Effects of magnetic center forces 1

[Mendit]

Does any body know the answers ? What are forces of magnetic centering of motor rotors on sleeve bearing machines. If the rotor were off center by as much as 1/4" how much force would there be trying to correct this.
We have 8000hp 4160v motor and the DE bearing is fixed position (only .016"axial clearance) The motor has failed the thrust face of the radial bearing (Renk). We are told by the manufacturer that the extruder can not apply any thrust to the motor.

 

[DougMSOE]

Just so we are all on the same page.
I assume you mean axial magnetic alignment not radial?

 

[Mendit]

Yes I do mean axail forces, sorry about nor being clear.

 

[DougMSOE]

Axial forces from my 30+ years of experience do not go above about 150 Lbs, empirical data.
However, the constant thrusting movement against the thrust face will destroy it.
This thrust, axial, is changed by the amount of slip, motor theory and empirical data.
Also, radial thrust, depending on several factors, one the stator/rotor air gap can and has been proven to be tons of force. Several technical papers and an induction motor design book address this.

 

[Mendit]

Thank for you input Doug, Had you ever read the TN-15 from EASA, this describes in detail how axial forcs are generated and also shows how axial forces increase as a result of radial airgaps in the iron. These actualy make the rotor iron pack in to a series of short iron cores and each one would be an axial force generator if they were miss-aligned. Many are miss-aligned on purpose to cut down on electrical noise, but even the miss-aligned are symetrical from end to end of the core.
The Tech Note however does not give force values.

 

[electricpete]

Nailen says the force does not exceed 200 - 300 pounds.

 

[vanstoja]

Presumably reputable calculations done for axial magnetic pull force on a 4-pole, 440 volt induction motor of about 90HP with a 0.158 inch axial offset of magnetic centers yielded a force of 21.2 pounds. For a 0.250 inch offset, a direct ratio gives a force of 33.5 lbs. Believed to be based on Bradford,C.E. & Rhudy,R.G., "Axial Magnetic Forces on Induction Machine Rotors", AIEE Paper 53-124, the force equation is given as:
F=B^2*V*(1/l)*dl/dy/(2*72.13x10^6)
where B=Flux density (lines)
V=Magnetic gap volume=pi*D*l*(g_e/k)
g_e=Magnetic gap*k_e (inches)
k_e=1-((3*g)/(2*l_s))-((l_r-l_s)/(5*l_s))
l,l_r,l_s=maggap,rotor,stator lengths(inches)
dl/dy=-[1-(2/pi)cot^-1(y/g)] for l_r=l_s
For the 440 volt motor:
B=32,300 lines
D=Rotor diameter=9.25 in.
g=0.125 in.
l_s=32 in.
y=0.158 in.


dl/dy=-[1-(2/pi)*ctn^-1*(y/g) for l_r=l_s
y=Axial offset of magnetic centers (inches)

 

[jbartos]

Suggestion: Visit

http://www.reliabilitydirect.com/alignmentproducts/DAM-D525.htm

for: No black magic and hocus-pocus

 

[macmil]

I had a problem several years ago with a 3700 HP mill 225 RPM mill motor that would go out of magnetic centre and trip on High temperature on one of the bearings (presumably the fixed one. After much investigation, we traced it down to a worn coupling between the motor and the mill pinion. It was an internally geared coupling, and despite being only a few years old, the gear teeth on one of the hubs had worn, presumably from lack of lubrication. The wear pattern on each of the teeth was a wedge shape which was enough to generate a thrust, enough to push the motor rotor out of magnetic centre. We changed the coupling and solved the problem.

 

[DougMSOE]

Mendit
To your question.
Yes I have read TN-15 from EASA. It is OK but I prefer to go back to the physics of the problem.
As far as iron noise, there was a design engineer at the old Allis-Chalmers company that designed a very efficient motor that was almost silent, but they were hand built and about 20 HP. Special project.
Don't forget that mechanical alignment will also cause some of the thrust problems.

electricpete, I at one time was employed by the same company as Nailen and did work with him on ocasion.

 

[rlpuck]

I dont know the answer to your question, but will offer this comment to perhaps help solve your problem. The problem is probably not the mag center but probably in the coupling. You never want to couple a motor in such a way that it can contact its thrust, and you use the extruder to hold the motor shaft from too much travel. I would think you would want to couple your motor to you extruder using a limited end float coupling. You would couple the motor is such a way that the coupling does not allow the motor to run against its thurst. The extruder should be rigid enough to hold the force of motor shaft. You will need to know how much float the extruder has, and how much float the motor shaft is allowed. (Float = axial travel). Place the coupling on the shafts of the two devices and coupling them in such a way so that the motor shaft float is limited in travel to say 50 to 100 thousandth short of bumping into the thrust of the bearing. ( in both directions) If you don't limit the shaft travel it will bump back and forth continuously and it will just be a mater of time before it fails.

