Can a Constant Torque VFD on Variable Torque Motor (Centrifugal Pump) cause failure?

[SewageSam]

We have a client 4 Vertical Turbine Pumps powered by the following motors:
Emerson, BF66, Type-RUSI, Frame 404TP
460v, 3PH, 1785 RPM, 60HZ
Class F Insulation, WP1

Motor Data Sheet Calls for VFD Load Type VT/PWM.

The VFDs are Altivar 71s. Which from the model number states it is a Constant Torque Drive.

The client has had 16+ motor rewinds in the past 3 years. The motor shop has evidence that they are all first-turn burn outs, indicating voltage spikes.

The Altivar Drives used to be 1000 feet away. They also have load reactors, dv/dt filters and terminators installed. There is also an accusine system is on the main feed.

All the motors seem to fail around the 800 hour mark and 90% of the time its on first startup of the day.

After about 13 or so failures, the drives were then moved the drives to 90 feet from the motor.
They have had 3 first turn burn outs since moving the drives and are very worried and they are down to 2 pumps.

Previous studies have shown voltage spikes in the 1200 range, but recently, they used a new fluke meter and had reading of over 2000 and some in the 3000. I cant fathom how that is possible.

With all things considered, could it be possible that the drives themselves could be the issue putting to much stress on the variable torque motor windings at low speeds? Or is there any settings in those drives that could be the issue if improperly programmed in the field?

Any help would be greatly appreciated. I'm just a civil engineer trying to help, so a laymans explanation would be greatly appreciated.

I can upload any pics if it would be helpful.

Thanks in advance.

 

[luktek]

This problem is being caused by reflected waves and the rapid rise and fall of the pwm waveform. You mentioned load reactors and dv/dt filters on the output of the VFDs...is it both? Do the cables travel through water at all? The readings for voltage you are getting on the output of the VFD - ie, 3000V don't really make sense. Where are you taking these measurements? What is the dc bus voltage of the VFD during normal operation? Have there been any VFD failures in the last 3 years?

The solution is probably going to be installing a sine wave filter on the VFD output.

Don't worry about the Constant Torque rating, all drives can be considered both constant torque and variable torque rated. The convention is for constant torque to indicate an allowable overload of 150% for 1min/5min, and variable torque is typically 110% overload for 1min/5min. So the same drive can be rated for 50HP variable torque and 40HP constant torque.

 

[Skogsgurra]

First, I didn't see the size of the motors. I could google it, of course, but a number included in the question would have been nice.

It is not easy to measure motor voltage with any instrument. The reason is that both the hot and the cold input of the instrument are subject to very fast voltage edges and that the cold input is NOT designed to handle such signals. The result is a very unreliable measurement that shouldn't have been made at all in the first place.

I use either a differential measurement where I measure two motor phases to ground and use the math (if it is a scope) function to calculate and show motor voltage. Or, I use a differential probe to do the measurement.

I made a write-up on the problems that I had with this. Read it here:

http://www.google.se/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&ved=0CDoQFjAB&url=http://

http://www.gke.org/pub/files/Using%...Op5Vy7H9s8K8kgsDQeSp45g&bvm=bv.46340616,d.bGE

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mikekilroy]

- how old is the application
- did it used to work for 10 yrs and then started to fail recently?
- multiple filters on outputs?
- are output reactors, filters, etc, located at the vfd or motor?
- hp? voltage?
- motor wire type? shielded? individually shielded for each phase?
- what other devices (connectors, terminal strips, etc) are between vfd and motors?
- all failures on DIFFERENT vfds or all on same one? two?
- how were the spikes measured
- any heavy vibration involved?

additionally to var torq mode to peak rating is var torque v/hz curve is not linear at lower speeds it goes down to better match the reduced torque pull of var torq loads as speed goes down. running CT vfd mode on VT application simply means the motor will run a bit warmer at slower speeds, but will have zero effect on spikes. that drive likely has VT v/hz mode so do not assume it is not set that way either.

 

[SewageSam]

Thanks for the responses. I'll try my best to answer any questions.

