
Sine-wave filters
Provides a sinusoidal phase-to-phase
motor terminal voltage. Complies with
IEC 60034-17 1 and NEMA-MG1
requirements for general purpose
motors with cables up to 500m (1km for
VLT frame size D and above).
Reduces bearing currents caused by
circulating currents. Does not reduce
common-mode currents (shaft
currents).
Eliminates motor cable ringing. Does
not change the emission class. Does not
allow longer motor cables as specified
for the frequency converter's built-in
RFI filter.
With guaranteed EMC performance:
150m screened and 300m unscreened.
Without guaranteed EMC performance:
up to 500m (1km for VLT frame size D
and above)
Eliminates acoustic switching noise
from the motor caused by magneto-
striction.
4-10%
Advantages
•
Protects the motor against high dU/dt values and
voltage peaks, hence prolongs the lifetime of the
motor
•
Allows the use of motors which are not specifically
designed for converter operation, for example in
retrofit applications
Application areas
Danfoss recommends the use of dU/dt filters in the following
applications:
•
Applications with frequent regenerative braking
•
Motors that are not rated for frequency converter
operation and not complying with IEC 600034-25
•
Motors placed in aggressive environments or
running at high temperatures
•
Applications with risk of flash over
®
is a registered Danfoss trademark
High-frequency common-mode filters
Does not reduce motor insulation stress
Reduces bearing stress by limiting
common-mode high-frequency
currents
Reduces high-frequency emissions
(above 1MHz). Does not change the
emission class of the RFI filter. Does not
allow longer motor cables as specified
for the frequency converter.
150m screened (frame size A, B, C), 300
m screened (frame size D, E, F), 300 m
unscreened
Does not eliminate acoustic switching
noise.
5 - 15%
none