Danfoss VLT Design Manual page 51

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Drive Selection
VLT Refrigeration Drive Design Guide
3.1.16 Output Filters
The high speed switching of the frequency converter
produces some secondary effects, which influence the
motor and the enclosed environment. These side effects
are addressed by two different filter types, the du/dt and
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the Sine-wave filter.
dU/dt filters
Motor insulation stresses are often caused by the
combination of rapid voltage and current increase. The
rapid energy changes can also be reflected back to the
DC-line in the inverter and cause shut down. The du/dt
filter is designed to reduce the voltage rise time/the rapid
energy change in the motor and by that intervention avoid
premature aging and flashover in the motor insulation.
du/dt filters have a positive influence on the radiation of
magnetic noise in the cable that connects the frequency
converter to the motor. The voltage wave form is still
pulse shaped but the du/dt ratio is reduced in comparison
with the installation without filter.
Sine-wave filters
Sine-wave filters are designed to let only low frequencies
pass. High frequencies are consequently shunted away
which results in a sinusoidal phase to phase voltage
waveform and sinusoidal current waveforms.
With the sinusoidal waveforms the use of special frequency
converter motors with reinforced insulation is no longer
needed. The acoustic noise from the motor is also damped
as a consequence of the wave condition.
Besides the features of the du/dt filter, the sine-wave filter
also reduces insulation stress and bearing currents in the
motor thus leading to prolonged motor lifetime and longer
periods between services. Sine-wave filters enable use of
longer motor cables in applications where the motor is
installed far from the frequency converter. The length is
unfortunately limited because the filter does not reduce
leakage currents in the cables.
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50 MG16G102 - VLT is a registered Danfoss trademark