Effects of magnetic center forces
Effects of magnetic center forces
(OP)
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.
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.





RE: Effects of magnetic center forces
I assume you mean axial magnetic alignment not radial?
RE: Effects of magnetic center forces
RE: Effects of magnetic center forces
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.
RE: Effects of magnetic center forces
The Tech Note however does not give force values.
RE: Effects of magnetic center forces
RE: Effects of magnetic center forces
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)
RE: Effects of magnetic center forces
http://www.reliabilitydirect.com/alignmentproducts/DAM-...
for: No black magic and hocus-pocus
RE: Effects of magnetic center forces
RE: Effects of magnetic center forces
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.
RE: Effects of magnetic center forces
RE: Effects of magnetic center forces
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?
RE: Effects of magnetic center forces
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.
RE: Effects of magnetic center forces
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
RE: Effects of magnetic center forces
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.
RE: Effects of magnetic center forces
RE: Effects of magnetic center forces
RE: Effects of magnetic center forces
RE: Effects of magnetic center forces
RE: Effects of magnetic center forces
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
RE: Effects of magnetic center forces
http://www.ekc.kth.se/eme/publ/kelvin_dr.html
for: Modelling and Analysis of Magnetic Noise in Squirrel-Cage Induction Motors