Converting from WPII to TEFC Motors 3

[JJPellin]

I will start with a disclaimer. I am a rotating machinery engineer with expertise regarding pumps and compressors. I know relatively little about motors.

We have been converting a number of motors from the original WPII configuration to TEFC. These motors tend to be in the range of 300 to 1200 HP, two pole (3600 rpm) and 4160 volt. These motors are all sleeve bearing designs. We seem to be seeing two sorts of problems after these conversions.

Some of these motors have been experiencing chronic axial shuttling. Our couplings are disk-pack designs (Thomas Series-71). We see a persistent axial bounce in the motor shaft which would typically be associated with a motor fighting to get back to magnetic center. We have repeatedly verified mag center and coupling hub spacing. But, the problem continues.

Some of these installations have experienced repeated coupling disk-pack failures. The failures include cracks in the outer disks at the edge of the washer. The coupling manufacturer’s literature would classify this as a failure indicative of excessive radial misalignment.

I suspect that two issues may be coming into play with these problems. First, I am suspicious that the vertical thermal growth of the new motors is significantly different than the old motor. The arrangement with the cooling fan on the non-drive end blowing toward the drive end seems like it would have a strong impact. Just placing my hands on the motor feet and end housings, I can feel a big difference in temperature between drive end and non-drive end. We should be able to take support temperatures and recalculate our cold alignment offsets to adjust for this. The flow path of the cooling air may also be affecting the thermal growth of the driven pump. The air blowing against the coupling end pedestals is probably cooling them down more than the thrust end pedestals. With the motor hotter on the inboard and the pump hotter on the outboard, a significant misalignment may be occurring in service.

The other issue I have considered is axial thermal growth of the motor rotor. Some of these motors are very long (perhaps as much as 8 feet long shafts). Even through the motor is a sleeve bearing design with about one-half inch mechanical float, there has to be some issue with axial growth of the motor rotor and shaft toward the coupling. If the magnetic center locks the rotor at the center point, then half of the axial growth would be directed toward the coupling and half would be directed away from the coupling. We do not pre-stretch these coupling spacers to accommodate this axial growth. I am beginning to think that we should. I will probably ask our mechanics to stretch the coupling spacer on one of these machines by 0.030 inch to see if that stops the shuttling.

I am interested if anyone else has seen similar problems when converting from WPII to TEFC motors. And, I would like for motor experts to comment on my assumptions regarding vertical and axial thermal growth. Any help would be much appreciated.


Johnny Pellin

 

[electricpete]

I have never been involved in conversion of ODP / WP to TEFC.

We do have some sleeve bearing motors with shim pack couplings that experience axial shuttling. Ours are 2500hp 1800rpm horizontal sleeve bearing motors driving single stage centrifugal pump through a shim pack coupling. In our case the shuttling occurs very predictably during plant startup and shutdown when there are different fluid conditions (lower flow rates, higher temperatures closer to saturation). It never occurs during normal full flow operation and doesn't change when we make axial position adjustments of the motor. The motor shaft moves about 1/8" at a frequency of about once per second if memory serves me right.

I conclude in our case the source of the oscillation is small random movement within the pump thrust bearing clearance (~ 0.020") possibly impacting against each limit of pump travel, which creates broadband excitation that excites the axial resonant frequency of the system.
Pump Position === Coupling/Spring === Motor mass

You can imagine the resonant frequency of large motor rotor mass several thousand pounds with coupling spring is very low. 1hz or so doesn't seem unreasonable to me.

I post it only for information. I don't know if it has anythign to do with your problem. It doesn't sound like any of the factors relevant to our condition (pump clearances and behavior, coupling type, motor rotor mass) would change during your conversion from open to TEFC.


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[electricpete]

Rexnord / Lovejoy has a very detailed document for inspection of shim pack couplings. I'm guessing it's the same one you looked at.

See if this link works

http://www.maintenance.org/fileSend...9590942964786646/ThomasShimPackInspection.pdf

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[electricpete]

If the magnetic center locks the rotor at the center point, then half of the axial growth would be directed toward the coupling and half would be directed away from the coupling.

