vfd problems

[quinn77]

I recently installed a vfd system for a new wastewater project in Tiki Island, Texas where the engineer had spec'd individual THHN conductors for 4 7.5 hp drive motors in one 2" pvc conduit. The engineer's design also had the last motor about 220' from the vfd. After approx. 90 days, the conductors had started to fail. The water table there sits at 18", and the labor that I had been provided failed to follow specs given for fitting the conduit, so; in replacing the feeders I had found the conduit to be full of water and what looked like "nicks" and holes blown in the insulation of the conductors. After doing research on vfd installation, I'm starting to think the engineer has made multiple mistakes on the design of this system. He also seems to think that filters and other electrical components will resolve this problem. I'm feeling the responsibility on this one and would like an engineer's feedback on this.

 

[ptonsparky]

Not an engineer.

Some place in the fine print, the engineer is not at fault.

 

[drbond24]

I'm confused. Are you saying that water in the conduit and nicks in the insulation are the engineer's fault? You seemed to be saying that there were some installation problems, and then you said the engineer made mistakes. Are there two separate problems there, or are they somehow related?

 

[wirenut1980]

Could you be more specific on what part of the design you feel is incorrect? 220' seems like a pretty long run from the VFD to the motor, but the VFD manual will tell you what the maximum length is suggested to be. THHN wire is dual rated THWN, so water in the conduit should not be the cause for the failed wire. Just a few random thought from an engineer.

 

[TxEngr]

The first place to look is at the manufacturers manual for the drives. For example, ABB specifies the type and length of cables required for their drives. I would look there first and suspect you wll find that the design engineer made a mistake.

In my experience with drives, 220 feet is too far to run without taking special precautions. First he should have specified load side filters at the drive. Second, he should have specified cable designed for VFDs which is generally a 3/C cable with 3 symmetrically arranged grounds contained in a continuous armor. Belden, Okonite, Anixter and others all have this type of cable and you can look it up. That might have eliminated the problems. Another bit of added help can be a filter applied at the motor, e.g. an Allen Bradley motor terminator. These are used to protect the motor.

The issue is the possibiliy of standing wave reflections that can develop on the cable which can cause the voltages to be 2X line voltage. On a 480V system, this can be almost 1400V. This may be what was stressing your cable. I'm surprised your motors weren't dying if this was going on unless the motors were inverter duty rated.

TxEngr

 

[jcole]

The issue is the possibiliy of standing wave reflections that can develop on the cable which can cause the voltages to be 2X line voltage. On a 480V system, this can be almost 1400V.


Never heard of standing wave reflections. Could you explain this to me?

 

[drbond24]

The issue is the possibiliy of standing wave reflections that can develop on the cable which can cause the voltages to be 2X line voltage. On a 480V system, this can be almost 1400V. This may be what was stressing your cable. I'm surprised your motors weren't dying if this was going on unless the motors were inverter duty rated.

This is possible, especially with the THHN insulation. I'm rustly on drive stuff and didn't think of this.

In my opinion, if reflected waves were the problem to begin with, the extra filters and stuff the engineer is recommending would probably help as they would decrease or prevent the waves from building up. However, using properly constructed VFD cable instead of THHN singles would also clear up the problem and would be simpler. XHHW insulation has a much higher voltage impluse withstand than THHN and wouldn't care even if there were waves propogating on the line.

As far as whether your engineer is at fault or not, I don't see enough information to have an opinion. I can tell you, though, that if you are admitting the cable was improperly installed you will have a hard time convincing anyone the cable's demise was anything but the installer's fault regardless of what the truth is. If you say "bad design" you'll get "bad installation" thrown right back at you. I'm not taking sides, just attempting to predict how that conversation would go.

I also think Besoeker should weigh in on this. If he doesn't find this thread on his own, you should PM him and ask what he thinks. He deals with this stuff every day.

 

[Besoeker]

Second, he should have specified cable designed for VFDs which is generally a 3/C cable with 3 symmetrically arranged grounds contained in a continuous armor.

I agree.
Individual conductors in conduit....
Not so good.

 

[Jraef]

The first big mistake is in having multiple VFD output cables in the same conduit. If there were no VFDs, running multiple cables in the same conduit is irrelevant, because mutual induction is impossible if everyone is in the same frequency and phase relationships. But if the motors are running at different speeds, the relative differences in output frequencies can wreak havoc on cables due to induced voltages alone. Add the standing wave phenomenon to that and you are asking for trouble. I have seen this over and over; every time there is a failure somewhere, usually the weakest link. The cables, the motor windings, the output transistors of the VFDs, it's a race to see who will be the first to go.

The solution, short of separate conduits, is what was mentioned above: the "VFD cables" that are triplexed conductors with an overall shield, and long lead filters on those VFD outputs (unless you have a VFD that has it built-in).

 

[ibew501ed]

for what it worth I totally agree with txengr
single stranded conductors in short run in individual metal conduits you can get away with .
All in 1 conduit for 200+' probably not

 

[Strahan]

Had a similiar problem here at our plant except we were burning motors. Reactors solved this problem for us. AB specifies distance and also specifies shielded VFD cable as well.

