Motor guru's I need your help...

[icky812]

Dick,
Thanks for keeping this in the forum. From this thread I've learned more about how a VFD works than I've managed to drag out of my vendors in the last 10 years.

I really do appreciate it!!

 

[Russ]

Russ

Thank you DickDV!!!!!
I'm still waiting to hear back from weg concerning the most recent motor failure. They had a bearing specialist come down from canada.
He pointed out that the shaft side race was blackened from heat (the bearings were very deformed). He also pointed out something I believe he called "Fretting." Which looked to be little gouges along the outside edge of the end bell race (where it had bumped metal-metal). The end bell race also had some discoloration, but it wasn't over the entire race (like the shaft side).
There's a lot of electrical noise coming from these motors. Are these signs of induced currents in the rotor? Is there any way to observe this (without lots of expensive equipment)?
Thank you again for all your help. I remember over a year ago someone mentioned insulated bearings (don't remember the context, but I think it had to do with the initial over-heating problems). Had we gone that way we might have saved ourselves a lot of discomfort.



Thanks again,
Russ

 

[Tom Jenkins]

Bearing fretting (or spalling) could be caused by a number of things. In your application the two most likely sources are:

1) Vibrations from operating equipment causing slight movements between the balls and the races on a piece of equipment that is stationary. This normally takes several weeks to occur, and can be avoided by manually rotating the shaft slightly every couple of days.

2) The stray currents I mentioned before, being conducted from the rotor through the bearings to the motor frame and then to ground.

 

[DickDV]

Motor-Drive Discussion

Russ, by electrical noise I assume you mean radio frequency interference, electro-magnetic interference, or electrical noise induced into nearby instrument and control loops. All of this is quite normal and requires additional measures to suppress or eliminate. It doesn't sound like your employer is terribly eager to cough up serious $ to deal with this. The good news is that it has no bearing (no pun intended!) on the motor problems you brought to this forum.

You can tell from my analysis above that I am not at all clear on how you determined that the motors were too hot. Would you please be more specific. I am particularly interested in whether the thermistors indicated high temp. If they did, then we probably have other problems that require further study.

Hope I have helped you in some way, DickDV

 

[Russ]

Russ

Motor temperatures..
When the motors failed due to over-heating it was the thermistor that opened up every time.
I would shoot the motors with a temp gun at 9 spots (per side)... noting the temperatures. The most recent temperatures showed 151F at the mid point (center of side of frame). The temperature near the shaft side is normally around 160'sF. I have had shaft side temperatures in excess of 230F (around 235-236F) before. But not with the current motors.
It was suggested that I paint the spots with flat black paint to eliminate reflection and to have the specific points marked out for reproducibility. I will be on site this afternoon and will have a new set of temps (for both standby and online conditions).


Thank you,
Russ

 

[Russ]

Russ

I have some temperatures. I took them using a Raytek Ranger MX temp gun. It was calibrated 6-20-02. The emmissivity was set at 0.93
I broke the motor into 6 sections (3 per side) and did a top/mid/bot for each section. Here's how I broke out the motor:

    ___
   |   |
   |   | Shaft
 __|   |__
 |       |
 |A     D|
 |       |___
 |           |
 |B      E   |
 |        ___|
 |        |
 |        |
 |C      F|
 |________| End Bell

The temps I read were:

Loc.  Top    Middle   Bottom (in F)
 A    206.6  209.4    210.6
 B    188    195.8    200.6
 C    121.8  120.6    122.4
 D    212    213.8    213.2
 E    229.6   N/A     239.2
 F    123.8  128.2    176

this motor was running at 95%CV, 57.4Hz, 1719RPM(estimate came off of vfd screen), 439V, 375-385A.
The inlet pressure to the fan this motor was driving was -15.5"WC, while the discharge was +14.1"WC. The temperature the fan was seeing was 304F.

the other motor was in standby mode it's temps were:

 Loc.   Top    Middle   Bottom  (in F)
  A     189.2  171      172.4
  B     157    159.4    159.4
  C     114.2  115.6    118.2
  D     171.2  172.6    172.8
  E     180.8   N/A     190.4  (the N/A is due to the junction box)
  F     115.4   117.2   122.2

This motor was running at 43.67%, 25.9Hz, 777RPM(est), 198V, 120-126A.
The inlet pressure to the fan this motor was driving was -7.3"WC, while the discharge was +2.5"WC. The temperature the fan was seeing was 104F.

Hope this data helps.


Thanks again,
Russ

 

[DickDV]

Motor-Drive Discussion

Russ, I am not an expert on motor thermal profiles. These motors certainly are running hot but probably not too hot. I'll bet the ambient temperature is over 100 degrees F too. This motor is probably rated for 40 degrees C which is 104 F.