- hp? voltage?
The 4 motors are 100 HP, 460V
Clarification. There used to be load reactors, and then replaced by dv/dt filters. There are line reactors.
Where are you taking these measurements?
190-204 Fluke Portable O Scope at Motor and VFD. Transients at both.
- how old is the application
Application is approximately 3 years old. Was a retrofit of a previous pump station with no prior issues.
- motor wire type? shielded? individually shielded for each phase?
Wire is THHN in PVC conduit. No shields
Do cables travel through water?
Application is near the coast, appoximately 1 mile. High ground water. Submerged wires is a definite.
- all failures on DIFFERENT vfds or all on same one? two?
All motors are on a dedicated VFD and all have failed. The motor furthest away from the drive has failed the most at 6 rewinds.
- any heavy vibration involved?
There is vibration. Motors vibrate out of the accepted range from 54 HZ to 60 HZ. Thats another issue being addressed. Motors are locked out at those frequencies current. One motor is being sent back to factor for vibration testing.
- multiple filters on outputs? Motor Terminators and TCI Dv/Dt Filters
Dv/Dts are at the VFD, Motor Terminators are at the Motor
- what other devices (connectors, terminal strips, etc) are between vfd and motors?
Non fused disconnect, No terminal strips.
What is the dc bus voltage of the VFD during normal operation?
698V

It may be of some interest that there are some (4) 100 HP motors on mechanical aerators that are 300 feet away from the same type of drive using the same cable and conduit. No failures to date. Im getting info on those motors. Those motors are seperated a by good distance from each other and other equipment as well.

Also wanted to ask if there could be any issues with the grounding that should be checked in detail.

 

[Skogsgurra]

"Previous studies have shown voltage spikes in the 1200 range, but recently, they used a new fluke meter and had reading of over 2000 and some in the 3000. I cant fathom how that is possible"

It isn't. The 190-204 doesn't stand up to that kind of measurements. See my notes in previous post - it was exactly that instrument I tried to use. The 1200 V peaks are more in line with a reflection factor at 1.7 - 1.8. Which is normal if there are no motor reactors, but much too high if you have reactors or du/dt filters. Are they really fitted? And correctly connected? What about Common Mode Filters?



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[SewageSam]

It isn't. The 190-204 doesn't stand up to that kind of measurements. See my notes in previous post - it was exactly that instrument I tried to use. The 1200 V peaks are more in line with a reflection factor at 1.7 - 1.8. Which is normal if there are no motor reactors, but much too high if you have reactors or du/dt filters. Are they really fitted? And correctly connected? What about Common Mode Filters

Sorry, I should have mentioned, those readings were with the filters offline. The filters should take it below 1000V so Ive read in the manual.

Unfortunately, this arrangement is the exact setup as before they moved the drives closer and there were still failures. Only thing different is the cable length.

Filters are TCI KLCUL130A and are within 6 feet of the drive.

 

[jraef]

Wire is THHN in PVC conduit. No shields

Bad idea. Either THHN in STEEL conduit, or shielded cable in any other kind of conduit. Think of those VFD output cables as being very powerful radio transmitter antennae. Your cables are now able to pick up stray RFI/EMI signals, even from adjacent drive conduits, right through that PVC conduit. This may be contributing to the problem, especially since it is likely that your TCI filters are at the VFD end. The other installation that you say is identical with no issues may have different routing / proximity of the conduits and cables and by chance ended up OK.

Secondly, having a high carrier frequency, which is sometimes used to reduce the noise created in the motor, can exacerbate spike generation and it's possible that it could be overwhelming the efforts to attenuate it. People sometimes complain about the whine in the motors during commissioning, the technician responds by increasing the carrier frequency (which doesn't remove it, it just moves it out of human hearing), but does not consider other possible consequences. Could be that in your other installation, the CF was left as low as possible, but here, someone turned it up because of a noise complaint.

Also IIRC, when it first came out years ago the ATV71 was purported to have some sort of output feature that Schneider claimed for a while was capable of reducing or eliminating standing wave spikes on the output without the need for additional filtering. I have not heard of them pushing that issue in the past few years, because maybe it turned out to be an "overly optimistic" claim. The reason why I mention it is that if they have something that is trying to take care of it, then you add ANOTHER DV/DT output filter, it may end up interacting with this to CREATE more spikes than it was intended to mitigate. I have no proof or theory behind it, I just remember sitting in a training class with Schneider when the ATV71 was initially released, hearing their claim, and wondering if this might cause more problems than it solved if people were not aware of it.