The axial magnetic centering force is relatively weak and the pump endplay clearance is relatively small. I tend to view that the pump and coupling setup controls position of the motor rotor. So it will tend to grow away from the pump. The axial magnetic force might pull it back toward center just a little but it's a weak force and by my reckoning won't distort the shim pack much. So if motor rotor grows thermally it tends to push the rotor off-center in direction away from the pump to my thinking. I suppose motor off- of magnetic center could result in hunting according to traditional wisdom (Personally I've never been involved in a situation where hunting was attributed to setting rotor off-magnetic center even though we had rotor off center for a variety of reasons).


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[electricpete]

I will probably ask our mechanics to stretch the coupling spacer on one of these machines by 0.030 inch to see if that stops the shuttling.

I'm not aware of any way to prestretch the coupling since there is no way to apply any force on the motor side. You could couple up the motor a little bit off of magnetic center toward the pump in hopes that it grows back into magnetic center when hot.

I can imagine that even if you have similar stator temperatures after the modification that the rotor can still be hotter since cooling of rotor is more indirect in TEFC than open.

I can't think of any easy ways to examine the rotor temperature or axial growth. Perhaps look at shaft extension with thermography where it exits the motor (if you have comparison data at same location from motor at similar load before the mod). Maybe the behavior of shuttling over time after start and during different load conditions would give a clue. I doubt that shaft journal inspection would show the location of contact with stationary bearing that well. If you looked at bearings you could check for evidence of axial contact (absence of contact wouldn't prove anything but indication of continuous contact would be telling of severe growth and would probably be accompanied by other symptoms temperature and vibration).

There are of course plenty of tools to check your theory about change from off-line to running radial alignment. (example Permalign or Acculign)

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[MikeHalloran]

The coupling manufacturer’s literature would classify this as a failure indicative of excessive radial misalignment.

The obvious (not easy) response to that indication is moving something axially far enough to allow use of two couplings and a short driveshaft, or a double coupling assembly if available, so that any radial misalignment becomes a non-issue.

The additional axial compliance of a second coupling might also help with the axial drift, regardless of the cause.









Mike Halloran
Pembroke Pines, FL, USA

 

[JJPellin]

I am not concerned about the pull toward mag center causing a coupling failure, directly. I am concerned that the force could be contributing to the shuttling problem. We would pre-stretch the coupling the same way to do on all of our large machines. We would place the motor on magnetic center and then position the coupling hubs to a spacing 0.030 inch longer than the coupling spacer length. With the motor cold, there would be a pull away from the pump. But, if the motor shaft grows toward the pump, as I suspect, the motor rotor would move toward the neutral position when hot and the axial pull would drop.

Johnny Pellin

 

[electricpete]

We would pre-stretch the coupling the same way to do on all of our large machines. We would place the motor on magnetic center and then position the coupling hubs to a spacing 0.030 inch longer than the coupling spacer length. With the motor cold, there would be a pull away from the pump.But, if the motor shaft grows toward the pump, as I suspect, the motor rotor would move toward the neutral position when hot and the axial pull would drop.

ok, you threw me the first time with that terminology prestretch. Now I understand better what you're saying and it's the same thing I said in my own terms. ("couple up the motor a little bit off of magnetic center toward the pump in hopes that it grows back into magnetic center when hot."). By "growing back into magnetic center" I was referring to what happens to the rotor as a whole, not was it observable at the shaft extension. Anyway just terminology, sorry for the detour.

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[electricpete]

If you were inclined to try to measure the amount of shaft thermal growth, maybe there is something you can do by monitoring the ODE / fan end.

You could perhaps put some reference marks on the shaft at the ODE (fan end) and maybe cut out a small window of the fan shroud and replace the cutout with plexiglass in order to be able to observe how those marks move during operation. Or if that's too difficult then just leave the cutout window open with some protective screen if you are not concerned about the small loss of cooling air from modifying the shroud during such a test.

Or if the end of the shaft is visible through the shroud there might possibly be some way to measure distance between shroud and end of shaft using something like a prox probe if it can read that distance.

