Filter types vacon • 15
Local contacts: https://www.danfoss.com/en/contact-us/contacts-list/
Voltage stress
Figure 6. Voltage pulse reflection
The reflected voltage maximum value is 2 × DC-link voltage, in practice 1.8 ...1.9 × U DC. The voltage
is fully developed at cable lengths > 5 m.
Note that the DC-link voltage is increased if the supply is high or if an active front end maintains a
high DC-link voltage or if the drive is in a regenerative (braking) mode. If this voltage stress is too
high, it is recommended to use the dU/dt filters.
A dU/dt filter is an undammed serial LC filter, with a resonance frequency of about 120 kHz - this
will limit the voltage rise speed to < 1000 V/µs. If lower speeds are required, the capacitance can be
increased.
The filter, cable and motor form a complex high frequency circuit, which causes a voltage at the
motor with different rise times and peak values depending on the cable length, type, etc.
7123_uk
Incoming pulse U
i
Reflected pulse U
r
Total Voltage = U
i
+U
r
=U
i
(1+r)=2U
i
( )
Z
o
Z
i
r=
Z
i
–Z
o
Z
i
+Z
o
Z
i
Z
i
+Z
o
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