Danfoss VLT Series Design Manual page 77

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VLT HVAC Drive Design Guide 3. VLT HVAC Selection
3.8.14. 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 dif-
ferent filter types, -the du/dt and the Sine-wave filter.
du/dt filters
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Motor insulation stresses are often caused by the combination of rapid voltage and current in-
crease. 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 drive to the motor. The voltage wave form is still pulse shaped but the du/dt
ration 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 cur-
rent waveforms.
With the sinusoidal waveforms the use of special frequency converter motors with reinforced in-
sulation 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 drive. The length is unfortunately limited because the filter does not reduce
leakage currents in the cables.
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MG.11.B2.02 - VLT is a registered Danfoss trademark