What are line reactors?

[milemaker13]

My VFD says that if my supply xfmr is more than 10x the vfd I need 3% line reactors or an isolation xfmr. Since the xfmr is the incomming utility I assume it is much larger than this. What are line reactors and how are they typically installed?

 

[rcwilson]

Line reactor is a large coil, usually with an iron core, that is wired in series with the VFD power source. The reactor's impedance varies with frequency so it will block high frequency noise generated by the VFD but allow 60 Hz current through without minimal voltage drop. It's inductance also smoothes out the current.

They look like three phase dry type transformers and are wired similarily: three phase wires on input terminals and three phase wires on output. It is not a seperately derived source. There is no wye or delta connection, just a winding in each phase.

Search for Line Reactors for more information.

 

[milemaker13]

Yeah, I read more about them online. Thanks for the input. I have to check into this abit more.

 

[Besoeker]

My VFD says that if my supply xfmr is more than 10x the vfd I need 3% line reactors or an isolation xfmr. Since the xfmr is the incomming utility I assume it is much larger than this. What are line reactors and how are they typically installed?

They are installed in the input supply to the VFD.
Isolation transformers seem to me to be particular North American practice.
I've been involved in the design and manufacture of VSDs for gigayears. Other that changing the voltage level required by the VSD, I don't see any great merit in including the cost and bulk of an isolation transformer.

 

[Jraef]

They are installed in the input supply to the VFD.
Isolation transformers seem to me to be particular North American practice.
I've been involved in the design and manufacture of VSDs for gigayears. Other that changing the voltage level required by the VSD, I don't see any great merit in including the cost and bulk of an isolation transformer.

We need them sometimes because we have a lot of legacy ungrounded or corner gounded delta power systems, something you don't have to deal with as much over yonder. New installations are all supposed to be 4 wire Wye systems now, but in reality a lot of utilities ignore the rules. In far flung places it's too expensive for them to run all 3 lines so they run two and do a corner grounded delta. Don't forget, your entire country is geographically smaller than most of our states.

 

[mike_kilroy]

the reason ur vfd mfgr said to use a 3% choke or xfmr is to add impedance between the power source and ur vfd; ie., add sponge so the vfd can protect itself against short circuit by slowing down the rate of current rise.


Ask them directly if you want confirmation of this. the 10x rule is rather antiquated but some mfgrs may still stick to it out of inertia, resistance to change, or resistance to improving their short circuit design to switch off quick enough to protect itself. In the olden days (1990's), one had a 50-50 chance of a vfd shutting off on output short circuit with the fault protection or just blowing up. In the olden olden days (1970's), that ratio was more 90% chance of blow up. Nowadays a vfd should be able to shut off fast enough to protect itself so the 10x kva input vs drive kva size rule is normally history. But if you have an old or otherwise vfd that requires this 3"percent" (misnomer) Z, you can get it via the choke or a typical isolation transformer (5-ish % Z (real not misnomer) nominally).

 

[kwired]

JMO but for a small drive 10 hp or less - maybe even 25hp or less for some people, if you have a fault within the drive what is the reactor going to protect, that will not cost as much to repair as just replacing the drive?

Short circuit or ground fault on load side of drive is likley handled by the drive and a shut down happens with little incident as far as damaging levels of current go. Look at display of drive and it gives you a short circuit or ground fault error. Someone will tell me how wrong I am on this - but I may learn something if they do.

 

[mike_kilroy]

Short circuit or ground fault on load side of drive is likley handled by the drive and a shut down happens with little incident as far as damaging levels of cuent go. Look at display of drive and it gives you a short circuit or ground fault error.

that was my point; drive mfgrs USED to require SOME input impedance to allow their ground fault and short protection schemes to work - they never used to be quick enough to prevent blown output devices without at least a certain amount of system Z in front of them to slow the di/dt. remember that the outputs were not fast switching IGBTs as today, but simple bipolar transistors or even GTOs with turn off times in the msec; so current up front had to be slowed down to allow next cycle shut off before it rose to damaging levels. Since today we switch in .xx usec, this archaic 10x rule should not be required on a decently designed drive.

 

[Besoeker]

Don't forget, your entire country is geographically smaller than most of our states.

Yet, much less insular......
Actually, the system used in UK is that pretty adopted over all of Europe which somewhat negates the geographical argument.

As it happens we, our company that is, have designed, manufactured, and commissioned drives in many countries.
In doing so I personally have been to some far flung places like Hong Kong, the Malaysian jungle, the Sahara, etc.
A bit of an aside but still VSD related.....mods be kind...
The Hong Kong visit was quite interesting. The drives were located under the walkways at the harbour front. Access was by lifting a manhole cover then climbing down a vertical ladder. Not for the claustrophobic....and a bit of a pain if you wanted to carry test gear and documents down in to the bowels of the sea front. The VSDs were driving pumps that circulated sea water to the cooling systems of the high rise, mainly commercial, buildings. That meant that we couldn't do any testing during office hours. So it was down the hole after 8:00pm. The problem was that the VSDs had an inordinate number of failures of the input rectifier and I was asked by the main contractor, the pump manufacturer in this case, to take a look. The fix was relatively simple. The politics was far from so. Mainstream Japanese manufacturer. Potential loss of face if someone independent is brought in to expedite a fix. Wading through syrup comes to mind.

Back on topic...
Yes, I take your point about ungrounded and corner grounded delta systems. Not something I've come across.

 

[ramsy]

New installations are all supposed to be 4 wire Wye systems now, but in reality a lot of utilities ignore the rules.

Industries lacking enforceable regulation are exploited by other predators. Bad-faith insurers make their killing on this lack of enforceable regulation, since claims can be denied after investigation exposes any illegal practice or missing permit. The regulatory environment in some regions justify keeping your children away from those industries, out of harms way.

 

[kwired]

New installations are all supposed to be 4 wire Wye systems now, but in reality a lot of utilities ignore the rules. In far flung places it's too expensive for them to run all 3 lines so they run two and do a corner grounded delta.

Is this a NESC rule? Is NESC adopted as law in most places or just accepted as the industry standard?

There are a lot of delta connected services here new and old. Corner ground is non existant - they have updated any old corner ground systems to mid point of a phase grounded or to a wye connected system(and did include a fourth conductor). There is quite a bit of open delta systems - new and old.

 

[mivey]

Is this a NESC rule?

No. I suspect it is a California thing.

Is NESC adopted as law in most places or just accepted as the industry standard?

Some states adopt it. It is considered an industry standard, but is not necessarily required to be followed by a utility unless the state or their governing authority says so. Keep in mind that many utilities had safety practices developed and practiced long before the NESC put some of it in writing.

Personally, I think it is unwise to not follow the NESC in most cases unless you have a good way to defend your practices in court.

 

[kwired]

Personally, I think it is unwise to not follow the NESC in most cases unless you have a good way to defend your practices in court.

Totally agree with that. Even when I am working on something that doesn't require permits or inspections, following the NEC is still good practice for the same reasons. I don't think I could write my own standards and make it worthwhile, for one thing a lot of reasearch would be needed to make standards that would not be challenged in a lawsuit, NFPA has already done that for me, it is not perfect but is better than what I could do for the purpose. If I was to write standards I would need to get out of installations and dedicate all my time to writing standards and reasearch for these standards.