Or maybe you can come up with a better way. (just trying the "prestretch" you suggested might be easier)


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[electricpete]

I think the axial forces from rotor interaction with cooling air is also something to consider, especially since that flow path has presumably been modified. lf you can describe the internal flow path of the modified motor, it might trigger some ideas. If there is a mechanism where fluid axial force on rotor varies with rotor position, it might contribute to oscillation.

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[electricpete]

I am curious if you have ever witnessed axial hunting from motor off magnetic center yourself. We have seven different families of horizontal sleeve bearing motors with fleXible couplings at our plant (about 30 machines) that I have watched for 16 years. The only hunting I have seen is described above (not magnetic). I realize it is commonly discussed by millwrights but I have always wondered how common axial hunting from being off of magnetic center really is.

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[JJPellin]

Electricpete,

I have seen dozens of instances of axial shuttling in the past 25 years. At this moment, we probably have 3 or more that are chronically shuttling. We have solved a number of these by using improved practices for establishing magnetic center and careful adjustment of the coupling hub spacing. For some of these newer TEFC motors, we know the issue. When running the motor solo, the fan pulls the rotor off center and we mark that position as mag center. For these we have to remove the cooling fan and solo the motor to get a true center. I have considered just ignoring them if I could believe that this motion is not harmful to the motor, pump or coupling. But, it freaks out the operators and it seems like it should be easy to resolve.

As a point of reference, we probably have 2000 motors in operation. There are probably 200 or more with sleeve bearings. These motors were manufactured as long ago as 1955 and as recently as last month. In the past, our TEFC motors tended to be the smaller ones. Now, we are buying much larger TEFC motors and the problem is becoming more common.

Johnny Pellin

 

[desertfox]

Hi

Point 5 on this list of the link given below suggests another reason for axial shuttling:-

https://books.google.co.uk/books?id...&q=axial shuttling in electric motors&f=false

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[MikeHalloran]

Desertfox, please paraphrase point 5. I do not have permission to read the book.
Thanks.

Mike Halloran
Pembroke Pines, FL, USA

 

[electricpete]

Here's what's in the link posted by desertfox
[quote="Centrifugal Pumps" by Gulich]5. Axial rotor or bearing housing vibrations are frequently observed on single stage, double-entry pumps during partload operation. They are caused by unsteady impeller inlet and discharge flows. Axial rotor shuttling is particularly pronounced when flow recirculation at the impeller outlet influences in an unsteady way the flow in the impeller sidewall gaps. Such effects usually occur at low frequencies, so that axial rotor movements may be visually observed.[/quote]
That seems to match pretty closely what we see that I posted above. As mentioned above, ours is single stage pump and the problem of motor shaft axial shuttling only occurs at low pump flow (or "part load"). Now that I think about it, it's also double suction pump to balance thrust. I think the "unsteady flow" part is consistent with my theory that the low thrust may be reversing randomly causing bumping back and forth within the small pump clearance, creating broadband excitation which excites the low resonant frequency of large motor rotor mass and low coupling spring constant. They talk about axial movements being visually observed.... I imagine they are referring to motor shaft movement like ours because I think pump clearances are typically small enough that the associated pump shaft movement would be very hard to detect visibly (I can't detect our pump shaft moving at all when motor is moving 1/8"). It's a detour from the thread, but interesting to me... first time I've seen something similar to our situation described in a book.


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[electricpete]

I found a (not copyrighted) article from Baldor (took over from Reliance) with a lot of detail about "hunting"

http://www.baldorprospec.com/Assets/PDF/MotorPrimer_Part1.pdf

http://www.baldorprospec.com/Assets/PDF/MotorPrimer_Part1.pdf[/URL]
12. Why do the shafts on sleeve bearing motors “hunt” axially when running uncoupled?

It has often been observed that when motors equipped with sleeve type bearings are operated uncoupled, the shaft will “hunt” or oscillate axially. This axial motion often has the following characteristics:
• The frequency of the oscillation is relatively low, on the order of 30 to 70 times per minute
• The amplitude of the oscillation is between 0.030” and 0.125”
• The oscillation is often times not periodic; it may appear random or it may build up in amplitude and then suddenly stop, only to begin again.