If there is anything surprising in your data it is how warm the standby motor is running even tho it's only drawing 125 amps. One has to wonder if maybe the bearings are partly damaged and contributing to the extra heat.

If you are game, I think it would be worth experimenting with the V/Hz curve to cut the voltage to the motor at low speeds. Take a look at the drive parameters and see if you can find a group or a single parameter that offers different V/Hz and therefore torque profiles. By dropping the voltage, we effectively reduce the magnetizing amps which also cuts the available torque. Since the fan requires very little torque at these speeds, it could possibly reduce motor temp and also save your employer a little wasted energy. (Do I detect a promotion here?!!) We have to be careful tho. If we drop it too far, the amps may actually rise. This would occur if the torque is reduced too far and the shaft speed starts to slip beyond the nameplate full-load slip of 10 rpm.

If there is a parameter summary in the user's manual, you might want to fax a copy to me if you don't see what we want. I might be able to spot it. We will have to arrange a time so I can turn my fax machine on.

Let me know what you find. DickDV

 

[jrwb4gbm]

Thanks DickDV for all the posts. I too have learned a lot. The only things I have to add is perhaps if there is a cool location to get some air from with an additional external blower that would help the ambient conditions.

 

[DickDV]

Motor-Drive Discussion

Russ, I just took a look at Robicon's website material on the 454GT drive. I have three concerns. First, they state that their drive is capable of carrier frequencies up to 12KHz. I assure you that any attempt to run your motors at this high frequency will cause no end of trouble including motor insulation failure (almost always phase-to-phase), extra motor heating, and extra RFI and EMI. You don't need any of these things and I would select the lowest carrier frequency available. 1500 or 2000 Hz would not be too low if available.

Second, their literature implies that this drive is capable of "space vector control" and then, later, calls it sensorless vector control but with a speed regulation spec of 3%. This is an excellent example of the snake oil I referred to earlier. The motor itself, without the benefit of any drive intelligence is already a .6% speed regulator!

100-(1800-1790)/1800 = .006 = .6%

Ordinary slip compensation can usually cut this in half to .3% if necessary. On a fan application, this is already plenty tight so any kind of sophisticated speed management is unneeded. Sensorless vector is only suitable for constant torque loads. If your drive is programmed for "space" or "sensorless" vector control, the setup tech would have had to run the motors uncoupled to set up a motor ID run. It is a programming mistakeand needs to be fixed.

Third, they mention an energy saver option. We need to check to see if this is turned on or off.

Ideally, I need to get a copy of the Instruction Manual. I don't have any contacts left in the Robicon organization (I used to rep them 10 years ago). If someone can help here, please tell us how.

 

[ben]

If its any help, I know that your grease will break down at somewhere around 90C and it wont lubricate your bearing. This of course is dependant on your grease. Is it high temperature?
I remember that we once fit bearings that were supposed to be C3 clearance and they weren’t. The 3000-rpm drive didn't last long. A statement of the obvious but have you got the right bearings in there?
I have seen currents damage bearings before through static charge. This is common where your impeller is running in dusty conditions. As mentioned somewhere earlier in a previous post by someone else you can fit a carbon brush to rub on the motor shaft and ground this out. An interesting test would be to measure this with your test meter, from your motor shaft to earth. Obviously via the brush tag.
Magnetic thrust cannot be ironed out? Is this damaging your bearings?
Have you run the motor light? Disconnected from your fan?
Forced air-cooling is a common remedy on a drive that is getting warm. Some machine manufacturers incorporate them in the machine design and use colder air from outside.
Do you have a problem with your thermisters surely when they get hot they should trip your overtemp unit out. Before you damage the motor anyway.
I feel almost sure that this is a VFD issue and you can sort this problem out by changing your VFD parameters.

 

[Russ]

DickDV,
I called Robicon on the 4th (LOL) to find out what the carrier freq's are. In the literature it did state that for motors <800HP the max carrier freq is 3K.
I didn't get an answer. Well actually.... they suggested using an inverter duty motor. (LOL.. I love reflex answers).
I do need to check the energy saving feature (I believe it is turned off.. and I'm fairly certain the 7.66V/Hz ratio is linear).
But so far I haven't found a way to find the exact carrier freq, nor a way to change it. Seems Robicon likes to keep their secrets.
Like you said earlier, some of the robicon techs are good some aren't. Just like with Weg. One Weg rep said that the outside temperature should not ever go above 205F (actually two people said that), but yesterday a third Weg tech said it was fine.
From what I've seen so far the Weg and robicon techs seem to be pretty useless at finding the root of the problem, but are exceptional at deflecting any corporate responsibility for the problem itself.
At this rate I think my future issues will be simplified as I would strongly oppose ever purchasing either piece of equipment ever again for new sites. The small amount of savings has been more than offset by the penalties and multiplied 100X by the incredible amount of BULLSH_T both these companies have spewed. They should become poster children for ex-lax.



