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Converting from WPII to TEFC Motors
3

Converting from WPII to TEFC Motors

Converting from WPII to TEFC Motors

(OP)
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

RE: Converting from WPII to TEFC Motors

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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

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/fileSendAction/fcType/0...

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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

Quote:

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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

Quote:

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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

Quote (JJPellin)

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

RE: Converting from WPII to TEFC Motors

(OP)
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

RE: Converting from WPII to TEFC Motors

Quote:

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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

(OP)
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

RE: Converting from WPII to TEFC Motors

Hi

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

https://books.google.co.uk/books?id=eurAVC3_Pk4C&a...

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

RE: Converting from WPII to TEFC Motors

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

Mike Halloran
Pembroke Pines, FL, USA

RE: Converting from WPII to TEFC Motors

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.
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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

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/MotorPrime...

Quote ("A MOTOR PRIMER Part 1", by Donner (Texaco), Subler (Reliance) and Evou (Reliance) )


http://www.baldorprospec.com/Assets/PDF/MotorPrime...
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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

(OP)
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

RE: Converting from WPII to TEFC Motors

Another reference
http://www.eecoonline.com/wp-content/uploads/2014/...

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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

Quote (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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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

@ 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
www.edison.co.in

RE: Converting from WPII to TEFC Motors

JJPellin,

Axial hunting is not uncommon on large 2 pole TEFC motors with sleeve bearings when running uncoupled or running coupled with large axial clearance in the coupling. As described in the Reliance motor primer, the ODE cooling fan forces the rotor away from magnetic center. As the rotor moves away from magnetic center, the centering force increases and the fan aerodynamic force decreases until the point that the centering force overcomes the aerodynamic force. The result is usually an oscillation rather than operating at a point of equilibrium. This is due to the non-linear nature of the forces involved, the relatively large inertia of the rotating assembly, and the fact that the sleeve bearings offer no resistance (friction) to axial movement.

As a note, in the extreme case the fan will completely overcome the magnetic centering force and the rotor will thrust completely towards the ODE and will run against the bearing thrust face. I have seen a motor fail on commissioning because the millwright performing the alignment took this to be magnetic center.

RE: Converting from WPII to TEFC Motors

Quote (electricpete)

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.
Attached I tried to provide a graphical depiction of above under assumption the magnetic force is given by the expression above the coupling spring is linear. The particular numerical values I picked out (*) were xmagcenter = 0.2 (offset from the coupling force acting at x=0), Kmag = 1, Kcoupling = 1, also coupling spring completely linear (big assumption... I imagine it stiffens as it deflects). * These values are 100% fictional just to make the plots look good for illustration, I made no attempt to figure out realistic values. The first graph attached is Fmag vs x. It looks fairly much like a straight line everywhere except around x=0.2 where there is an inflection point. The next graph is the slope of the first labeled KeffMagnetic... the effective spring constant of magnetic force. It has a peak at x=0.2 which is MATHEMATICALLY infinite according to this form of the equation. Obviously the form of the equation must in reality change somewhere near this singular point.. The next graph shows magnetic force in red (equilibrium if this was the only force would be the zero-crossing at x=0.2), coupling force in green (equilibrium if this was the only force would be the zero crossing at x=0), and the sum/combination in blue (equilibrium is the zero crossing at x~0.1). The final graph is again the spring constants where the combination spring constant is sum of the two.

EDIT - I should have defined the forces with a negative sign: F = -Kcoupling*x and Fmag(x) = -Kmag*(x-xmagcenter)^0.75 so that all the forces act toward their equilibrium point instead of away from it. Use your imagination and flip the graphs horizontally.

What does all that prove? Nothing much other than illustrating that the functional form of the magnetic force given in the paper suggests there is an increase in the effective spring constant "near" the magnetic center. How near is near... would have to put some better numbers to it to characterize that better. Again, just trying to piece together a story that makes sense but I'm not sure I have the whole story.

Brainstorming what might be done, especially focusing on the mechanical side:
* stiffer coupling would help (if it's all about stiffness to resist aerodynamic forces as the articles seem to be suggesting)
* add an antifriction bearing in the housing on the outboard end if you have room (suggested in one of the articles)
* change the fan pitch was mentioned in attempt to reduce axial force to make the blades closer to a radial fan vs axial fan and reduce fan thrust. It might even be possible to go completely to a radial fan if the shroud shape (possibly redesigned) is capable or redirecting radial flow into axial flow. Or I can imagine using more powerful radial fan in attempt to compensate for reduced efficiency of the shroud/fan in delivering axial flow. Safe to say it requires some study by someone that knows what they're doing more than me.
* your practice of figuring out magnetic center the best way possible (by removing external fan) might help get it closer to magnetic center which should still help..
* your idea of offsetting from magnetic center when coupled in anticipation of what changes might happen would help in theory to the extent you can predict what thermal changes occur during operation. If you were lucky enough to be aware that there are remaining unbalanced fan force that cannot be removed when you scribe magnetic center, then you might attempt to compensate for those too when you establish coupled position, in attempt to land on true magnetic center during operation for strongest magnetic centering force.