No oscillation is observed when the motor is coupled to the driven equipment because the coupling between the motor and the driven equipment either locks the motor shaft axially or, due to the coupling axial stiffness, greatly reduces the magnitude of the oscillation. This “hunting” may be the result of an imbalance between the magnetic centering forces generated between the rotor and the stator of the motor and the aerodynamic forces generated by the various ventilating fans attached to the motor shaft. This type of “hunting” is most prevalent in the following types of motors:
• Motors without radial ventilation ducts in the rotor. This is typical of certain two pole motors and most TEFC motors.
• TEFC motors that are designed with low air gap flux densities.
• Motors with radial ventilation ducts in the rotor as well as the stator, but these ducts are not axially aligned.
• Motors that have unbalanced aerodynamic flow forces such as TEFC and TETC/TEAAC motors with shaft mounted external air circuit fans and motors with single end ventilation.
The magnetic centering force is a function of the magnetizing current of the motor (basically the no load amperage of the motor), air gap flux density, air gap radial distance, number of aligned rotor and stator segments (ends of rotor, and stator and radial air ducts), voltage, air gap axial length and axial misalignment between the rotor and stator. [22, 23] The magnetic centering force increases from zero when the motor is operating on its magnetic center (magnetic equilibrium), while the rotor is displaced axially relative to the stator. See Figure 15. Typically, at a 0.125” axial displacement, non-ducted rotors may develop 50 to 150 pounds of axial centering force, whereas rotors with 12 radial ventilating ducts aligned with stator duct may develop up to several hundred pounds. At start-up, these motors will develop axial forces up to three times these steady state values.
Rotating fans generate a low static pressure on their inlet and a high static pressure on their discharge. These pressures result in a net aerodynamic force equal to the pressure difference times the fan effective area. This fan force will tend to displace the motor shaft axially, see figure 16.
The motor rotor will move axially to effect a balance between the above two forces. The aerodynamic fan force changes as the fan moves toward and away from the stationary fan shrouds. As the fan moves away from the fan shroud the pressure differential across the fan decreases and the effective force decreases. In addition, as the rotor moves off of magnetic center, the magnetic centering force increases. Thus, the motor shaft “hunts” axially to maintain a force balance.
Another cause of this “hunting” can be a rotor with its ends and or its radial air ducts manufactured at an angle to its axis of rotation. This type of geometry in a rotor is referred to as a “wobble” or parallelogram. With this geometry, the rotor continually moves axially as the rotor end alignments vary relative to the stator as a function of the angle of rotation.

I've got to admit it's still somewhat mysterious to me. Some things that stand out to me initially:
1 - They seem to give almost as much weight to aerodynamic aspects as to magnetic aspects.
2 - Many of the characteristics associated with hungint in the article (low flux density, air ducts not aligned, 2-pole motors) would be exactly the characteristics that REDUCE the magnetic centering force. I'm not sure exactly why that is. I might guess (swag) that the magnetic centering force actually plays a role of stabilizing the aerodynamically-induced hunting rather than aggravating/creating it. Then again why does moving it off-center create this hunting? It's hard for me to understand because the magnetic centering force for a given rotor position is constant (doesn't change with time).
3 - The frequency in the neighborhood 30-70 per minute is mentioned in context of uncoupled hunting. That makes me thing that maybe I was wrong about resonant frequency of motor mass against coupling spring constant in the case of our motors although ours would seem to be quite a different animal.


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[JJPellin]

Thanks for the replies. I believe that the source of our axial hunting is coming from the motor side of the coupling, not from the pump side. As has been noted, the axial play in the pump bearings is in the range of about 0.002” to 0.004”. The axial play in the motor rotor is closer to 0.500”. Many of these pumps are multi-stage designs with opposed impellers that should produce stable and predictable thrust forces. These are not double suction impellers and we are not operating at reduced flows.

The motors that are giving us this trouble are TEFC motors which is prominently included in the last reference from electricpete. The fact that these motors are 5 kV rather than 480 V means that the amps are lower which means that the restoring force pulling the rotor toward magnetic center is also lower. The increased efficiency of these motors probably results in lower air gaps and increased chance of aerodynamic axial forces. All of this makes good sense to me. I am still not sure what to do to resolve it. Trying to do a better and better job of setting the motor rotor perfectly on mag center is not the solution for some of these. I don’t want to get into the business of trying to redesign the motor rotors or stators or cooling fans. All that leaves me is the coupling. I can make some changes to the axial stiffness of my coupling to try and detune this response. But, this would likely be a trial and error process. Any other suggestions?