Have you consulted TCI on this? They may have some experiences with the ATV71 that would be helpful.

"Will work for (the memory of) salami"

 

[SewageSam]

Think of those VFD output cables as being very powerful radio transmitter antennae. Your cables are now able to pick up stray RFI/EMI signals,

This seems like a strong indication of the issue. The other cables to the aerator motors have good seperation. Is there a way to test for that before recommending shielded cable? Doesnt sound cheap, however it would only be <100' this go round. They could try one and see if there is immediate improvement.

 

[jraef]

Is there a way to test for that before recommending shielded cable?

It needs no testing in my opinion and should not just be a "recommendation", it's an absolute requirement: either use steel conduit or shielded cable. If you can't use steel conduit because of corrosion issues, then use the shielded cable. Every VFD installation manual I have seen in the past 10 years now makes a recommendation for shielded cable, and puts in a mild disclaimer that says it MAY not be necessary under certain conditions. But if you look at grounding recommendations, they will show either the shielded cable or grounded steel conduit. My company no longer recommends anything but shielded cable regardless of the type of conduit.

"Will work for (the memory of) salami"

 

[controlsdude]

Voltage spike problems are either cable problem or distance problem.

Wet locations you need special rated VFD shielded cables not the usual THHN install.

 

[SewageSam]

Today with both the terminators and dv/dt filters back online, the max voltage observed was 740 volts. This same configuration at a 1000 ft distance had a max voltage of 1200 so moving the drives looks to have helped a good bit.

My question is if this motor is rated for NEMA Part 31, its supposedly capable of 1,600 Vpk at 0.1us rise time, why would it fail under either current or previous arrangement. Could there be that much Common Noise Voltage?

 

[jraef]

My question is if this motor is rated for NEMA Part 31, its supposedly capable of 1,600 Vpk at 0.1us rise time, why would it fail under either current or previous arrangement. Could there be that much Common Noise Voltage?

Yes, if the grounding is not done right and with the PVC conduit but no shielding, that can be very difficult to get just right. Your symptoms however are still classic for standing wave voltage spikes. I would suspect that your meter is not seeing the problem as it really exists, it may be filtering it for you, which is not doing you any favors in this case. I admit to not looking at the specs for that meter, but as I read it Gunnar (skogsgura) has warned you in an earlier post that he had similar issues with that meter not seeing the whole story.

As to the motor ratings, you have to remember that NEMA is a membership driven organization, not a 3rd party testing agency. Compliance is voluntary and members "self regulate" their testing and marketing claims. In other words, saying it meets Part 31 is one thing, how it is tested against that standard to say you are actually meeting it is another. Motor mfrs have been know to "exaggerate" in the past. We had a thread about one earlier this year where the mfr (from Brazil) told the user it was "inverter rated" but a look at their own specification sheets showed that they were using an extremely loose interpretation of that term.

"Will work for (the memory of) salami"

 

[controlsdude]

Also if you look at the manufacturer's spreadsheet on distance versus voltage classes.
Usually you see no solution (just wire from drive to motor), load reactor, or load reactor and some type of reistor inductor combo.

As HP or kw goes up the distance becomes longer for all different types of solutions. But as you drop in HP the distance drops in length.

But the 1488v and 1600v rated type motors usually have the same maximum distance. Its the 1200 and 1000v rated type motors where the maximum distance drops to a lower value.

But all these tables are different for the drives and the type of motor that is chosen. Seems to be a manufacturer of the drive and motor to what type of solution you can get away with on your system.

There is also the frequency that you use also in a drive. 2kHz versus 4kHZ
Some drives you can get a longer distance on the 2kHz freq then the 4kHz, but again this is manufacturer driven.

 

[controlsdude]

Also remember this from past job.

If you have multiple motors on 1 vfd, the distance is not the distance from drive to motor, but some lesser value. This should be a calculation in the VFD manual to determine this max distance. But this was out of a mitsu vfd manual and not sure if there is a max motor to VFD ratio that can be used, what that is in the alvatar world I am clueless?