Danfoss VLT Series Design Manual page 84

3. VLT HVAC Selection
3.6.14. Output Filters
The high speed switching of the adjustable frequency drive 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 the sine-wave filter.
du/dt filters
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Motor insulation stress is 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, it prevents premature aging and flashover in the motor insulation.
The 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 ratio
is reduced in comparison with the installation without filter.
Sine-wave filters
Sine-wave filters are designed to allow only low frequencies to pass. High frequencies are conse-
quently shunted away, which results in a sinusoidal phase-to-phase voltage waveform and
sinusoidal current waveforms.
With the sinusoidal waveforms, the use of special adjustable frequency drive motors with rein-
forced 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 be-
tween service. 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.B4.22 - VLT ® is a registered Danfoss trademark.
VLT ® HVAC Drive Design Guide