 

[TxEngr]

jcole

The standing wave (also known as reflected wave) is caused by an impedence mismatch between the cable and the motor combined with the fast switching time of a PWM VFD. This can cause the voltage to become twice the DC bus voltage of the drive as the voltage wave going to the drive is combined with the reflected wave from the motor. It is also related to the distance of the drive from the motor and the cable. A good reference is the AB Drive Engineering Handbook at http://literature.rockwellautomation.com/idc/groups/literature/documents/at/drive-at001_-en-p.pdf

TxEngr

 

[Besoeker]

The first big mistake is in having multiple VFD output cables in the same conduit. If there were no VFDs, running multiple cables in the same conduit is irrelevant, because mutual induction is impossible if everyone is in the same frequency and phase relationships. But if the motors are running at different speeds, the relative differences in output frequencies can wreak havoc on cables due to induced voltages alone.

I think the switching frequencies with their very high dv/dt are rather more of an issue that the power frequency components which have a dv/dt about three orders of magnitude lower.

 

[kadidle51]

In my experience, limited it may be, 220ft seems like a long run for a drive. In a perfect world you would want the drive to be as close to the load as possible. Correct me if I'm wrong here guys, but it is my understanding that these harmonics increase as the cable length increases. I would suggest VFD cable and/or line reactors. I could be wrong but in most cases a 7.5hp drive is not going to have onboard line reactors as a standard feature. VFD cable with XHHW-2 conductors would be my pick. If my understanding is correct the braid shielding that can be found on some of these cables is good for alleviating the reflective wave that comes back from the motor.

I am not an engineer.

 

[quinn77]

thank you all for your input. the water in the conduit was just thrown into my post for factual info ( im not an engineer, so i didnt know how that might effect the install...down here on the texas coast, it is pretty much impossible to keep water out of pvc underground due to the water table at 18" ). the manufacturer spec'd 100' max cable runs, however i didnt recieve these specs until after the wire had been pulled( engineers instructions ). The "nicks" in the wires are prob. the result of corona discharge right? the engineer now tells me that we will be installing load side filters, installing seperate conduits, and pulling a belden vfd cable. thank you to the engineers who responded.

quinn

 

[quinn77]

ps

ps

ps
the drives are sq/d altivar 31, the motors are 5 hp sew eurodrive inverter duty, the wire and conduits were installed per the "fine print" and per engineers supervision....and this is no rookie engineer...hes been designing and building sewer/water/lift/pump/scada for 40 years. hes an electrical engineer with a masters in physics...this is why im puzzled that he missed these details. anyway...not important; thanks again to you who responded!

quinn

 

[TxEngr]

Be warned that the Belden VFD cable is a lot harder to pull than single insulated conductors so you may want to use oversize conduit and use long radius 90s - no sharp turns. Allow extra time and manpower for the pull and watch your scale carefully when pulling. Is there anyway to go above ground instead of buried? Down your way you can get some real 'frog choker' rains and any underground cable is subject to flooding.

Don't be too hard on the engineer. When he first started doing this, he had SCR based drives that didn't have these problems. This is a relatively new (20 years) phenomonen since the advent of PWM drives and their fast switching frequencies. If you haven't dealt much with them, you're not aware of the issues. And most of us learned the hard way. And if that isn't a big enough problem, don't forget the circulating currents that can be caused by drives and can chew up bearings in the motor and gearboxes. I've had to use insulated couplings and special grounding brushes on some applications.

Good luck with the rework.

TxEngr

 

[quinn77]

thank you txengr,

well the engineer is a friend of mine and yes we are all learning about this the hard way. im going to use individual 2" conduits with sweeping elbows. only the first 50 ft will be underground and from there it will be above. this will hopefully give us a better result.

thanks again,
quinn

 

[weressl]

thank you txengr,

well the engineer is a friend of mine and yes we are all learning about this the hard way. im going to use individual 2" conduits with sweeping elbows. only the first 50 ft will be underground and from there it will be above. this will hopefully give us a better result.

thanks again,
quinn

If you can avoid the underground portion altogether that would be the best. Separate conduits for each power circuit AND separate for the controls as well.

Mind you that even wires rated for wet location are not designed for continuous submersion in water. All polymers - common name for insulators - will absorb water over time and result in eventual failure. Installing CLX cables that have a continuous metallic jacket would be your best solution. Make sure that it has triple grounding conductors. This has been working for me in hundreds of applications.

Altivar warns about problems with installations greater than 100', so it's best to let them tell you and specify what to do, reactors, etc. If you have 4 motors and they can be controlled together a single larger drive 20HP+ may be a better solution.

 

[quinn77]

thanks for the input weressl,
i suggested we use a fiber cable tray but "hurricane proofing" seems to take precedence. this treatment plant has 2 aerotor basins, each with two drive units. each basin is controlled by one dissolved oxygen meter/transmitter, so one drive here wouldn't be in "their" scope. the altivar drives were installed in the existing elevated blower room for "hurricane" protection. the engineers were on the right track; and when i suggested we move the drives to the catwalk of the basins they looked at me like i was crazy. so were just going to wait for the powers that be to come up with a solution. im just soaking all of this in and putting the data and research in my records for future use as a master/contractor.

quinn