VFD Chokes are Choking Me Out!

Basically it is whether or not a choke can handle the heat loss from AC core losses and DC wire resistance flowing through it or just the AC current flowing through it. Because don't forget a DC bias can easily saturate many core magnetic fields. A completely core less choke has no magnetic material to saturate so it ideally handles both but is often impractical to fabricate. At least this is AFAIK.

Very interesting, looking forward to hearing more responses

Thank you guys for answering. Much appreciated.

It sounds like you are having problems getting enough current. Is you voltage output suffering?

Harmonic current is supposed to be turned into heat inside the main step down transformers but sometimes a little gets transformed back out on the power lines where your power factor correction capacitors generally reside. Too much harmonic current can cause the power factor capacitors to burn up (literally on fire) because the harmonic content changes reactance and causes the voltage to increase. Just an interesting little observation. Weird but true.

Oh, and to answer your questions more completely, a choke is just an inductor who's impedance: Z = Ldi/dt which also equals 2*Pi*f*L where f is the frequency. Harmonic current is the sum of the currents at each of the harmonics of 60 Hz (or 50 Hz in some places). The sum is subject to vector analysis so that some harmonics add while others subtract. The result can be severe overheating of a motor if the 3rd, 7th, and 11th harmonic is too large.

Dear Mr. NotUrOrdinary Joe

I am not an Elec. Engineer (Mech. Engineering Graduate, with Post Graduate in Sugar Engg., but considerably picked up Elec.Engg.) and I want one doubt to be clarified/cleared by you/

During the year 2011, there was a serious problem of (4 nos., of 600 KW, 630 Volts, base speed 1000 RPM, VFD drives) and enormous over heating of the drives noticed and the internal components like control cards got burnt. The Design and supply of Motors with VFDs is a well known multinational supplier and their Engineers could not solve the problem.

They told that the Harmonics effect is very high and told that the Harmonic currents of 5^th, 7^th, 11^th 13^th and 15^th orders getting added ARITHMETICALLY (not VECTORIALLY) and resuts in over heating even though additional air conditioners installed.

After 7 days they brought some components and fitted in the VFD panel and the problem was under control but normalcy not reached. I find this statement of the Elec.Engineer from the supplier’s side is quite contradicting that in an electrical circuit all the current or voltages be VECTORIALLY added which we studied during our college days. When I raised this issue he replied that the harmonic frequencies are different and the vectorial addition will not apply and it is arithmetical addition only.

You have referred in your posting No.6 which reads as “Harmonic current is the sum of the currents at each of the harmonics of 60 Hz (or 50 Hz in some places). The sum is subject to vector analysis so that some harmonics add while others subtract. The result can be severe overheating of a motor if the 3rd, 7th, and 11th harmonic is too large.”

Pl. enlighten on this subject about it is vectorial addition/Arithmetical addition of current in the Elec. Circuits

Other CR4 Members also can post their comments.

Thanks,

DHAYANANDHAN.S

What a great bunch of info

Im still a little confused but seriously, what a great bunch of replies here

One thing i want to clarify. My original question is in regards to the separate components both line and load side of the VFD. I am no talking about the actual components on the SCCR

What i am getting at is , im trying to understand why an engineer before me in my job in a drawing set of ours installed both a Line reactor and a "choke" on the line side PRE VFD

Then has nothing connected to the load side.

I have come across transients before on the line side and actually had an SCCR develop a L-L short from harmonics and blow it self to pieces. (No line reactors)

Or perhaps was the scenario mentioned above about depleted caps and then an instantaneous current surge.

Really interesting stuff here.

^{"What i am getting at is , im trying to understand why an engineer before me in my job in a drawing set of ours installed both a Line reactor and a "choke" on the line side PRE VFD"}

A line reactor IS a "choke", so I'm confused by your statement of someone putting "both" on the line side of a VFD. That would be like saying there is a wire AND a conductor; they are two words for the same thing.

Unless perhaps there was a line reactor, PLUS a torroidal choke or ferrite bead choke, possibly to reduce CM (Common Mode) noise from passing up stream? It would have been unnecessary on the line side of the VFD, as CM noise is generally dealt with on the load side or the DC bus because it is created by the transistors firing, not the diode rectifier.

