3.1.14 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 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 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 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 drive. The length is unfortunately limited because the filter does not reduce leakage currents in the cables.
3 VLT HVAC Drive Selection
VLT
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HVAC Drive Design Guide
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MG.11.B9.02 - VLT
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