Motor guru's I need your help...

Russ

Guest
R
I haven't mentioned this because I thought this forumn was strictly plc, but thank gosh it's not....
Currently I have 2 weg motors (400hp non-inverter duty) hooked up to two Robicon vfds (454GT series.. the vfd was set up with a maximum variable current of 460Amps).
Initially there were weg 350's hooked up, but they kept failing (overheating, and also have had one phase-ground fault).
One month after the new units 400Hp was started it failed (phase-ground).. the bearings were heavily distorted due to heat.
Has anyone else had problems using a weg with an inverter? By the way the wire length from the vfd to the motor is 75ft.
For 2 units 4 motors (in total) have failed and had to be replaced since december 2000.
There is a lot of electrical noise, and the motor frames are always really warm-hot (150F + over most of the motor). They are TEFC as well.



Thanks,
Russ
 
Russ

What are the motors in control of and what kind of speeds (hz) do they normally run? If you know along with the speeds (hz) what is the current draw? Also how slow is the ramp speed?
I don't know if I can help out here but these questions will shed a little more light on hte happenings of the process.

And of course anyother particulars you think might help.

Drewcrew6
 
Yeah more info is needed.
1. Are the motors Inverter/Vector duty?
2. What size wire is being used to the motor?
I am assuming this is a 460v connection due to the 460 amp max set on the VFD. Since 350 HP motors were originally used the wire size may be a size too small. For the 460A at 75 foot I would use a size larger than needed. EX: Raceway, earth etc I would use 700MCM.Supposedly running in Free Air (exposed) you can use 300/350 MCM for that load...but I aint seen that to be reliable.
3. Are all connections good.
4. Wire tested not be making ground connection at any time?
5. Have you verified there isnt any mechanical problems that could make motor be loaded heavily for extended periods?

Basically what you need to do is determine if its the load causing the motor/inverter to work too hard. Is the motor getting hot due to load and causing the insulation to deteriorate etc. OR is the motor not designed for the inverter and application which its being used?

Lotta variables here, are they running it too slow all the time? too fast?
 
USMOTORS says this...


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You obviously have a heating problem. The distorted bearings and repeated failures to ground give proof of this. You might consider using an inverter-type motor with the VFD.
 
Since this problem began a number of the field techs recommended getting inverter-duty motors, but we were ignored.
Initially the 350Hp wegs (which had a 1.00Service factor and a IMax of 404A) were set to run in standby mode at 25%/15Hz (but the motor's couldn't take it. The load at 15Hz was 168A). So I increased the standby mode freq to 26Hz and it did not overheat. the motor is direct coupled via Faulk coupling/flex grid to a Mas fan. There are 2 fan bearings (with TC's measuring temp.. the fan bearings have always run cool.. average 120's-130's with 140's-low 150'sF highest temp)..
We're using 500MCM wire from the vfd to the motor. The wire was changed when the motors were upsized.
The weg motors are not inverter-duty motors. They have aluminum rotor bars and non of the extra's that inverter-duty motors possess. I tried to get inverter duty motors installed. But due to the fact that people are more concerned with the short term books, they kept going back to weg motors.
Weg has not been truthful with me or my company. Unfortunately as long as my company only see's the bottom line (which has now cost us over $42,000 in penalties/alignments/replacement weg motors) the only loser is the client.
The problem is that I have to make these pieces of **** run as well as possible.
With the 400Hp I've left the 26Hz minimum speed. Line 1 runs (when online at about 82% or 48-49Hz), while line 2 runs online right around 92% (54-55Hz). Due to the temperature of the air the current is in the mid 330'sA.
The best part is that in April when the new Weg 400HP melted my company decided to buy another Weg. Though for less than $4,000 more we could have had a top-of-the-line Baldor heavy duty inverter-duty motor. It is poor decision making at its best.




Russ
 
You can lead a horse to water.... you can't make him drink.

And I don't think you can trick him into drinking either.

That is, if someone is trying to operate the system outside of the normal limit window... well, you get whatever you can get. And usually, it ain't what you want.

If you are asking us how to do something that can't be done... within normal operating parameters... well, my only suggestion is to refrigerate the motors with 0% moisture air.

Good Luck!

If you do find a way around the normal constraints, please, let us know.
 
To Russ

Russ, I have been a Drive-Motor Application Engineer for 14+ years with some direct experience with Weg and Robicon. I will do my best to help you if you email me at [email protected].

I will need more data and will offer some things to try.To do it on the bbs here will get cumbersome plus it really is off-topic.
 
To DickDV,

You don't think that the rest of us deserve or need to know this info?

Your asking that further communication occur over E-Mail instead of On-Site is one of the Cardinal Sins!

