3.6.17 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
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, 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 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 adjustable frequency drive 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 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.
3 VLT AQUA Selection VLT
®
AQUA Drive Design Guide
3-46
MG.20.N5.22 - VLT
®
is a registered Danfoss trademark
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