Intermediate Section - Danfoss VLT Series Service Manual

Intermediate Section

Following the rectifier section, voltage passes to the
intermediate section. (see Figure 2-5). This rectified voltage is
smoothed by an LC filter circuit consisting of the DC bus
inductor and the DC bus capacitor bank.
The DC bus inductor provides series impedance to changing
current. This aids the filtering process while reducing harmonic
distortion to the input AC current waveform normally inherent
in rectifier circuits.
The DC capacitor bank assembly consists of up to eight
capacitors arranged in series/parallel configuration. Higher
power units have two capacitor banks assemblies. Also
contained within the assembly is the bleeder/balance circuitry.
This circuitry maintains equal voltage drops across each
capacitor and provides a current path for discharging the
capacitors once power has been removed from the drive.
Also located in the intermediate section is the high frequency
(HF) filter card. It contains a high frequency filter circuit to
reduce naturally occurring currents in the HF range to prevent
interference with other sensitive equipment in the area. The
circuit, as with other RFI filter circuitry, can be sensitive to
unbalanced phase-to-ground voltages in the three-phase AC
input line. This can occasionally result in nuisance overvoltage
alarms. For this reason, the HF filter card on 380 - 500 V
range drives, contains a set of relay contacts in the ground
connection of the filter capacitors. The relay is tied into the
RFI/HF switch on the interface card, which can be manually
switched off. This disconnects the ground references to all
filters should unbalanced phase-to-ground voltages create
nuisance overvoltage conditions.
For 525-600/690 V drives, there are no relay contacts that
disconnect to ground.
2-6
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