If you think it is hard to develop your answers on the On-Site posting tool, then you should consider simply writing your answer in Word, then doing a Cut-n-Paste. That's how most of us do our more involved responses.

The real point being, WE ALL WOULD LIKE TO KNOW THESE ANSWERS!!!!!

So please be a PLCS.net kinda guy and keep all communications On-Board!

Unless, of course, you are just trying to sell Russ a "Bill of Goods".

Russ,
I encourage you to Register with PLCS.net, become one of us AND also, as I do, insist that DickDV keep his info above-board!

SHARE THE INFO!!! That's why we are here!!!
 
Personally I am in consensus and think the problem may be the fact that non iverter duty motors are being used.

My main clue came with the fact that it doesnt operate efficient at 15HZ, for whatever reason the biggest problems with inverters are related to lower frequenciesand or noise...from what I have seen.

I agree, if you can offer more advice we are willing to listen...I am especially.

Please do your best to answer here so we may all see.
 
Russ

I think what Dick is concerned about it that the Robicon vfd has a lot of parameters, and listing those would take a lot of space.
The volts/hertz ratio is 7.66, and is linear. No one output phase is markedly higher than the others. The thing that perplexes me is that "supposedly" the weg motors work fine with the allen bradley 1336A drive. I'm not sure if it is cleaner, has more step (the robicon is a 6 step pwm), or if maybe its a carrier frequency issue.
My gut feeling (which is about as accurate as the Cleveland indian bullpen :)) is that the robicon might have a wider range of tolerance on its output signals to the motor, making the motor hotter the allen bradley model, and the resultant spikes just eat up the insulation and heat the aluminum rotor bars. Someone mentioned carrier frequencies, but I really am leaning torward the robicon drive. Not that the drive is at fault, it would work beautifully with a Baldor or another inverter-duty motor. But with the frailty of the Weg motors the Robicon just beats them down over time.
But again I'm not a motor guy... just a guy in the unenviable position of trying to find a working solution.
Weg came out here this past week and I truly feel that I will not get a straight answer out of them. I believe when the motor went down in April they purposely said it was our fault to save warranty costs (though the fan/fan bearings are all in good shape with .043 vibration). So..I dunno..



Russ
 
Russ, I am a guy in the same position as you. Gotta listen to the BS of suppliers and follow the bottom line (costs). I went to the WEG web site and did not find a 350/400 hp motor that was inverter rated.

ALL BS aside, if it aint rated for the job (just like us) then it should NOT be in the position.

Overall a vfd is a vfd, they all got a bunch of parameters. As Terry said if the motor is rated to work with them then it shouldnt be a problem unless you have some gawd awfully wrong settings.

When it comes to drives dont feel bad, recently I had a fan (cooling) motor lose its blades and fall onto the connections for a DC motor. It blew the drive...i mean BOOOOMMMM. I replaced the drive but after loading the parameters it wouldnt work right (affected another drive too). I called Eurotherm in and eventually we got it to work.

FOR A DAY.

That night the machine went to do a changeover and when restarted it wouldnt work. I was at a loss as to what to do.

Anyway they had a safety meeting and were down for a few hours, so I reloaded the program. IN the process I noticed that when done uploading from a computer you are suppose to reset the comm setting. Originally this wasnt done because drive tech said settings were saved automatically. I concluded that the program was loaded successfully and ran but when shutdown for changeover it lost the last setting change...ie the communication setting. This time I saved it after loading and changing.

I state all this because whether DC or AC drive there are so many variables that there is no way to know them all. Live and learn.

Overall the best solution is your first choice...get a motor that is inverted rated. Then go from there if more problems occur.

This is a plc site but so many things are associated with plc's that there is a fine line on what to discuss. VFD's etc to me are a prime example of a device that is closely associated and should be discussed.

As was stated, this is a forum. Once the subject has been brought up then let alll see the answers to all the questions. Please.
 
VFD-Motor Discussion

WEll, folks, I thought I was being polite by asking that this off-topic subject be taken to direct email. I guess not!

So, Russ, here goes. First, I need motor nameplate data for the motors. Everything on there but the bearing numbers.

Second, tell me a bit about the fan the motor is driving. Are there dampers that move on the suction or discharge side? And, if you know, does the HP curve for the fan follow the usual cube-rule characteristic?

Third, what is the vintage of the Robicon drive? You mention 6 step-PWM which would make it a rather old unit. I suspect that it is straight PWM. Please confirm.

Fourth, normally a fan is a very easy starting machine--bearing friction only. Is that true for your unit or might it be a "sticky start". If a sticky start, please explain why.
 
I'm not an expert, and I'm not familiar with Weg motors, but I have a couple of comments.

