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12 & 18 Pulse VFDs

12 & 18 Pulse VFDs

12 & 18 Pulse VFDs

For a total load that is 90% from VFDs it appears that a 12 pulse drive may satisfy IEEE 519, but an 18 pulse drive will without question.  What is the relative cost differential between a 12 and an 18?  Is it reasonable to think of a 6 pulse drive as needing a lot of linear load dilution, a 12-pulse needing less and an 18 pulse needing almost none?

RE: 12 & 18 Pulse VFDs

Hi, I've never heard of an 18 pulse drive, maybe you could explain what you think it is.

RE: 12 & 18 Pulse VFDs

18 pulse drives certainly exist, along with 24 pulse, etc.  It is just another three-phase bridge rectifier that is operated at a phase angle difference from the incoming three-phase power, just like a 12-pulse drive.  This is done with an isolation transformer designed to provide the necessary phase shift.

I think it's safe to say that an 18-pulse drive should eliminate nearly any concern regarding harmonics at the primary of your drive transformers.  But it's probably overkill for most applications.  

We have seen recent systems on very large drives utilizing a 12-pulse drive in conjunction with a four-winding transformer.  The fourth winding is used for harmonic filtering.  This is probably less expensive than an 18 pulse drive in medium-voltage applications.  Similar arrangements are also used for large dc rectifiers used in smelting operations.

There are such wide-ranging applications for adjustable-frequency drives that it is a little hard to generalize.


RE: 12 & 18 Pulse VFDs

Clarification by post originator

The drives are low voltage (at or below 4160).

My model of an 18 pulse drive is combination of three 6-pulse drives, one bridge operating at the line voltage phase angle, a second at 120 deg and a third at 240 deg.

RE: 12 & 18 Pulse VFDs

We generally consider "low-voltage" to apply up 1000V only, at least here in the U.S.  A 4160V or 2300V drive is considered "medium-voltage".  

But I think the phase angle separation for the 18 pulse drive is more like 40 degrees, not 120.  This generally requires a special transformer. Westinghouse/Cutler Hammer used to have something called a "differential delta" transformer to do this, although I've never really understood how it works.  

As to your original question, I think you could put in an 18-pulse drive and not have to give much thought to harmonic distortion problems.  You just have to find room for the drive and transformer!

RE: 12 & 18 Pulse VFDs

Try www.robicon.com ; They offer drives up to 30-pulse at 4160 V and should be able to answer your questions and provide price information.

RE: 12 & 18 Pulse VFDs

These drives work using PAM (pulse amplitude modulation) instead of the more familiar PWM (pulse width modulation) technique. Any number of steps can be created fairly easy if it is a multiple of six, they can simply hook the drives up in parallel or design them in one package. These drives are extremely common in the oilfield. Due to the close proximity of the drives, IEEE 519 is starting to be a factor to these oil companies because they are starting to regulate the distortion based on 519. I wouldn't assume that a 18 pulse drive has less distortion than a 12 pulse drive. It would all depend on the converter section and not the inverter section (which is what the number of pulses is referring too). One of the largest drive manufacturers for the oil field is Centrift (division of baker hughes). You might try contacting them to see what they think.

RE: 12 & 18 Pulse VFDs

Hi again, I have done a little more research but i still cannot find an 18 pulse drive, or the mentioned transformer to give the phase shifts needed. Can anyone provide a web link to such equipment. Thanks in advance

RE: 12 & 18 Pulse VFDs

If you want to have more background information take a look at:

Paice, Derek A.:"Power Electronic Converter Harmonics, Multipulse Methods for Clean Power", IEEE Press, New York, ISBN 0-7803-1137-X

RE: 12 & 18 Pulse VFDs

Cutler Hammer (Eaton) makes an good 18 pulse that is an IED.

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