 

[electricpete]

vonstoja - That is an interesting formula. I have to study it a little more to understand the geometry and flux distrubtion they are assuming.

Doug - Wow. So you have worked with the big man himself. I am impressed. on the subject of motors, it seems like there is no-one who has published more or quoted more than Nailen.

Mendit - I have to say now that I think about the question rlpuck has hit the nail on the head (star for him). The question is not how much force is being applied by magnetic centering force, but how did this force come to be applied on the thurst face of a radial bearing. As rlpuck says that is what needs to be answered.

I don't 100% understand your setup. What is a Renk bearing. Can you describe the bearings on both ends of the motor, and possibly the driven machine. And what type coupling?

 

[Mendit]

Thanks to all on this. Answers to questions, and problem update.
Motor is fitted with Renk DIN standard sleeve bearings, these are now offered as sleeve bearings to almost all large motor manufactures .
The Motor rotor is positioned by the drive end bearing only 0.016 of axial float. This is a special application where the normal 1/2" of float can not be allowed.
The Extruder is extruding plastic feed stock in a chemical plant.
The coupling is like a Holset coupling, a flat disc about 30" dia with a 24 of drive pins approx 1-1/4 diameter, each pin is fitted with a barrel shaped rubber composite bushing approx 3" dia and 4" long. These bushings fit into a similar sized disc with holes to accept the bushings, the holes being 0.005 larger than the bushings.
The extruded gear box has fixed thrust bearings and can not impose load on the motor.
Solving problem
To my way of thinking the load on the motor was increased when product of a lower temperature and viscosity was processed, this required greater torque at low speed (100rpm).The extra toque defromed the coupling and created axial forces. The bearing even though it was forced lube could not form a hydrodynamic oil film due to the low speed, to protect the bearing. Bearing is 180mm diameter and 125mm long.

 

[gsimson]

For large motors with sleeve bearing the axial thrust required to move the shaft is negligible. The shaft is always pushed using a pencil for marking the magnetic centre wrt to the physical centere ( Of stator & rotor).

Another 2 factors normally consudered are the CG of the rotor or the level of the shaft and the coupling gap.

If the coupling is wornout even a little, the thrust point, either labrynth or bearing or coupling, will wearout fast due to the mass of the rotor even if the acceleration is very small.

The standard procedure is
- Check the level of the shaft
- Check and mark the magnetic centre
- Shift the stator or rotor to match physical center and the magnetic centre
- Adjust the coupling gap leaving only the permitted float especially wrt to the thrust point

 

[electricpete]

"Motor is fitted with Renk DIN standard sleeve bearings, these are now offered as sleeve bearings to almost all large motor manufactures .
The Motor rotor is positioned by the drive end bearing only 0.016 of axial float. This is a special application where the normal 1/2" of float can not be allowed."

It does not sound reasonable to expect a radial sleeve bearing to do the normal job of axial positioning. The thrust shoulders are intended only for intermittent contact, not continuous contact.

So to prevent continuous contact you would need impeccable setting of coupling gap dead-on mechanical center 0.008" each side. Then if you add up the total possible movement of driven machine thrust bearing clearance (a few mils at least), thermal expansion, movement of the flex coupling under load etc you must have less then 0.008"

Unless I am misunderstanding something (which is very possible) it just sounds like a bad design with no room for error. Can you explain how there is margin to ensure the radial bearing does not continuously run against it's thrust face.

 

[macmil]

from your description of the coupling, and the very small allowable axial float, I would concentrate my investigation on the coupling, unless you want to alter the fixed bearing. I agree with pete's assessment of the design.

 

[Mendit]

The thrust load design is limit 6688N (1503 Lbf) direct on the bearing.

 

[electricpete]

Is the bearing designed for continuous thrust loading?

 

[Mendit]

Pete,it does look similar to other bearings but has tapered land oil pockets on the face and the forced lube is pumped into the face at the horizontal joint.Check the new technology on BBI web page, there is picture of DIN bearing with Tilt pads.

http://www.babbitt-inc.com/

 

[jbartos]

Suggestion: Visit

http://www.tu.kielce.pl/SME2002/papers.htm

for:
Paper 20:
FRYDRYCHOWICZ-JASTRZEBSKA G., Politechnika Poznanska
One - sided magnetic pull force and centrifugal force in conditions of eccentricity and unbalance of rotor