First off, I try to make 30 Hz my minimum speed as a general rule to avoid overheating. If the cooling fan on your motor isn't quite up to snuff you may need to make your minimum speed even higher. It also makes a difference if your motor is TEFC (Totally Enclosed Fan Cooled) or ODP (Open Drip Proof)

Second, Inverter duty motors have better insulation, but I'm not convinced that they will run any cooler. I've used Robicon VFDs successfully on non-inverter duty motors running centrifugal blowers. Because of pressure constraints we hardly ever get below 40 Hz, and so heating hasn't been a problem.

Third, most VFD suppliers like to keep the motor leads down to 50 feet, primarily to minimize harmonics. If your motors are experiencing problems you should investigate putting Reflective Wave Traps (RWT) at the motor and/or line reactors at the drive. This is something I would do even if I had inverter duty motors, and the RWTs aren't very expensive. Cutler Hammer has ben very helpful in the past for me with this kind of problem.

Fourth, bearings can be destroyed in some applications because of ground currents circulating through the motor bearings from the frame. Check your grounding. There are even some products out there (basically brushes with grounded conductors) that are designed specifically to eliminate ground currents from the shaft through the bearings.

Finally, Robicon tech support has some real duds and also a few top notch guys that have helped me out in the past. Contact Robicon corporate in Pittsburgh and keep pushing until you get someone that is using his brain to help you out, not cover his butt.
 
Could you change the coupling to the fan and make it belt driven or chain driven? Then you could increase the frequency to your motor and by using a different gear/pulley ratio you could run the fans at the usual speed. This may not be an option, but its a thought.
 
Motor-Drive Discussion

While Russ is getting the data I requested, there is an opportunity to clear up some terminology and other items.

First, the term "inverter duty" is mostly snake oil due to its loose definition and irresponsible application by some manufacturers. For example, Baldor will put an inverter duty sticker on an ordinary TEFC or ODP motor for no better reason than that it has Class F insulation. At the other extreme is Reliance which reserves such a title for motors with auxiliary blowers or those like TENV motors capable of continuous rated torque duty at near zero speed. To protect yourself, you need to match the motor to the drive output technology as well as select the motor design to survive thermally at all expected speeds and loads, especially low speeds.

Second, while there are some variations between manufacturers, the general rule is that a motor that meets MG1.31 insulation standards will not need motor lead reactors or wave traps unless lead length exceeds 250 feet. There are some exceptions. Also, you can expect line reactor manufacturers and drive manufacturers selling reactors to recommend shorter lengths but, again, this is largely snake oil.

Third, variable torque loads like most fans and centrifugal pumps have a horsepower-speed characteristic curve that declines as a function of the cube of the speed and a torque-speed curve that declines as a function of the square of the speed. This means that at one-half speed, the horsepower is down to one-eighth and torque is down to one-quarter of what it was at full speed. In view of this rule, a fan or pump turning at one-quarter speed is down to one-sixtyfourth HP and one-sixteenth torque which, for the motor, is very close to operating at no load. There is absolutely no load-related reason why an ordinary TEFC motor should run hot or even warm at one-quarter speed on this kind of load. Sure, the motor's shaft-driven fan is not turning very fast but the motor is doing almost no work.

Fourth, motor amps is a poor way to measure motor load. This is due to the lead amps consisting of two components, magnetizing amps and torque-producing amps. These components are vectors at 90 degrees from each other so they do not add arithmetically. You can find magnetizing amps on the job-site by uncoupling the motor and running it at full voltage and frequency. This no-load current usually comes to about 20-25% (premium efficient motors are at the low end, standard efficient are near the high end) of nameplate amps. Calculating torque-producing amps from the total amps requires triangulation to solve. This is what "vector drives" do to measure torque. A far better way to measure motor shaft load or torque is to use a strobe light and find shaft speed. Assuming 60HZ, a four pole motor will turn right at sync speed of 1800 rpm with no load, a two pole will be at 3600 rpm, etc. As the motor load increases, the shaft speed will drop linearly until, at rated output, the speed will be right at nameplate rpm and amps will also be right at nameplate.
For example, the nameplate of a 100hp motor reads 1760 rpm. Since this is clearly a four pole motor, no load speed will be at 1800 rpm and full load speed will be at 1760 rpm. Torque calculates to 300 ft-lbs at full load. If you measure shaft speed while working and find the speed is 1780 rpm, you can be sure that the load is taking 150 ft-lbs torque out of the motor--one-half the speed drop will be one-half the rated torque. Since most modern drives will do a rough calculation of torque for you and display it, the above measurement technique is really only useful for motors on across-the-line starters.

Let's see what Russ comes back with for nameplate data and we'll take this further at that time.
 

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