Difficult to have harmonics of DC, but I guess you might mean the ripple from the rectfier which is at double the line frequency. Do you have any reference for the 'special concern'?
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If Scotty has corrrectly interpretted that we are talking about harmonics of line frequency present in the dc ripple, then 6*LF is normal, 2*LF is not. 2*LF (plus harmonics of 2*LF) would be associated with an asymmetry on the ac input of 3-phase full wave bridge rectifier such that one phase is seen differently than the other two... either unbalanced voltage, or open fuse or other component associated with one of the phase inputs.
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The application is a 3-phase to 1-phase power supply.
The "special concern" arose from the following comments in a report:
Limitations on the DC link due to increased 2nd harmonic (if any):
Doubling DC link capacitance with active front end should give no limitation on DC performance. Longevity may be shortened (capacitor life).
Quite unusual to have 3-phase to 1-phase power supply for a drive. What kind of motor do you use ?
The second harmonic of the output frequency on the DC-link will create further harmonics in the output voltage of the inverter as well as in the input current of the rectifier.
