2nd Harmonics caused by a three pulse rectifier

[polyphase]

What remediation techniques are available to manage the 2nd harmonics being injected into the AC supply by a three pulse rectifier for a DC motor? Reportedly, the total harmonics on this service exceeds the IEEE 519 limits. The harmonics are predominantly 2nd, 3rd and 5th. If the load increases as proposed, is it likely that the 2nd harmonic could cause problems in the three phase supply transformer?

 

[Skogsgurra]

There should not be any 2nd at all. And if you have it, it is a siggn that the rectifier is faukty. Is it a controlled (thyristor) rectifier or an uncontrolled (just diodes) one?

Second harmonic will definitely caus problems in the transformer since it will magnetize unsymmetrical and tends to bring the core more or less into saturation.

Third, fifth and seventh are OK, but not second. No way.

By the wat, is it a three-pulse rectifier? Six pulse is more common in drives.

 

[electricuwe]

Regarding the statement that three-pulse rectifiers are not common I agree to skogsgurra. Regarding the statement that existence of a 2nd harmonic indicates a faulty rectifier I have to disagree. Three-pulse rectifiers will always cause 2nd harmonic current in the line. That's the reason why the are not in use any longer.

 

[Skogsgurra]

Yes, of course. Uwe is right. A three pulse rectifier causes as much 2nd harmonics as you can get since it lets only one half of the sinewave through. Sorry for the slip.

If you really have a three pulse rectifier, then there isn't much to do about it. Except changing it to a six pulse rectifier.

 

[electricpete]

I can see there would be symmetry in the input current waveform for 6-pulse rectifier, drawing current twice per cycle (per phase). One half period of the waveform (sometimes loosely referred to as the positive half) is the negative of the other half period of the waveform (sometimes loosely referred to as the negative half).

For 3-pulse rectifier firing only once per cycle per phase the two halves of the current waveform are different (only draws current during one half.

Odd harmonics have that particular type of symmetery. Even harmonics don't.

Can someone remind me what is the name for that type of symmetry?

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[itsmoked]

Bipolar?

 

[electricpete]

Hey. Just because I'm asking questions, don't go making fun of my mental state ;-)

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[itsmoked]

Your mental state has never been in question.............
HA! LOL.

That answer might also be "two quadrant" symetry.

 

[electricpete]

Never in question could be taken two different ways. Hmmm.

I think the word I was looking for was half-wave symmetry. A waveform with half-wave symmetry has only odd harmonics.

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