Russ

 

[DickDV]

Drive-Motor Discussion

Well, Russ, I can understand your disgust at the lack of support you seem to be getting. I've had direct experience with both Robicon and Weg. I personally never had a problem with Weg and always liked their product. I was lucky, apparently, in not having to depend upon them for much technical support because the organization I was part of had many very good motor people in it. My years of rep'ing Robicon 10 years ago taught me that they were a professional embarassment and I was glad to leave them so as not to have them tainting my reputation. I have built my reputation, instead, on ABB and Reliance drives, and Reliance, Weg, and Marathon motors and have had 10 years of good experience with them. But enough of that. We need to get your situation in hand, if we can.

I think that it is essential that I get in my hands a copy of the Robicon 454GT instruction manual. You said you have more than one of these drives so I assume that you have more than one of their manuals. I'm asking you to mail me one of the manuals. I'm on record here promising to mail it back to you when we are done with this or whenever you ask for it back. Or, if you can get an extra copy from your Robicon rep, send that. It would be good if you would note in it as much as possible what parameters have been modified and what the custom settings are. I will get back to you with specific changes.

Just so no one thinks this thread is somehow going underground, I'm also on record that my recommendations will be reported here on this forum, as well.

Let me know if and how you can get a manual to me.

 

[ewloeff]

motor failure

Russ, I am a motor / drive tech. The key to your problem seems to be inspecting the failed motor. We can usually tell you if a failed motor is due to votage spikes from the VFD, overheating, or a phase unbalance,by looking at where the coil shorted. If you would like some non-biased info, I perform the specs and install for motors and frives, I can give you usefull info to correct the problems. I am also curious to know what WEG has to say. I deal in other brands but have worked on WEG motors and drives. I can provide engineering data to prove or disprove some of the blame.

 

[Russ]

All help is greatly appreciated. Weg came out the Wednesday (June 26th). They promised to have a report to me by the beginning of the following week (I figured by July 3rd).
Well.. here it is the 10th and still nothing. This isn't the first time Weg has failed to produce a critical report. The last time they did a report I never actually saw it (though I repeatedly asked for it before it was "finished." Months went by without a report, then I complained (something I guess I am pretty good at )...the Birmingham rep brought me the "supposed report" the next week, but it wasn't even for the same motor.
In essence Weg is so incompetent they couldn't even fabricate a bogus report without screwing it up. Much to my dismay.
So to me Weg has an incredible amount of integrity issues to resolve before I would ever recommend them to anyone. I've had a motor sitting on site since mid-January that they have yet to pick up. Their number 1 warranty company (Jay Electic) won't pick it up since Weg has been very very late (months and months) on paying them for work done. On top of which I have been promised an RMA number for quite some time (I believe for over 5 weeks) from Weg and it has yet to materialize. Their bearing specialist took almost 2 months to get onsite (initially they were supposed to be on site at the end of April/beginning of May, but arrived June 26th!!!). Meanwhile their service group set up the replacement 400HP motor with 4 soft feet (in April).
I will contact you Dick about getting you a manual. Thank you EwLoeff for the offer. At this time I'd like to proceed with one course of action, but I will definitely keep your offer in consideration.



Russ

 

[DickDV]

Motor-Drive Discussion

Russ, I contacted an acquaintance of mine that works in a large EASA motor shop and also WEG motor distributor. He basically confirms that WEG's technical support department has been decimated and is largely dysfunctional. He also confirms that their products are, generally, still of good quality altho delivery has gotten to be a problem lately.

Of greater concern is his comment that, about nine months to a year ago, WEG apparently received a batch of magnetic steel lamination material that was not properly insulated. The result was that excessive eddy currents can flow in the laminations causing excess heat and reduced motor efficiency. Now, I don't know if this relates in any way to your motors and I don't want to needlessly alarm you. Also, unless you are able to talk directly to the motor shop people who evaluate your failed motors, I doubt that you would ever get the straight story on this from WEG sales people.

You may want to approach WEG with this in a general way but, if you please, you didn't hear it from me! Just tell them you've done a little research and are aware that they were having this problem. Who knows! Maybe you will scare them sober!

Personally, I'm doubtful that this is a factor in your motors since you seem to be having the same problem with the 350hp's as well as the two 400's. Pretty unlikely that the problem would have spread over this much time and different size ranges.

Let me know how you want to handle getting me a manual.