But maybe this was an old old drive? Or maybe an old old Engineer who had been used to old "Six-Step" thyristor based VFDs from 30+ years ago? Hard to say now.

Regardless the issue now would be that there is no point in having two reactors in series ahead of a drive, in fact the voltage drop that would create may interfere with the motor operation.

I agree with JReaf that there is no point in having both a line reactor and another choke on the line side. But a larger line reactor would amount to the same thing anyway. It could be that the installed reactor is undersized, but it is more likely that your harmonics from other nearby devices is already near "critical mass". That is a tough call and hard to measure because harmonic currents tend to flow all around a network depending on what else is connected at that instant. Consequently, as things turn on and off, the harmonic current will seek its own path instantly as well.

All of that being said, it seems that few people are aware of a special version of transformer where a few windings are borrowed from an adjacent transformer to mitigate harmonics. In a three phase power system, three transformers are nearly always used but a special version can mitigate harmonics very effectively by mixing a little of the adjacent power onto the windings of a single phase transformer. That is a really cool trick and it apparently works very well. Leave the load side alone.

I only know of one vendor who does this, but there are others and this is an established method for calming down dirty power. You can email me directly through clicking on my pseudo-name for further recommendations.

ah ok

i see what your saying

Here is some more info then

Ill give you guys the part numbers and links and maybe you can tell me what the difference is.

Everything is SEW and this is an SEW MoviTrac system

They had an upstream breaker then

SEW # ND045-013

on page 255 they call this one a "LINE CHOKE"

Here but go to page 255

Then continuing in series again PRE VFD they put a

SEW NF048-503

Click Here to see

Then the VFD

Then a motor disconnect and finally the motor

So if what you guys say is correct that a choke is a Line reactor what is SEW talking about

OK, the NF 0480503 device is what's called an "EMC Filter", EMC stands for Electro-Magnetic Compatibility, a requirement for power electronic devices that are used in IEC conforming countries in order to attain CE compliance. But yes, that device does ALSO have a choke / reactor inside, along with other components, but is designed strictly for high frequency emissions, i.e. EMI/RFI radiation into the surrounding environment. That one has nothing to do with protecting the VFD or reducing harmonics.

You don't indicate where you are, but if you are in North America, that device is totally optional. If you are basically anywhere else, it may not be.

Take a look a this link, http://www.es.eaton.com/HMT/ (sorry, link no longer available) or harmonics mitigating transformers. It would be preferable to too many chokes. Each inductor reduces the available line voltage as well as increasing the power factor load. A transformer, on the other hand, deals with the issues directly but it will be a more expensive solution.

Too much inductance from chokes or line reactors (different names for a series inductor) will make it hard to charge up the capacitor bank in the VFD. That DC voltage in the capacitor bank is the power source from which the VFD recreates a synthetic voltage at the desired frequency. If it suffers from "Low Voltage" it will disable the VFD.

Your fix might be as simple as removing the additional line reactor but it may also be a matter of replacing the VFD. Removing the line reactor may destroy the VFD and then you will be able to buy one that matches up with your motor. By the way, the motor manufacturers have a certain type meant to work at "other" frequencies via a VFD. Pairing up the two is always best.

Upgrading other DC power supplies is also a way to remove harmonics. The type used in computers changed years ago to low harmonic content designs. But very few other DC supplies have been treated the same. Harmonic current also burns up neutral lines in a three phase system. If you read the frequency on the neutral, you are likely to find the fundamental frequency is the 3rd harmonic. No surprise for us EE types. Most of the time the problems are created within the facility.

Just use your favorite search engine with these magic words, "Mitigating Harmonic Distortion".

Lastly, click on the "Rating" box and apply "Good Answers" for those you feel were helpful. Good luck with your issue.

Thank you very much sir

What a great bunch of info here. I hope this thread also helps others learn more about this topic. I wish they taught more of this in school honestly. Im a recent grad in my second year of work so still struggling to learn all the applications of the basics taught in EE.

You guys have been very helpful to the new engineer.