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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

Quote (electricpete)

EDIT - I should have defined the forces with a negative sign: F = -Kcoupling*x and Fmag(x) = -Kmag*(x-xmagcenter)^0.75 so that all the forces act toward their equilibrium point instead of away from it. Use your imagination and flip the graphs horizontally.
Fixed in attachment here

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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

Below is linked an article which goes to considerable effort examining many variables involved in magnetic centering force.
"The Influence of Axial Magnetic Centering Forces on Sleeve Bearing Induction Motors" from IEEE/IAS/PCIC conference 2006
http://ieeexplore.ieee.org/document/4199057/

Safe to say, the IEEE article is nowhere as simple as the x^0.75 relationship given in the EASA-based document above.
I don't fully understand what's going on the IEEE document and I can't post it (it's copyrighted... although it's free to me as member of IAS).
The simplest expression that I could find for relative variation of magnetic centering force vs position was given by equation 3 of the IEEE article:
F(hu) ~ dLeffective/dhu = 1-2/pi*arccot(hu)
where
hu = h / g
h = offset from center position
g = gap distance accross the airgap
I think this equation applies to a situation of no ducts and identical length rotor and stator.

Attached I tried to compare this simplest IEEE expression to the previous one from EASA. By changing the relative scaling of the two forms I couldnt' make them match at large values of hu but I could make them match pretty closely for |hu| < 1. Maybe the EASA expression was a more approximate expression for use on small distances up to one airgap distance offset. Also when we take derivative of F to compute effective spring constant Keffective = dF/dx, the IEEE equation does not suffer from the problem of going to infinity at x=0 like the EASA equation did.

These aspects are plotted attached (IEEE red, EASA blue). I'd say it's nowhere near perfect, but it seems that the red IEEE curve in the 2nd plot is a better starting point to understand how the magnetic stifness falls off with distance (for this particular case no ducts, rotor and stator same length) than the blue EASA curve. Max stiffness at center, about half that at 1 airgap length offset, about quarter that at two airgap lengths offset. Obviously anyone reading this would be waaay better off going directly to the article than accepting my interpretation of it from my limited understanding. And there's a whole lot more there in that article.... particularly a lot of examination of effects of many variations of rotor and stator vent duct configurations.

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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

I read the quoted material that EP found from Baldor and it made perfect sense to me. They attribute the hunting (of an unloaded rotor) to variable fan thrust. They also associated it with the centering force on the rotor. In my reading, when the rotor is centered, the stabilizing force is at a *minimum* and the shaft must be forced off-center to develop any magnetic force that will return it to center.

So (according to the Baldor document alone) you have an oscillating thrust from the cooling fan, pushing against a weak centering force from the rotor, which requires a significant displacement (your stated 1/8") before the force can arrest the fan thrust force. When the fan's thrust breaks down because the shaft has moved, the thrust drops and the shaft returns for another cycle.

I also agree that the fan thrust can be variable, because the TEFC motors I've used (admittedly, up to a piddly 10HP!) the effectiveness of the fan is completely dependent on its proximity to the stator housing. If that fan moved away from the housing, then it would lose effectiveness, static pressure on the outlet would drop, upstream the static low pressure on the inlet would drop, and the thrust force would be reduced. I never considered this a problem because small TEFC motors I've used do not have enough axial play relative to the gap between fan fins and the stator housing. It sounds like yours does. If the fans in your TEFC motors look anything like the small ones (not a safe assumption for me to make) then the fan blades may also be swaying all over the place, adding a *third* mode of displacement, driven by the shaft oscillation.

Take another look at your cooling fans.

STF

RE: Converting from WPII to TEFC Motors

Quote:

In my reading, when the rotor is centered, the stabilizing force is at a *minimum* and the shaft must be forced off-center to develop any magnetic force that will return it to center.
At magnetic center, the magnetic centering force Fmag is at a minimum (0) but the associated effective stiffness is a maximum (stiffness is the slope of the Force vs position curve). I believe this variation in effective stiffness (as position changes) is the reason why the magnetic force provides the most effective stabilization when on-center where effective magnetic stiffness is maximum. That is the one-sentence punchline for all the rambling of my last few posts.

Sorry for monopolizing this thread. Good comments about external fan movement… makes good sense to me fwiw.


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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

(OP)
Thanks to all of you for the very interesting discussion. Much of the electrical details went right over my head. Much of the literature on the subject focusses on axial shutting when the motor is unloaded or uncoupled. Strangely, our motors are perfectly stable when uncoupled and only shuttle when running fully loaded, coupled up to a pump running at full rate. Our shuttling is generally about 1/16" rather than the 1/8" referenced by electricpete. I am going to add a pre-stretch to our alignment practices for these motor as a first attempt to resolve this. I am going to ask for documented magnetic center on new motors established with the cooling fan removed.

There has not been much discussion about vertical thermal growth on TEFC motors. I have always considered vertical growth on motors to be uniform on drive end and non-drive end. For these larger TEFC motor, I don't believe that is appropriate. I will get temperatures on the supports and calculate the thermal growth and make changes to our cold offset alignment targets.

Thanks again,

Johnny Pellin

RE: Converting from WPII to TEFC Motors

Quote:

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.
More curiosity:
In your previous history (aside from the modified motors you descrbed in this post):
Is hunting more likely to occur while coupled/loaded or while uncoupled?
Have you seen it on any motors that don't have axial fans?


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(2B)+(2B)' ?

RE: Converting from WPII to TEFC Motors

(OP)
I am only aware of any axial shuttling on motors running coupled and loaded. We have seen this on WPII motors that I don't think have axial fans. On those motors, we tended to find problems with magnetic center that were more extreme. In one instance that I can think of, the mechanics set the motor on the center of mechanical float. On a later check, we found that mag center was about 1/4" different than mechanical center. When we properly positioned the motor rotor relative to mag center, the shuttling problem went away. In another motor, we changed from a sleeve bearing design to a ball bearing design because we could not get it to stop hunting.

Johnny Pellin

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