Johnny Pellin

 

[electricpete]

Another reference

http://www.eecoonline.com/wp-content/uploads/2014/03/Sleeve-Bearing-Repair-EASA0607.pdf

See Appendix II (page 12) of the link, which is "Axial Hunting of 2-pole motors - causes and cures"

Some observations:
1 - the first two sentences reinforces the idea that it is the machines with weak magnetic centering force are the ones that are susceptible to hunting. So again it suggests the magnetic centering force has a stabilizing effect. Revisiting my own question (last post) of why being off-center should cause a problem, I think I can cobble together a better proposed answer. The magnetic centering force has a stabilizing effect in pulling rotor back to its original position IF the original position is cenetered. In the centered case, regardless of direction of perturbation the mag force will pull back to original position. It's like being at a valley of the potential function (stable). But when way off-center the response to a perturbation is always the same direction (center) regardless of direction of perturbation. It's like being on a sloped part of the potential function rather than local minimum. Not as stabilizing. Maybe.

there's a lot more in this article to read. (Table 2 - causes and cures of axial hunting)


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[electricpete]

Revisiting my own question (last post) of why being off-center should cause a problem, I think I can cobble together a better proposed answer. The magnetic centering force has a stabilizing effect in pulling rotor back to its original position IF the original position is cenetered. In the centered case, regardless of direction of perturbation the mag force will pull back to original position. It's like being at a valley of the potential function (stable). But when way off-center the response to a perturbation is always the same direction (center) regardless of direction of perturbation. It's like being on a sloped part of the potential function rather than local minimum. Not as stabilizing. Maybe.

This explanation would not work if we viewed the magnetic centering force as a linear spring (Fmag = Kmag*(x-xmagcenter)). In this case if we combine it with another linear spring Fcoupling=Kcoupling*(x-x0), the combined characteristic F vs x is a straight line with slope Kmag+Kcoupling. Restoring force for perturbation has spring constant Kmag+Kcoupling. The effect of non-zero xmagcenter (representing set off of magnetic center) is simply to shift the equilibrium point, but not to change the characteristics of the combined spring constant (which is the restoring force).

But magnetic force is not a linear spring. In the latest link it suggests Fmag = Kmag*(x-xmagcenter)^0.75. The slope of this function Fmag(x) vs x (representing the effective spring constant from magnetic force) is KmagEffective = d/dx(Fmag) = 0.75*Kmag*(x-xmagcenter)^(-0.25). This effective magnetic spring constant KmagEffective is highest when x-xmagcenter =0 (when actual position x is near magnetic center xmagcenter) and decreases as x moves away from xmagcenter. If the combined action of coupling and magnetic force is an equilibrium position x different than xmagcenter, then the effective spring constant from magnetic force is lower. If coupling spring force is weak and we're relying on added magnetic centering force to limit response to perturbing forces, then it is not as effective when the equilibrium position is off of magnetic center. That represents a more tenable explanation although I'm not sure it's the whole story.

Kmag may have been unfortunate choice of symbols since it is not strictly a spring constant when used in the non linear equation (because units of Kmag in non linear. equation above would have to be N/m^0.75 rather than N/m). However units of KmagEffective would still be N/m consistent with an effective spring constant.

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[edison123]

@ Johnny Pellin

The voltage rating of the motor has got nothing to do with the motor flux density. Whether 480 V or 5 KV, the air-gap flux density remains almost the same.

I once solved this problem of hunting axial movement of the rotor (which was perfectly located to the magnetic center as per theory) by moving the stator back and forth along the axis. Of course, this took 5 to 6 times of starting and stopping the motor and the load, in addition to locking the stator so that it did not move across the axis. The bearings were located in separate pedestals and hence moving the stator was not a problem. If the bearings are mounted on the stator end brackets, this is a bust.

Muthu

http://www.edison.co.in