Following the rectifier section, voltage passes to the intermediate section. (See following illustration). This rectified voltage is smoothed by an LC filter circuit

consisting of the DC bus inductor and the DC bus capacitor banks per each inverter module.

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.

Each inverter module contains two DC capacitor bank assemblies consisting of up to eight capacitors arranged in series/parallel configuration. 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 frequency converter.

Also located in the intermediate section is the high frequency (HF) filter card for each inverter module. 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-earth voltages in the three-phase AC input line. This can occasionally result in nuisance overvoltage alarms. For this reason, the high

frequency filter card on 380–500 V range frequency converters, contains a set of relay contacts in the earth connection of the filter capacitors. The relay is tied

into the RFI/HF switch, which can be switched on or off in par. SP-50 RFI Filter. This disconnects the earth references to all filters to eliminate nuisance overvoltage

conditions created by an unbalanced phase-to-earth voltages.

For 525–690 V frequency converters, the customer may not open the relay contacts to disconnect the earthing via par. SP-50 RFI Filter, but the relay automatically

opens based on the DC bus voltage to protect the drive.

High Power Service Manual for Unit Sizes 6x

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